Biomechanics and Treatment of Dentofacial Deformity










389389
CHAPTER
Patients in need of signicant dentofacial skeletal changes involving surgery, distraction
osteogenesis, and implants require detailed work-ups and treatment planning. The prob-
lem list should be specic so all problems can be addressed. The ideal goal list must be nar-
rowed to the nal practical goals. The biomechanics of leveling in a patient requiring surgi-
cal mandibular advancement can inuence the nal outcome. The decompensation of axial
inclinations necessary to achieve desirable surgical changes is limited by orthodontic tooth
movement considerations. The use of bone plates solves many of these problems, and their
use and biomechanical principles involved are discussed in this chapter. The biomechanics
of combined orthodontic and orthognathic surgery for difcult cases such as open bites,
facial asymmetries, and stable skeletal width expansion are also explored. In addition, con-
siderations for selecting bone or tooth anchors are presented. Surgical patients are not
always stable, and the role of muscle and soft tissue factors must be considered in order to
achieve the best stability. If the surgery fails to obtain the established goals, the solution
may be immediate intervention after surgery; the biomechanical principles and the appli-
ances used in such situations are therefore described in detail in this chapter.
OVERVIEW
“The happiness of a man in this life does not consist in the absence
but in the mastery of his passions.”
— Alfred Lord Tennyson
“I am a part of all that I have met.”
— Alfred Lord Tennyson
Biomechanics
and Treatment
of Dentofacial
Deformity
17
Mithran Goonewardene
Brent Allan

17
Biomechanics and Treatment of Dentofacial Deformity
390
Special considerations in treatment planning are re-
quired for orthodontic patients in need of surgery.
These considerations are usually tailored for the
specic type of treatment. In all patients, however,
a series of diagnostic procedures is conducted to es-
tablish a database from which a problem list can be
outlined. This will usually include the following:
A primary complaint
Medical and dental history with any associated
psychologic issues
Facial and soft tissue problems
Skeletal problems, which can be further subdivid-
ed into:
– Anteroposterior (AP) discrepancies
– Vertical discrepancies
– Transverse discrepancies
Dental problems
After the problem list is reviewed, an essential
component of treatment planning involves priori-
tizing this list to ensure that the clinician can identi-
fy the most important issues. It is essential that this
prioritization process is developed in close consul-
tation with the patient. Often treatment plans are
developed that do not address each problem, and it
should be clear from the outset which issues need to
be addressed ahead of other issues to improve the
likelihood of satisfying the patient’s desires.
Once the prioritized problem list has been estab-
lished, the clinician should develop a series of ideal
goals associated to address each problem. Often it
is impossible to achieve each ideal goal, so several
treatment options are usually explored with mod-
ied goals and relative advantages and disadvan-
tages. This is a complex process that necessitates full
engagement of the patient and wisdom from the
treating practitioner (Fig 17-1).
In combined surgical-orthodontic treatment, it is
necessary to have a series of guidelines to establish
optimal facial form and dental esthetics. It is possible
to reorient the skeletal and dental structures to ideal
positions, and there are many mechanisms that can
assist the practitioner and patient in decision making.
A sequence of processes should be considered:
1. AP projection of the midface
2. Vertical position of the maxillary incisor
3. AP position of the maxillary incisor
4. AP position of the mandible
5. AP position of the mandibular dentition
6. AP and vertical position of the chin
7. Transverse relationship of the maxilla, zygomatic
projections, gonial angles, and chin
8. Maxillary arch width
A number of methods have been employed to
assist the clinician in decision-making related to re-
orientation of the hard and soft tissue structures.
Routine anthropometry and cephalometrics can
be utilized and compared with historical norms.
1–4
These cephalometric analyses can be utilized in
template form, such as the Bolton standards or
Moorrees mesh diagram, and overlaid in two di-
Ideal treatment
goals
• Advantages
• Disadvantages
Costs, etc
• Advantages
• Disadvantages
Costs, etc
• Advantages
• Disadvantages
Costs, etc
• Advantages
• Disadvantages
Costs, etc
Final treatment
plan
Considerations
4
3
Fig 17-1 Algorithm outlin-
ing the process of develop-
ing a treatment plan.

391
Biomechanics and Treatment of Dentofacial Deformity
mensions to appraise facial form and establish
morphologic goals
5
(Fig 17-2). More sophisticated
three-dimensional (3D) facial masks of average faces
can also be overlaid on a photograph of the pa-
tient’s face or skeleton to identify any deviations
6
(Fig 17-3). As with any type of analysis, these masks
can be customized for age, sex, and facial width and
height and can be modied according to numerous
possible parameters.
Once specic treatment goals have been identi-
ed and outlined for a treatment plan, it is essential
for the clinician to progress through a systematic
plan of dental and skeletal movements.
The ultimate goal of treatment for any dentofa-
cial deformity is ideally to place the skeletal struc-
tures (maxilla, mandible, and chin) in the optimal
positions to reect an ideal soft tissue form. For this
to be achieved, it is often necessary to place the
maxillary and mandibular teeth in the ideal position
within their respective jaws in all three dimensions.
Teeth have been demonstrated to compensate in all
three dimensions in patients with aberrant skeletal
jaw base relationships.
7
In Class II skeletal patterns,
the mandibular teeth often compensate forward
and procline while the maxillary teeth may compen-
sate backward and retrocline (Fig 17-4). Converse-
ly, in Class III skeletal patterns, compensations are
characterized by maxillary incisor proclination and
mandibular incisor retroclination. In the transverse-
ly narrow maxilla, the mandibular posterior teeth
Fig 17-2 A Moorrees mesh diagram superimposed over a cephalo-
metric radiograph to act as a guide for planning surgical and dental
movements. The relative proportions of the jaws can be compared
with the norm.
Fig 17-3 Comparison of a patient’s face in three dimensions (a)
with a constructed normalized face (b).
Fig 17-4 Frequently encountered decom-
pensations include the need to retract the
mandibular incisors (a) or procline the max-
illary incisors (b) in patients with Class II
skeletal problems, retract the maxillary and
procline the mandibular incisors in patients
with Class III skeletal problems (c), and in-
trude the incisors in open bite patients (d).
a b
a b
c d

17
Biomechanics and Treatment of Dentofacial Deformity
392
tend to compensate by tipping lingually, while the
maxillary teeth may compensate by tipping buccal-
ly (Fig 17-5). It has been shown that the objective
of removing all compensation in three dimensions
is generally difcult to achieve in most combined
surgical-orthodontic patients due to structural and
biomechanical limitations.
8,9
This invariably results in
a less than ideal relationship of the skeletal struc-
tures, which may lead to slight deviations from
ideal facial form. If the teeth are inadequately de-
compensated, adjunctive surgical procedures such
as genioplasty or maxillary autogenous or synthetic
onlays may be required.
Therefore, the following objectives should be con-
sidered in the treatment-planning stage:
1. Identify a treatment occlusal plane around which
the teeth will be moved in all three planes.
2. Determine the extent of vertical movements of
the anterior and posterior teeth relative to the
occlusal plane.
3. Determine the relative AP tooth movements of
the maxillary and mandibular anterior teeth on
their respective skeletal bases.
4. Establish the optimal vertical and AP positions of
the maxillary and mandibular skeletal bases, in-
cluding reorientation of the occlusal plane.
5. Determine the extent of dental and skeletal
transverse changes to achieve a balanced occlusal
relationship and smile width.
6. Evaluate the nal chin position and possible need
for chin modication in the AP and vertical di-
mensions.
General Considerations in
Orthodontic Mechanotherapy
Combined surgical-orthodontic treatment usually
progresses through the following sequence
10,11
:
1. Presurgical orthodontic treatment
2. Presurgical reassessment and progress records
3. Prediction processes and model surgery/surgical
simulation with preparation of surgical splints
4. Denitive surgery
5. Postsurgical orthodontics
6. Retention
Presurgical Orthodontic
Treatment
In most circumstances, clinicians use contemporary
preadjusted appliances with the dimensions of either
0.018 × 0.025 inch or 0.022 × 0.028 inch. These pre-
adjusted appliances facilitate the objectives of achiev-
ing ideal inclinations of teeth in all three dimensions
within the respective jaws. The second-order and
third-order adjustments will assist in moving the
teeth toward their ideal inclinations, unlike many
routine orthodontic treatments in which the clini-
cian considers compensatory tooth movements to
camouage a skeletal problem.
In general, bands are better selections for at least
the rst and/or second molars; bonded molar attach-
Fig 17-5 Transverse compensations for a narrow maxilla with buc-
cally tipped maxillary posterior teeth and lingually tipped mandib-
ular posterior teeth.

393
Presurgical Orthodontic Treatment
ments may be more likely to become debonded
during surgery when the surgeon is xing the arch
into the surgical splint. In addition to the routine
attachments, palatal sheaths should be considered
routinely on the maxillary bands, and occasionally
mandibular lingual attachments may be considered
to facilitate active expansion or contraction as indi-
cated. The hinge cap attachment is recommended
because it facilitates placement of lingual arches
even when mouth opening may be limited in the im-
mediate postoperative period
12
(see Fig 13-7). Max-
illary horseshoe palatal arches are simple to place
after surgical segmental maxillary expansion, even
with limited opening, to stabilize surgical healing.
The Goshgarian-type lingual attachment can also be
used, but placement of lingual arches, particularly in
the immediate postoperative period, may be more
challenging (Fig 17-6).
The immediate requirements during presurgical
orthodontics usually include:
1. Alignment
2. Leveling
3. Decompensation (removal of any compensations)
in the dentition for the skeletal discrepancy in all
three planes of space
Alignment
Alignment of arches is often efciently achieved us-
ing round, superelastic nickel-titanium (Ni-Ti) arch-
wires, although small multistranded or looped steel
wires may be used based on the clinician’s individ-
ual preference. The clinician may then progress to
rectangular Ni-Ti through to titanium-molybdenum
alloy (TMA) or large steel archwires to initiate arch
form coordination.
13
Although most orthodontists
do not progress beyond a 0.019 × 0.025–inch arch-
wire, in open bite patients it is important to prog-
ress through to full-dimensional archwires such as
0.021 × 0.025–inch TMA wires to ensure that ide-
al axial inclinations or third-order corrections are
achieved before surgery.
10,11
This is an important is-
sue because correction of axial inclinations will of-
ten result in vertical biomechanical side effects such
as extrusion and intrusion. These side effects may be
quite undesirable in the immediate postsurgical pe-
riod if the orthodontist needs to correct signicant
axial inclination deviations. Most orthodontic tooth
movements will inevitably result in some extrusive
side effects, but minimizing these adverse effects
during nishing in patients with previous open bite
malocclusion may be quite challenging.
14
In other
words, patients with open bites should be prepared
to t as closely as possible to ideal occlusion at the
time of surgery. In deep bite patients, on the other
hand, it is possible that these side effects may work
in their favor to improve the occlusal relationships.
As a consequence, it is sometimes possible with
these patients to proceed to surgery at an earlier
stage in the orthodontic treatment.
In patients with problematic occlusal relationships,
temporary anchors may be placed as an adjunct to
control the vertical dimension during nishing pro-
cedures (Fig 17-7).
Leveling
The fundamental issue in developing a biomechan-
ical plan in orthodontics is to establish treatment
goals based on the diagnostic ndings and problem
list. In surgical orthodontics, as in routine nonsurgi-
cal treatment, establishment of a treatment occlusal
plane is the rst stage in planning (Fig 17-8).
The decision to level the curve of Spee will de-
pend on the specic treatment goals.
15
In patients
with deep bite, the geometry of the curve of Spee
is essential to determine the specic nature of tooth
Fig 17-6 Goshgarian-type transpalatal arch
and sheaths.
a
Fig 17-7 Temporary anchors such as bone plates may be placed at the time of jaw surgery
in cases in which extrusive movement may be of concern during postsurgical nishing. The
bone plates are placed lateral to the molars, and extrusive forces may be placed from the
molars to the bone plates.
b

17
Biomechanics and Treatment of Dentofacial Deformity
394
movements (Box 17-1). As with all deep bite correc-
tions, the tooth-to-lip relationships, incisor inclina-
tions, and locations of steps in the occlusal plane
must be evaluated carefully.
On the other hand, open bite patients may pre-
sent with multiple-level occlusal planes, and careful
consideration of the locations of the steps and inci-
sor inclinations are necessary before developing a
biomechanical strategy to level the occlusal plane
(Fig 17-9). Leveling of the occlusal planes may be
considered prior to surgery, at the time of surgery,
and after surgery.
Leveling of the occlusal plane prior to
surgery
In many circumstances, leveling of the occlusal plane
is performed prior to surgery. The orthodontist will
consider the desired tooth movements. An essential
goal of treatment is to prepare the anterior and pos-
terior alveoli to the ideal vertical dimension prior to
surgical procedures to idealize the jaw relationships.
For example, a patient may present with the need to
intrude and are the maxillary incisors as part of a
Class II, division 2 malocclusion, and the signicant
Box 17-1 Leveling options
• Maxillary incisor intrusion
• Mandibular incisor intrusion
• Maxillary incisor proclination
• Mandibular incisor proclination
• Posterior extrusion
Fig 17-8 This patient presented with a Class II skeletal pattern but did not wish to consider a surgical plan. Once the treatment occlusal
plane had been established, tooth movements could be planned related to the occlusal plane. The mandibular incisors would be intruded
(a) prior to maxillary incisor retraction (b) if the vertical dimension was to remain constant. If the patient had selected a surgical plan, again
a treatment occlusal plane would be considered and the mandibular incisor intruded (a) before surgical mandibular advancement (c) if
vertical facial dimensions were to be maintained.
a
Fig 17-9 Open bites may present in many geometries. Steps may be found between the incisors and canines (a) or between the canines
and premolars (b), or the occlusal planes may diverge completely anteriorly (c).
b c
a b c

395
Presurgical Orthodontic Treatment
component of the problem may be the mandibular
retrognathism with minimal vertical problems. The
maxillary incisor positions alone may be the major
contributor to the vertical occlusal problems, and af-
ter these teeth are repositioned to their ideal posi-
tions in the maxilla, all that is needed is a mandibu-
lar advancement without signicant vertical change
(Fig 17-10). The required force system is a simple
anterior force (see Fig 17-10f). In other circumstanc-
es, vertical and AP changes may be required in both
anterior segments as the treatment goals dictate.
a b c d
Fig 17-10 (a to d) A woman with a skeletal Class II malocclusion characterized by a retrognathic mandible and normal facial height. The
maxillary incisors are retroclined and relatively extruded with resultant gingival display on smiling. (e) Leveling is achieved by a simple force
at the brackets (A), which will rotate the anterior teeth counterclockwise, decreasing the vertical projection (B). (f) The required force system
at the bracket (A) with equivalent force system at the center of resistance (B). This may be achieved by placing a straight wire with or with-
out an overlay piggyback wire that can be tied down to the brackets (C). (g and h) The presurgical cephalometric radiograph and intraoral
photograph demonstrate good leveling of the maxillary curve of Spee. (i and j) The postsurgical cephalometric radiograph and intraoral
photograph reveal an occlusion that will be easy to detail and nish. (k to n) Clinical photographs of the nal outcome reveal an excellent
occlusion and improvement in both the prole and smile line.
e f
g
i
h j
k l m n
CBA
BA

17
Biomechanics and Treatment of Dentofacial Deformity
396
Open bite patients remain one of the signicant
challenges in surgical orthodontics. A systematic re-
view demonstrated that nearly half of all patients
who undergo combined surgery and orthodontics
will not retain incisor contact in the long term.
16
This
leaves the clinician with the dilemma of carefully as-
sessing what will inuence the success of treatment.
If it is essentially the maintenance of open bite cor-
rection, there must be careful consideration of the
risks of treatment with the associated gains. If, on
the other hand, signicant facial changes are envis-
aged, such as changes in facial height and gingival
display or changes in prole convexity, then there
may be a number of issues that together will dene
a successful outcome.
The timing of occlusal plane leveling is a con-
troversial issue. Some clinicians recommend that
stepped occlusal planes be leveled prior to surgery
via extrusive movements of the incisors in order to
simplify the surgical procedures, thereby eliminat-
ing more complex and risky segmental surgery
17
(Fig 17-11). Other clinicians have suggested that ex-
trusive movements of the incisors performed prior
to surgery increase the likelihood of postsurgical
orthodontic relapse, with recurrence of the open
bite.
11,18
Leveling of the occlusal plane at the time of
surgery
Signicant steps in the occlusal plane may be pres-
ent in both open bite and deep bite patients. His-
torically, segmental subapical osteotomies were per-
formed to intrude and possibly retract the maxillary
and mandibular anterior segments in patients with
severe anterior open bite or increased horizontal
overlap (also known as overjet).
19
This represented
a time when orthodontic appliances were not rou-
tinely utilized as part of the surgical correction and
when effectiveness of orthodontic mechanotherapy
was more limited. The use of leveling appliances
with or without the use of temporary anchors has
expanded the range of possible tooth movements.
This has reduced the likelihood of these segmen-
tal procedures being universally applied, thereby
reducing their associated morbidities such as root
damage with possible periodontal and pulpal com-
promise.
20,21
As mentioned previously, open bite malocclusions
can present with a variety of occlusal planes, from
at maxillary and mandibular occlusal planes that
diverge anteriorly to complex steps between anteri-
or and posterior teeth that may commonly pre sent
between lateral incisors and canines or between
canines and premolars. Many clinicians believe that
stepped occlusal planes should be retained during
presurgical orthodontics and segmental surgi-
cal procedures utilized to level the occlusal plane.
Moreover, it has been suggested that the open bite
should be worsened prior to surgery to encourage
orthodontic relapse almost as a mechanism for com-
pensating for any postsurgical skeletal changes.
18
However, it has been recommended that clinicians
be somewhat conservative in exaggerating steps,
because the extrusive side effects on the posterior
teeth have been observed to relapse in retention
with posterior teeth moving out of occlusion.
11
Fig 17-11 Options for leveling a stepped
occlusal plane include extrusion of the
maxillary incisors with orthodontic tooth
movement or surgical repositioning of the
posterior teeth superiorly (A), the anterior
teeth inferiorly (B), or both.
Pretreatment
Surgical leveling
Presurgical orthodontic leveling

397
Presurgical Orthodontic Treatment
Leveling of the occlusal plane after surgery
Leveling of the curve of Spee is often considered
as part of the postsurgical orthodontics. In patients
with a skeletal Class II malocclusion with deep bite
and decreased facial dimension, mandibular surgery
will often advance the mandible with a clockwise
rotation of the distal segment of the mandible (Figs
17-12a to 17-12c). This achieves two objectives. First,
a vertical space is opened between the maxillary
and mandibular posterior teeth that can be readily
used to extrude the mandibular posterior teeth (Figs
17-12d to 17-12i) and establish a new vertical di-
mension of the face.
11,22
Second, the clockwise rota-
tion of the distal segment projects what usually is a
relatively prominent chin to assume a more vertical
position. Moreover, this may have the added benet
of unfolding a deep mentolabial sulcus. Overall, the
effect is frequently observed as a softening of the
chin-to-lip curves and a steepening of the mandibu-
lar border (Figs 17-12j and 17-12k).
The presurgical orthodontic goals in short-faced pa-
tients with Class II skeletal problems include leveling
and aligning the maxillary arch to idealize maxillary
dental positions within the face. A large-dimension
rectangular arch such as a 0.019 × 0.025–inch stain-
a b c
Fig 17-12 (a to c) A woman with signicantly increased vertical overlap (also known as overbite) and mandibular retrognathism with re-
duced facial height. (d and e) The presurgical cephalometric radiograph and intraoral photograph reveal the mandibular retrognathism and
decreased facial height with vertical overlap; note that the mandibular curve of Spee remains as in the pretreatment relationship. (f and g)
After mandibular surgery to advance and rotate the mandible clockwise, the posterior teeth are kept out of occlusion. (h and i) Following
surgery, the posterior teeth are extruded using an extrusion arch. The green line indicates the postsurgical position, and the yellow molar
represents the anticipated molar extrusion. (j and k) The postsurgical facial outcome demonstrates a favorable increase in facial height and
softening of the chin projection due to the clockwise rotation of the mandible. The posterior teeth have been extruded into occlusion.
d
f
e
g
h i j k

17
Biomechanics and Treatment of Dentofacial Deformity
398
less steel wire is used in the maxillary arch. The max-
illary incisors will become the focal point for reposi-
tioning the mandibular incisor during mandibular
surgery. During this surgery, the existing curve of
Spee or steps will remain in the mandibular arch
without attempts made to level them. The wire may
be left in segments or as one uniform wire, if de-
sired. Although it is usually unnecessary to progress
to a large rectangular wire, arch forms are better
coordinated if larger wires are used (Fig 17-13). If
the mandibular posterior teeth present in an up-
right relationship, it may be necessary to leave a
small space behind the canines to provide the neces-
sary space for leveling. However, if the posterior
teeth are tipped, space may be gained during up-
righting of these teeth after surgery.
15
Following mandibular repositioning and splint
removal, usually a progress radiograph is taken to
assess the surgical outcome and ensure that the de-
sired position has been realized. The postsurgical
orthodontics may then commence with the use of
an extrusion arch constructed from 0.018 × 0.025–
inch stainless steel. Elastic bands will be used as an
adjunct to assist with extrusion of the mandibular
Fig 17-13 The sequence of leveling the
mandibular curve of Spee after surgery.
(a) The arches are prepared, and the man-
dibular curve of Spee is maintained with a
0.019 × 0.025–inch stainless steel wire.
(b) The mandible is advanced, and the
patient is left to function in the splint. (c)
After 4 weeks, the mandibular archwire
is sectioned, and a 0.017 × 0.025–inch
stainless steel extrusion archwire is placed
in the auxiliary tube and tied to the ante-
rior teeth. (d) With the assistance of elas-
tics to facilitate the extrusive side effects
on the molars, the molars extrude within
a month. (e) A continuous, low-load Ni-Ti
wire is used to realign the arches prior to
the placement of nishing wires to detail
the occlusion. (f to i) Diagrammatic repre-
sentation of the mechanics. (f) The man-
dibular arch remains in two levels prior to
surgery. (g) After surgery, the mandible is
rotated clockwise, and the posterior teeth
are brought into occlusion via an extru-
sion spring after the mandibular archwire
is segmented. The arrow represents the
force on the wire to engage it in the anteri-
or segment. (h) The posterior teeth extrude
with a counterclockwise moment with the
help of a seating elastic. (i) The extrusive
effect on the mandibular molars may tip
them lingually, and a lingual arch may be
required to assist in controlling the trans-
verse dimension.
a b c
d e
f g
h i

399
Presurgical Orthodontic Treatment
posterior teeth into occlusion. This process is usually
very rapid, with the segments aligning within a 6- to
8-week period. Once the posterior segments have
been extruded, a wire with a low load deection
may be used to facilitate alignment, followed by a
rectangular TMA wire as desired to detail and nish
the treatment (see Fig 17-13).
In patients who require signicant extrusion,
transverse side effects may be encountered from the
joint vertical extrusive forces of the extrusive arch
and elastics. The mandibular posterior teeth may tip
lingually, necessitating insertion of a lingual arch
into the hinge cap attachments to control the trans-
verse dimension.
Decompensation
Decompensation of the dentition is an essential com-
ponent of the orthodontics in combined surgical-
orthodontic treatment. Historically, the goal of pre-
surgical orthodontics was to completely remove any
compensations in the dentition that had occurred
secondary to the skeletal discrepancy in all three
dimensions.
7
Philosophically, this meant that the
discrepancy at the dental level represented the un-
derlying skeletal discrepancy. Unfortunately, howev-
er, the limited space acquired from extractions and
auxiliary force-delivery systems such as headgears
and elastics resulted in less than ideal decompensa-
tions in a signicant number of patients.
8,9
This of-
ten resulted in the need for adjunctive procedures
such as genioplasty or onlay grafting in the parana-
sal and midfacial regions in circumstances of inade-
quate decompensation in the mandibular and max-
illary arches, respectively.
Skeletal Class II malocclusions usually have com-
pensations of either maxillary incisor retroclination
or mandibular incisor proclination (or both). Howev-
er, it is possible that patients present with signicant
deviations from the anticipated compensations. De-
compensation of retroclined maxillary incisors usu-
ally requires a force to advance the incisor crowns,
which will undoubtedly change the vertical position
of the incisal edge as the tooth rotates close to the
center of resistance. If further intrusive movements
are required, additional intrusion arch mechanics
may be instituted. Decompensation of proclined
mandibular incisors in skeletal Class II patients is of-
ten challenging because of added complexities of
leveling the mandibular curve of Spee. Mandibular
rst premolars are often removed to create space to
decompensate the mandibular incisors to a relative-
ly normal position within the mandibular skeletal
base. Unfortunately, when the mandible is ultimate-
ly advanced, the maxillary second molars remain un-
opposed and will most likely require extraction (Fig
17-14).
Bone plates for decompensation of Class II
malocclusions
The introduction of temporary anchors to the or-
thodontic armamentarium has extended the range
of tooth movement that can be considered in com-
bined surgical-orthodontic treatment. Dr Junji Suga-
wara from Sendai, Japan, has popularized this tech-
nique with the Sendai skeletal anchorage system.
23
Because many patients require extraction of the
third molars prior to orthognathic surgery, there is
an opportunity to place bilateral bone anchors si-
multaneously, immediately lateral to the mandibu-
lar rst molars (Fig 17-15). The bone plates may be
used as anchors to retract the entire mandibular
arch and level the curve of Spee simultaneously (Fig
17-16).
Fixed appliances are placed prior to placement of
the bone plates to ensure that the surgeon is able to
place the plates lateral to the mandibular rst mo-
lars. It is important to ensure that the bone plates
are readily accessible for cleaning and that the sul-
cus depth is adequate so that there is enough ex-
tension of the plate from the soft tissue. If the plate
is placed too far posteriorly, cleaning is difcult,
and access to the plate may be uncomfortable and
problematic. This failure to maintain excellent oral
hygiene around the bone plates facilitates propa-
gation of bacterial products below the soft tissue
along the bone plates. This may lead to pain, tooth
mobility,
and ultimate failure of the bone plate. Pa-
tients are therefore encouraged to use small inter-
dental brushes around the bone plates with topical
chlorhexidine gel.
Bone plates can also be used in patients who have
previously undergone mandibular extraction. More-
over, treatment times appear to be shorter because
treatment does not require the closure of extraction
sites and the alignment of roots, and the maxillary
second molars do not need to be considered for ex-
traction. This technique progresses toward the lim-
itations of routine orthodontics in achieving ideal
inclinations of teeth prior to orthodontics, as de-
scribed by Potts et al in 2009.
9

17
Biomechanics and Treatment of Dentofacial Deformity
400
a b c
d
Fig 17-14 (a to c) A patient with a skeletal Class II malocclusion who requested an improvement in chin denition. The mandibular teeth
have clearly compensated for the mandibular retrognathism by proclining. (d and e) Decompensation was performed by extracting two
mandibular rst premolars and tipping the mandibular incisors backward. (f and g) The mandible and chin were then advanced. The post-
surgical occlusion required minor detailing to nish in a Class III molar relationship. (h to j) After treatment, an excellent occlusal outcome
was observed, although the maxillary second molars would be extracted because they were no longer nonfunctional. Decompensation of
the mandibular teeth enabled a pleasing change in the facial prole with the addition of an advancement genioplasty.
e f g
h i j
Fig 17-15 Bone plates are
placed buccal to the rst molar.
The design of the plates enables
the force from an elastic chain
to be placed from an extension
arm attached to the archwire to
the bone plate at a point that
approximates the center of re-
sistance of the mandibular arch
(A). This will enable the entire
mandibular arch to be translated
distally (B).

401
Presurgical Orthodontic Treatment
c d e
f g h
l
m n
a b
j k
Fig 17-16 (a and b) This teenage boy presented with a Class II, division 2 malocclusion with mild maxillary retrognathism and moderate
mandibular retrognathism. (c to e) The mandibular teeth were slightly proclined and crowded with an increased curve of Spee. (f to h) Usu-
ally the mandibular premolars would be extracted to create space to upright and level the mandibular arch, but bone plates were placed at
the time of third molar removal to be used as anchors to upright the mandibular arch. (i) The presurgical cephalometric radiograph reveals
good control of the mandibular incisor inclinations and the bone plates in position. The maxillary incisors have also been decompensated.
(j) The maxilla and mandible were both advanced, and a simultaneous advancement genioplasty was performed. (k to n) After treatment,
the facial and dental outcomes were excellent.
i

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389389CHAPTERPatients in need of signicant dentofacial skeletal changes involving surgery, distraction osteogenesis, and implants require detailed work-ups and treatment planning. The prob-lem list should be specic so all problems can be addressed. The ideal goal list must be nar-rowed to the nal practical goals. The biomechanics of leveling in a patient requiring surgi-cal mandibular advancement can inuence the nal outcome. The decompensation of axial inclinations necessary to achieve desirable surgical changes is limited by orthodontic tooth movement considerations. The use of bone plates solves many of these problems, and their use and biomechanical principles involved are discussed in this chapter. The biomechanics of combined orthodontic and orthognathic surgery for difcult cases such as open bites, facial asymmetries, and stable skeletal width expansion are also explored. In addition, con-siderations for selecting bone or tooth anchors are presented. Surgical patients are not always stable, and the role of muscle and soft tissue factors must be considered in order to achieve the best stability. If the surgery fails to obtain the established goals, the solution may be immediate intervention after surgery; the biomechanical principles and the appli-ances used in such situations are therefore described in detail in this chapter.OVERVIEW“The happiness of a man in this life does not consist in the absence but in the mastery of his passions.” — Alfred Lord Tennyson“I am a part of all that I have met.” — Alfred Lord TennysonBiomechanics and Treatment of Dentofacial Deformity 17Mithran GoonewardeneBrent Allan 17Biomechanics and Treatment of Dentofacial Deformity 390Special considerations in treatment planning are re-quired for orthodontic patients in need of surgery. These considerations are usually tailored for the specic type of treatment. In all patients, however, a series of diagnostic procedures is conducted to es-tablish a database from which a problem list can be outlined. This will usually include the following:• A primary complaint• Medical and dental history with any associated psychologic issues• Facial and soft tissue problems• Skeletal problems, which can be further subdivid-ed into:– Anteroposterior (AP) discrepancies– Vertical discrepancies– Transverse discrepancies• Dental problemsAfter the problem list is reviewed, an essential component of treatment planning involves priori-tizing this list to ensure that the clinician can identi-fy the most important issues. It is essential that this prioritization process is developed in close consul-tation with the patient. Often treatment plans are developed that do not address each problem, and it should be clear from the outset which issues need to be addressed ahead of other issues to improve the likelihood of satisfying the patient’s desires. Once the prioritized problem list has been estab-lished, the clinician should develop a series of ideal goals associated to address each problem. Often it is impossible to achieve each ideal goal, so several treatment options are usually explored with mod-ied goals and relative advantages and disadvan-tages. This is a complex process that necessitates full engagement of the patient and wisdom from the treating practitioner (Fig 17-1).In combined surgical-orthodontic treatment, it is necessary to have a series of guidelines to establish optimal facial form and dental esthetics. It is possible to reorient the skeletal and dental structures to ideal positions, and there are many mechanisms that can assist the practitioner and patient in decision making.A sequence of processes should be considered:1. AP projection of the midface2. Vertical position of the maxillary incisor3. AP position of the maxillary incisor4. AP position of the mandible5. AP position of the mandibular dentition6. AP and vertical position of the chin7. Transverse relationship of the maxilla, zygomatic projections, gonial angles, and chin8. Maxillary arch width A number of methods have been employed to assist the clinician in decision-making related to re-orientation of the hard and soft tissue structures. Routine anthropometry and cephalometrics can be utilized and compared with historical norms.1–4 These cephalometric analyses can be utilized in template form, such as the Bolton standards or Moorrees mesh diagram, and overlaid in two di-Ideal treatment goals• Advantages• Disadvantages Costs, etc• Advantages• Disadvantages Costs, etc• Advantages• Disadvantages Costs, etc• Advantages• Disadvantages Costs, etcFinal treatment planConsiderations43Fig 17-1 Algorithm outlin-ing the process of develop-ing a treatment plan. 391Biomechanics and Treatment of Dentofacial Deformity mensions to appraise facial form and establish morphologic goals5 (Fig 17-2). More sophisticated three-dimensional (3D) facial masks of average faces can also be overlaid on a photograph of the pa-tient’s face or skeleton to identify any deviations6 (Fig 17-3). As with any type of analysis, these masks can be customized for age, sex, and facial width and height and can be modied according to numerous possible parameters.Once specic treatment goals have been identi-ed and outlined for a treatment plan, it is essential for the clinician to progress through a systematic plan of dental and skeletal movements. The ultimate goal of treatment for any dentofa-cial deformity is ideally to place the skeletal struc-tures (maxilla, mandible, and chin) in the optimal positions to reect an ideal soft tissue form. For this to be achieved, it is often necessary to place the maxillary and mandibular teeth in the ideal position within their respective jaws in all three dimensions. Teeth have been demonstrated to compensate in all three dimensions in patients with aberrant skeletal jaw base relationships.7 In Class II skeletal patterns, the mandibular teeth often compensate forward and procline while the maxillary teeth may compen-sate backward and retrocline (Fig 17-4). Converse-ly, in Class III skeletal patterns, compensations are characterized by maxillary incisor proclination and mandibular incisor retroclination. In the transverse-ly narrow maxilla, the mandibular posterior teeth Fig 17-2 A Moorrees mesh diagram superimposed over a cephalo-metric radiograph to act as a guide for planning surgical and dental movements. The relative proportions of the jaws can be compared with the norm.Fig 17-3 Comparison of a patient’s face in three dimensions (a) with a constructed normalized face (b).Fig 17-4 Frequently encountered decom-pensations include the need to retract the mandibular incisors (a) or procline the max-illary incisors (b) in patients with Class II skeletal problems, retract the maxillary and procline the mandibular incisors in patients with Class III skeletal problems (c), and in-trude the incisors in open bite patients (d).a ba bc d 17Biomechanics and Treatment of Dentofacial Deformity 392tend to compensate by tipping lingually, while the maxillary teeth may compensate by tipping buccal-ly (Fig 17-5). It has been shown that the objective of removing all compensation in three dimensions is generally difcult to achieve in most combined surgical-orthodontic patients due to structural and biomechanical limitations.8,9 This invariably results in a less than ideal relationship of the skeletal struc-tures, which may lead to slight deviations from ideal facial form. If the teeth are inadequately de-compensated, adjunctive surgical procedures such as genioplasty or maxillary autogenous or synthetic onlays may be required. Therefore, the following objectives should be con-sidered in the treatment-planning stage:1. Identify a treatment occlusal plane around which the teeth will be moved in all three planes.2. Determine the extent of vertical movements of the anterior and posterior teeth relative to the occlusal plane.3. Determine the relative AP tooth movements of the maxillary and mandibular anterior teeth on their respective skeletal bases.4. Establish the optimal vertical and AP positions of the maxillary and mandibular skeletal bases, in-cluding reorientation of the occlusal plane.5. Determine the extent of dental and skeletal transverse changes to achieve a balanced occlusal relationship and smile width.6. Evaluate the nal chin position and possible need for chin modication in the AP and vertical di-mensions.General Considerations in Orthodontic MechanotherapyCombined surgical-orthodontic treatment usually progresses through the following sequence10,11:1. Presurgical orthodontic treatment2. Presurgical reassessment and progress records3. Prediction processes and model surgery/surgical simulation with preparation of surgical splints4. Denitive surgery5. Postsurgical orthodontics6. RetentionPresurgical Orthodontic TreatmentIn most circumstances, clinicians use contemporary preadjusted appliances with the dimensions of either 0.018 × 0.025 inch or 0.022 × 0.028 inch. These pre-adjusted appliances facilitate the objectives of achiev-ing ideal inclinations of teeth in all three dimensions within the respective jaws. The second-order and third-order adjustments will assist in moving the teeth toward their ideal inclinations, unlike many routine orthodontic treatments in which the clini-cian considers compensatory tooth movements to camouage a skeletal problem.In general, bands are better selections for at least the rst and/or second molars; bonded molar attach-Fig 17-5 Transverse compensations for a narrow maxilla with buc-cally tipped maxillary posterior teeth and lingually tipped mandib-ular posterior teeth. 393Presurgical Orthodontic Treatmentments may be more likely to become debonded during surgery when the surgeon is xing the arch into the surgical splint. In addition to the routine attachments, palatal sheaths should be considered routinely on the maxillary bands, and occasionally mandibular lingual attachments may be considered to facilitate active expansion or contraction as indi-cated. The hinge cap attachment is recommended because it facilitates placement of lingual arches even when mouth opening may be limited in the im-mediate postoperative period12 (see Fig 13-7). Max-illary horseshoe palatal arches are simple to place after surgical segmental maxillary expansion, even with limited opening, to stabilize surgical healing. The Goshgarian-type lingual attachment can also be used, but placement of lingual arches, particularly in the immediate postoperative period, may be more challenging (Fig 17-6).The immediate requirements during presurgical orthodontics usually include:1. Alignment 2. Leveling 3. Decompensation (removal of any compensations) in the dentition for the skeletal discrepancy in all three planes of space AlignmentAlignment of arches is often efciently achieved us-ing round, superelastic nickel-titanium (Ni-Ti) arch-wires, although small multistranded or looped steel wires may be used based on the clinician’s individ-ual preference. The clinician may then progress to rectangular Ni-Ti through to titanium-molybdenum alloy (TMA) or large steel archwires to initiate arch form coordination.13 Although most orthodontists do not progress beyond a 0.019 × 0.025–inch arch-wire, in open bite patients it is important to prog-ress through to full-dimensional archwires such as 0.021 × 0.025–inch TMA wires to ensure that ide-al axial inclinations or third-order corrections are achieved before surgery.10,11 This is an important is-sue because correction of axial inclinations will of-ten result in vertical biomechanical side effects such as extrusion and intrusion. These side effects may be quite undesirable in the immediate postsurgical pe-riod if the orthodontist needs to correct signicant axial inclination deviations. Most orthodontic tooth movements will inevitably result in some extrusive side effects, but minimizing these adverse effects during nishing in patients with previous open bite malocclusion may be quite challenging.14 In other words, patients with open bites should be prepared to t as closely as possible to ideal occlusion at the time of surgery. In deep bite patients, on the other hand, it is possible that these side effects may work in their favor to improve the occlusal relationships. As a consequence, it is sometimes possible with these patients to proceed to surgery at an earlier stage in the orthodontic treatment.In patients with problematic occlusal relationships, temporary anchors may be placed as an adjunct to control the vertical dimension during nishing pro-cedures (Fig 17-7).LevelingThe fundamental issue in developing a biomechan-ical plan in orthodontics is to establish treatment goals based on the diagnostic ndings and problem list. In surgical orthodontics, as in routine nonsurgi-cal treatment, establishment of a treatment occlusal plane is the rst stage in planning (Fig 17-8). The decision to level the curve of Spee will de-pend on the specic treatment goals.15 In patients with deep bite, the geometry of the curve of Spee is essential to determine the specic nature of tooth Fig 17-6 Goshgarian-type transpalatal arch and sheaths.aFig 17-7 Temporary anchors such as bone plates may be placed at the time of jaw surgery in cases in which extrusive movement may be of concern during postsurgical nishing. The bone plates are placed lateral to the molars, and extrusive forces may be placed from the molars to the bone plates. b 17Biomechanics and Treatment of Dentofacial Deformity 394movements (Box 17-1). As with all deep bite correc-tions, the tooth-to-lip relationships, incisor inclina-tions, and locations of steps in the occlusal plane must be evaluated carefully. On the other hand, open bite patients may pre-sent with multiple-level occlusal planes, and careful consideration of the locations of the steps and inci-sor inclinations are necessary before developing a biomechanical strategy to level the occlusal plane (Fig 17-9). Leveling of the occlusal planes may be considered prior to surgery, at the time of surgery, and after surgery.Leveling of the occlusal plane prior to surgeryIn many circumstances, leveling of the occlusal plane is performed prior to surgery. The orthodontist will consider the desired tooth movements. An essential goal of treatment is to prepare the anterior and pos-terior alveoli to the ideal vertical dimension prior to surgical procedures to idealize the jaw relationships. For example, a patient may present with the need to intrude and are the maxillary incisors as part of a Class II, division 2 malocclusion, and the signicant Box 17-1 Leveling options• Maxillary incisor intrusion• Mandibular incisor intrusion• Maxillary incisor proclination• Mandibular incisor proclination• Posterior extrusionFig 17-8 This patient presented with a Class II skeletal pattern but did not wish to consider a surgical plan. Once the treatment occlusal plane had been established, tooth movements could be planned related to the occlusal plane. The mandibular incisors would be intruded (a) prior to maxillary incisor retraction (b) if the vertical dimension was to remain constant. If the patient had selected a surgical plan, again a treatment occlusal plane would be considered and the mandibular incisor intruded (a) before surgical mandibular advancement (c) if vertical facial dimensions were to be maintained.aFig 17-9 Open bites may present in many geometries. Steps may be found between the incisors and canines (a) or between the canines and premolars (b), or the occlusal planes may diverge completely anteriorly (c).b ca b c 395Presurgical Orthodontic Treatmentcomponent of the problem may be the mandibular retrognathism with minimal vertical problems. The maxillary incisor positions alone may be the major contributor to the vertical occlusal problems, and af-ter these teeth are repositioned to their ideal posi-tions in the maxilla, all that is needed is a mandibu-lar advancement without signicant vertical change (Fig 17-10). The required force system is a simple anterior force (see Fig 17-10f). In other circumstanc-es, vertical and AP changes may be required in both anterior segments as the treatment goals dictate. a b c dFig 17-10 (a to d) A woman with a skeletal Class II malocclusion characterized by a retrognathic mandible and normal facial height. The maxillary incisors are retroclined and relatively extruded with resultant gingival display on smiling. (e) Leveling is achieved by a simple force at the brackets (A), which will rotate the anterior teeth counterclockwise, decreasing the vertical projection (B). (f) The required force system at the bracket (A) with equivalent force system at the center of resistance (B). This may be achieved by placing a straight wire with or with-out an overlay piggyback wire that can be tied down to the brackets (C). (g and h) The presurgical cephalometric radiograph and intraoral photograph demonstrate good leveling of the maxillary curve of Spee. (i and j) The postsurgical cephalometric radiograph and intraoral photograph reveal an occlusion that will be easy to detail and nish. (k to n) Clinical photographs of the nal outcome reveal an excellent occlusion and improvement in both the prole and smile line.e fgih jk l m nCBABA 17Biomechanics and Treatment of Dentofacial Deformity 396Open bite patients remain one of the signicant challenges in surgical orthodontics. A systematic re-view demonstrated that nearly half of all patients who undergo combined surgery and orthodontics will not retain incisor contact in the long term.16 This leaves the clinician with the dilemma of carefully as-sessing what will inuence the success of treatment. If it is essentially the maintenance of open bite cor-rection, there must be careful consideration of the risks of treatment with the associated gains. If, on the other hand, signicant facial changes are envis-aged, such as changes in facial height and gingival display or changes in prole convexity, then there may be a number of issues that together will dene a successful outcome. The timing of occlusal plane leveling is a con-troversial issue. Some clinicians recommend that stepped occlusal planes be leveled prior to surgery via extrusive movements of the incisors in order to simplify the surgical procedures, thereby eliminat-ing more complex and risky segmental surgery17 (Fig 17-11). Other clinicians have suggested that ex-trusive movements of the incisors performed prior to surgery increase the likelihood of postsurgical orthodontic relapse, with recurrence of the open bite.11,18 Leveling of the occlusal plane at the time of surgerySignicant steps in the occlusal plane may be pres-ent in both open bite and deep bite patients. His-torically, segmental subapical osteotomies were per-formed to intrude and possibly retract the maxillary and mandibular anterior segments in patients with severe anterior open bite or increased horizontal overlap (also known as overjet).19 This represented a time when orthodontic appliances were not rou-tinely utilized as part of the surgical correction and when effectiveness of orthodontic mechanotherapy was more limited. The use of leveling appliances with or without the use of temporary anchors has expanded the range of possible tooth movements. This has reduced the likelihood of these segmen-tal procedures being universally applied, thereby reducing their associated morbidities such as root damage with possible periodontal and pulpal com-promise.20,21As mentioned previously, open bite malocclusions can present with a variety of occlusal planes, from at maxillary and mandibular occlusal planes that diverge anteriorly to complex steps between anteri-or and posterior teeth that may commonly pre sent between lateral incisors and canines or between canines and premolars. Many clinicians believe that stepped occlusal planes should be retained during presurgical orthodontics and segmental surgi-cal procedures utilized to level the occlusal plane. Moreover, it has been suggested that the open bite should be worsened prior to surgery to encourage orthodontic relapse almost as a mechanism for com-pensating for any postsurgical skeletal changes.18 However, it has been recommended that clinicians be somewhat conservative in exaggerating steps, because the extrusive side effects on the posterior teeth have been observed to relapse in retention with posterior teeth moving out of occlusion.11Fig 17-11 Options for leveling a stepped occlusal plane include extrusion of the maxillary incisors with orthodontic tooth movement or surgical repositioning of the posterior teeth superiorly (A), the anterior teeth inferiorly (B), or both. PretreatmentSurgical levelingPresurgical orthodontic leveling 397Presurgical Orthodontic TreatmentLeveling of the occlusal plane after surgeryLeveling of the curve of Spee is often considered as part of the postsurgical orthodontics. In patients with a skeletal Class II malocclusion with deep bite and decreased facial dimension, mandibular surgery will often advance the mandible with a clockwise rotation of the distal segment of the mandible (Figs 17-12a to 17-12c). This achieves two objectives. First, a vertical space is opened between the maxillary and mandibular posterior teeth that can be readily used to extrude the mandibular posterior teeth (Figs 17-12d to 17-12i) and establish a new vertical di-mension of the face.11,22 Second, the clockwise rota-tion of the distal segment projects what usually is a relatively prominent chin to assume a more vertical position. Moreover, this may have the added benet of unfolding a deep mentolabial sulcus. Overall, the effect is frequently observed as a softening of the chin-to-lip curves and a steepening of the mandibu-lar border (Figs 17-12j and 17-12k).The presurgical orthodontic goals in short-faced pa-tients with Class II skeletal problems include leveling and aligning the maxillary arch to idealize maxillary dental positions within the face. A large-dimension rectangular arch such as a 0.019 × 0.025–inch stain-a b cFig 17-12 (a to c) A woman with signicantly increased vertical overlap (also known as overbite) and mandibular retrognathism with re-duced facial height. (d and e) The presurgical cephalometric radiograph and intraoral photograph reveal the mandibular retrognathism and decreased facial height with vertical overlap; note that the mandibular curve of Spee remains as in the pretreatment relationship. (f and g) After mandibular surgery to advance and rotate the mandible clockwise, the posterior teeth are kept out of occlusion. (h and i) Following surgery, the posterior teeth are extruded using an extrusion arch. The green line indicates the postsurgical position, and the yellow molar represents the anticipated molar extrusion. (j and k) The postsurgical facial outcome demonstrates a favorable increase in facial height and softening of the chin projection due to the clockwise rotation of the mandible. The posterior teeth have been extruded into occlusion.dfegh i j k 17Biomechanics and Treatment of Dentofacial Deformity 398less steel wire is used in the maxillary arch. The max-illary incisors will become the focal point for reposi-tioning the mandibular incisor during mandibular surgery. During this surgery, the existing curve of Spee or steps will remain in the mandibular arch without attempts made to level them. The wire may be left in segments or as one uniform wire, if de-sired. Although it is usually unnecessary to progress to a large rectangular wire, arch forms are better coordinated if larger wires are used (Fig 17-13). If the mandibular posterior teeth present in an up-right relationship, it may be necessary to leave a small space behind the canines to provide the neces-sary space for leveling. However, if the posterior teeth are tipped, space may be gained during up-righting of these teeth after surgery.15Following mandibular repositioning and splint removal, usually a progress radiograph is taken to assess the surgical outcome and ensure that the de-sired position has been realized. The postsurgical orthodontics may then commence with the use of an extrusion arch constructed from 0.018 × 0.025–inch stainless steel. Elastic bands will be used as an adjunct to assist with extrusion of the mandibular Fig 17-13 The sequence of leveling the mandibular curve of Spee after surgery. (a) The arches are prepared, and the man-dibular curve of Spee is maintained with a 0.019 × 0.025–inch stainless steel wire. (b) The mandible is advanced, and the patient is left to function in the splint. (c) After 4 weeks, the mandibular archwire is sectioned, and a 0.017 × 0.025–inch stainless steel extrusion archwire is placed in the auxiliary tube and tied to the ante-rior teeth. (d) With the assistance of elas-tics to facilitate the extrusive side effects on the molars, the molars extrude within a month. (e) A continuous, low-load Ni-Ti wire is used to realign the arches prior to the placement of nishing wires to detail the occlusion. (f to i) Diagrammatic repre-sentation of the mechanics. (f) The man-dibular arch remains in two levels prior to surgery. (g) After surgery, the mandible is rotated clockwise, and the posterior teeth are brought into occlusion via an extru-sion spring after the mandibular archwire is segmented. The arrow represents the force on the wire to engage it in the anteri-or segment. (h) The posterior teeth extrude with a counterclockwise moment with the help of a seating elastic. (i) The extrusive effect on the mandibular molars may tip them lingually, and a lingual arch may be required to assist in controlling the trans-verse dimension.a b cd ef gh i 399Presurgical Orthodontic Treatmentposterior teeth into occlusion. This process is usually very rapid, with the segments aligning within a 6- to 8-week period. Once the posterior segments have been extruded, a wire with a low load deection may be used to facilitate alignment, followed by a rectangular TMA wire as desired to detail and nish the treatment (see Fig 17-13).In patients who require signicant extrusion, transverse side effects may be encountered from the joint vertical extrusive forces of the extrusive arch and elastics. The mandibular posterior teeth may tip lingually, necessitating insertion of a lingual arch into the hinge cap attachments to control the trans-verse dimension. DecompensationDecompensation of the dentition is an essential com-ponent of the orthodontics in combined surgical- orthodontic treatment. Historically, the goal of pre-surgical orthodontics was to completely remove any compensations in the dentition that had occurred secondary to the skeletal discrepancy in all three dimensions.7 Philosophically, this meant that the discrepancy at the dental level represented the un-derlying skeletal discrepancy. Unfortunately, howev-er, the limited space acquired from extractions and auxiliary force-delivery systems such as headgears and elastics resulted in less than ideal decompensa-tions in a signicant number of patients.8,9 This of-ten resulted in the need for adjunctive procedures such as genioplasty or onlay grafting in the parana-sal and midfacial regions in circumstances of inade-quate decompensation in the mandibular and max-illary arches, respectively.Skeletal Class II malocclusions usually have com-pensations of either maxillary incisor retroclination or mandibular incisor proclination (or both). Howev-er, it is possible that patients present with signicant deviations from the anticipated compensations. De-compensation of retroclined maxillary incisors usu-ally requires a force to advance the incisor crowns, which will undoubtedly change the vertical position of the incisal edge as the tooth rotates close to the center of resistance. If further intrusive movements are required, additional intrusion arch mechanics may be instituted. Decompensation of proclined mandibular incisors in skeletal Class II patients is of-ten challenging because of added complexities of leveling the mandibular curve of Spee. Mandibular rst premolars are often removed to create space to decompensate the mandibular incisors to a relative-ly normal position within the mandibular skeletal base. Unfortunately, when the mandible is ultimate-ly advanced, the maxillary second molars remain un-opposed and will most likely require extraction (Fig 17-14).Bone plates for decompensation of Class II malocclusionsThe introduction of temporary anchors to the or-thodontic armamentarium has extended the range of tooth movement that can be considered in com-bined surgical-orthodontic treatment. Dr Junji Suga-wara from Sendai, Japan, has popularized this tech-nique with the Sendai skeletal anchorage system.23 Because many patients require extraction of the third molars prior to orthognathic surgery, there is an opportunity to place bilateral bone anchors si-multaneously, immediately lateral to the mandibu-lar rst molars (Fig 17-15). The bone plates may be used as anchors to retract the entire mandibular arch and level the curve of Spee simultaneously (Fig 17-16).Fixed appliances are placed prior to placement of the bone plates to ensure that the surgeon is able to place the plates lateral to the mandibular rst mo-lars. It is important to ensure that the bone plates are readily accessible for cleaning and that the sul-cus depth is adequate so that there is enough ex-tension of the plate from the soft tissue. If the plate is placed too far posteriorly, cleaning is difcult, and access to the plate may be uncomfortable and problematic. This failure to maintain excellent oral hygiene around the bone plates facilitates propa-gation of bacterial products below the soft tissue along the bone plates. This may lead to pain, tooth mobility, and ultimate failure of the bone plate. Pa-tients are therefore encouraged to use small inter-dental brushes around the bone plates with topical chlorhexidine gel.Bone plates can also be used in patients who have previously undergone mandibular extraction. More-over, treatment times appear to be shorter because treatment does not require the closure of extraction sites and the alignment of roots, and the maxillary second molars do not need to be considered for ex-traction. This technique progresses toward the lim-itations of routine orthodontics in achieving ideal inclinations of teeth prior to orthodontics, as de-scribed by Potts et al in 2009.9 17Biomechanics and Treatment of Dentofacial Deformity 400a b cdFig 17-14 (a to c) A patient with a skeletal Class II malocclusion who requested an improvement in chin denition. The mandibular teeth have clearly compensated for the mandibular retrognathism by proclining. (d and e) Decompensation was performed by extracting two mandibular rst premolars and tipping the mandibular incisors backward. (f and g) The mandible and chin were then advanced. The post-surgical occlusion required minor detailing to nish in a Class III molar relationship. (h to j) After treatment, an excellent occlusal outcome was observed, although the maxillary second molars would be extracted because they were no longer nonfunctional. Decompensation of the mandibular teeth enabled a pleasing change in the facial prole with the addition of an advancement genioplasty.e f gh i jFig 17-15 Bone plates are placed buccal to the rst molar. The design of the plates enables the force from an elastic chain to be placed from an extension arm attached to the archwire to the bone plate at a point that approximates the center of re-sistance of the mandibular arch (A). This will enable the entire mandibular arch to be translated distally (B). 401Presurgical Orthodontic Treatmentc d ef g hlm na bj kFig 17-16 (a and b) This teenage boy presented with a Class II, division 2 malocclusion with mild maxillary retrognathism and moderate mandibular retrognathism. (c to e) The mandibular teeth were slightly proclined and crowded with an increased curve of Spee. (f to h) Usu-ally the mandibular premolars would be extracted to create space to upright and level the mandibular arch, but bone plates were placed at the time of third molar removal to be used as anchors to upright the mandibular arch. (i) The presurgical cephalometric radiograph reveals good control of the mandibular incisor inclinations and the bone plates in position. The maxillary incisors have also been decompensated. (j) The maxilla and mandible were both advanced, and a simultaneous advancement genioplasty was performed. (k to n) After treatment, the facial and dental outcomes were excellent.i 17Biomechanics and Treatment of Dentofacial Deformity 402c d ea bf gFig 17-17 (a to g) This young adult pre-sented with a Class III malocclusion with signicant crowding in the maxillary arch and a Class III skeletal relationship charac-terized by maxillary retrognathism. Histor-ically this problem may have been treated with extraction of the maxillary rst pre-molars to create space for both decom-pensation and alignment of the maxillary arch. The need for extraction of the third molars provided an opportunity to place bone plates in the maxilla to facilitate space creation. (h) The cephalometric ra-diograph reveals the true skeletal maxillary retrognathism after the bone plates were placed buccal to the maxillary rst molars. (i) A 0.019 × 0.025–inch TMA segment was placed in the maxillary posterior teeth with an extension arm to deliver a force as close to the center of resistance as possi-ble to move the posterior teeth backward with possible translatory tooth movement. (j and k) The mandibular arch was bonded, but no attempt was made to engage the canines until space was created.h ij k 403Presurgical Orthodontic TreatmentBone plates for decompensation of Class III malocclusionsIt is also possible to consider bone plates to simulta-neously create space for alignment and retraction of irregular maxillary teeth in patients who would his-torically be treated with extraction of maxillary pre-molars only. As with the Class II patients, extraction of third molars in Class III patients offers an oppor-tunity to place bone plates lateral to the maxillary rst molars. The bone plate will extend into the zy-gomatic buttress region, and similar considerations related to hygiene access and sulcus depth must be addressed.Buccal segments with extension arms are placed on the posterior teeth. It is important to extend arms above the occlusal plane to direct the force as close as possible to the center of resistance of the posterior tooth segments. This will minimize tipping and facilitate translatory tooth movements. The bone plates offer a quick solution to creating space for alignment and retraction and are only removed at the time of denitive surgery (Fig 17-17).Fig 17-17 (cont) (l to o) The maxilla was advanced after space was gained. Align-ment and detailing were then performed. (p to v) After treatment, the facial and occlusal outcomes reveal an improved midface projection and a normal occlusal relationship. mlqn ot u vp r s 17Biomechanics and Treatment of Dentofacial Deformity 404Bone plates can also be utilized to assist in decom-pensating mandibular teeth by facilitating mandib-ular arch protraction. Again, placement of exten-sion arms is necessary to direct the forces on the posterior teeth as close to the center of resistance as possible. This enables the mandibular spaces to be closed while protraction of the mandibular arch occurs. The mandibular incisors may be placed in the ideal position prior to denitive surgical planning.Surgical Correction of Skeletal Discrepancy Followed by Dental CompensationThe development of sophisticated imaging and sur-gical planning software has opened the door for new approaches to managing dentofacial deformities. In some specic types of malocclusions such as Class III and open bites, it may be possible to carefully ana-lyze and determine the ideal position of the skeletal bases. The bony maxilla or mandible may be placed in an ideal position in three planes without any at-tempt to decompensate the respective teeth.24 Bone plates are placed at the same time as the denitive orthognathic surgery and are used to compensate the teeth after a period of initial healing.Complex tooth movements can also be consid-ered, which may include intrusive changes in the posterior dentition as well as retraction and protrac-tion of targeted arches. Maxillomandibular elastics can also be incorporated into the mechanical plan. The principle of regional acceleratory phenomena (RAP),25 by which metabolic activity in bone is sig-nicantly enhanced, contributes to acceleration of the compensatory tooth movements, signicantly reducing treatment time.24 Moreover, patients can undergo surgery at the beginning of treatment or early in the orthodontics phase and do not have to suffer through the progressively worsening esthetic issues related to fully decompensating the dentition (Fig 17-18).ca bd eFig 17-18 (a and b) This patient presented with a Class III malocclusion on a Class III skeletal base characterized by moderate maxillary ret-rognathism and mandibular prognathism. (c to e) The maxillary and mandibular teeth had compensated by maxillary incisor proclination and mandibular incisor retroclination. Surgical maxillary advancement was considered in isolation because of the issues related to mandibular surgery, and surgery was planned to reposition the maxilla into an ideal position without decompensating the teeth, instead using bone plates to compensate the teeth after surgery. The goals of treatment were primarily related to positioning the skeletal maxilla and mandible ideally relative to one another. Bone plates would be placed simultaneously and would be used to move the maxillary and mandibular teeth to their ideal inclinations. 405Surgical Correction of Skeletal Discrepancy Followed by Dental Compensationf g hi j kl m nqo pr sFig 17-18 (cont) (f to h) Prior to surgery, no attempt was made to align the maxillary canines because this would further procline the maxillary teeth. While Ni-Ti wires avoided these teeth, the mandibular curve of Spee was leveled by mandibular incisor proclination, as this was part of the treatment goal. (i to k) Because there was still time before the surgery was scheduled, a passive, rectangular stainless steel wire was placed in the maxillary arch, and a 0.019 × 0.025–inch stainless steel wire was placed in the mandibular arch. (l to n) Whereas the pretreatment cephalometric radiograph (l) shows the compensations in the dentition with proclined maxillary and retroclined mandibular incisors, the immediate postsurgical cephalometric radiograph (m) shows the maxillary dentition related in a Class II relationship and bone plates placed in both arches. (n) The maxillary arch has been retracted to reduce the projection of the maxillary dentition with bone plate anchorage. (o) The presurgical facial prole reveals the signicant midface deciency. (p) The immediate postsurgical photograph shows a signicantly protrusive upper lip because the teeth have been set into a signicant Class II relationship (q to s). 17Biomechanics and Treatment of Dentofacial Deformity 406Class III malocclusions with signicant open bites may now be treated by placement of appliances and surgical repositioning of the jaw(s) to an ideal ge-ometry. Careful consideration must be given to the impact of closure of the vertical dimension when any intrusive movements have been calculated. The observed speed of treatment with xed appliances following the surgery has made this an appealing process to consider (Fig 17-19).In some patients, third molar extractions must be performed at least 6 months prior to denitive jaw surgery, and these extractions provide an opportu-nity for placement of bone plates to commence de-compensation of the dentition. Following denitive surgery, if decompensation is incomplete, the bone plates may remain and be utilized for the postsurgi-cal period, taking advantage of the RAP.tuvwxyz aabb cc ddFig 17-18 (cont) (t to v) Initial traction commenced with an elastic chain from the bone plates to the canines with simultaneous Class II elastics. The occlusion corrected rapidly, and slight seating elastics then attempted to improve the intercuspation. (w to y) As the occlusion settled, an intrusive spring attached to the bone plate on the maxillary right side was used to level a slight occlusal plane cant. (z to dd) After treatment, the facial outcome included a signicantly improved midface projection, and the occlusal relationships at debanding were excellent. 407Surgical Correction of Skeletal Discrepancy Followed by Dental Compensationa bfgc d ehijFig 17-19 (a to e) This patient presented with a Class III malocclusion on a Class III skeletal base characterized by moderate maxillary ret-rognathism and mandibular prognathism but the additional complication of increased lower facial height with an anterior open bite. (f) The maxillary teeth were signicantly crowded, and the maxilla was skeletally narrow. (g) The sequence of treatment and goals included initially placing appliances on the teeth in a passive relationship (A). The maxilla would then be placed in a signicant Class II relationship. The advancement of the maxilla would consider the anticipated autorotation following intrusion and retraction of the maxillary posterior teeth. Bone plates would be placed lateral to the maxillary molars, and vertically directed forces would be applied to the posterior teeth from the bone plates. Simultaneously, elastics would be applied to retract the maxillary arch (B). When the open bite had resolved, retrac-tion of the maxillary arch would be refocused by applying a force through the center of resistance by attaching an extension arm to a rigid rectangular wire (C). (h to j) Fixed appliances and rectangular 0.018 × 0.025–inch stainless steel wires were placed just before surgery, and during surgery the maxilla was repositioned into a signicant Class II relationship. The patient functioned in the surgical splint for at least 2 to 3 weeks before active orthodontic tooth movement was initiated. After 2 to 3 weeks, retraction of the maxillary canines began with the use of bone plates as anchors. 17Biomechanics and Treatment of Dentofacial Deformity 408k l mn o pq r s tFig 17-19 (cont) (k to m) Intrusive forces were added to the posterior teeth via vertically directed forces applied to the posterior teeth. (n to p) When the open bite had closed, extension arms were added to 0.019 × 0.025–inch rectangular archwires to facilitate bodily retraction of both arches. (q and r) Pretreatment (q) and immediately presurgical (r) cephalometric radiographs reveal the Class III open bite relationship. (s) Immediate postsurgical cephalometric radiograph shows a Class II open bite relationship with the increased vertical dimension. (t) Closure of the open bite and reduction in facial height as well as Class II correction are achieved rapidly by using the bone plates in the maxillary and mandibular arches. (u to z) After treatment. Signicant improvement in facial form and occlusion have been achieved in less than 12 months of treatment. u v wx y z 409Special Considerations in Managing Class III ProblemsSpecial Considerations in Managing Class III ProblemsClass III problems can present with either maxil-lary retrognathism or mandibular prognathism (or both), with over half of the white population pre-senting with some degree of maxillary retrogna-thism.26 It is important to appreciate that there has been a historical trend to treat the majority of Class III patients with maxillary surgery either in isolation or combined with mandibular setback, even in the presence of mandibular retrognathism.27 There are some very important issues to consider when plan-ning surgical treatment for Class III patients (Table 17-1).Growth and the Class III patientThe cephalocaudal gradient of growth describes the fact that mandibular growth will probably contin-ue for longer periods even when maxillary growth has diminished. This is particularly signicant in male patients. During this growth, it is important for the orthodontist to monitor the magnitude and direction of growth, because signicant deleterious dental compensations may occur. This is particular-ly relevant for mandibular incisor retroclination, which in severe cases can inuence the dimension of the alveolar process above the chin, making fu-ture decompensation problematic because the AP bony dimension of the chin may be too thin. Serial superimpositions are recommended to determine if unfavorable inclinations occur; if so, a xed lingual arch is recommended to maintain the mandibular incisor position and arch length (Fig 17-20). Annu-al serial cephalometric superimpositions should be evaluated to determine when growth has ceased; only then should denitive surgery be considered.28It is also important to recognize that many Class III patients’ mandibles grow asymmetrically, and the chin is often deviated to the left.29 This is important to appreciate because if clinicians do not want to operate on the mandible for some of the aforemen-tioned reasons, it may be necessary to compensate Table 17-1 Issues to consider when planning surgery for Class III patientsMaxillary advancement Mandibular setbackSetback More stable Less stableNerve effectsMild temporary effects: Nasopalatine resection and numbnessPossible inferior dental injuryBleeding Signicant MinimalTMJ effects Minimal Condylar resorption (5% to 10% of the time)Facial esthetics Facelift effect Possible redundant tissueNasal esthetics Possible secondary nasal effects NoneAirway Possible enhanced airow Possible diminished airowTMJ, temporomandibular joint.Fig 17-20 (a) In growing Class III pa-tients, often the mandibular incisors compensate by retroclining signicant-ly, which affects the way that the chin and alveoli remodel. (b) A lingual arch is indicated in patients who exhibit this growth change to maintain arch length. a b 17Biomechanics and Treatment of Dentofacial Deformity 410the dentition in the mandibular or maxillary arch to achieve a satisfactory occlusion. It is possible to mod-ify the maxillary midline by 1 to 2 mm without ap-preciable deviations, but temporary anchors should be considered to address mandibular midlines if the midline deviation is greater than 2 mm and the cli-nician wishes to avoid operating on the mandible.It has been reported that early surgery on the mandible can be considered if signicant psycholog-ic impact is a concern. The likelihood of repeat sur-gery is high, but supplementation of a minor con-dylectomy or condylar shave has been suggested to prevent further overgrowth if required.30 Orthodontic mechanotherapyIt is important to take a set of progress models just prior to surgery to assess the anticipated occlusal relationships. At initial banding, the mandibular brackets must be sufciently low on the labial of the teeth to enable adequate vertical overlap to be achieved at the time of surgery. In addition, it is a signicant advantage to proceed to surgery with a mild mandibular Bolton discrepancy. This will ensure that incisor overlap is encouraged, particularly in the event of any minor postsurgical occlusal changes. The combination of bracket positions and mandib-ular Bolton discrepancy will act as a type of escape valve in the event of unexpected changes. Slight in-terproximal enamel reduction may be considered in the mandibular arch to achieve the desired Bolton discrepancy. Some clinicians like to open spaces dis-tal to the maxillary lateral incisors to achieve the de-sired outcome, but these spaces may be difcult to close in some circumstances.Biomechanics of Class III surgical correctionThere are reported signicant changes in muscular force after correction of the Class III relationship. These changes have been compared to changes ob-served in long-faced patients when facial heights are adjusted toward normal dimensions. When the maxilla and mandible are repositioned, even to ef-fect reduction in mechanical advantage according to lever principles, it appears that the relatively larger mandibles are accompanied by larger muscle masses; therefore, geometry alone does not explain changes in possible masticatory efciency.31Stability of mandibular setbackThe stability of mandibular surgical setback is gener-ally unpredictable, and special caution must be tak-en by the surgeon to ensure that the proximal seg-ment is not rotated back during xation, although this is difcult to achieve on a routine basis.32,33Special Considerations in Managing Open Bites and Long-Face ProblemsGrowth and long-face problemsYoung children with long faces are often identied early in their preadolescent years. These children continue to develop in unique ways, often express-ing signicant vertical growth changes in the max-illa and mandible. The etiology of these problems has been the subject of controversy, with theories of form and function interactions such as diminished muscle force, aberrant obstructive respiration, and nonnutritive oral habits. Unlike children with nor-mal facial height, growing children with long fac-es do not exhibit the increased ability to deliver larger forces through their teeth as they progress through puberty.34 However, facial height changes appear to be well established before the magnitude of muscle forces diverge. Increased nasal airway re-sistance has been shown to be associated with long-faced children, and changes in airway dimensions by maxillary expansion or adenoidectomy have been shown to have short-term effects on facial growth direction.35,36 Very little high-level evidence suggests a signicant role of aberrant function on facial growth changes.The role of genetics in determining orofacial form is complex, and efforts are being focused on attempting to comprehend facial growth changes from the underlying genome.37 No denitive con-clusions have been forthcoming, and the clinician is still confronted with the patient who presents with minimal vertical overlap and a slightly long face in preadolescence. The treatment usually progresses to a satisfactory conclusion, yet in the postadolescent period, signicant vertical growth continues until late adulthood, often resulting in maxillary vertical excess and anterior open bite. These growth chang-es do not seem to be consistent, and various com-ponents of the face may contribute in a range of magnitudes to the nal outcome.38 411Special Considerations in Managing Open Bites and Long-Face ProblemsThe implications for the orthodontist are signi-cant in that treatment, which may include maxillary and mandibular surgery, should be delayed until growth has ceased. Similar to the strategy present-ed for Class III patients, annual serial cephalometric superimpositions should be evaluated for stability before denitive surgical planning is undertaken.As mentioned previously, the reduced occlusal forces dictate that postsurgical orthodontics should be minimized in both magnitude of tooth move-ments and treatment time to minimize any extru-sive side effects, which inevitably accompany most orthodontic tooth movement.14Orthodontic mechanotherapy and sequencingPatients must be prepared carefully for surgery, and all alignment and leveling should be complete be-fore proceeding to surgery. The only exception is open bite patients who exhibit multileveled occlusal planes, when surgical leveling of the occlusal plane is planned (Fig 17-21). From the orthodontist's per-spective, all third-order corrections, marginal ridge discrepancies, and root second-order corrections must be achieved before surgery. Clinicians should progress through to large-dimension rectangular wires such as 0.021 × 0.025–inch TMA wires in both arches to achieve full expression of the preadjusted appliance.It is also important to ensure that when progress models are articulated, either in one piece or seg-mented as required, that the models “snap” togeth-er at the time of surgery (Box 17-2). It is possible that the surgeon and orthodontist will prescribe spot grinding to achieve this goal. Placement of the man-dibular brackets must provide sufcient clearance for the maxillary incisors so they are not locked into a complete vertical overlap. Moreover, it is import-ant to create a mild mandibular Bolton deciency by either interproximal reduction or opening spaces distal to the maxillary lateral incisors (Fig 17-22).Stability of surgical management in long-faced patientsIt is extremely difcult to guarantee stability of open bite treatment in children because of the un-certainties associated with postadolescent growth. Children with a history of nonnutritive sucking hab-its appear to enjoy success if the habit is ceased in a timely manner.Surgical correction of long-faced patients has re-sulted in a demonstrable increase in bite force mag-nitude.39–41 The theory for this outcome has been attributed to biomechanical changes in the mechan-ical advantage afforded by the reoriented muscles of mastication42 (Fig 17-23). Care must be taken in attempting to model complex biomechanical sys-tems, which include a multitude of tissue types with complex articulations in the craniofacial regions that cannot be interpreted like tooth movement. The changes in bite force for other types of skeletal corrections are not so predictable from biomechan-ical models with patients demonstrating similar, increased, and decreased bite force following man-dibular advancements and setbacks and maxillary inferior repositioning. In summary, it appears that generally it is difcult to determine the outcome of orthognathic surgery on bite force. Moreover, there is some weak evidence that an increase in the num-ber of occlusal contacts and bite force magnitude may contribute to enhanced stability of orthodontic treatment.43 Stability of open bite treatment in adolescents or adults with surgical and nonsurgical treatment modes has been the subject of systematic reviews. It appears that both modes of treatment are equal-ly effective (or ineffective) if success is measured by attainment of a complete vertical overlap.16 Signi-cant changes in the vertical overlap are observed in both groups, and caution must be taken in offering patients a guarantee of success unless other factors such as change in gingival display during smiling, lip incompetency, and improvement in the soft tissue facial prole are added into the equation. 17Biomechanics and Treatment of Dentofacial Deformity 412a bc d ef g hj kl m niFig 17-21 (a to e) This adult patient presented with an anterior open bite with normal tooth display on smiling. A surgical correction was planned to segment the maxilla and intrude the posterior teeth. The maxilla would be advanced slightly to address the issues associated with the mandible rotating forward. (f to i) The maxillary arch was aligned in two levels, with care taken to open spaces distal to the lateral incisors and prepare sufcient space between the roots for the osteotomy, minimizing the likelihood of root damage. The Bolton discrep-ancy was carefully checked, and the mandibular anterior brackets provided adequate room for the teeth to overlap and establish a positive vertical overlap at the time of surgery. (j to n) After treatment, the nal occlusion and esthetic outcome were pleasing. 413Special Considerations in Managing Open Bites and Long-Face ProblemsBox 17-2 Progress check for presurgical models in open bite patients1. Third-order corrections complete2. Marginal ridges level3. Root angulations in second order complete4. Mandibular brackets clear of maxillary incisor incisal edges5. Mandibular Bolton deciency• Interproximal reduction• Open spaces distal to the maxillary lateral incisorsFig 17-22 It is critical to take progress records before scheduling surgery. (a to c) In this case, progress models reveal that a Class I canine relationship cannot be achieved without an edge-to-edge occlusion. A decision has to be made to open up space for larger lateral incisors or to perform interproximal reduction on the mandibular anterior teeth. It is important for the arches of open bite patients to t precisely at the time of surgery to minimize postsurgical orthodontics. a b cFig 17-23 The mathematic modeling components considered by Throckmorton et al,42 who attempted to predict changes in muscle force. Condyle(fulcrum)TemporalisMasseterBite forceTemporalis MasseterBite forceCondyle(fulcrum) 17Biomechanics and Treatment of Dentofacial Deformity 414Special Considerations in Managing Skeletal AsymmetryThe management of skeletal asymmetry is complex, and a thorough discussion of diagnosis and treat-ment is beyond the scope of this chapter. Following is a brief overview of the important considerations regarding skeletal asymmetry.Skeletal asymmetry is present, to some extent, in all individuals. There appears to be a trend toward a signicant directional asymmetry in the craniofa-cial region, with most adults exhibiting right-sided dominance or chin deviation to the left.29,44,45 These ndings have been reported to be age and sex spe-cic, with the average younger child exhibiting left- sided dominance in prepubertal years, equivalence at or around puberty, and right-sided dominance in adulthood.44,45 Janson et al46 have suggested that variation in size of the right and left hemimandibles of more than 3 to 4 mm may be considered to be of signicance and may require further follow-up.The etiology of facial asymmetry is complex and may include:• Functional shifts of the mandible associated with occlusal interferences• Craniofacial anomalies such as hemifacial micro-somia and plagiocephaly• Trauma to the condyle and altered function• Condylar hyperplasia• Hemimandibular hyperplasia• Degenerative conditions of the joints such as rheu-matoid arthritis and other autoimmune diseases• Ankylosis of the temporomandibular joint sec-ondary to infection• Hemifacial atrophy disordersContemporary imaging techniques are now avail-able to add signicant value to the diagnosis and management of facial asymmetry. The important issues to comprehend are consistent with other den-tofacial deformities except that the interactions be-tween hard and soft tissues are even more complex in three dimensions. The goals of presurgical ortho-dontics are to remove dental compensations in all three dimensions followed by surgical treatment to idealize the relationships of the skeletal structures (Fig 17-24). Signicant effort has been directed to development of software to assist with planning and facilitating surgical procedures (Fig 17-25).Fig 17-24 Asymmetries present complex-ities in compensations that are best en-visioned by 3D imaging techniques. Cor-rection of these complex compensations is essential to maximize the impact of the skeletal correction. Fig 17-25 The surgical movement of lev-eling and advancing the maxilla and rotat-ing and advancing the mandible can be planned out on the computer, and splints can be constructed to assist in positioning during surgery. (a) Asymmetry before sur-gery. (b) After surgery.a b 415Special Considerations in Managing Skeletal AsymmetryMaxillary expansionMaxillary arch expansion has been performed in children for more than a century. Although doubt was originally cast over the suggestion that Dr E. H. Angle had succeeded in separating the maxilla along the midpalatal suture, it has become a rou-tine procedure to manage crowding and posterior reverse articulation (also known as crossbite) rela-tionships.It is important to recognize that like AP skeletal problems, transverse maxillary deciency is also ac-companied by transverse dental compensations. The mandibular teeth tend to tip lingually and the max-illary teeth buccally to a variable degree. Skeletal expansion is usually achieved in the child by separating the midpalatal suture, and expan-sion progresses until slight overexpansion has been achieved. This is to account for the anticipated re-lapse demonstrated in many studies and the need to address the transverse compensations in the den-tition. As the child progresses beyond puberty, the circummaxillary sutures become more interdigitated and continue to increase in complexity throughout adulthood.47,48 These changes make it more dif-cult, if not impossible, to achieve skeletal transverse changes in the adult.Many adult patients present with dentofacial de-formity that may be characterized by maxillary skel-etal narrowing independently or in combination with other skeletal problems. The clinician has the choice of surgically assisted maxillary expansion or a segmental Le Fort I procedure (Table 17-2). The surgically assisted expansion requires an additional procedure with a large maxillary expansion device tted to the maxillary arch prior to placement of edgewise appliances. Segmental Le Fort I surgery may be performed at the same time as other den-itive procedures. The choice of surgical procedure may be based on the surgical preference of the sur-geon or orthodontist, the goals of treatment, the need for presurgical extractions (eg, third molars), and cost. It appears that both procedures are equal-ly stable (or unstable), being at the lower end of the hierarchy of stability as outlined by Proft et al.33Surgically assisted maxillary expansionSurgically assisted maxillary expansion can be con-sidered in adult patients with transverse skeletal narrowing in isolation or those with more complex skeletal problems who require other general anes-thesia procedures prior to placement of xed ortho-dontic appliances. The appliance is either cement-ed to bands on the posterior teeth or attached to the lateral palatal walls using temporary anchorage devices. Early studies do not demonstrate any sig-nicant advantage in the outcome of the skeletally anchored expander, although it could be placed at the time of the osteotomy. An osteotomy is performed in the maxilla with variations in clinical protocols ranging from com-plete lateral corticotomy of the maxilla toward the piriform aperture to separation of the midpalatal suture to disarticulation at the pterygoid plates49–51 (Fig 17-26). It is recommended that the surgeon acti-vate the jackscrew at least 5 mm at the time of sur-gery to ensure that the maxilla is truly disarticulated so that symmetric expansion can take place.The expansion procedure progresses along the protocols of distraction osteogenesis for 4 to 5 days after surgery, with two activations per day, until the desired expansion is achieved. Similar to expansion in children, expansion is ceased when the maxillary arch is overexpanded to account for relapse and re-moval of transverse compensations. It is important to observe a large diastema developing between the central incisors and inform the patient about the ne-cessity for excellent oral hygiene immediately adja-cent to the central incisors. The sulcular epithelium Table 17-2 Differences between surgically assisted maxillary expansion and segmental Le Fort I maxillary expansionSurgically assisted maxillary expansion Segmental Le Fort I maxillary expansionAppliance Banded or bone-anchored appliance NoneCost Additional procedure: general anesthesia/hospitalization, may require tooth extractionsSmall additional cost for segmental procedureTime Additional 3 to 5 months Leveling and expansion performed at the same time (may be more rapid)Discomfort (speech) Bulky appliance Have to wear occlusal wafer for longer period (4 to 6 weeks)Arch length Gains arch length for crowding Small increase in arch lengthRisks Routine bone surgical risks (minimal) Periodontal attachment loss 17Biomechanics and Treatment of Dentofacial Deformity 416will be exposed as the teeth separate (Atherton’s patch), and if symmetric expansion is occurring, the sulci will be equally exposed (Fig 17-27). A complication may occur during activation that is not encountered as frequently in children. As a consequence of the signicant mobility in the pala-tal segments combined with the goal of placing the screw deep in the palate to direct forces closer to the center of resistance of the maxilla, the appliance may experience signicant exing, which may result in binding of the screw (Fig 17-28). This is a dif-cult problem to overcome because it will probably require removal and redesign to a more supercial depth. Although less than ideal biomechanically, this may be required to ensure that the screw func-tions adequately.The biomechanics of maxillary expansion have been studied in nite element models with and without surgery, and the variations in osteotomy de-sign add confounders to the predictive modeling. Naturally, there are greater translatory movements of the palatal halves. The appliance is usually left in place for at least 3 to 4 months to provide time for consolidation of the osteoidlike tissue. The expand-er is then removed and replaced with bands and a 0.032 × 0.032–inch hinge cap palatal attachment with a horseshoe palatal arch to retain the expan-sion while alignment procedures are performed (Fig 17-29).A round 0.032-inch stainless steel palatal arch provides enough rigidity yet enables pure crown tipping palatally to remove the previously compen-sated maxillary molars (Fig 17-30). A similarly fabri-Fig 17-26 The type of osteotomy that is performed to assist in surgically assisted rapid maxillary expansion varies depending on the clinician. 1. Lateral maxillary ostectomy3. Midline ostectomy2. PterygoidreleaseFig 17-27 It is important to observe equal amounts of sulcular ep-ithelium being exposed on both central incisors during expansion (arrows). This exposure is known as Atherton’s patch and indicates that the expansion is proceeding symmetrically.Fig 17-28 Placement of the expansion screw in surgical expansion patients may be problematic if placed too high in the palatal vault. The extreme exibility of the osteotomized segments may result in signicant exure not seen in pediatric patients, and this may bend the appliance, causing the screw to bind. This is extremely disap-pointing for the clinician and the patient because often the appli-ance will have to be removed and remade with the screw placed more supercially. 417Special Considerations in Managing Skeletal AsymmetryFig 17-29 (a) Following active expansion, the screw can be tied off with a segment of brass wire (arrow) to prevent it from turning back. (b) The expander is left in place for 4 to 6 months before removal and placement of a hinge cap palatal arch, an essential retention mechanism.a bda b ce fg h ij kFig 17-30 An adult patient presents with a primary complaint of irregular teeth. (a to e) The maxilla is observed to be nar-row, and the dental relationships exhibit a posterior reverse articulation and lingually compensated posterior teeth in the man-dibular arch. (f) Surgery is performed and the appliance activated. (g to k) The maxil-lary arch has been expanded and the teeth aligned to a pleasing esthetic and function-al outcome. The mandibular arch compen-sations have also been addressed. 17Biomechanics and Treatment of Dentofacial Deformity 418cated mandibular lingual arch may also be used to tip the lingually compensated mandibular posterior teeth to their correct positions. A repositioning os-teotomy of the maxilla is required. Usually in these circumstances, open bite patients are leveled with xed appliances, avoiding the need for segmental osteotomies and the associated periodontal and root transection risks. Moreover, surgically assisted expansion can provide additional space for irregular teeth that may have necessitated tooth extraction or possible periodontal compromise if the crowding was treated with dentoalveolar expansion.Segmental maxillary expansionSegmental surgery of the maxilla may be required to address transverse discrepancies of the maxilla or to ensure that stepped occlusal planes are main-tained. The design of the osteotomy depends on the specic requirements of the patient. The magnitude of relative expansion of the canines, premolars, and molars will inuence the type of osteotomy, ranging from a two- to four-piece maxillary osteotomy (Fig 17-31). In addition, the leveling of the stepped oc-clusal planes may be simultaneously achieved during expansion. Signicant increase in intercanine width may not be possible with segmental expansion, so when it is required, surgically assisted expansion should be considered; there is a fundamental differ-ence in the response to expansion between these two approaches (Fig 17-32).It is important to ensure that the patient is pre-pared adequately with proper coordination of the arch forms prior to surgery. It is necessary to co-ordinate the maxillary arch form from the ideal dimension of the mandibular archwire. The nec-essary V-bends or steps should be placed between the brackets associated with the osteotomy site so that the segments of the arch are aligned to the complete mandibular arch. Furthermore, adequate space must be developed between the roots to fa-cilitate the osteotomy. This may be achieved by di-verging the roots or opening space with compressed coils. Divergence of the roots may be achieved by either placing adjustment bands in the wires or bonding the brackets (Fig 17-33). Brackets will then have to be rebonded after surgery. Extended post-surgical orthodontics in open bite patients may be problematic because of the associated extrusive side effects (as discussed earlier), so in these patients mi-nor tooth inclination corrections should be planned along with minimal root divergence.The model surgery must overexpand the maxillary arch in segments by at least 10% to 15% to compen-sate for the signicant relapse that is to be antic-ipated.33 Following segmental surgery, the patient will require an occlusal wafer wired to the maxillary teeth for at least 4 to 6 weeks to minimize relapse. The wafer can then be removed and a low–load deection continuous maxillary archwire placed together with a palatal arch. It is possible to acti-vate the palatal arch to narrow or broaden for ne adjustments. Minor irregularities in levels and arch form can then be re-evaluated before proceeding to nishing procedures.Fig 17-31 The design of the segmental osteotomy depends to a large extent on the desired expansion needs. If more posterior ex-pansion is required than anterior expansion, then a two-piece oste-otomy may be required. If there are steps in the occlusal plane, it is possible that this will inuence where the osteotomy is performed. Fig 17-32 The nature of expansion varies between surgically as-sisted rapid maxillary expansion (more anterior) (a) and two-piece Le Fort I expansion (b), as indicated diagrammatically. a b 419Special Considerations in Distraction OsteogenesisSpecial Considerations in Distraction OsteogenesisManipulation of fractured bony limbs following trauma was reported as early as the ancient Greeks, with Hippocrates placing broken legs in traction. Over 100 years ago, Codavilla reported a technique to lengthen legs by creating a fracture in the bone and extending it with subsequent traction. Ilizarov, a 20th-century Russian physician, developed specic techniques and protocols from rudimentary materi-als available to him for limb lengthening, which have since been adapted and improved by the develop-ment of more sophisticated force-delivery devices.The rst application to the craniofacial region was reported by Snyder et al in 1973.52 He used an expan-sion screw to lengthen the mandible of a dog. Sub-sequently, Guerrero et al53 reported the rst human application of distraction osteogenesis in which he expanded the midline of the mandible with a modi-ed palatal expansion screw.A signicant period of rapid application of distrac-tion osteogenesis to various craniofacial abnormal-ities followed with research to evaluate the cellular and biologic mechanisms. This technique was sug-gested to potentially replace conventional orthog-nathic surgery. Understanding the biomechanics of distraction osteogenesis, particularly in the mandi-ble, became a theoretical challenge because of the complex mechanisms by which the mandible artic-ulates. As a consequence, distraction of the maxilla and particularly the mandible became quite unpre-dictable, often requiring revision procedures to add nesse to the nal positional changes. Moreover, the need for multiple procedures and the accompa-nying discomfort and inconvenience of a protracted period of traction has resulted in renement of its application to specically challenging craniofacial deformities.Mandibular expansionTransverse distraction of the mandibular symphy-sis has been reported in the orthodontic literature to address signicant transverse discrepancies be-tween the mandible and maxilla.53 This expansion of basal bone has also been suggested to improve arch length for tooth alignment. It has also been recommended to work toward improving the oro-pharyngeal airway. Mandibular symphyseal distraction can be achieved by placing a tooth-borne expansion screw or a bone-borne expander attached to the bony symphysis af-ter extending a low-impact osteotomy through the symphysis midline (Fig 17-34). The midline cut en-ables symmetric expansion of the chin, but it may be stepped to extend between teeth with sufcient space to create the osteotomy with minimal root damage (Fig 17-35).Although the force from the distraction screw is controlled by the guides in the screw mechanism, there are many issues to be considered when plac-ing expansion screws, including exure of the appli-ance, bone bending, tooth movement, and the com-plexity of the attachment and articulation of the mandibular tooth-supporting structures. If bone-Compressed coil Compressed coil and stepAngular bends Brackets angleda bc dFig 17-33 Creating space for segmental osteotomies is essential to minimize the risk to tooth roots during sur-gery. This may be achieved with either push coils (a and b) or placement of artistic bends (c) or bracket angula-tions (d) to diverge the roots. Brackets will then have to be rebonded after surgery. 17Biomechanics and Treatment of Dentofacial Deformity 420borne expanders are used, the added issue of rigid-ity of the xation screws or lack thereof also has to be considered (Fig 17-36). This procedure appears to be effective in changing the anterior mandibular dimensions without signif-icant compromise to the temporomandibular joints, but the long-term effects have yet to be reported.Mandibular lengtheningMandibular-advancement osteotomies have many inherent limitations and risks, including inferior dental nerve damage, instability, and condylar re-sorption. Distraction osteogenesis was initially pro-moted to potentially extend the range of attainable mandibular advancement. However, the concept of histogenesis appeared more attractive to surgeons because it was proposed that the stretched tissues proliferated such that muscular, brous, and neural elements would be stimulated to extend and devel-op beyond what could be anticipated from acute surgical advancements.The immediate application of this new technolo-gy to craniofacial anomalies such as hemifacial mi-crosomia challenged conventional treatments, and new protocols were suggested for management of Fig 17-34 Broadening of a narrow man-dible (a) can be achieved by distraction os-teogenesis, in this case with a tooth-borne expander (b). (c) Following a consolidation period, the appliance is removed, and a 0.032-inch Burstone lingual arch is placed to maintain the expansion while xed ap-pliances align the teeth. (d) The nal out-come is excellent.a bc dFig 17-35 The ostectomy in the chin is often placed directly in the midline to ensure that the chin expands symmetrically. If roots in the midline are too closely related, it is possible to consider a step-like ostectomy so that the line of the cut can pass through the region with the most space between the roots. Fig 17-36 The use of bone-borne distractors or, in this case, a hybrid-type attachment (attached to the bone and the teeth) aims to deliver the force lower in the mandible in an attempt to reduce torquing effects on the segments. Biomechanically, the force is lower on the mandible; however, as with tooth-borne distractors, the biomechanics are difcult to interpret. 421Special Considerations in Distraction Osteogenesisthese problems (Fig 17-37). These procedures target-ed both the ramus and body lengthening, either in isolation or in combination with dual osteotomies and two active distraction movements per side de-pendent on the morphologic needs.Comprehension of the biomechanics of these changes was complex, with many investigators pro-posing biomechanical models for comprehension of changes in animal models.54 In clinical practice, these complex movements are problematic to effect accu-rately because of issues related to the anatomy, ar-ticulations, and support structures surrounding the maxilla and mandible.Placement of the distractor may rely on complex imaging and guidance techniques, but once the ap-pliance is activated, variations in tissue response, x-ation rigidity, and appliance design often mean that the patient will require revision surgery to ne-tune the outcome. Distraction devices have even been designed to enable reorientation during activation if an unpredictable movement is encountered (Fig 17-38).The long-term stimulation of tissue supposedly re-sulting from histogenesis and the secondary impact on subsequent postsurgical growth have been dis-appointing, resulting in more specic targeting of indications and patients who may benet from this procedure rather than a universal application of this technology.a b c dFig 17-37 (a) This young girl presented with left-sided hemifacial microsomia with associated maxillary and mandibular deciencies. (b) The plan was developed to osteotomize the maxilla and man-dible and place a distractor in the vertical ramus to elongate the mandible. The maxilla would be xed to the mandible by maxillo-mandibular xation, and the maxilla would be drawn down with the force delivered to the mandible. (c to e) Progress of the distrac-tion can be seen in the vertical displacement of the mandible and maxilla, which were extended nearly 2 cm. (f to h) There has been a marked improvement in the cant of occlusion. e f gh 17Biomechanics and Treatment of Dentofacial Deformity 422Fig 17-38 Distraction is particularly useful in patients with signicant craniofacial deformities, such as this man with Treacher Collins syndrome (a and b). (c) The mandible was distracted downward and forward along the vector established by computer software. The aim of the distractor was to extend the mandible as close to the ideal size as possible before orthodontics, and revision orthognathics would address the maxillary and zygomatic deciencies. (d and e) A stereolithographic model was used to assess the details of the morphology and construct a guide to position the distractor at the time of surgery to achieve the desired vector. (f to i) This guide was used at the time of surgery to locate the distractor accurately on the osteotomized mandible. (j and k) After orthodontics and renement orthognathics involving the maxilla, mandible, and chin, the facial and occlusal outcomes were pleasing. a b cd e fg h ij k 423Special Considerations in Distraction OsteogenesisMaxillary advancementMaxillary advancement with distraction can be per-formed with an appliance directly attached to the maxilla and supporting cranial bones superior and posterior to the maxilla. Usually the zygomatic struc-tures or temporal regions are used as anchorage to protract the maxilla. Alternatively, a rigid frame may be skeletally xated to the cranium, from which a screw device may be attached to progressively move the maxilla forward and downward. The maxilla is used to anchor an intraoral device, and the direction of traction may be modied to conceptually deliver forces relative to a theoretical center of resistance of the osteotomized maxilla. The advantage of this device is that modication of the vector of force is easy to achieve as the patient’s individual response is reviewed (Fig 17-39).Distraction osteogenesis of the maxilla is particu-larly useful in cleft lip and cleft palate patients with many purported advantages. However, there are a number of intraoral complications with the distract-ers, such as pain and infection. A recent comparison of distraction and orthognathic surgery revealed similar surgical and speech outcomes with both tech-niques.55 Distraction osteogenesis therefore appears to be indicated for the larger advancements that would challenge the tissue compliance during an acute advancement.a b cfd eFig 17-39 (a and b) A rigid external frame has been used to anchor an externally activated screw device to distract the osteotomized maxilla. (c) Pretreatment occlusion. (d) A xed appliance is cemented to the maxilla. This device has arms extending from the mouth, which will be attached to the screw mechanism. The screw device has the ability to be adjusted to modify the displacement during treatment. (e and f) Forces are delivered relative to the center of resistance of the osteotomized maxilla to aid in planning movement, but there are still many confounding factors. 17Biomechanics and Treatment of Dentofacial Deformity 424Management of Postoperative Morphologic ComplicationsThere are a number of routine consequences associ-ated with any intervention, and the purpose of this section is not to explore these in detail. However, such issues as pain, hemorrhage, swelling, infection, antral health issues, reduced range of motion, and neurosensory disturbances are encountered rela-tively frequently. Bad splits of the osteotomy site, root transection or damage, periodontal damage, and oronasal stulae secondary to segmental sur-gery are less frequently encountered.Complications arising from inappropriate diag-nosis and treatment planning are often difcult to manage and may only present either prior to or after surgery. It is possible that the surgical proce-dure may not achieve the desired esthetic outcome or occlusal relationship. Moreover, patients require appropriate psychologic counseling to ensure that they are prepared for the issues associated with the orthodontics and surgery. The immediate postoper-ative period requires a signicant support network from both the clinician and the patient’s home en-vironment. Patients with possible psychologic prob-lems such as depression, somatoform disorders, or body dysmorphic disorders need to be carefully con-sidered and counseled before a decision is made to subject them to this type of treatment. Clinicians will often regret proceeding to surgery early, which inevitably results in extended treatment time with associated iatrogenic issues including possible signif-icant psychologic issues.56Complications related to errors of positioning or postsurgical movement of osteotomized fragments are encountered less frequently, and fortunately most are often overcome by timely diagnosis and management. Immediately following discharge from surgery, the t of the maxillary and mandib-ular teeth into the surgical splint must be assessed. It is essential for clinicians to review the immediate postsurgical outcome radiographically. The presur-gical prediction tracing should be overlaid on the postsurgical radiograph to assess the outcome and accuracy of the surgery. First, the condylar seating in the glenoid fossa must be conrmed. The clini-cian should then compare the angulation of the ramus and consider if it has been retropositioned or inclined forward. This may affect the postoper-ative response as the patient is released from wire or elastic xation. The orientation of the fragments can then be appraised and compared with the pre-diction tracing. In maxillomandibular surgery, the position of the maxilla in all three planes must be reviewed, and particular attention should be direct-ed to assessment of the vertical position of the max-illary incisor and molar. Maxillary repositioningErrors in maxillary positioning are probably most signicant in the vertical dimension, with the impact on nal tooth display anteriorly, and in the posteri-or dimension, which will inuence the cant of the occlusal plane. Signicant steepening of the occlu-sal plane posteriorly may have a deleterious effect on the relative chin projection. This may inuence patient satisfaction if chin projection was an import-ant consideration in the patient’s treatment goals. If the occlusion is related well in the surgical splint but signicant errors are evident in positioning of the maxilla, then it is unlikely that any signicant changes in position can be achieved by postsurgi-cal orthodontics, and the clinician and patient must decide whether they are prepared to accept this dis-crepancy. If not, it may be necessary to return to the operating room.If the teeth do not t into the splint immediately following surgery, it is important to identify where the discrepancy lies and consider whether a period of elastic traction with or without some type of ex-traoral traction is required (Fig 17-40). This will de-pend on the goals of the postsurgical orthodontics. If signicant effort has been made to reduce tooth display by superior repositioning of the maxilla, and the patient were to return with an anterior open bite, it is probably unwise to simply run vertical elas-tics to the anterior teeth and indiscriminately ex-trude the maxillary and mandibular teeth. A J-pull headgear may be considered with light seating elastics to control the mandibular incisor position differentially. However, incisor display may need to be increased, in which case elastics and a headgear with long outer bow should be considered to rotate the occlusal plane downward anteriorly. In summa-ry, the selection of facebow headgear for the maxil-lary arch or J-pull headgear for either arch with ap-propriately directed elastic forces relative to the center of resistance and associated elastic traction should be routinely considered to facilitate goal- oriented postsurgical tooth movements. 425Management of Postoperative Morphologic Complicationsa b cde fg hi j kFig 17-40 (a to c) This woman presented for what was considered a routine maxillary advancement to address her midface deciency and Class III relationship. (d) The teeth were aligned, and a relatively simple plan was established by the surgical team. Unfortunately, the patient had a signicant bleed in the operating room from a rare bleeding disorder that was not detected by routine blood screening. The surgical team fought to keep her alive and were more concerned with controlling the bleeding than correcting the jaw position. (e to h) As a result, on splint removal the occlusion (g and h) looked similar to the presurgical position (e and f). (i) The surgeons had no desire to put her back in the operating room, so with the osteotomized maxilla, a reverse headgear was prescribed in an attempt to protract the maxilla. (j) The maxilla moved quite rapidly, and the small right lateral incisor was restored to normal size and form. (k to n) The nal outcome was excellent, although this was not envisaged in the immediate postoperative period. l nm 17Biomechanics and Treatment of Dentofacial Deformity 426Mandibular repositioningIt is essential for the surgeon to accurately place the condyles in the glenoid fossa prior to applying skel-etal xation. Methods have been proposed to assist in locating the condyle in its preoperative position, but they have been unpopular because they add op-erating time to the procedure.57 With the develop-ment of rigid xation, this issue is more important because minor variations in absolute or angular lo-cation in the joint will reect in postsurgical move-ments at tooth level as the condyle aims to assume its original orientation. This may reect as nonocclu-sion or AP discrepancies in occlusion.58Condylar sagCondylar sag was a term used to describe a problem encountered with xation with wire osteosynthesis. The condyle would be located out of the glenoid fossa because of traction applied during xation or edema encountered during the operation or post-operatively. This has been described as “central” condylar sag when the condyle is completely dis-placed from the fossa or “peripheral” condylar sag when the condyle is displaced onto the medial, lat-eral, anterior, or posterior walls and interferes with full seating of the condyle (Figs 17-41 and 17-42). This appears to be due to inadvertent twisting of the condyle during xation.59 The condyle slowly adapts by displacement and resorption in the weeks following surgery. In these patients, it is essential to examine them within the rst 2 to 3 weeks follow-ing surgery and possibly consider removing their x-ation. Elastics can then be used to guide their teeth into occlusion while the bony fragments are manip-ulated.Ramus angular errorsAccurate angular positioning of the condyle is essen-tial to minimize postoperative positional changes in the occlusion. If the ramus is displaced backward, the mandible will slowly move forward following removal of xation. This will result in an edge-to-edge occlusal relationship extending to a negative horizontal overlap (Fig 17-43). This problem must be differentially diagnosed from failure of xation, which may require different management. Con-versely, if the ramus is displaced forward at the time of xation, the opposite problem may occur with progression toward Class II relationships at the den-tal level. Angular ramal positioning errors may occur unilaterally or bilaterally. This problem can general-ly be managed with traction to the mandibular arch with elastics in most circumstances, but in severe cases, the patient may have to be returned to the operating room. In the retropositioned ramus, it is possible to even consider a J-pull headgear to facil-itate movement of the mandibular arch (Fig 17-44).Fig 17-41 Errors in positioning the condyle in the glenoid fossa at the time of placing xation wires or screws will re-sult in signicant discrepancies between retruded contact position and intercuspal position. In severe cases, if the condyles are displaced signicantly, when the mandible re-turns to its position in the fossa, the vertical displacement results in premature contacts on the posterior teeth, which will result in an anterior open bite. 427Management of Postoperative Morphologic Complicationsa bFig 17-42 (a) This patient returned from having maxillary and mandibular surgery with a genioplasty. (b) Review of the cephalometric radiograph reveals signicant differences in the position of the posterior ramus between the right and left sides. (c to e) The intraoral pho-tographs reveal a relatively symmetric occlusal relationship. (f and g) Further evaluation of the condylar position demonstrated that the right joint (f) was displaced 5 mm from the glenoid fossa. With 1 to 3 mm of displacement, it is possible to compensate the teeth by removing the xation and running seating elastics; however, the magnitude of this problem probably requires further surgery and repositioning of the ramus back into the fossa. c d ef gFig 17-43 Control of the proximal fragment is one of the most crucial aspects of positioning the mandible accurately following jaw surgery. In some circumstances, surgeons are determined to push the condyle back into the glenoid fossa, which may rotate the proximal segment backward. Although this may be of little signicance in the immediate postoperative period, once xation is released, the muscles have a tendency to move the ramus back to its original position, which rotates the anterior mandible forward, often leading to a negative hori-zontal overlap. 17Biomechanics and Treatment of Dentofacial Deformity 428Condylar degenerative changesCondylar resorption is an undesirable consequence of mandibular surgery that has been reported to occur in 3% to 10% of patients and may occur throughout the postoperative period.60,61 The length of the mandible has been shown to decrease in 10% of patients who underwent mandibular ramus surgery (by less than 2 mm in one sample group). However, in half of these patients, no discernable change in occlusion was observed because the den-tition compensated for these changes. In the short term, compensatory strategies may be considered, but many of these changes may occur after removal of appliances with few long-term functional conse-quences.Signicant condylar breakdown can be observed when the occlusion undergoes signicant change, excluding as a result of systemic conditions such as rheumatoid arthritis and other autoimmune con-ditions. Splint therapy should be considered to re-duce the loads on the joint. This condition usually stabilizes, and then the clinician must consider the risk-to-benet ratio of reoperating, with further un-certainty in the joints following surgery (Fig 17-45). Alternatively, the maxilla can be set back if it had been advanced as part of the initial surgery, or an advancement genioplasty can be considered if es-thetic concerns are expressed by the patient.Failure of xationFailure of xation is a signicant problem that can result in the osteotomized maxilla or mandible mov-ing following surgery. Fixation screws may not have been placed securely in the bone, or signicant mus-cular or soft tissue tension may have mobilized the screws. It is important for the clinician to differen-tially diagnose this condition and comprehend the possible implications on both short-term and long-term stability (Fig 17-46). In some circumstances, fail-ure is complete, and signicant movement occurs; in such cases, the patient will have to be returned to the operating room. However, in a number of patients, this movement is more subtle, which may be observed as minor to moderate changes in the Fig 17-44 (a) Presurgical cephalometric radiograph of a Class II patient who requires mandibular surgery. (b and c) Postsurgical cephalomet-ric radiograph with tracing showing that the proximal fragment has been rotated backward, which resulted in the mandibular teeth moving into a negative horizontal overlap. (d and e) A J-pull headgear and elastic traction were instituted to address the positional issue with the mandible, and the problem was resolved rapidly. (f) Final occlusion.a b cd e fJ-pull headgearClass III elastic 429Management of Postoperative Morphologic Complicationsocclusion. Early identication and an appropriately directed strategy can avoid the need for addition-al surgery. It is important for both the orthodontist and the surgeon to examine the patient together or at similar times to facilitate the appropriate man-agement protocol.The most frequently encountered problem with failure of xation is encountered after mandibular osteotomies following a sagittal split osteotomy. The muscular pull of the medial pterygoid muscle will pull the distal fragment upward, rotating the man-dibular incisors forward and downward (Fig 17-47). The strategy for managing this problem ranges from replacement of maxillomandibular xation with wires or strong elastics in isolated mandibular surgery to a combination of extraoral traction and elastics to the mandibular arch. If the patient has had signicant vertical maxillary impaction, it is probably unwise to a b c dFig 17-45 (a) A woman who initially presented with a Class II malocclusion was managed by maxillomandibular surgery to advance and impact her maxilla and advance her mandible to an acceptable outcome. (b and c) Two years later, there were signicant changes in her jaws, which continued due to condylar resorption. (d) The patient opted for further surgery after signicant consideration of the relative risks and gains. The positions of the jaws remained stable. Fig 17-46 Failure of xation is a signicant problem that requires early diagnosis and timely management. It is the result of failure, either partial or total, of the hardware securing the osteotomized mandible. The upward pull on the distal fragment by the medial pterygoid and the downward pull on the chin may effect a clockwise rotation of the distal fragment. Concomitantly, the temporalis muscle may pull upward on the cor-onoid process. This has to be differentially diagnosed from posterior ramus positioning.Fig 17-47 This patient had distraction of the mandible followed by renement orthognath-ics. Fixation screws were not held in the bone as securely as with normal bone, so the muscu-lar pull rotated the distal fragment clockwise. Because the maxilla had been impacted to re-duce the gummy smile, elastic bands should be used with caution because the maxillary incisors could be pulled down again. The negative horizontal overlap could also reect mandibular ramus positioning errors. Accurate interpretation of the cephalometric radiograph displays the etiology of the problem (ie, failure of xation). 17Biomechanics and Treatment of Dentofacial Deformity 430run signicant elastic traction from the maxillary to the mandibular anterior teeth, as this may bring the maxillary anterior teeth downward. It may be wiser to consider a mandibular J-pull headgear attached to the rigid surgical wire in the mandible to rotate the mandibular distal fragment superiorly (Fig 17-48). Elastics can be simultaneously considered, but the force levels must be more moderate.SummaryComprehension of biomechanical principles associ-ated with tooth movement and dentofacial ortho-pedics have to be applied to combined surgical- orthodontic treatment to ensure that effective pre- and postsurgical goals are achieved. The intro-duction of temporary anchors to facilitate pre- and postoperative tooth movement demands thorough comprehension of their effects as the clinician ex-tends the envelopes of possible tooth movements. This should inevitably shorten treatment times and move toward more predictable, excellent outcomes. Moreover, the use of distraction osteogenesis devic-es with their underlying complexities leave more questions to be answered as the clinician strives to determine the appropriate application of this tech-nology.Fig 17-48 (a and b) This patient present-ed with an asymmetric Class III malocclu-sion with negative horizontal overlap that required fairly straightforward maxillo-mandibular surgery, as outlined in the pre-diction tracing (c). (d) Complete failure of xation was observed on discharge from the hospital, with signicant counterclock-wise rotation of the proximal fragment from the temporalis muscle and clockwise rotation of the distal fragment from the medial pterygoid and suprahyoid muscles. This required reoperation, but the patient and surgeon did not want to redo the sur-gery. (e and f) Elastics were continued by the surgeon. (g) The effects of the rotation of the distal fragment and the impact on bringing the anterior maxilla downward are marked on the patient’s smile line, and most individuals would be quite dissatis-ed with the outcome. Early recognition of this problem should have resulted in imme-diate return to the operating room.a bc defg 431ReferencesReferences1. Farkas LG. Anthropometry of the head and face in medi-cine. New York: Elsevier, 1981.2. Burstone CJ, James RB, Legan H, Murphy GA, Norton LA. Cephalometrics for orthognathic surgery. J Oral Surg 1978; 36:269–277.3. Steiner CC. 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