Mandibular Anatomic Implications for Dental Implant Surgery










737
30
Mandibular Anatomic
Implications for Dental
Implant Surgery
RANDOLPH R. RESNIK
I
n dental implantology today it is imperative the clinician have
a strong understanding of the surgical anatomy and variations
with respect to implant placement in the mandible. Before the
commencement of dental implant surgery, a careful and detailed
evaluation should be completed of the mandibular vital structures.
is may be accomplished by including a clinical evaluation, a
visual examination, along with palpation of the anatomic areas.
e clinician should have a clear and concise three-dimensional
(3D) vision of the anatomic structures in relation to the intended
implant surgical procedure.
A thorough radiographic examination needs to be completed
to provide information concerning the location and topography
of the 3D anatomy. In this chapter a comprehensive evaluation
of the important mandibular anatomic areas will be discussed,
together with their clinical relevance in dental implant surgery
(Fig. 30.1).
Mandibular Anterior
Hourglass Anterior Mandibles
e mandibular anterior region has historically been considered
one of the safest and most predictable regions for implant place-
ment. e predictability stems from the favorable quality of bone
(i.e., thick cortical and dense trabecular bone) most commonly
present in this area. e morphology of the mandibular anterior
has been classied as the following shapes: hourglass, ovoid, pear,
sickle, and triangular. e pear shape, which is usually abundant
with bone, has been shown to be the most common among eden-
tulous and dentate patients.
1
However, this anatomic area may be compromised by a nar-
row alveolar width or severe osseous constriction. ese types
of bony variations have been termed an hourglass eect, which
is usually indicative of a developmental abnormality. Hourglass
mandibles, which have been shown to have an incidence rate
of approximately 4%, should always be concerning to the cli-
nician because of possible perforations during implant place-
ment surgery.
2
e position of the alveolar constriction may
vary signicantly because they have been shown to be high,
low, or variable within the alveolus. A thorough 3D cone beam
computed tomographic (CBCT) examination should be com-
pleted to prevent complications in this area, and guided surgery
is recommended to minimize the possibility of perforations
3
(Fig. 30.2).
Butura etal.
4
classied mandibular anterior constrictions as:
(1) facial constriction, (2) lingual constriction, and (3) hourglass
constriction. ey discussed various treatment options to include
alveoplasty to a level beyond the constriction, staged bone graft
reconstruction, posterior and anterior angled implants to avoid
the site, and extra-long implants to bypass the constriction and
engage the inferior border of the mandible.
4
Clinical Relevance
Due to the variable expression of hourglass mandibles, treatment
strategies are based on the location and extent of the undercut.
In some cases implant placement in this area will be contrain-
dicated. In less severe constrictions an osteoplasty may be per-
formed, together with implant placement. However, the crown
height space may be increased signicantly, leading to possible
biomechanical issues. In addition, other constrictions may require
grafting procedures to increase bone volume for implant place-
ment (Fig. 30.3).
If the positioning of a dental implant leads to perforation of
the bony mandibular plates, possible life-threatening hemorrhage
episodes may occur. ese events have been reported when a drill
perforates the lingual plate of the sublingual region of the man-
dible and traumatizes a sublingual or submental artery, especially
in the canine region.
5,6
If perforation of the lingual cortical plate
is associated with arterial bleeding, it is critical to identify its ori-
gin and treat aggressively. e origin of bleeding in the oor of
the anterior region of the mouth may be from the lingual artery,
facial artery, or one of its branches. Perforation of either the sub-
mental artery (originates from the facial artery) or the sublingual
artery (originates from the lingual artery) may lead to bony perfo-
ration and bleeding, causing an expanding ecchymosis (sublingual
hematoma) and compromising the airway. If this should occur,
the patient should be repositioned in an upright position, and
bimanual pressure should be applied to the area of bleeding. If the
airway is compromised, immediate emergency assistance should
be summoned (Fig. 30.4).

738
PART VI Implant Surgery
Median Vascular Canal
In the mandibular midline, radiographic examination often reveals
the presence of a radiolucent canal, which is termed a median vas-
cular canal. is canal houses the bilateral sublingual arteries that
enter the lingual foramen, which is located on the lingual aspect of
the mandible. e lingual foramen is seen as a radiopacity below
the genial tubercles, which is visible on approximately 52% of
CBCT scans.
7
is arterial anastomosis may transverse anteriorly,
inferiorly, or superiorly within the anterior mandible, in some
instances exiting the facial aspect of the symphysis area. Various
studies have shown median vascular canals to be present in 100%
of cases, detected on CBCT examinations. Two-dimensional (2D)
panoramic radiographs observe their presence only 4.2% of the
time.
8
is is most likely due to the superimposition of the cervi-
cal vertebrae and to the orientation of the panoramic beam in
relation to the position of the canals. Gahleitner etal.
9
reported
one to ve canals per patient with an average diameter of 0.7 mm,
with a range of 0.4 to 1.5 mm (Fig. 30.5).
e presence and size of the sublingual anastomosis and the
median vascular canal are easily seen on a cross-sectional or axial
image of a CBCT scan. In approximately 31% of lingual vascular
canals the diameter exceeds at least 1 mm.
10
e sublingual artery
is a branch of the lingual artery that originates from the external
carotid artery. e lingual artery courses medially to the greater
horn of the hyoid bone and crosses inferiorly and facially around the
hypoglossal nerve. It then transverses deep to the digastric and stylo-
hyoid muscles, and courses between the hyoglossus and genioglos-
sus muscles. ere exist four main branches of the lingual artery: the
suprahyoid, dorsal lingual, deep lingual, and sublingual (Fig. 30.6).
Clinical Relevance
When planning implants in the anterior mandible, if a large anasto-
mosis is present, the position may be modied to prevent encroach-
ment on the structure. If this area is violated, excessive bleeding may
result. e intraosseous bleeding is usually well controlled by plac-
ing an implant, direction indicator, or surgical bur in the osteotomy
site. ere will be no neurosensory issue with encroaching on this
area because there are no sensory bers within the canal (Fig. 30.7).
Severely Angled Anterior Mandible
ere exists one uncommon subcategory of Division C, namely
C−a (i.e. Division C bone with excessive angulation). In this cate-
gory, available bone is adequate in height, but angulation is greater
than 30 degrees regardless of implant placement. When present,
this condition is most often found in the anterior mandible. For
ideal implant placement, usually bone augmentation is required.
However, a diagnostic wax-up should be completed rst because
Division C−a mandibles are usually associated with skeletal Class
III patients (Fig. 30.8).
A
B
C
Fig. 30.1 Variable Mandibular Anatomy. The mandibular arch varies
dramatically with respect to the amount of hard and soft tissue resorption.
(A) Mandibular arch with a signicant amount of bone and keratinized tis-
sue. (B) As bone resorption occurs, the loss of the attached tissue results.
(C) Advanced resorption of the hard and soft tissues resulting in a severely
atrophic mandible with minimal attached tissue.
A
B
Fig. 30.2 Constricted Mandibular Anterior. (A) Cone beam computed
tomographic panoramic image that depicts a signicant amount of avail-
able bone; however, it does not indicate a constriction is present. (B)
Cross-sectional images showing the hourglass appearance of the anterior
mandible, which contraindicates implant placement.

739
CHAPTER 30 Mandibular Anatomic Implications for Dental Implant Surgery
Clinical Relevance
Root form implants placed in Division C−a will lead to poorly posi-
tioned implants that will most likely be nonrestorable for a xed
prosthesis. is will most likely result in an overcontoured pros-
thesis, speech diculty, compromised tongue space, and inability
to obtain an ideal occlusion. erefore in most cases, a staged bone
graft and implant treatment plan should be formulated.
Lack of Keratinized Tissue
As the mandibular osseous process progresses, the presence of kera-
tinized tissue becomes more compromised. In general, implants
are healthiest when there exists sucient keratinized tissue. Some
reports indicate the lack of keratinized tissue may contribute to
implant failure.
11
Mobile, nonkeratinized mucosa has been shown
to exhibit greater probing depths, which has been conrmed his-
tologically. e absence of keratinized mucosa also increases the
susceptibility of peri-implant regions to plaque-induced destruc-
tion.
12
Additional studies have shown that mobile mucosa may
disrupt the implant-epithelial attachment zone and contribute
to an increased risk for inammation from plaque.
13
For larger
edentulous ridges the zone of attached tissue on the facial ap
(mandible) provides greater resistance for the sutures against ten-
sion of the mentalis muscle in the anterior region and the buccina-
tor muscle in the molar and premolar regions, which often cause
incision line opening. As a result, an incision made facial to the
attached tissue may cause partial ischemia to some of the crestal
tissue. In addition, the incision in unkeratinized facial tissue may
sever larger blood vessels, which increases bleeding and decreases
vision during surgery, while also potentially complicating nal
suturing (Fig. 30.9).
Clinical Relevance
For implant sites, an evaluation of the quality and quantity of kera-
tinized tissue should be completed. If insucient attached tissue
is present, tissue augmentation procedures should be completed
before implant placement. For larger edentulous sites, especially
in the mandible, the incision may be modied to maintain the
attached tissue in some cases. If the crest of the ridge is above the
oor of the mouth, and there exists greater than 3 mm of attached,
keratinized gingiva on the crest of the ridge, a full-thickness inci-
sion is made, bisecting the attached tissue. If less than 3 mm of
attached gingiva exists on the ridge, the full-thickness incision is
made more to the lingual so that at least 1.5 mm of the attached
tissue is to the facial aspect of the incision line. Another treatment
option includes the use of an acellular dermis (e.g., OraCell; Sal-
vin Dental Corp.). Acellular dermis may be placed at the time of
implant placement, thereby increasing the thickness and quality
of the tissue while the integration of the implants is taking place
(Fig. 30.10).
a. Pear
shape
1. Ridge
augmentation
Or
2. Osteoplasty
1. Ridge
augmentation
1. Ridge
augmentation
1. Ridge
augmentation
1. Ridge
augmentation
1. OsteoplastyNo
treatment
b. Sickle
shape
e. Buccal
constriction
f. Lingual
constriction
g. Hourglassc. Ovoid d. Triangular
Treatment
Fig. 30.3 Anterior Mandible Cross-Sectional Morphology and Treatment. (A) Pear shape. (B) Sickle
shape. (C) Ovoid. (D) Triangular. (E) Buccal constriction. (F) Lingual constriction. (G) Hourglass.

740
PART VI Implant Surgery
ABC
DE
Fig. 30.4 Mandibular Anterior Perforation. (A) Implant treatment plan showing perforation of the inferior
border of the mandible. (B) Lingual constriction that may lead to bleeding complications, along with chronic
tissue irritation. (C to E) Implant placement in a constricted ridge leading to nonideal implant placement.

741
CHAPTER 30 Mandibular Anatomic Implications for Dental Implant Surgery
A
B
Fig. 30.5 Median Vascular Canal. (A) Canal extending close to buccal
plate and then superiorly to almost the crestal area. (B) Canal extending
inferiorly and then superiorly.
Fig. 30.6 Rare off-midline vascular canal.
Fig. 30.7 Implant placement into the canal may result in bleeding epi-
sodes.
A
B
Fig. 30.8 Division C−a Mandible. (A) Mandibular anterior Division C−a with
extreme angulation. (B) Cross-sectional image of a Division C−a mandible.

742
PART VI Implant Surgery
Inadequate Width of Bone
A common consequence of tooth loss and bone remodeling in
the mandibular anterior region is the resultant narrowing and
knife-edge conguration of the mandibular bony ridges (i.e.,
Division B available bone). Pietrokovski etal.
14
evaluated eden-
tulous ridges in human jaws and found that 43% of mandibular
anterior ridges were knife-edge and 38% in the premolar region.
Nishimura et al.
15
reported a higher incidence of mandibular
knife-edge ridges in females compared with males, mainly because
of increased osteopenia changes with unfavorable bone mineral
density values.
Clinical Relevance
erefore before implant placement, it is often necessary to reduce
the bone width (i.e., osteoplasty), which results in an increased
horizontal width of available bone. An osteoplasty may be carried
out with various methods, including osteoplasty burs, barrel burs
(i.e., acrylic burs), rongeurs, bone chisels, and Piezosurgery units.
By increasing the width of bone, dental implants may be placed
with sucient bone on the buccal (2.0 mm) and on the lingual
(1.0 mm).
Although an osteoplasty increases available bone for implant
placement, many detrimental eects may result. Reduction of a
knifelike ridge will decrease the amount of cortical bone present.
e cortical bone is a crucial component for primary stability of
the implant and is responsible for a greater stress distribution. In
addition, as the bony height is reduced, the crown height space
increases. e increased crown/implant ratio results in greater
strain at the peri-implant interface, which predisposes the implant
to biomechanical complications.
e amount of osteoplasty required should be determined
before surgery via the use of CBCT treatment planning. In gen-
eral, if a xed prosthesis (e.g., FP-1, FP-2, FP-3) is indicated, a
minimal osteoplasty is recommended. By minimizing the reduc-
tion of bone height, the possibility of a crown/implant ratio prob-
lem resulting is decreased. In some cases bone augmentation may
be required to maintain the height of bone and increase the bone
width, rather than reducing the ridge by means of an osteoplasty.
If a removable prosthesis (e.g. RP-4, RP-5) is indicated, a more
aggressive osteoplasty is recommended, because this will allow
for increased space for the removable prosthesis (i.e., thickness of
acrylic, attachment space). In general, the greater the interocclusal
space, the less likely a prosthesis or attachment fracture can occur
(Box 30.1 and Fig. 30.11).
Mandibular Posterior
Although the posterior mandible is considered a predictable ana-
tomic area for implant placement, there are many drawbacks that
include compromised available bone in height and width, sig-
nicant undercuts, dicult access, and numerous vital structures
that may be damaged (e.g., inferior alveolar nerve [IAN], mental
nerve, submandibular gland). Iatrogenic violation of these vital
structures may result in neurosensory disturbance, pain, infec-
tion, excessive bleeding, and compromised implant positioning
(Fig.30.12).
Lack of Bone Height
e posterior mandible resorbs from buccal to lingual, transform-
ing from a Division A to a Division B rather rapidly. Because of
the trajectory of the posterior mandible, implant placement in
an ideal position for prosthetic rehabilitation may be dicult.
When limited alveolar ridge height exists, four options are usu-
ally available: (1) no treatment, (2) vertical ridge augmentation
with delayed implant placement, (3) vertical bone augmentation
with simultaneous implant placement, and (4) the use of short
implants.
16-19
Vertical Bone Augmentation (With Simultaneous or
Delayed Implant Placement)
With severely resorbed alveolar ridges in the posterior mandible,
the available bone height for standard implant placement is often
limited by the proximity of the mandibular canal. Vertical bone
augmentation is an option for increasing the ridge dimensions,
thereby allowing for placement of standard-length implants. By
increasing the bone height, esthetics and biomechanical compli-
cations are less likely to complicate the longevity of the implant
prosthesis.
However, increasing bone height in the posterior mandible
is one of the most challenging procedures in implant dentistry.
To increase the available bone, various techniques, including
autogenous block grafts, guided bone regeneration, and distrac-
tion osteogenesis, have been discussed in the literature. However,
an increased rate of surgical complications and enhanced patient
morbidity have been associated with these types of procedures.
Shorter Implants
Recently the use of short implants (8 mm) in the atrophic poste-
rior mandible has been introduced to circumvent the need for ver-
tical bone augmentation. Because the loss of vertical bone height
is often associated with inadequate available bone for implant
A
B
Fig. 30.9 (A) Maxillary anterior ridge with compromised attached tis-
sue. (B) When less than 3 mm of attached gingiva is present, the incision
for a full-arch mandible should include making the incision more lingual
and extending to the lingual aspect of the mandible when there is nerve
dehiscence.

743
CHAPTER 30 Mandibular Anatomic Implications for Dental Implant Surgery
A
B
D
E
F
C
G
Fig. 30.10 (A) Anterior mandible with minimal attached, keratinized tissue and implant placement. (B and
C) OrACELL acellular dermis. (D) Four-millimeter holes are made with a biopsy punch to t over the neck
of the implants. (E) Acellular dermis placed with healing abutments. This technique has the advantage of
increasing tissue quantity in conjunction with implant healing. (F and G) Before and after case of implant
placement and acellular dermis.

744
PART VI Implant Surgery
placement, the safety zone (2-mm distance between the implant
and nerve canal) is sometimes compromised with conventional
length implants. If this occurs, an increased possibility of a neuro-
sensory impairment may result.
erefore the use of shorter length implants oers the clinician
many advantages in comparison with vertical bone augmentation:
it is a less invasive surgery, less surgical experience is required, it is
less expensive, and it has a faster treatment time. However, shorter
implants do have drawbacks: they may result in an increased
crown height space, less surface area in comparison with standard
length implants, and a possible higher rate of biological and tech-
nical complications from occlusal overload.
In summary, studies on the use of short implants (8 mm) are
promising.
20,21
Many factors should be evaluated when deciding
whether short implants should be used instead of vertical bone aug-
mentation. Force factors (e.g., opposing occlusion, parafunction)
must be favorable, and an implant-protected occlusion should
adhered to. In addition, with short implants, the widest diameter
implant possible should be selected, along with an increased num-
ber of implants. e nal prosthesis involving multiple implants
should always be splinted for greater force distribution (Fig. 30.13).
Mandibular Deformation (Flexure of the
Mandible)
Full-arch implant-supported prostheses with a rigid substructure
have become controversial in implant dentistry because of the
associated increased strain at the bone-implant interface. Because
of the rigid bone-implant interface that is associated with dental
implants, jaw deformation may transmit excessive stress, which can
result in complications. In the literature, pain has been associated
with full-arch rigidly splinted prostheses.
22,23
Gates and Nicholls
24
reported on deformation of impression material when full-arch
impressions are taken with the mouth wide open. e inaccuracies
may result in ill-tting or nonpassive superstructures in dierent
jaw positions. In addition, mandibular deformation has been asso-
ciated with loosening of full-arch implant-supported prostheses
and possible fractures of prostheses during mastication.
25,26
Etiology
Flexure. e body of the mandible exes distal to the foramen
on opening and has torsion during heavy biting, with potential
clinical signicance for full-arch implant prostheses. Many reports
have addressed the dimensional changes of the mandible during
jaw activity as a result of masticatory muscle action. Five dierent
movements have been postulated. Medial convergence is the one
most commonly addressed.
27
e mandible between the mental
foramina is stable relative to exure and torsion. However, distal
to the foramina, the mandible exhibits considerable movement
toward the midline on opening.
28
is movement is caused pri-
marily by the attachment of the internal pterygoid muscles on the
medial ramus of the mandible. e distortion of the mandible
occurs early in the opening cycle, and the maximum changes may
occur with as little as 28% opening (or about 12 mm). is exure
has also been observed during protrusive jaw movements.
29
e
greater the active opening and protrusive movements, the greater
the amplitude of mandibular exion. e amount of movement
varies among individuals and depends on the density and volume
of bone and the location of the site in question. In general the
more distal the sites, the more medial is exure. e amplitude
of the mandibular body exure toward the midline has been mea-
sured to be as much as 800 μm in the rst molar-to-rst molar
region to as much as 1500 μm in the ramus-to-ramus sites.
Torsion. Torsion of the mandibular body distal to the foramina
has also been documented in both animal and human studies.
Hylander
30
evaluated larger members of the rhesus monkey family
(macaque) and found the mandible twisted on the working side
and bent in the parasagittal plane on the balancing side during the
power stroke of mastication and unilateral molar biting. Parasagit-
tal bending of the human jaw during unilateral biting was con-
rmed by Abdel-Latif etal.,
31
who showed patients with implant
prostheses measured up to 19 degrees of dorsoventral shear.
e torsion during parafunction is caused primarily by forceful
contraction of the masseter muscle attachments. Parafunctional
bruxism and clenching may cause torsion-related problems in the
implant support system and prosthesis when the mandibular teeth
are splinted from the molar-to-molar regions.
Implants placed in front of the foramina and splinted
together or implants in one posterior quadrant joined to ante-
rior implants have not shown these complications related to the
exure or torsion of the mandible. Complete implant-supported
xed restorations can halt the posterior bone loss associated with
edentulism, improve psychological health, and produce fewer
prosthetic complications than removable restorations. All eden-
tulous mandibular patients should be given the option of having
a xed prosthesis. However, the increase in forces of mastication,
increase in force with patients of greater force factors (e.g., para-
function, crown height space, opposing arch type), or reduced
bone density in the implant sites warrant an increase in implant
number or implant position in anterior and posterior implant
sites (Fig. 30.14).
Prevention
e concept of exure and torsion does not aect the maxilla,
where all implants are often splinted together, regardless of their
positions in the arch. Prevention of mandibular exure should
include the following treatment plans:
Bilateral posterior implants: If implants are positioned bilaterally in
the premolar/molar regions of the mandible, the nal prosthe-
sis should be fabricated with two sections. is will minimize
the possibility of exure/torsion issues. Usually, the prosthesis
is. splint in the premolar area.
Anterior implants with unilateral implants posterior: With implant
support on only one posterior side, full-arch splinted prosthe-
ses will not be subject to the exure/torsion problems.
Anterior implants with no posterior implants: With no posterior im-
plant support, full-arch splinted prostheses may be fabricated
without concern regarding exure/torsion problems.
Treatment
If a full-arch splinted prosthesis is fabricated and the patient
exhibits complications (e.g. pain, diculty opening, posterior
1. Fixed implant prosthesis (FP-1, FP-2, FP-3)
a. Porcelain fused to metal: 10 mm
b. Zirconia: 8 mm
2. Removable implant prosthesis (RP-4, RP-5)
a. Attachments (no bar): 9 mm (e.g., Locator)
b. Bar + attachment: 15 mm
BOX
30.1
Approximate Minimum Interocclusal
Space (Bone Level to Incisal Edge)

745
CHAPTER 30 Mandibular Anatomic Implications for Dental Implant Surgery
A B
RP-4/5
FP-1/2/3
C
D
Fig. 30.11 Osteoplasty. (A) Because of bone resorption and resultant Division B ridge, it is often neces-
sary to reduce the height of bone via osteoplasty. (B) Osteoplasty of the mandibular anterior ridge to gain
width of bone and increase interocclusal space. (C) In general, for a xed prosthesis (e.g., FP-1, FP-2,
FP-3), minimal osteoplasty should be completed to minimize crown height space issues. For a removable
prosthesis (e.g., RP-4, RP-5), a more aggressive osteoplasty should be performed to increase space for
prosthetic rehabilitation. (D) Implant placement for a removable implant overdenture with insufcient inter-
occlusal space leading to the nonrestorability of the implants.

746
PART VI Implant Surgery
bone loss) related to the exure/torsion of the mandible, the pros-
thesis should ideally be re-fabricated to allow for stress relieve for
exure and torsion forces. is is most likely completed by mak-
ing the prosthesis in more than one piece.
Bony Anatomic Areas
Posterior Lingual Undercut
In the posterior mandible, it is imperative the implant clinician
have detailed knowledge of the three-dimensional anatomy of the
area. A lingual undercut is often present, which may lead to com-
plications with life-threatening consequences.
Parnia et al.
32
classied posterior lingual concavities into
three types: type 1 (20%): at depressions less than 2 mm in
depth, type 2 (52%) occur with 2 to 3 mm in depth, type 3
(28%) showed signicant concavities of more than 3 mm.
Nickenig etal.
33
classied posterior mandible morphology to be
U-shaped (undercut), P-shaped (parallel), and C-shaped (con-
vex). Lingual undercuts had a prevalence rate of 68% in the
molar region, with the prevalence rate far greater in the second
24.91
Fig. 30.12 Posterior Mandible. Because of the bone morphology and
resorptive patterns of the posterior mandible, this anatomic area is often
difcult to treat.
C
DE
A
B
Fig. 30.13 Augmentation Versus Short Implants. (A) Due to extensive atrophy, posterior resorption
results in a Division D ridge that contraindicates bone augmentation. (B) Short implant placement that pre-
vents nerve impairment, but predisposes patient to biomechanical issues. (C to E) Vertical augmentation
graft: large defect augmented with autogenous bone, with delayed implant placement.

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73730Mandibular Anatomic Implications for Dental Implant SurgeryRANDOLPH R. RESNIKIn dental implantology today it is imperative the clinician have a strong understanding of the surgical anatomy and variations with respect to implant placement in the mandible. Before the commencement of dental implant surgery, a careful and detailed evaluation should be completed of the mandibular vital structures. is may be accomplished by including a clinical evaluation, a visual examination, along with palpation of the anatomic areas. e clinician should have a clear and concise three-dimensional (3D) vision of the anatomic structures in relation to the intended implant surgical procedure.A thorough radiographic examination needs to be completed to provide information concerning the location and topography of the 3D anatomy. In this chapter a comprehensive evaluation of the important mandibular anatomic areas will be discussed, together with their clinical relevance in dental implant surgery (Fig. 30.1).Mandibular AnteriorHourglass Anterior Mandiblese mandibular anterior region has historically been considered one of the safest and most predictable regions for implant place-ment. e predictability stems from the favorable quality of bone (i.e., thick cortical and dense trabecular bone) most commonly present in this area. e morphology of the mandibular anterior has been classied as the following shapes: hourglass, ovoid, pear, sickle, and triangular. e pear shape, which is usually abundant with bone, has been shown to be the most common among eden-tulous and dentate patients.1However, this anatomic area may be compromised by a nar-row alveolar width or severe osseous constriction. ese types of bony variations have been termed an hourglass eect, which is usually indicative of a developmental abnormality. Hourglass mandibles, which have been shown to have an incidence rate of approximately 4%, should always be concerning to the cli-nician because of possible perforations during implant place-ment surgery.2 e position of the alveolar constriction may vary signicantly because they have been shown to be high, low, or variable within the alveolus. A thorough 3D cone beam computed tomographic (CBCT) examination should be com-pleted to prevent complications in this area, and guided surgery is recommended to minimize the possibility of perforations3 (Fig. 30.2).Butura etal.4 classied mandibular anterior constrictions as: (1) facial constriction, (2) lingual constriction, and (3) hourglass constriction. ey discussed various treatment options to include alveoplasty to a level beyond the constriction, staged bone graft reconstruction, posterior and anterior angled implants to avoid the site, and extra-long implants to bypass the constriction and engage the inferior border of the mandible.4Clinical RelevanceDue to the variable expression of hourglass mandibles, treatment strategies are based on the location and extent of the undercut. In some cases implant placement in this area will be contrain-dicated. In less severe constrictions an osteoplasty may be per-formed, together with implant placement. However, the crown height space may be increased signicantly, leading to possible biomechanical issues. In addition, other constrictions may require grafting procedures to increase bone volume for implant place-ment (Fig. 30.3).If the positioning of a dental implant leads to perforation of the bony mandibular plates, possible life-threatening hemorrhage episodes may occur. ese events have been reported when a drill perforates the lingual plate of the sublingual region of the man-dible and traumatizes a sublingual or submental artery, especially in the canine region.5,6 If perforation of the lingual cortical plate is associated with arterial bleeding, it is critical to identify its ori-gin and treat aggressively. e origin of bleeding in the oor of the anterior region of the mouth may be from the lingual artery, facial artery, or one of its branches. Perforation of either the sub-mental artery (originates from the facial artery) or the sublingual artery (originates from the lingual artery) may lead to bony perfo-ration and bleeding, causing an expanding ecchymosis (sublingual hematoma) and compromising the airway. If this should occur, the patient should be repositioned in an upright position, and bimanual pressure should be applied to the area of bleeding. If the airway is compromised, immediate emergency assistance should be summoned (Fig. 30.4).  738PART VI Implant SurgeryMedian Vascular CanalIn the mandibular midline, radiographic examination often reveals the presence of a radiolucent canal, which is termed a median vas-cular canal. is canal houses the bilateral sublingual arteries that enter the lingual foramen, which is located on the lingual aspect of the mandible. e lingual foramen is seen as a radiopacity below the genial tubercles, which is visible on approximately 52% of CBCT scans.7 is arterial anastomosis may transverse anteriorly, inferiorly, or superiorly within the anterior mandible, in some instances exiting the facial aspect of the symphysis area. Various studies have shown median vascular canals to be present in 100% of cases, detected on CBCT examinations. Two-dimensional (2D) panoramic radiographs observe their presence only 4.2% of the time.8 is is most likely due to the superimposition of the cervi-cal vertebrae and to the orientation of the panoramic beam in relation to the position of the canals. Gahleitner etal.9 reported one to ve canals per patient with an average diameter of 0.7 mm, with a range of 0.4 to 1.5 mm (Fig. 30.5).e presence and size of the sublingual anastomosis and the median vascular canal are easily seen on a cross-sectional or axial image of a CBCT scan. In approximately 31% of lingual vascular canals the diameter exceeds at least 1 mm.10 e sublingual artery is a branch of the lingual artery that originates from the external carotid artery. e lingual artery courses medially to the greater horn of the hyoid bone and crosses inferiorly and facially around the hypoglossal nerve. It then transverses deep to the digastric and stylo-hyoid muscles, and courses between the hyoglossus and genioglos-sus muscles. ere exist four main branches of the lingual artery: the suprahyoid, dorsal lingual, deep lingual, and sublingual (Fig. 30.6).Clinical RelevanceWhen planning implants in the anterior mandible, if a large anasto-mosis is present, the position may be modied to prevent encroach-ment on the structure. If this area is violated, excessive bleeding may result. e intraosseous bleeding is usually well controlled by plac-ing an implant, direction indicator, or surgical bur in the osteotomy site. ere will be no neurosensory issue with encroaching on this area because there are no sensory bers within the canal (Fig. 30.7). Severely Angled Anterior Mandibleere exists one uncommon subcategory of Division C, namely C−a (i.e. Division C bone with excessive angulation). In this cate-gory, available bone is adequate in height, but angulation is greater than 30 degrees regardless of implant placement. When present, this condition is most often found in the anterior mandible. For ideal implant placement, usually bone augmentation is required. However, a diagnostic wax-up should be completed rst because Division C−a mandibles are usually associated with skeletal Class III patients (Fig. 30.8).ABC• Fig. 30.1 Variable Mandibular Anatomy. The mandibular arch varies dramatically with respect to the amount of hard and soft tissue resorption. (A) Mandibular arch with a signicant amount of bone and keratinized tis-sue. (B) As bone resorption occurs, the loss of the attached tissue results. (C) Advanced resorption of the hard and soft tissues resulting in a severely atrophic mandible with minimal attached tissue.AB• Fig. 30.2 Constricted Mandibular Anterior. (A) Cone beam computed tomographic panoramic image that depicts a signicant amount of avail-able bone; however, it does not indicate a constriction is present. (B) Cross-sectional images showing the hourglass appearance of the anterior mandible, which contraindicates implant placement. 739CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryClinical RelevanceRoot form implants placed in Division C−a will lead to poorly posi-tioned implants that will most likely be nonrestorable for a xed prosthesis. is will most likely result in an overcontoured pros-thesis, speech diculty, compromised tongue space, and inability to obtain an ideal occlusion. erefore in most cases, a staged bone graft and implant treatment plan should be formulated. Lack of Keratinized TissueAs the mandibular osseous process progresses, the presence of kera-tinized tissue becomes more compromised. In general, implants are healthiest when there exists sucient keratinized tissue. Some reports indicate the lack of keratinized tissue may contribute to implant failure.11 Mobile, nonkeratinized mucosa has been shown to exhibit greater probing depths, which has been conrmed his-tologically. e absence of keratinized mucosa also increases the susceptibility of peri-implant regions to plaque-induced destruc-tion.12 Additional studies have shown that mobile mucosa may disrupt the implant-epithelial attachment zone and contribute to an increased risk for inammation from plaque.13 For larger edentulous ridges the zone of attached tissue on the facial ap (mandible) provides greater resistance for the sutures against ten-sion of the mentalis muscle in the anterior region and the buccina-tor muscle in the molar and premolar regions, which often cause incision line opening. As a result, an incision made facial to the attached tissue may cause partial ischemia to some of the crestal tissue. In addition, the incision in unkeratinized facial tissue may sever larger blood vessels, which increases bleeding and decreases vision during surgery, while also potentially complicating nal suturing (Fig. 30.9).Clinical RelevanceFor implant sites, an evaluation of the quality and quantity of kera-tinized tissue should be completed. If insucient attached tissue is present, tissue augmentation procedures should be completed before implant placement. For larger edentulous sites, especially in the mandible, the incision may be modied to maintain the attached tissue in some cases. If the crest of the ridge is above the oor of the mouth, and there exists greater than 3 mm of attached, keratinized gingiva on the crest of the ridge, a full-thickness inci-sion is made, bisecting the attached tissue. If less than 3 mm of attached gingiva exists on the ridge, the full-thickness incision is made more to the lingual so that at least 1.5 mm of the attached tissue is to the facial aspect of the incision line. Another treatment option includes the use of an acellular dermis (e.g., OraCell; Sal-vin Dental Corp.). Acellular dermis may be placed at the time of implant placement, thereby increasing the thickness and quality of the tissue while the integration of the implants is taking place (Fig. 30.10). a. Pearshape1. RidgeaugmentationOr2. Osteoplasty1. Ridgeaugmentation1. Ridgeaugmentation1. Ridgeaugmentation1. Ridgeaugmentation1. OsteoplastyNotreatmentb. Sickleshapee. Buccalconstrictionf. Lingualconstrictiong. Hourglassc. Ovoid d. TriangularTreatment• Fig. 30.3 Anterior Mandible Cross-Sectional Morphology and Treatment. (A) Pear shape. (B) Sickle shape. (C) Ovoid. (D) Triangular. (E) Buccal constriction. (F) Lingual constriction. (G) Hourglass. 740PART VI Implant SurgeryABCDE• Fig. 30.4 Mandibular Anterior Perforation. (A) Implant treatment plan showing perforation of the inferior border of the mandible. (B) Lingual constriction that may lead to bleeding complications, along with chronic tissue irritation. (C to E) Implant placement in a constricted ridge leading to nonideal implant placement. 741CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryAB• Fig. 30.5 Median Vascular Canal. (A) Canal extending close to buccal plate and then superiorly to almost the crestal area. (B) Canal extending inferiorly and then superiorly.• Fig. 30.6 Rare off-midline vascular canal.• Fig. 30.7 Implant placement into the canal may result in bleeding epi-sodes.AB• Fig. 30.8 Division C−a Mandible. (A) Mandibular anterior Division C−a with extreme angulation. (B) Cross-sectional image of a Division C−a mandible. 742PART VI Implant SurgeryInadequate Width of BoneA common consequence of tooth loss and bone remodeling in the mandibular anterior region is the resultant narrowing and knife-edge conguration of the mandibular bony ridges (i.e., Division B available bone). Pietrokovski etal.14 evaluated eden-tulous ridges in human jaws and found that 43% of mandibular anterior ridges were knife-edge and 38% in the premolar region. Nishimura et al.15 reported a higher incidence of mandibular knife-edge ridges in females compared with males, mainly because of increased osteopenia changes with unfavorable bone mineral density values.Clinical Relevanceerefore before implant placement, it is often necessary to reduce the bone width (i.e., osteoplasty), which results in an increased horizontal width of available bone. An osteoplasty may be carried out with various methods, including osteoplasty burs, barrel burs (i.e., acrylic burs), rongeurs, bone chisels, and Piezosurgery units. By increasing the width of bone, dental implants may be placed with sucient bone on the buccal (∼2.0 mm) and on the lingual (∼1.0 mm).Although an osteoplasty increases available bone for implant placement, many detrimental eects may result. Reduction of a knifelike ridge will decrease the amount of cortical bone present. e cortical bone is a crucial component for primary stability of the implant and is responsible for a greater stress distribution. In addition, as the bony height is reduced, the crown height space increases. e increased crown/implant ratio results in greater strain at the peri-implant interface, which predisposes the implant to biomechanical complications.e amount of osteoplasty required should be determined before surgery via the use of CBCT treatment planning. In gen-eral, if a xed prosthesis (e.g., FP-1, FP-2, FP-3) is indicated, a minimal osteoplasty is recommended. By minimizing the reduc-tion of bone height, the possibility of a crown/implant ratio prob-lem resulting is decreased. In some cases bone augmentation may be required to maintain the height of bone and increase the bone width, rather than reducing the ridge by means of an osteoplasty. If a removable prosthesis (e.g. RP-4, RP-5) is indicated, a more aggressive osteoplasty is recommended, because this will allow for increased space for the removable prosthesis (i.e., thickness of acrylic, attachment space). In general, the greater the interocclusal space, the less likely a prosthesis or attachment fracture can occur (Box 30.1 and Fig. 30.11). Mandibular PosteriorAlthough the posterior mandible is considered a predictable ana-tomic area for implant placement, there are many drawbacks that include compromised available bone in height and width, sig-nicant undercuts, dicult access, and numerous vital structures that may be damaged (e.g., inferior alveolar nerve [IAN], mental nerve, submandibular gland). Iatrogenic violation of these vital structures may result in neurosensory disturbance, pain, infec-tion, excessive bleeding, and compromised implant positioning (Fig.30.12). Lack of Bone Heighte posterior mandible resorbs from buccal to lingual, transform-ing from a Division A to a Division B rather rapidly. Because of the trajectory of the posterior mandible, implant placement in an ideal position for prosthetic rehabilitation may be dicult. When limited alveolar ridge height exists, four options are usu-ally available: (1) no treatment, (2) vertical ridge augmentation with delayed implant placement, (3) vertical bone augmentation with simultaneous implant placement, and (4) the use of short implants.16-19Vertical Bone Augmentation (With Simultaneous or Delayed Implant Placement)With severely resorbed alveolar ridges in the posterior mandible, the available bone height for standard implant placement is often limited by the proximity of the mandibular canal. Vertical bone augmentation is an option for increasing the ridge dimensions, thereby allowing for placement of standard-length implants. By increasing the bone height, esthetics and biomechanical compli-cations are less likely to complicate the longevity of the implant prosthesis.However, increasing bone height in the posterior mandible is one of the most challenging procedures in implant dentistry. To increase the available bone, various techniques, including autogenous block grafts, guided bone regeneration, and distrac-tion osteogenesis, have been discussed in the literature. However, an increased rate of surgical complications and enhanced patient morbidity have been associated with these types of procedures. Shorter ImplantsRecently the use of short implants (∼8 mm) in the atrophic poste-rior mandible has been introduced to circumvent the need for ver-tical bone augmentation. Because the loss of vertical bone height is often associated with inadequate available bone for implant AB• Fig. 30.9 (A) Maxillary anterior ridge with compromised attached tis-sue. (B) When less than 3 mm of attached gingiva is present, the incision for a full-arch mandible should include making the incision more lingual and extending to the lingual aspect of the mandible when there is nerve dehiscence. 743CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryABDEFCG• Fig. 30.10 (A) Anterior mandible with minimal attached, keratinized tissue and implant placement. (B and C) OrACELL acellular dermis. (D) Four-millimeter holes are made with a biopsy punch to t over the neck of the implants. (E) Acellular dermis placed with healing abutments. This technique has the advantage of increasing tissue quantity in conjunction with implant healing. (F and G) Before and after case of implant placement and acellular dermis. 744PART VI Implant Surgeryplacement, the safety zone (2-mm distance between the implant and nerve canal) is sometimes compromised with conventional length implants. If this occurs, an increased possibility of a neuro-sensory impairment may result.erefore the use of shorter length implants oers the clinician many advantages in comparison with vertical bone augmentation: it is a less invasive surgery, less surgical experience is required, it is less expensive, and it has a faster treatment time. However, shorter implants do have drawbacks: they may result in an increased crown height space, less surface area in comparison with standard length implants, and a possible higher rate of biological and tech-nical complications from occlusal overload.In summary, studies on the use of short implants (∼8 mm) are promising.20,21 Many factors should be evaluated when deciding whether short implants should be used instead of vertical bone aug-mentation. Force factors (e.g., opposing occlusion, parafunction) must be favorable, and an implant-protected occlusion should adhered to. In addition, with short implants, the widest diameter implant possible should be selected, along with an increased num-ber of implants. e nal prosthesis involving multiple implants should always be splinted for greater force distribution (Fig. 30.13). Mandibular Deformation (Flexure of the Mandible)Full-arch implant-supported prostheses with a rigid substructure have become controversial in implant dentistry because of the associated increased strain at the bone-implant interface. Because of the rigid bone-implant interface that is associated with dental implants, jaw deformation may transmit excessive stress, which can result in complications. In the literature, pain has been associated with full-arch rigidly splinted prostheses.22,23 Gates and Nicholls24 reported on deformation of impression material when full-arch impressions are taken with the mouth wide open. e inaccuracies may result in ill-tting or nonpassive superstructures in dierent jaw positions. In addition, mandibular deformation has been asso-ciated with loosening of full-arch implant-supported prostheses and possible fractures of prostheses during mastication.25,26EtiologyFlexure. e body of the mandible exes distal to the foramen on opening and has torsion during heavy biting, with potential clinical signicance for full-arch implant prostheses. Many reports have addressed the dimensional changes of the mandible during jaw activity as a result of masticatory muscle action. Five dierent movements have been postulated. Medial convergence is the one most commonly addressed.27 e mandible between the mental foramina is stable relative to exure and torsion. However, distal to the foramina, the mandible exhibits considerable movement toward the midline on opening.28 is movement is caused pri-marily by the attachment of the internal pterygoid muscles on the medial ramus of the mandible. e distortion of the mandible occurs early in the opening cycle, and the maximum changes may occur with as little as 28% opening (or about 12 mm). is exure has also been observed during protrusive jaw movements.29 e greater the active opening and protrusive movements, the greater the amplitude of mandibular exion. e amount of movement varies among individuals and depends on the density and volume of bone and the location of the site in question. In general the more distal the sites, the more medial is exure. e amplitude of the mandibular body exure toward the midline has been mea-sured to be as much as 800 μm in the rst molar-to-rst molar region to as much as 1500 μm in the ramus-to-ramus sites. Torsion. Torsion of the mandibular body distal to the foramina has also been documented in both animal and human studies. Hylander30 evaluated larger members of the rhesus monkey family (macaque) and found the mandible twisted on the working side and bent in the parasagittal plane on the balancing side during the power stroke of mastication and unilateral molar biting. Parasagit-tal bending of the human jaw during unilateral biting was con-rmed by Abdel-Latif etal.,31 who showed patients with implant prostheses measured up to 19 degrees of dorsoventral shear.e torsion during parafunction is caused primarily by forceful contraction of the masseter muscle attachments. Parafunctional bruxism and clenching may cause torsion-related problems in the implant support system and prosthesis when the mandibular teeth are splinted from the molar-to-molar regions.Implants placed in front of the foramina and splinted together or implants in one posterior quadrant joined to ante-rior implants have not shown these complications related to the exure or torsion of the mandible. Complete implant-supported xed restorations can halt the posterior bone loss associated with edentulism, improve psychological health, and produce fewer prosthetic complications than removable restorations. All eden-tulous mandibular patients should be given the option of having a xed prosthesis. However, the increase in forces of mastication, increase in force with patients of greater force factors (e.g., para-function, crown height space, opposing arch type), or reduced bone density in the implant sites warrant an increase in implant number or implant position in anterior and posterior implant sites (Fig. 30.14). Preventione concept of exure and torsion does not aect the maxilla, where all implants are often splinted together, regardless of their positions in the arch. Prevention of mandibular exure should include the following treatment plans: Bilateral posterior implants: If implants are positioned bilaterally in the premolar/molar regions of the mandible, the nal prosthe-sis should be fabricated with two sections. is will minimize the possibility of exure/torsion issues. Usually, the prosthesis is. splint in the premolar area.Anterior implants with unilateral implants posterior: With implant support on only one posterior side, full-arch splinted prosthe-ses will not be subject to the exure/torsion problems.Anterior implants with no posterior implants: With no posterior im-plant support, full-arch splinted prostheses may be fabricated without concern regarding exure/torsion problems. TreatmentIf a full-arch splinted prosthesis is fabricated and the patient exhibits complications (e.g. pain, diculty opening, posterior 1. Fixed implant prosthesis (FP-1, FP-2, FP-3) a. Porcelain fused to metal: 10 mm b. Zirconia: 8 mm 2. Removable implant prosthesis (RP-4, RP-5) a. Attachments (no bar): 9 mm (e.g., Locator) b. Bar + attachment: 15 mm • BOX 30.1 Approximate Minimum Interocclusal Space (Bone Level to Incisal Edge) 745CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryA BRP-4/5FP-1/2/3CD• Fig. 30.11 Osteoplasty. (A) Because of bone resorption and resultant Division B ridge, it is often neces-sary to reduce the height of bone via osteoplasty. (B) Osteoplasty of the mandibular anterior ridge to gain width of bone and increase interocclusal space. (C) In general, for a xed prosthesis (e.g., FP-1, FP-2, FP-3), minimal osteoplasty should be completed to minimize crown height space issues. For a removable prosthesis (e.g., RP-4, RP-5), a more aggressive osteoplasty should be performed to increase space for prosthetic rehabilitation. (D) Implant placement for a removable implant overdenture with insufcient inter-occlusal space leading to the nonrestorability of the implants. 746PART VI Implant Surgerybone loss) related to the exure/torsion of the mandible, the pros-thesis should ideally be re-fabricated to allow for stress relieve for exure and torsion forces. is is most likely completed by mak-ing the prosthesis in more than one piece. Bony Anatomic AreasPosterior Lingual UndercutIn the posterior mandible, it is imperative the implant clinician have detailed knowledge of the three-dimensional anatomy of the area. A lingual undercut is often present, which may lead to com-plications with life-threatening consequences.Parnia et al.32 classied posterior lingual concavities into three types: type 1 (20%): at depressions less than 2 mm in depth, type 2 (52%) occur with 2 to 3 mm in depth, type 3 (28%) showed signicant concavities of more than 3 mm. Nickenig etal.33 classied posterior mandible morphology to be U-shaped (undercut), P-shaped (parallel), and C-shaped (con-vex). Lingual undercuts had a prevalence rate of 68% in the molar region, with the prevalence rate far greater in the second 24.91• Fig. 30.12 Posterior Mandible. Because of the bone morphology and resorptive patterns of the posterior mandible, this anatomic area is often difcult to treat.CDEAB• Fig. 30.13 Augmentation Versus Short Implants. (A) Due to extensive atrophy, posterior resorption results in a Division D ridge that contraindicates bone augmentation. (B) Short implant placement that pre-vents nerve impairment, but predisposes patient to biomechanical issues. (C to E) Vertical augmentation graft: large defect augmented with autogenous bone, with delayed implant placement. 0.8 mmBAContraction ExpansionC• Fig. 30.14 (A) The mandible exes toward the midline on opening or during protrusive movements as a result of the internal pterygoid muscle attachments on the ramus. The mandible also torques, with the inferior border rotating out and up, and the crestal region rotating lingually. The movement is caused by the masseter muscles during forceful biting or parafunction. (B) The amount of exure depends on the amount of the bone volume and the sites in question. The medial movement from the rst molar to the rst molar region may be 800 μm. (C) Unilateral molar biting causes the mandible to undergo torsion with the bottom of the mandible expanding outward and the crest of the mandible rotating medially. (Adapted from Hylander WL. Mandibular function in Galago crassicaudatus and Macaca fascicularis: an invivo approach to stress analysis of the mandible. J Morphol. 1979;159:253-296.) 748PART VI Implant Surgerymolar region (90%) than in the rst molar region (56%).33 Other studies have shown that lingual undercuts occur in approximately 66% of the population, with a mean undercut of 2.4 mm (Fig. 30.15).34 Clinical Relevance If perforation of the lingual plate is made with either the surgical drill or implant placement, life-threatening situations may result from sublingual bleeding. Within the lingual undercut area, the sublingual and submental arteries are present. Trauma to either of these arteries can result in a sublingual hematoma and airway compromise.If the perforation were to occur above the mylohyoid muscle, damage to the lingual nerve may result in a neurosensory impair-ment. If an implant is inserted in this area that extends into the undercut, constant irritation from the extruded implant in the soft tissue may cause the patient chronic pain. In some cases viola-tion may predispose the patient to infection. A clinical examina-tion should always be carried out to determine whether an osseous undercut exists. is may be conrmed with a CBCT examina-tion because cross-sectional images are an eective way of observ-ing lingual undercuts. Violation of this area may cause infection or constant irritation from the extruded implant in the soft tissue.In addition to the blood vessels in the sublingual area, two sali-vary glands are also present. e submandibular fossa is a depres-sion on the medial surface of the posterior mandible, which is inferior to the mylohyoid ridge. Within this fossa the subman-dibular gland is present. Anterior to the submandibular gland is the sublingual fossa, which is present on both sides of the men-tal spine. e sublingual gland is found in the sublingual fossa. e submandibular and sublingual fossae should be palpated and evaluated before implant osteotomies. In this area, perforation of the lingual plate may damage either of the glands, resulting in possible infection.Accurate measurements must be determined to prevent over-preparation of the osteotomy site in the posterior mandible. is is most easily completed with a CBCT examination. A clinical examination and palpation of the bone ridge at the proposed implant sites should also be completed. Osteotomy angulation should be carefully evaluated because improper drilling angulation may also lead to perforations. Perforations may lead to infection that can spread to the parapharyngeal and retropharyngeal space. Infections in these spaces progress to severe complications, such as mediastinitis, mycotic aneurysm, internal jugular vein thrombo-sis, or upper airway obstruction.35 ese complications may occur immediately or can be delayed and should be treated aggressively.Shorter implants with a tapered design have been shown to be benecial in avoiding lingual bone perforations.36 Ideally, implants should always be positioned along the long axis of the occlusal forces, therefore implants should not be placed at an excessive angulation (>30 degrees) to avoid undercuts. de Souza etal.37 has shown that the submandibular fossa has a direct inu-ence on implant placement (i.e., implant size, position, and angu-lation) 20% of the time (Fig. 30.16). Vascular ConsiderationsIncisive Canal Vesselse incisive artery is the second terminal branch of the inferior alveolar artery, which is a branch of the maxillary artery. e inci-sal branch continues anteriorly after supplying the mandibular rst molar area, where it innervates the incisor teeth and anasto-moses with the contralateral incisal artery. In rare cases the inci-sive canal is large, lending to greater bleeding during osteotomy preparation or bone-grafting procedures. e exact location of the incisive canal is easily determined via a CBCT evaluation in the panoramic or sagittal views.Clinical RelevanceClinicians often confuse the incisive canal with an anterior loop of the mental nerve. e nerve, artery, and vein within this canal may cause bleeding episodes if traumatized. Usually the place-ment of the implant, a direction indicator, or surgical bur can be placed into osteotomy to apply pressure to allow for the clotting process. Inferior Alveolar Arterye inferior alveolar artery is a branch of the maxillary artery, one of the two terminal branches of the external carotid. Before enter-ing the mandibular foramen, it gives o the mylohyoid artery. In approximately the rst molar region, it divides into the mental and incisal branches. e mental branch exits the mental foramen and supplies the chin and lower lip, where it eventually will anas-tomose with the submental and inferior labial arteries. e exact location of the inferior alveolar artery is easily determined via a CBCT evaluation in the panoramic or sagittal views.Clinical RelevanceNormally the inferior alveolar artery is located superiorly to the Inferior alveolar nerve (IAN) IAN within the bony mandibular canal. Drilling or placing an implant into the inferior alveolar canal may predispose to signicant bleeding. Some authors have recommended the placement of an implant or direction indica-tor short of the canal to control the bleeding; however, this may lead to possible neurosensory disturbances from hematoma or local irritation to the IAN canal. A 2.0-mm safety zone should be established to prevent complications in this area. If bleeding does occur, follow-up postoperative care is essential because hematoma formation within the canal may lead to a neurosensory impair-ment. is condition should be monitored because it may prog-ress to respiratory depression via a dissecting hematoma in the oor of the mouth. Buccal ArteryA common donor site for autogenous grafting is the lateral ramus area in the posterior mandible. When making the incision lateral to the retromolar pad, a common blood vessel to sever is the buc-cal artery. e buccal artery is a branch of the maxillary artery and will most likely cause a signicant bleeding episode. is artery runs obliquely between the internal pterygoid and the insertion of the temporalis on the outer surface of the buccinator.Clinical RelevanceIn most cases, damage to the buccal artery is very dicult to avoid. Incision and reection will usually encompass the area of buccal artery location. When performing surgery in this area, a hemostat should always be available for immediate access to clamp the vessel. A curved hemostat should be used to clamp the vessel, thus decreasing the bleeding. It should be left in place for 3 to 5 minutes. If bleeding persists, a ligature may be placed with Vicryl suture material (i.e., resorbable) (Fig. 30.18).  749CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryABCDE• Fig. 30.15 Posterior Lingual Undercut. (A) Large sublingual posterior undercut. (B) Of concern is the possibility of lingual perforation. (C and D) Implant dimension measurements should never be made from cone beam computed tomographic panoramic views because third dimension of bone needs to be deter-mined. (E) Implant placement without regard to sublingual undercut results in migration of the dental implant into sublingual space. 750PART VI Implant SurgeryFacial Arterye facial artery is a branch of the external carotid artery, lying superior to the lingual artery and medial to the ramus of the mandible. It courses below the digastric and stylohyoid muscles, and passes through a groove in the submandibular gland before it becomes supercial around the inferior border of the mandible. ere are two main branches of the facial artery: the facial and cervical. e facial branch encompasses ve branches, which sup-ply the eye, nose, and lips. ere are four branches of the cervi-cal region, supplying the pharynx, soft palate, auditory tube, and submandibular gland.Clinical RelevanceExcessive retraction in this area may lead to trauma to the facial artery. If bleeding from the facial artery exists, pressure should immediately be applied to the angle of the mandible over the vessel. Usually immediate medical assistance will need to be summoned. Neural ConsiderationsLingual Nervee lingual nerve is a branch of the trigeminal nerve that pro-vides sensory innervation to the mandibular lingual tissue and the ABCDEF• Fig. 30.16 Variable Posterior Bony Anatomy. (A) Straight, no angulation. (B) Slight angulation with mini-mal lingual undercut. (C) Larger lingual undercut resulting from more extensive atrophy. (D) More signicant angulation resulting from buccal bone resorption. (E and F) Extensive lingual undercuts. AB3rd Molar(17)2nd Molar(18)Low Lingual Nerve1stMolar2ndMolarLNSuperior Lingual Alveolar CrestHigh Lingual Nerve1st Molar(19)• Fig. 30.17 Lingual Nerve Anatomy and Variant Positions. (A,B) Note the proximity to the crest of the ridge in the “high” variant position. A lingually placed incision or excessive retraction may cause damage to the lingual nerve. (From Benninger B, Kloenne J, Horn JL. Clinical anatomy of the lingual nerve and identification with ultrasonography. Br J Oral Maxillofac Surg. 2013;51:541-544.)Externalcarotid arteryBuccalarterySuperficialtemporal arteryMaxillaryartery Mylohyoid arteryMentalarteryIncisorbranch• Fig. 30.18 Buccal artery location and most common arteries in the head and neck. 752PART VI Implant Surgeryanterior two-thirds of the tongue. e lingual nerve is a concern for implant clinicians because it may be damaged during reection of the lingual ap. e lingual nerve is most commonly found 3 mm apical to the alveolar crest and 2 mm horizontal from the lingual cortical plate. However, 22% of the time, it may contact the lingual cortical plate.38 Variations of this nerve have reported it to be located lingual to the third molar area, at or above the crest of the bone.39Clinical RelevanceWhen elevating tissue in the posterior mandible, always maintain the retractor on the bone and minimize stretching of the tissue on the lingual aspect of the mandible. e lingual nerve is very sus-ceptible to neuropraxia types of nerve impairments. In addition, no lingual vertical release incisions should be used because of the variant lingual nerve anatomy. In addition, in the posterior ramus area, incisions shoudl always be lateral lateral to the retromolar pad because the lingual nerve transverses this area in 10% of cases (Fig. 30.17). Inferior Alveolar NervePrevention of iatrogenic injuries to the third division of the tri-geminal nerve is paramount in implant dentistry today. A resultant neurosensory impairment in the head and neck region may aect the patient’s quality of life and may present potentially signi-cant medicolegal problems for the clinician. To prevent damage to these vital nerve structures, it is imperative for the implant den-tist to have a comprehensive radiographic survey of the region, thorough knowledge of the normal versus variant anatomy, and awareness of intraoperative surgical techniques to minimize the possibility of nerve impairment (Fig. 30.19). Radiographic ConsiderationsTwo-Dimensional RadiographyToday the use of 2D radiography is becoming less common for dental implant treatment planning. Two- dimensional radio-graphs, mainly panoramic, have many inherent disadvantages in evaluating potential implant sites. All panoramic (2D) radio-graphs exhibit some degree of distortion, nonuniform magnica-tion, and image superimposition, which can potentially lead to incorrect measurement and assessment of neural structures. Stud-ies have shown periapical and panoramic radiography to be unreli-able in assessing the true location of the inferior alveolar canal and the mental foramen.40 Extreme caution should be exercised when using 2D radiography as the only modality for implant site evalu-ation (Fig. 30.20). Two-dimensional radiographs may be used for initial assessment of potential implants sites.Even with the generalized acceptance of CBCT radiography in the diagnosis and treatment planning of dental implants, numerous manufacturers still make available to implant dentists magnication guides and digital software programs for intraoral 12 %20 – 45 %10 - 70 %40 %20 %50 %˜˜AB• Fig. 30.20 Two-Dimensional Radiographs. (A) Panoramic radiographs are inherently inaccurate and do not show the accurate position of the inferior alveolar canal or mental foramen. (B) Panoramic radiographs have a variable magnication factor in the vertical and horizontal planes.AB• Fig. 30.19 Posterior Mandible Complication. (A) Implant osteotomy violating the inferior alveolar canal from poor preoperative planning. (B) Implant placement resulting in a neurosensory decit. 753CHAPTER 30 Mandibular Anatomic Implications for Dental Implant Surgeryradiographs to assist in the placement of implants over vital struc-tures. e clinician should be aware that panoramic radiographs have variable magnication (i.e., not 25% as related by many implant and panoramic companies), and even calibrated intraoral software programs cannot accurately assess true distances because of their 2D origin. Both periapical and panoramic radiography are associated with magnication that is inconsistent and dicult to determine. Schropp etal.41 have shown that in more than 70% of cases in which implant size was initially determined via panoramic radiographs, the implant size had to be altered after CBCT evalua-tion. Magnication guides should never be used as the sole criteria for implant site evaluation because they may lead to overestima-tion of available bone dimensions. Three-Dimensional RadiographyIn most cases a 3D radiographic modality is recommended for evaluation of the mandibular arch and related nerve anatomy. Studies have shown approximately 50% of nerve injuries resulted from inadequate radiographic assessment.42,43 erefore to deter-mine the ideal location and measurement parameters associated with the dental implant placement, the clinician must be able to accurately measure the distance between the alveolar crest and the superior border of the mandibular canal, as well as the width of bone in the proposed implant site. Medical slice computed tomography (MSCT) and CBCT images have been shown to be the most accurate radiographic modalities in the assessment of available bone and identication of the IAN.44 A thorough knowledge of the relative 3D position of the IAN is crucial in preventing mandibular nerve impairment before implant place-ment (Fig. 30.21).Because MSCT and CBCT have been shown to be 1:1 (no magnication), the implant dentist has the ability to place implants, measure available bone, evaluate bone density, deter-mine the prosthetic treatment plan, and order surgical templates directly from their computerized treatment plan. Interactive treat-ment planning software programs available today contain libraries of most implants systems, which allow the clinician to accurately access the size, type, and ideal placement of the implant in relation to anatomic structures. is virtual treatment plan may then be transferred to the patient’s surgery by means of a surgical template or computer-assisted navigation system.For clinicians early on their learning curve, the fabrication of a bone model can be an invaluable preoperative diagnostic tool. Bone models are made directly from the CBCT dicom (.dcm) data, which are easily fabricated with in-oce 3D printers. e AB• Fig. 30.21 (A) Cone beam computed tomographic image depicting cross-sectional, axial, panoramic, and three-dimensional images. (B) Virtual treatment plan showing the mandibular nerve in relation to place-ment of an implant. 754PART VI Implant Surgeryclinician is able to evaluate the exact osseous morphology (width of bone, undercuts, bony landmarks) and location of vital structures (color coded within the model) before the actual surgery. Implant osteotomies may be performed in a laboratory setting to allow the implant dentist to complete the procedure before surgery.Neurosensory impairment issues are most frequently an inad-vertent sequela of improper diagnosis, treatment planning, or sur-gical technique. Many of these complications can be overcome by using 3D surgical guides for the ideal positioning and placement of implants. Basically the surgical guide is the conduit for trans-ferring the interactive treatment plan from the computer to the patient’s actual surgical procedure. is allows the implant dentist to be able to place the implants in the exact location as per the treatment plan. Surgical guides are categorized based on method of retainment: tooth, bone, or mucosa supported. In addition, guides are distinguished by the surgical technique involved: fully guided—all osteotomies and implant placement is completed through the guide; universal guide—all osteotomies except for the nal drill and implant placement are completed through the guide; and pilot—only the rst or initial drill is used through the guide. Guided surgery with surgical templates has been reported to improve the accuracy of implant placement in clinical situa-tions in comparison with conventional surgical methods (Fig. 30.22).45 Nickenig et al.46 showed that implants placed with surgical templates were within 0.9 mm of the planned positions, whereas free-hand placement resulted in deviations of approxi-mately 2 to 3mm. Anatomic ConsiderationsTo avoid damage to the IAN, the clinician must have a thorough understanding of the normal versus variant anatomy of the pos-terior mandible.Inferior Alveolar CanalInferior-Superior Planeere is a common belief that the vertical position of the IAN is relatively constant within the mandible. Normally the IAN runs a concave path from posterior to anterior, with anterior ter-minal segments exiting the mental foramen (mental nerve) and a branch that ascends to the midline of the mandible (incisive nerve). However, numerous anatomic studies have conrmed the inferior-superior (vertical) positions of the IAN are not consis-tent.47,48 An early classication of the vertical positions of the course of the alveolar nerve was reported by Carter and Keen.49 ey described three distinct types: (1) in close approximation to the apices of the teeth, (2) a large nerve approximately in the middle of the mandible with individual nerves supplying the mandibular teeth, and (3) a nerve trunk close to the inferior cor-tical plate with large plexuses to the mandibular teeth. In type 1 nerves, impairment is common because of the close proxim-ity to the nerve bundle. ree percent of patients can have the IAN directly contacting one or both of the roots of the mandibu-lar rst molar.50 It is highly recommended that a comprehensive radiographic survey be completed to evaluate the IAN in a vertical plane, especially with type 1 and 2 nerves (Fig. 30.23). Juodzbalys etal.51 categorized the inferior-superior positioning as either: (1) • Fig. 30.22 Guided-Implant Surgery. With the use of surgical templates, implants may be positioned more accurately, thereby avoiding possible nerve complications.ABC• Fig. 30.23 Inferior Alveolar Nerve (Inferior-Superior Plane). (A) Type 1 (high): positioned close to tooth apex. (B) Type 2 (intermediate): most com-mon position within the middle of the mandible. (C) Type 3 (low): positioned in the inferior border of the mandible. 755CHAPTER 30 Mandibular Anatomic Implications for Dental Implant Surgeryhigh, within 2 mm of teeth apices; (2) intermediate; or (3) low. Heasman52 reported that 68% of patients exhibit an intermediate zone path of the IAN canal, with an average distance of 3.5 to 5.4 mm from the rst and second molar roots. Buccal-Lingual PlaneStudies have shown the buccal-lingual location of the IAN as it progresses anteriorly is not constant. e nerve paths have been described in a buccal-lingual direction with a high degree of vari-ability and are dependent on the amount of bone resorption, as well as age and race variables.53Kim etal.54 evaluated and classied the buccolingual IAN loca-tion into three types: type 1, IAN canal is in close proximity to the lingual plate (∼70%); type 2, IAN canal follows the middle of the ramus from the second molars to rst molars (∼15%); and type 3, IAN canal follows the middle or lingual thirds of the mandible from the ramus to the body (∼15%).In addition, older and Caucasian patient groups have shown less distance between the buccal aspect of the nerve and the infe-rior border of the mandible. Other studies have shown the most common area for the IAN to be in the middle of the buccal and lingual cortical plates is the rst molar region.55 us, in the buc-cal-lingual plane, the IAN is highly variable, therefore 3D cross-sectional images should be used to determine the true position of the nerve (Fig. 30.24). Mental ForamenDetermining the exact location of the mental foramen is crucial when placing implants in the posterior mandible. Although the foramen has been thought to be symmetric to the contralateral side in most patients, the location has been shown to be highly variable.56 e mental nerve passes through the mental foramen with three to four nerve branches that exit with an average diam-eter of 1 mm.57 is nerve will innervate the skin of the mental area, and the other two proceed to innervate the skin of the lower lip, mucous membranes, and the gingiva as far posteriorly as the second premolar. Any trauma to this nerve may result in neuro-sensory impairment in this area.e size, shape, location, and opening angulation of the mental foramen are variable. Usually the mental nerve exits the mental foramen from the mental canal. e mental canal is most com-monly angled in a superior direction from the mandibular canal (i.e., average is approximately 50 degrees, with a range from 11 to 70 degrees).58 e size of the mental foramen in the literature has been reported to range from 2.5 to 5.5 mm. e most common shape is ovoid (∼65%) and round (∼23%).59e positioning of the mental foramen is also extremely vari-able in the vertical and horizontal planes. Clinically there are many dierent techniques in identifying the foramen, with a wide variation of predictability.Visualization of the Mental NerveTwo-dimensional Radiographs. Studies have shown that in more than 50% of periapical and panoramic radiographs the mental fora-men is not in the location depicted on the 2D image.60 Conven-tional 2D radiography should never be used as the sole diagnostic modality in evaluating the foramen position (Fig. 30.25). ree-dimensional Radiography. e literature has shown that 3D imaging is the most accurate diagnostic tool to ascertain the exact location of the mental foramen. CBCT panoramic and cross-sectional images, together with 3D images, are the easiest and most accurate techniques in determining the exact foramen location (Fig. 30.26).61 Palpation. In rare cases the clinician may be able to palpate the location of the mental foramen. Most notably, when bone resorp-tion has caused the nerve to be exposed on the residual ridge, the concavity formed by the exposure of the nerve can be determined. In these cases the location of the mental foramen may be marked with a surgical pen. When the nerve is located on the buccal surface of the mandible, the palpation method of identication has very low utility. Anatomic Eandmarks. In the literature, many authors have postulated that landmarks such as teeth and mandibular bony areas may help identify the location of the mental foramen. With respect to teeth, the location cannot conclusively be associated with a particular tooth (e.g., rst premolar, second premolar, between apices of the premolars) because studies have shown the location to be dependent on gender, age, and race.62 In addition, patients exhibit dierent types of facial and skeletal growth, along with orthodontic factors that make dentition landmarks com-pletely inaccurate. Numerous studies have shown a high correla-tion between mental foramen location and race. However, most of these studies associate the location of the mental foramen with a specic tooth63-66 (Fig. 30.27). AB• Fig. 30.24 Buccal-Lingual Inferior Alveolar Nerve Canal Position. The position in the buccal-lingual position is variable: (A) buccal positioned; (B) lingual positioned.• Fig. 30.25 Two-dimensional radiograph that has been shown to depict the true location of the mental foramen only 50% of the time. 756PART VI Implant SurgeryABC• Fig. 30.26 (A and B) Cone beam computed tomographic three-dimensional image showing true position and size of mental foramen. (C) Complete dehiscence of mandibular canal.A B• Fig. 30.27 (A) The position of mental foramen does not correspond to a specic anatomic landmark (e.g., rst premolar, second premolar). The mental foramen may be positioned as far anterior as the cuspid and as far posterior as the rst molar. (B) In the literature a vertical line drawn from the pupil of the eye and infra-orbital foramen will be in close approximation to the mental foramen. However, this technique has inherent inaccuracies because many patients have different skeletal relationships. 757CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryDirect Evaluation. e most precise technique available today to determine the exact location of the mental foramen is by direct evaluation. Exposing the mental foramen may be intimidating to some clinicians, especially early on their learning curve. is can be accomplished with very low morbidity; how-ever, the technique’s success depends on the clinician’s training and experience. Technique to Expose Mental Foramen 1. Crestal incision is made from the canine position (mesial) to the rst molar position, with vertical 45-degree release inci-sions anterior and posterior (Fig. 30.28). 2. Full-thickness reection is completed below the mucogingival junction: A moist 4 × 4 gauze is placed over the index nger, and the ap is elevated apically until the superior aspect of the foramen is located (Fig. 30.29). 3. e gauze may be used anterior and posterior to the foramen to conrm the foramen location. 4. Once the foramen is located, a periodontal probe may be used to measure the ridge height. e implant length is usually 2 mm less than the measured distance.ree-dimensional Ultrasound. e most promising imag-ing technique for the future is ultrasound. Ultrasound has the advantage of no ionizing radiation and the ability to reconstruct 3D images of bone surfaces to within an accuracy level of 24 μm. At this time, ultrasound units are not available specically for den-tal use.67 Mental Nerve VariantsAccessory and Double Foramen. Studies have shown that in approximately 6.62% to 12.5% of patients, an accessory foramen is present.68,69 In the majority of cases, small accessory foramina usually contain a small branch of the mental nerve or a nutrient branch that supplies the teeth. ese are usually not problematic because of cross-innervation or actually contain nutrient branches and no sensory bers to the soft tissue. Accessory foramens are usually radiographically dierentiated from a double foramen in the accessory foramen, which will be seen on a CBCT as a very small foramen, usually anterior to the larger main mental foramen.However, in a small percentage of cases, a larger branch of the mental nerve (equal or larger-size foramen) may exit the second mental foramen, which is termed a double foramen. Special care should be extended in this situation because it may contain com-ponents of one of the three branches of the mental nerve. Acces-sory foramina are believed to be the result of early branching of the IAN, before exiting the mental foramen during the 12th week of gestation.70 Double foramens are easily seen in 3D images, or the coronal CBCT images are depicted as two larger size fora-mens, often being of the same size (Fig. 30.30). Anterior Loops of the Mental Nerve. As the mental nerve pro-ceeds anteriorly in the mandible, it sometimes runs inferior and anterior to the mental foramen. is anterior and caudal compo-nent of the mental nerve will curve cranially back to the mental foramen and is termed the anterior loop.71 Recently, CBCT and dissection studies have shown a rather high (70%) prevalence rate of anterior loops, with a mean of 1.16 mm distance anteriorly. e anterior loop may be depicted most predictably on axial CBCT images, with 2D radiographs being totally unreliable.Determining the presence of an anterior loop is critical when placing implants anterior to the mental foramen. Inability to ascertain the presence of an anterior loop may result in damage to the mental nerve (Fig. 30.31). e anterior loop measurement should be added to the safe zone to avoid damaging the mental nerve. For example, if a 1.0-mm anterior loop is present, then the safety zone should be calculated to be 3.0 mm (1.0 mm anterior loop + 2.0 mm safety zone). Incisive Nerve Branch. e incisive nerve branch, a continu-ation and terminal branch of the IAN, supplies the mandibular canine and incisor teeth, and is seen as a radiolucent canal in the anterior mandible. e canal is most commonly present in the middle third of the mandible and narrows toward the midline, reaching the midline only 18% of the time.72 e incisive nerve is often mistaken for an anterior loop in the mandible. Because there is no soft tissue sensory component to this nerve, implants may be placed in proximity to it without nerve impairment. Studies have shown incisive canals have a mean diameter of 1.8 mm and loca-tion 9.7 mm from the lower cortical border.73 e incisive nerve has been recognized as an important anatomic structure that must be taken into consideration when performing surgery in this area. Excessive bleeding has been reported as a signicant intraoperative complication in this area when it is perforated during osteotomy preparation (Fig. 30.32). However, this is usually remedied by placing the implant, direction indicator, or surgical bur into the osteotomy site. AB• Fig. 30.28 (A) Incision outline to expose foramen consists of a crestal incision to the mesial of the canine and distal of the molar, with an anterior and a posterior vertical release incision. (B) After foramen exposure the height of available bone may be determined. 758PART VI Implant SurgeryABCD E FGH• Fig. 30.29 Exposure of the Mental Foramen. (A) General incision outline with mental foramen high-lighted in yellow. (B) Anterior full-thickness reection. (C) Posterior full-thickness reection. (D) Moist 4 × 4 gauze placed over index nger. (E) Tissue anterior and posterior (green arrows) is reected apically with gauze. (F) Tissue reected apically with gauze. (G) Superior margin of foramen identied. (H) Verication of exposed foramen. 759CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryACBD• Fig. 30.30 (A and B) Accessory foramen, depicted with a small and a large foramen. (C and D) Double foramen, depicted with two large foramina. 760PART VI Implant SurgerySurgical Principles to Decrease Neurosensory ComplicationsSafety ZoneA 2-mm safety zone with osteotomy preparation and nal implant placement is paramount in preventing neurosensory impair-ments.74 erefore the nal implant position should always maintain a minimum distance of 2 mm from the IAN canal. Com-pression-related injuries (neuropraxia) can occur by encroaching on the IAN without actual contact. Nerve impairments have been reported when implants are placed less than 2 mm from the canal without actual invasion of the canal.Bleeding and resultant hematomas have been shown to cause nerve damage because of nal positioning of the implant too close to the neurovascular canal.75 In addition, the IAN superior corti-cal bone can be compressed, causing pressure necrosis with resul-tant nerve impairment.76 Interactive treatment planning software programs allow the implant clinician to accurately assess the ideal placement with respect to this vital structure (Fig. 30.33). Always Take Into Account the Y Dimension of the Implant BursCare should always be exercised in knowing the exact drilling depth when performing osteotomies over vital structures, espe-cially in the posterior mandible. e implant clinician should double-check the marking depth on the burs before initiating the osteotomy. e principle of “measure twice, drill once” should be followed to prevent iatrogenic overpreparation of the implant site. In addition, the Y dimension of the implant system being used must be known. With many implant surgical systems, the depth of the millimeter lines inscribed on surgical drills do not always coincide with the actual depth of the drill. Most drills contain a V-shaped apical portion designed for cutting eciency (Y dimen-sion). Usually the wider the drill, the greater the Y dimension. e implant clinician should always evaluate the manufacturer’s drill length with respect to the length of the implant before performing the osteotomy. If this concept is not adhered to, overpreparation of the site may occur, resulting in nerve damage (Fig. 30.34). Use Drill Stop Burs to Prevent OverpreparationAn additional technique to prevent overpreparation of the oste-otomy site is the use of stop drills. ese drills have a predeter-mined depth marking that prevents overpreparation. Stop drills are benecial in the mandibular posterior area, especially when visibility and access are compromised. Generic drill stop kits are also available that may be used with most implant surgical systems (Salvin Dental Corp.). ese autoclavable, reusable kits 4 mm2 mm2 mmABCDE4 mmNo anterior loopAnterior loopAB• Fig. 30.31 Anterior Loop. (A) Anterior loop of mental nerve that is consistent with the mental nerve anterior to the mental foramen. (B) The anterior loop measurement should be added to the 2-mm safety zone to ensure adequate space between the implant and the foramen.AB• Fig. 30.32 Incisive Nerve. (A) Three-dimensional image depicting the incisive nerve, which is the second terminal branch of the inferior alveolar nerve. (B) Cone beam computed tomographic panoramic and cross-sec-tional images showing the incisive nerve canal (green arrows). 761CHAPTER 30 Mandibular Anatomic Implications for Dental Implant Surgerymay be used for any size length implant and corresponding drill (Fig. 30.35). Some surgical implant systems have specic depth burs that coincide with the actual implant depth (e.g., Hahn Implants; Glidewell Corp.). Understand Bony Crest AnatomyDue to resultant bone resorption after extraction, the alveolar ridge becomes compromised in width (Division B bone) at the expense of the buccal plate. When measuring available bone height, special consideration should be given to the nal location of the superior aspect of the implant platform, not the existing crest of the ridge. It will often appear there is adequate vertical height for implant placement; however, when the osteotomy is initiated, the thin crest will be lost (i.e., because the diameter of the drill exceeds the width of the bone) and the implant will be placed inferior to where it was originally intended. is can lead to unexpected depth drilling and an implant that is placed too close to the vital structure. e clinician should either augment the ridge to main-tain vertical height or reduce the height calculation by the amount of osteotomy-induced osteoplasty (Fig. 30.36). Maintain Total Control of the HandpieceWhen performing osteotomies in the posterior mandible, special care should be noted to maintain complete control of the surgi-cal handpiece. Large marrow spaces (i.e., where there is a lack of or thin trabecular bone) are often present, which may allow the osteotomy site to become deeper than intended. is will result in the implant being placed more apically, leading to neurosen-sory impairment. A CBCT comprehensive evaluation allows the implant dentist to view the bone quality before surgery. Most soft-ware programs associated with CBCT units allow the clinician to ascertain the density in the intended site. e implant clinician may also determine the bone density by tactile sensation when drilling. In addition, when drilling the osteotomy near the mental foramen, care should be exercised not to bend the wrist. is can • Fig. 30.33 Safety Zone. A 2-mm safety zone should always be present between the implant and the inferior alveolar canal.2.0 mm1.5 mm 2.5 mm 3.0 mm 3.4 mm1.5 mm2.0 mm2.5 mm3.0 mm3.2 mm3.4 mm3.7 mm4.0 mm4.2 mm4.4 mm4.7 mm0.43 mm0.58 mm0.74 mm0.86 mm0.94 mm0.99 mm1.07 mm1.17 mm1.22 mm1.27 mm1.35 mmYDrilldiameterYdimensionAB• Fig. 30.34 “Y” Dimension. (A) All surgical drills have an inherent Y dimension, which results in a greater drill length to each drill. (B) Y dimen-sion increases as the surgical drills increase in size.• Fig. 30.35 Drill Stops. To prevent overdrilling in depth, special drills have predetermined depth stops that correspond to the intended implant length. 762PART VI Implant Surgerypotentially redirect the drill or implant placement in an unwanted direction (e.g., near the mental foramen, into a tooth root). Surgi-cal templates and guides are benecial in preventing this malposi-tioning complication. Do Not Place Bone Graft Material in Close Approximation to NerveAfter tooth extractions, especially in the mandibular premo-lar areas, care should be exercised in placing bone graft mate-rial (autologous, allogenic, xenogenic) in direct contact with an exposed IAN. Whether socket grafting or in conjunction with implant placement, case studies have shown resultant neurosen-sory impairment from bone graft material causing compression, crushing, or chemical burn injuries.77 When grafting sockets with an exposed IAN canal, excessive pressure should be avoided. In addition, a small piece of fast resorbing collagen (e.g., OraTape, OraPlug) may be placed before the addition of grafting material. is will decrease the chance of particulate graft being in direct approximation to the nerve canal (Fig. 30.37). Use Copious Amounts of IrrigationOverheating the bone during osteotomy preparation may pro-duce thermal stimuli that may lead to peri-implant necrosis and secondary postoperative nerve damage. Neural tissue is extremely sensitive and may be damaged by heat stimuli. e thickness of the necrotic area is proportional to the amount of heat generated during preparation.78 e implant dentist must be cautious not to overheat the bone. is can be minimized by “bone dancing,” which involves drilling in short intervals and allowing irrigation to enter the osteotomy, preventing heat gen-eration. In addition, new (sharp) and intermediate-size drill burs may be used to reduce heat generation. is is more crucial with harder bone density (e.g., D1 or D2) or bone with compromised vascularity. Avoid Incision-Related InjuriesAvoid incision-related injuries when making incisions in close approximation to the mental foramen and associated nerve struc-tures in the posterior mandible. In cases of severe bone atrophy the presence of nerve dehiscence may inadvertently result in a transected nerve during the initial incision (i.e., making the inci-sion on the crest of the ridge). Anatomic landmarks, 3D models, accurate measurements from CBCT scan, and palpation of the nerves are ways to avoid this complication. In addition, incisions in the posterior of the oral cavity should never be made over the retromolar pad. is can result in possible injury to the lingual nerve, which in 10% of cases transects this area79 (Fig. 30.38). Avoid Flap/Retraction–Related InjuriesNeurosensory impairments may also occur from overzealous use or incorrect placement of retractors. Broad-base (not sharp) retrac-tors should be used to retract tissue that is not directly over the mental foramen because excessive stretching of the nerve trunk may cause irreversible damage. It is imperative that the mental foramen and associated branches of the mental nerve be identi-ed in this area when placing retractors. Retractors should always be placed and held on the bone to prevent slippage or excessive soft tissue pressure, which can lead to a neuropraxia type of nerve damage (Fig. 30.39). Excessive stretching of the tissue may also lead to neurosensory impairments. It has been shown the perineu-rium protects the fascicles; however, if greater than 30% elonga-tion of the nerve occurs, structural damage will occur to the nerve bers.80 Use Special Care When Releasing Periosteum Over Mental ForamenIt is a common procedure during closure after implant placement or bone grafting to stretch the periosteal tissue to allow primary and “tension-free” closure.AB C• Fig. 30.36 Division B Ridge. (A) Incorrect measurement from superior crest to inferior alveolar nerve (IAN) canal. (B) Because the thin crest was not taken into consideration, implant placement will lead to encroachment of the IAN canal. (C) Ideal selection of implant length and positioning. 763CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryABCD• Fig. 30.37 Postextraction Site. (A) Care should be taken when grafting an extraction site in close approximation to the inferior alveolar nerve. (B) A curette should be used with caution because direct dam-age to the nerve may occur. (C) Grafting in close approximation to the canal may lead to nerve trauma. (D) Bone graft material placed into an extraction socket resulting in a nerve impairment.Various techniques are used to “release” the tissue to improve vascularization of the incision line and adhesion of the margins to prevent incision line opening. e submucosal technique devel-oped by Misch in 1988 is an eective method to expand the tis-sue. is procedure involves the use of a #15 scalpel blade and soft tissue scissors (i.e., Metzenbaum) to create a blunt dissection. Knowledge of the location of the three mental nerve branches is necessary because inadvertent incisions over the mental nerve branches may result in neurotmesis (transection) types of nerve injuries (Fig. 30.40). Careful SuturingWhen the mental nerve is exposed, care should be exercised to prevent nerve tissue from being entrapped within the sutures. e mental nerve emerges from the mental foramen and divides into three branches below the depressor anguli oris muscle. Caution must be exercised to prevent any of the mental nerve branches from becoming entrapped within the suture material, potentially causing a neuropraxia (compression) type of nerve injury. In addition, nerve bers may be damaged from the passage of the extremely sharp suture needle through the tissue.  764PART VI Implant SurgeryVerify Correct Positioning of CBCT SurgiGuidesStudies have shown that the most precise and accurate surgical templates are tooth supported. When using bone- or tissue-sup-ported surgical guides, care must be exercised to correctly posi-tion the guide because an error in placement may result in direct damage to the IAN. Tooth-supported guides should always be the rst choice if possible because they are clinically proven to give rise to the fewest positioning errors. e least accurate is the mucosa supported, which are usually used for apless surgery.66 Studies have shown that apless surgical guides consistently show deviations of implant positions from ideal locations. Perforations of the buccal plate can be found in more than 50% of the apless cases.81 A very minor discrepancy (anteroposterior) in the place-ment of the guide can lead to impingement on vital structures. erefore surgical templates should always be xated and the ideal position veried. Surgical Procedures That Increase Neurosensory ComplicationsImmediate Implants in the Mandibular Premolar AreaImmediate implants have gained overwhelming popularity in impl ant dentistry today. Extreme caution must be exercised when extracting and immediately placing implants in the man-dibular premolar area. As noted earlier, many variables dictate the position of the mental foramen, with the foramen being highly variable. Studies have shown that 25% to 38% of the time the mental foramen is superior to the premolars apex.82 Because most immediate implant osteotomy sites involve drill-ing the osteotomy site deeper for stability (∼2–4.0 mm), the odds of nerve trauma are greatly increased. Because of this the implant clinician must be very selective in cases involving extraction and immediate implant placement in this anatomic area (Fig. 30.41). • Fig. 30.38 Incision-Related Injuries. In patients with signicant mandib-ular atrophy and dehiscence of the nerve canal, possible incision-related injuries may result. The incision should be modied to avoid exposed nerves by extending to the lingual when approaching the exposed nerves.• Fig. 30.39 Flap Retraction–Related Injuries. Retractors should be carefully positioned to avoid stretching or damaging the inferior alveolar nerve.AB• Fig. 30.40 (A and B) Periosteal release incision (A) and blunt dissection (B) should never be completed in close approximation to the mandibular nerve. 765CHAPTER 30 Mandibular Anatomic Implications for Dental Implant SurgeryA BC• Fig. 30.41 Mandibular Premolar Immediate Implants. (A) Approximately one-third of mandibular pre-molars root apexes are inferior to the mental foramen. (B and C) Implant placement into the mental foram-ina leading to neurosensory impairment.AB• Fig. 30.42 Superior Cortical Plate of Inferior Alveolar Nerve (IAN) Canal. (A) Mandibular canal with thick cortical plate, which is uncommon. (B) Thin to no cortical bone is present over the IAN canal.Drill Until the Superior Cortical Plate Is “Felt”It has been advocated in the literature that the osteotomy depth may be determined by “feeling” the superior cortical plate of the inferior alveolar canal. A 2-mm safety zone should always be adhered to because research has shown that in approximately 28% of posterior mandibles there is no superior cortical plate over the inferior alveolar canal.83 In addition, studies have shown it to be impossible to use tactile sense to ascertain the presence of superior cortical bone surrounding the mandibular canal. Clinical reports have revealed hemorrhage into the canal, or bone fragments may cause compression or ischemia of the nerve from engaging the superior cortical plate. Dependence on the ability to “feel” the superior cortical plate through tactile sense increases the likeli-hood of nerve complications (Fig. 30.42). Infiltration TechniqueAn alternative technique in placing implants in the posterior mandible is not using mandibular nerve block anesthesia. Instead, inltration is accomplished in the soft tissue surrounding the oste-otomy site, and the patient is asked to alert the implant clinician 766PART VI Implant SurgeryAB• Fig. 30.43 Lingual Placed Implant. (A and B) Implants should never be placed lingual to the inferior alveolar nerve canal because nerve injury or perforation of the lingual plate may occur.• Fig. 30.44 In type 1 nerves, placement of implants at the apex of the adjacent tooth may result in direct nerve trauma.on the proximity of the drill to the nerve bundle.84 is alternative technique results in a very high degree of subjectivity concern-ing patients’ responses, because of varying degrees of pain thresh-olds. In addition, disadvantages of this surgical method include inconsistent mandibular nerve anatomy, with varying locations of dental-alveolar nerve branches. With the success of CBCT radiog-raphy in implant dentistry today in determining the exact location of the IAN, this technique should be avoided because of the high degree of false-negative and false-positive results from patients. Etoz etal.85 showed this supraperiosteal inltration technique to be safe in 91% of cases. However, according to this study, approxi-mately 1 patient in 10 ended up with a neurosensory decit. Placing Implants Lingual to the Inferior Alveolar Nerve Canal or ForamenMany authors have advocated placing implants lingual to the neuro-vascular bundle (Kumar; Stellar). As stated previously, the buccal-lin-gual nerve position within the mandible is extremely variable, along with the incidence and trajectory of lingual osseous concavities. Attempting to place implants buccal or lingual to the inferior alve-olar canal or mental foramen is associated with a high degree of morbidity, even with the use of CBCT-guided surgery. In addition, perforation of the cortical plate can occur, which may lead to sublin-gual bleeding or formation of a sublingual hematoma (Fig. 30.43). Place Implants at the Depth of the Adjacent Root ApexesMany implant clinicians use the location and length of the adjacent teeth as a guide in determining the size (length) of the implant to be placed. Usually a Panorex or periapical radiograph is used in determination of this length. When this technique is used in anatomic type 2 or 3 (i.e., more apically positioned in the vertical dimension) nerve courses, the incidence of nerve impair-ment is low. However, in mandibles that exhibit a type 1 nerve course (close to root apex), close approximation of the implant to the canal is likely, leading to a higher probability of neurosensory impairment. Ideally the implant clinician should ascertain the available bone above the mandibular canal via 3D radiographic analysis (Fig. 30.44).  767CHAPTER 30 Mandibular Anatomic Implications for Dental Implant Surgery“As Long as There Is Not Excessive Bleeding, the Mandibular Canal Has Not Been Violated”Another unconventional technique in avoiding nerve impairment is the evaluation of the amount of bleeding from the osteotomy site. Many practitioners correlate the amount of hemorrhage with the proximity of the neurovascular bundle (IAN, artery, vein, and lymphatic vessels). Anatomic studies have shown that the inferior alveolar artery may lie parallel to the nerve and lingual as it traverses anteriorly. Its position varies with respect to the IAN within the man-dibular canal. Other studies show the inferior alveolar artery appears to be solitary and lies superior and lingual to the IAN, slightly above the horizontal position.86 In addition, there exist multiple inferior alveolar veins positioned superior to the nerve, which may cause venous oozing if directly traumatized. A false-positive result may occur if this area is damaged because large marrow spaces, which can cause excessive bleeding, are common in the posterior mandible (D3 bone). e degree of bleeding should not be used as an indica-tion of nerve proximity or violation of the mandibular canal. Replacing Second Molarsere are many prosthetic and surgical disadvantages when eval-uating edentulous, second mandibular molar sites for implant placement. Disadvantages include high incidence of sublingual bony undercuts, which can result in perforation of the lingual plate or angulation issues, decreased interocclusal space (espe-cially with supraeruption of the adjacent tooth), dicult access for surgery and prosthetic component insertion, and the fact that there is 10% greater occlusal force on the second molar versus the rst molar. Function is not a primary reason for replacement because 90% of masticatory eciency is generated anterior to the mesial half of the mandibular rst molar, and cheek biting is more common in this area because of the proximity of the buccina-tors muscle. One of the most important disadvantages is the close approximation of the mandibular canal in the second molar area, which leads to diculty in placement of implants in this area. When implants are placed, usually the available bone present is compromised in height. As a result the second molar is often not replaced when the only posterior teeth missing are the second and third molars. e primary disadvantage of not replacing the sec-ond molar is extrusion of the opposing maxillary second molar. If extrusion is a signicant concern, a full-coverage crown on the mandibular rst molar may include occlusal contact with the mesial marginal ridge of the maxillary second molar (Fig. 30.45).Nerve RepositioningTreatment of patients who exhibit compromised alveolar crest height in the posterior mandibular area can be very challenging. Techniques include the use of shorter implants, which become biomechanically compromised, or the use of bone grafting to increase available bone for future implant placement. An alterna-tive technique is to reposition the IAN laterally, either by nerve lateralization or nerve transposition. In nerve lateralization the IAN is exposed and retracted laterally while the dental implants are placed. e transposition technique, rst published in 1987 by Jensen and Nock,87 includes the mental foramen in the oste-otomy, resulting in the IAN being positioned more posterior. e inherent risk with these complex procedures is neurosensory impairment (anesthesia, paresthesia, or dysesthesia) to the mental nerve branch. Although this is a valid treatment option in sig-nicantly atrophied cases, this technique should be reserved for practitioners with advanced training and experience with these procedures (Fig. 30.46).ConclusionPrior to implant or bone grafting procedures in the mandible, a careful and detailed clinical and radiographic evaluation is para-mount to identify vital structures in the mandible. e use of cone beam computed tomography is essential to determine the location of normal and variant anatomic structures such as bone undercuts, poor bone density, extreme bony angulation, blood vessels, and the mandibular canal and mental foramen. e com-plications that may result can range from very minor issues to life AB• Fig. 30.45 Second Molar Implants. (A and B) Implants in the second molar region have a high inci-dence of nerve trauma because of the location of the mandibular nerve in relation to the second molar implant site. 768PART VI Implant Surgerythreatening circumstances. erefore, the clinician must under-stand the possible sequalae and management if violation of one of these mandibular vital structures is compromised. References 1. Wright DMD, Roberta A. An Analysis of Anterior Mandibular Anat-omy Using Cone Beam Computed Tomography: A Study of Dentate and Edentulous Mandibles; 2016. 2. Butura CC, Galindo DF, Cottam J, Adams M, Jensen O. Hour-glass mandibular anatomic variant incidence and treatment consid-erations for all-on-four implant therapy: report of 10 cases. J Oral Maxillofacial Surg. 2011;69(8):2135–2143. 3. Greenstein G, Cavallaro J, Tarnow D. Practical applica-tion of anatomy for the dental implant surgeon. J Periodontol. 2008;79(10):1833–1846. 4. Butura CC, Galindo DF, Cottam J, Adams M, Jensen O. 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