The Biomechanics of Miniscrews










433
The
Biomechanics
of Miniscrews
18
433
CHAPTER
OVERVIEW
Although miniscrews can be an important adjunct in anchorage control, it is still important
to understand their biomechanics. Posterior segment movement can occur if a continuous
archwire connects anterior and posterior segments. There are advantages to a direct pull
between a temporary anchorage device (TAD) and an anterior segment during space clo-
sure and other types of segmental movement: The system is statically determinate, and
the line of force is easily visualized. For anterior retraction, a favorable intrusive force to
the incisors is possible. If lever arms are not far enough apically because of anatomical lim-
itations, compensating moments can be placed in a continuous archwire. These moments
can lead to friction in the appliance, reduce anterior retraction, and produce total-arch
movements. Miniscrews are very versatile and can be placed in many locations. Therefore,
many novel biomechanical solutions with TADs are presented in this chapter. In addition,
the possibilities of total-arch movement with miniscrews instead of segmental movement
are explored.
“You are never too old to set another goal or to dream a new dream.”
— C. S. Lewis
“By failing to prepare, you are preparing to fail.”
— Benjamin Franklin
“It is no use saying, ‘We are doing our best.’ You have got to succeed in
doing what is necessary.”
— Winston Churchill
Kee-Joon Lee
Young Chel Park

18
The Biomechanics of Miniscrews
434
Rationale for TADs in Clinical
Orthodontics
Bone-borne TADs have enlarged the scope of con-
temporary orthodontics by enabling arbitrary tooth
movement in three dimensions. In particular, mono-
cortical mini screw-type TADs possess versatility, easi-
ness in insertion and removal, and low cost. Howev-
er, to maximize the effectiveness of miniscrews and
to make the appliance deliver the desired force sys-
tem, it is crucial to understand the characteristics of
the force system created by miniscrews. This chapter
describes how mini screws help to overcome conven-
tional biomechanical limitations in practical ways.
Characteristics of Miniscrew-
Driven Orthodontics
Major biomechanical characteristics of miniscrews
are outlined in the following sections.
Elimination of movement of the
reactive unit
Consideration of the active and reactive compo-
nents of tooth movement
has been a major issue in
clinical orthodontics. In particular, statically indeter-
minate force systems along a continuous archwire
make it hard to anticipate the pattern of tooth
movement. When a force is applied to a tooth or a
segment of an active unit from a miniscrew inserted
in the alveolar bone, the reactive unit is expressed at
the miniscrew. The fear of “anchorage loss” is there-
fore resolved.
One may have to simply focus on the
relationship between the force vector(s) and the ac-
tive unit (Fig 18-1). In contrast, if elastic components
connect the anterior and posterior units on a contin-
uous archwire, possible movement of the posterior
teeth must be considered at the same time.
Predictable force system
The force exerted from a miniscrew is usually a sin-
gle force with a line of action that is identical to the
elastic chain or coil spring connecting the miniscrew
head and the attachment(s). Orthodontists are able
to construct and design a predictable force system
because it is statically determinate. The force system
is efcient because the magnitude of force can be
measured and the line of action of the force can be
easily visualized.
Intrusive component of force
Miniscrews inserted in the interradicular area func-
tion at the points of force application and create
intrusive force vectors without extruding the poste-
rior teeth, unlike conventional intra-arch appliances
(see Fig 18-1). This can be a helpful feature for the
correction of the hyperdivergent facial type, be-
cause very few appliances have been proven to be
effective in the control of “long face.”
Various points of force application
Miniscrews, unlike other types of bone-borne an-
chorage devices such as miniplates or onplants, can
be placed in various sites, including interradicular
spaces, the retromolar area, the midpalatal suture,
the palatal slope, and the anterior subapical area.
Various insertion sites mean various points of force
application, which create various force vectors de-
pending on the desired type of movement and the
location of the active unit, thus enabling arbitrary
tooth movement in all three dimensions.
Fig 18-1 Force vectors created by an interradicular miniscrew. The
active unit is the dentition, and the reactive unit is the miniscrew.
The line of force can be resolved into horizontal and vertical com-
ponents of force (red arrows).

435
Force-Driven Approach Applied to TADs
Force-Driven Approach Applied
to TADs
Despite the latest progress in appliance technologies,
such as self-ligating brackets and computer-guided
indirect bonding systems, conventional biomechan-
ical principles are still valid. The treatment goals
must be dened using clear terms instead of vague
words such as “good occlusion” or “tooth align-
ment.” Predictable movement of the active unit and
proper interpretation of the result are performed
according to the following steps:
1. Determination of the active unit force system
2. Estimation of the required force system
3. Appliance design and application
4. Clinical monitoring and troubleshooting
Determination of the active unit
force system
The rst step of appliance construction is determi-
nation of the active unit movement and the line of
force required to achieve it. The type of tooth move-
ment is determined by the relationship between the
force vector and the active unit, where the active
unit movement is determined by the equivalent
force system at the center of resistance (CR). For ex-
ample, an elastic chain from the miniscrew at the
interradicular space between the second premolar
and rst molar can easily cause clockwise rotation of
the anterior segment due to the distance between
the line of force and the CR of the anterior segment
(Fig 18-2a). In contrast, the active unit could be the
entire arch if a rigid continuous wire is placed with
a stop mesial to the rst molar (Fig 18-2b). The same
line of force may cause much less rotation when the
archwire is tied to the entire arch for two reasons:
(1) the full arch with more teeth is less sensitive to
rotation, and (2) the distance between the line of
action and the estimated location of the CR is short-
er in the full arch (D
T
) than the distance measured
at the anterior segment (D
A
). Instead of rotation of
the anterior segment only, a smaller rotation of the
entire occlusal plane can result. The type of tooth
movement should be determined from both clini-
cal and theoretical standpoints. For example, with
translation of incisors in any direction, the direction
of CR movement is parallel to the line of action of
the force; this is also true of the incisal edge and
the bracket (Fig 18-3a). Note in Fig 18-3b that the
correct line of force direction is still parallel to a line
connecting the movement of the CR; however, in-
Fig 18-2 Active unit dened by the extension of the
archwire. (a) Anterior segment as the active unit. (b) En-
tire arch as the active unit. When a continuous archwire is
used, the active unit can be the entire arch, not a partic-
ular segment. CR
A
, center of resistance of anterior teeth;
D
A
, distance between the force vector and CR
A
; CR
T
, cen-
ter of resistance of total arch; D
T
, distance between the
force vector and CR
T
.
Fig 18-3 Type of tooth movement. (a) Translation. (b) Rotation. For trans-
lation, the correct force vector is a line connecting the CR before and after
movement. With rotation, the connecting bracket or incisal edge points do
not give the correct force direction. Green circles indicate the types of tooth
movement that can be clinically interpreted as extrusion of the incisal edge.
a
b a b

18
The Biomechanics of Miniscrews
436
cisal edge or bracket movement would be an incor-
rect guide.
Therefore, the clinical treatment goals must be
precisely dened before appliance selection. Not
only the type of tooth movement of the anterior
segment but also the rotation of the occlusal plane
should be considered, because the entire arch is con-
sidered an active unit in the continuous archwire.
Estimation of the required force
system
The force system consists of a force and/or a mo-
ment. If placement of the desired force vector is not
possible, an equivalent force system comprising a
force and a moment is used instead. The selection
of an equivalent force system is decided according
to anatomical or technical limitations. For example,
if translation of the anterior segment is needed par-
allel to the occlusal plane (Fig 18-4a), a single force
is placed at the CR parallel to the occlusal plane in-
stead of a force and moment at the level of the inci-
sor bracket (Figs 18-4b and 18-4c).
Appliance design and application
In miniscrew-driven orthodontics, appliance design
includes selection of the insertion site and exten-
sion of the archwire to determine the point of force
application. The movement can be ne-tuned by
varying the insertion points of the miniscrews and
the position of the hooks or attachments on the
archwire. In Fig 18-4, the miniscrew is placed high
enough so that the previously determined force is
produced.
Clinical monitoring and
troubleshooting
Although the force system has been precisely ap-
plied, other variables may affect the predicted tooth
movement. The CR may vary depending on the al-
veolar bone height, root length, root shape, and
tooth inclination. It is often difcult to pinpoint the
CR if the segment extends to either the mesial or
distal side within the arch. Moreover, in the case of
movement of a segment, deformation of the arch-
wire or splint can be another variable. Hence, the
displacement pattern must be carefully interpreted
chairside using appropriate diagnostic tools includ-
ing cephalometric radiographs.
Transition from Single-Tooth to
Total-Arch Movement
According to conventional sliding mechanics in pre-
molar extraction cases, movement of the anterior
segment affects that of the posterior segment. The
line of force is normally placed along the archwire,
below the possible CRs of respective anterior and
posterior segments, for which either a reverse curve
of Spee or gable bend is needed to compensate
for the possible lingual tipping of the anterior seg-
ment and forward tipping of the posterior segment.
However, sliding mechanics using an interradicular
miniscrew differs from the conventional mechanics
in two major aspects: (1) the line of force is inclined
to the gingival side, creating an intrusive retraction
force, and (2) the reciprocal force on the posterior
Fig 18-4 Appliance design procedure according to the force-driven approach. (a) Step 1: Determine the type of tooth movement (in this
case, translation of the incisor). (b) Step 2: Establish the line of force (in this case, a single force that passes through the CR of the incisor).
(c) Step 3: Determine the insertion site and point of force application of the appliance.
ba c

437
Transition from Single-Tooth to Total-Arch Movement
segment is nonexistent. The question remaining is
how the line of force would displace the active unit.
Regardless of the presence of the miniscrew, a line
of force below the CR causes lingual tipping of the
incisors. The incisal edge may be extruded as a result
of the rotation. If a continuous archwire is in place,
the wire may form a Class III geometry between
the anterior and posterior segments, leading to
forward tipping of the posterior segment. A rough
estimate of the treatment result is depicted in Fig
18-5, not considering friction between the wire and
the brackets. The less stiff the continuous archwire,
the greater will be the mesial tipping as the anteri-
or segment goes through phases I and II of tipping.
Greater distal force will also increase the anterior
tipping, which causes posterior tipping because the
posterior teeth are connected to the archwire. Lee
et al
1
reported that using sliding mechanics with
interradicular miniscrews between the second pre-
molar and rst molar did not induce signicant ex-
trusive displacement of the incisal edge, eliminating
the need for additional archwire intrusion mechan-
ics. However, the side effect of mesial tipping of
the buccal segments still exists and could be more
problematic. An elastically deformed continuous
archwire is shown in Fig 18-5. The posterior segment
could upright as the archwire levels. With any mesial
force (friction), anchorage loss occurs, which TADs
are designed to prevent. Rotating a buccal segment
back and forth cannot be desirable for TAD stability.
Two solutions are possible. First, a more rigid arch-
wire and a lower magnitude of force could be used;
also, reactivation should be minimized so tipping is
reduced. The difculty with this method is that high
friction occurs during phase III translation. A second
method is to eliminate the full continuous archwire
and deliver a line of action of the force only to the
anterior segment through the CR.
Additionally, Upadhyay et al
2
compared the treat-
ment effects between anterior retraction using a
miniscrew and conventional retraction without use
of a miniscrew in both the maxilla and mandible.
The miniscrew group displayed signicantly greater
retraction and reduction of the mandibular plane
angle. If, during space closure, much friction is pres-
ent or the space has fully closed, the line of force
may well rotate the whole arch as a unit. Depending
on the distance between the force vector and the
CR of the full arch, it is likely that the entire maxil-
lary arch may be rotated and the plane of occlusion
steepened (Fig 18-6).
Recent clinical studies and case reports provide
ample evidence of the possibility of the reduction
of vertical dimension even in adult patients.
3
This is
very encouraging because control of the hyperdiver-
gent facial type has been considered very challeng-
ing. Very few of the conventional appliances, such
as high-pull headgear, have been shown to be effec-
tive in controlling the vertical dimension, especially
during growth.
Fig 18-6 Displacement and rotation of the total arch with a mini-
screw. If, during space closure, friction is present or the space is
closed, the applied force should be related to the CR of the full
arch, not that of the anterior segment.
Fig 18-5 (a and b) If the anterior teeth
tip lingually with a continuous archwire in
place, the elastic deformation of the con-
tinuous archwire changes the geometry be-
tween the anterior and posterior segments.
This can produce a side effect on the poste-
rior teeth even if a TAD is used. The poste-
rior teeth tip mesially.
a
b

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433The Biomechanics of Miniscrews18433CHAPTEROVERVIEWAlthough miniscrews can be an important adjunct in anchorage control, it is still important to understand their biomechanics. Posterior segment movement can occur if a continuous archwire connects anterior and posterior segments. There are advantages to a direct pull between a temporary anchorage device (TAD) and an anterior segment during space clo-sure and other types of segmental movement: The system is statically determinate, and the line of force is easily visualized. For anterior retraction, a favorable intrusive force to the incisors is possible. If lever arms are not far enough apically because of anatomical lim-itations, compensating moments can be placed in a continuous archwire. These moments can lead to friction in the appliance, reduce anterior retraction, and produce total-arch movements. Miniscrews are very versatile and can be placed in many locations. Therefore, many novel biomechanical solutions with TADs are presented in this chapter. In addition, the possibilities of total-arch movement with miniscrews instead of segmental movement are explored.“You are never too old to set another goal or to dream a new dream.” — C. S. Lewis“By failing to prepare, you are preparing to fail.” — Benjamin Franklin“It is no use saying, ‘We are doing our best.’ You have got to succeed in doing what is necessary.” — Winston ChurchillKee-Joon LeeYoung Chel Park 18The Biomechanics of Miniscrews434Rationale for TADs in Clinical OrthodonticsBone-borne TADs have enlarged the scope of con-temporary orthodontics by enabling arbitrary tooth movement in three dimensions. In particular, mono-cortical mini screw-type TADs possess versatility, easi-ness in insertion and removal, and low cost. Howev-er, to maximize the effectiveness of miniscrews and to make the appliance deliver the desired force sys-tem, it is crucial to understand the characteristics of the force system created by miniscrews. This chapter describes how mini screws help to overcome conven-tional biomechanical limitations in practical ways.Characteristics of Miniscrew- Driven OrthodonticsMajor biomechanical characteristics of miniscrews are outlined in the following sections. Elimination of movement of the reactive unitConsideration of the active and reactive compo-nents of tooth movement has been a major issue in clinical orthodontics. In particular, statically indeter-minate force systems along a continuous archwire make it hard to anticipate the pattern of tooth movement. When a force is applied to a tooth or a segment of an active unit from a miniscrew inserted in the alveolar bone, the reactive unit is expressed at the miniscrew. The fear of “anchorage loss” is there-fore resolved. One may have to simply focus on the relationship between the force vector(s) and the ac-tive unit (Fig 18-1). In contrast, if elastic components connect the anterior and posterior units on a contin-uous archwire, possible movement of the posterior teeth must be considered at the same time.Predictable force systemThe force exerted from a miniscrew is usually a sin-gle force with a line of action that is identical to the elastic chain or coil spring connecting the miniscrew head and the attachment(s). Orthodontists are able to construct and design a predictable force system because it is statically determinate. The force system is efcient because the magnitude of force can be measured and the line of action of the force can be easily visualized.Intrusive component of forceMiniscrews inserted in the interradicular area func-tion at the points of force application and create intrusive force vectors without extruding the poste-rior teeth, unlike conventional intra-arch appliances (see Fig 18-1). This can be a helpful feature for the correction of the hyperdivergent facial type, be-cause very few appliances have been proven to be effective in the control of “long face.”Various points of force applicationMiniscrews, unlike other types of bone-borne an-chorage devices such as miniplates or onplants, can be placed in various sites, including interradicular spaces, the retromolar area, the midpalatal suture, the palatal slope, and the anterior subapical area. Various insertion sites mean various points of force application, which create various force vectors de-pending on the desired type of movement and the location of the active unit, thus enabling arbitrary tooth movement in all three dimensions. Fig 18-1 Force vectors created by an interradicular miniscrew. The active unit is the dentition, and the reactive unit is the miniscrew. The line of force can be resolved into horizontal and vertical com-ponents of force (red arrows). 435Force-Driven Approach Applied to TADsForce-Driven Approach Applied to TADsDespite the latest progress in appliance technologies, such as self-ligating brackets and computer-guided indirect bonding systems, conventional biomechan-ical principles are still valid. The treatment goals must be dened using clear terms instead of vague words such as “good occlusion” or “tooth align-ment.” Predictable movement of the active unit and proper interpretation of the result are performed according to the following steps:1. Determination of the active unit force system2. Estimation of the required force system3. Appliance design and application4. Clinical monitoring and troubleshootingDetermination of the active unit force systemThe rst step of appliance construction is determi-nation of the active unit movement and the line of force required to achieve it. The type of tooth move-ment is determined by the relationship between the force vector and the active unit, where the active unit movement is determined by the equivalent force system at the center of resistance (CR). For ex-ample, an elastic chain from the miniscrew at the interradicular space between the second premolar and rst molar can easily cause clockwise rotation of the anterior segment due to the distance between the line of force and the CR of the anterior segment (Fig 18-2a). In contrast, the active unit could be the entire arch if a rigid continuous wire is placed with a stop mesial to the rst molar (Fig 18-2b). The same line of force may cause much less rotation when the archwire is tied to the entire arch for two reasons: (1) the full arch with more teeth is less sensitive to rotation, and (2) the distance between the line of action and the estimated location of the CR is short-er in the full arch (DT) than the distance measured at the anterior segment (DA). Instead of rotation of the anterior segment only, a smaller rotation of the entire occlusal plane can result. The type of tooth movement should be determined from both clini-cal and theoretical standpoints. For example, with translation of incisors in any direction, the direction of CR movement is parallel to the line of action of the force; this is also true of the incisal edge and the bracket (Fig 18-3a). Note in Fig 18-3b that the correct line of force direction is still parallel to a line connecting the movement of the CR; however, in-Fig 18-2 Active unit dened by the extension of the archwire. (a) Anterior segment as the active unit. (b) En-tire arch as the active unit. When a continuous archwire is used, the active unit can be the entire arch, not a partic-ular segment. CRA, center of resistance of anterior teeth; DA, distance between the force vector and CRA; CRT, cen-ter of resistance of total arch; DT, distance between the force vector and CRT.Fig 18-3 Type of tooth movement. (a) Translation. (b) Rotation. For trans-lation, the correct force vector is a line connecting the CR before and after movement. With rotation, the connecting bracket or incisal edge points do not give the correct force direction. Green circles indicate the types of tooth movement that can be clinically interpreted as extrusion of the incisal edge.ab a b 18The Biomechanics of Miniscrews436cisal edge or bracket movement would be an incor-rect guide. Therefore, the clinical treatment goals must be precisely dened before appliance selection. Not only the type of tooth movement of the anterior segment but also the rotation of the occlusal plane should be considered, because the entire arch is con-sidered an active unit in the continuous archwire. Estimation of the required force systemThe force system consists of a force and/or a mo-ment. If placement of the desired force vector is not possible, an equivalent force system comprising a force and a moment is used instead. The selection of an equivalent force system is decided according to anatomical or technical limitations. For example, if translation of the anterior segment is needed par-allel to the occlusal plane (Fig 18-4a), a single force is placed at the CR parallel to the occlusal plane in-stead of a force and moment at the level of the inci-sor bracket (Figs 18-4b and 18-4c).Appliance design and applicationIn miniscrew-driven orthodontics, appliance design includes selection of the insertion site and exten-sion of the archwire to determine the point of force application. The movement can be ne-tuned by varying the insertion points of the miniscrews and the position of the hooks or attachments on the archwire. In Fig 18-4, the miniscrew is placed high enough so that the previously determined force is produced.Clinical monitoring and troubleshootingAlthough the force system has been precisely ap-plied, other variables may affect the predicted tooth movement. The CR may vary depending on the al-veolar bone height, root length, root shape, and tooth inclination. It is often difcult to pinpoint the CR if the segment extends to either the mesial or distal side within the arch. Moreover, in the case of movement of a segment, deformation of the arch-wire or splint can be another variable. Hence, the displacement pattern must be carefully interpreted chairside using appropriate diagnostic tools includ-ing cephalometric radiographs.Transition from Single-Tooth to Total-Arch MovementAccording to conventional sliding mechanics in pre-molar extraction cases, movement of the anterior segment affects that of the posterior segment. The line of force is normally placed along the archwire, below the possible CRs of respective anterior and posterior segments, for which either a reverse curve of Spee or gable bend is needed to compensate for the possible lingual tipping of the anterior seg-ment and forward tipping of the posterior segment. However, sliding mechanics using an interradicular miniscrew differs from the conventional mechanics in two major aspects: (1) the line of force is inclined to the gingival side, creating an intrusive retraction force, and (2) the reciprocal force on the posterior Fig 18-4 Appliance design procedure according to the force-driven approach. (a) Step 1: Determine the type of tooth movement (in this case, translation of the incisor). (b) Step 2: Establish the line of force (in this case, a single force that passes through the CR of the incisor). (c) Step 3: Determine the insertion site and point of force application of the appliance.ba c 437Transition from Single-Tooth to Total-Arch Movementsegment is nonexistent. The question remaining is how the line of force would displace the active unit.Regardless of the presence of the miniscrew, a line of force below the CR causes lingual tipping of the incisors. The incisal edge may be extruded as a result of the rotation. If a continuous archwire is in place, the wire may form a Class III geometry between the anterior and posterior segments, leading to forward tipping of the posterior segment. A rough estimate of the treatment result is depicted in Fig 18-5, not considering friction between the wire and the brackets. The less stiff the continuous archwire, the greater will be the mesial tipping as the anteri-or segment goes through phases I and II of tipping. Greater distal force will also increase the anterior tipping, which causes posterior tipping because the posterior teeth are connected to the archwire. Lee et al1 reported that using sliding mechanics with interradicular miniscrews between the second pre-molar and rst molar did not induce signicant ex-trusive displacement of the incisal edge, eliminating the need for additional archwire intrusion mechan-ics. However, the side effect of mesial tipping of the buccal segments still exists and could be more problematic. An elastically deformed continuous archwire is shown in Fig 18-5. The posterior segment could upright as the archwire levels. With any mesial force (friction), anchorage loss occurs, which TADs are designed to prevent. Rotating a buccal segment back and forth cannot be desirable for TAD stability. Two solutions are possible. First, a more rigid arch-wire and a lower magnitude of force could be used; also, reactivation should be minimized so tipping is reduced. The difculty with this method is that high friction occurs during phase III translation. A second method is to eliminate the full continuous archwire and deliver a line of action of the force only to the anterior segment through the CR.Additionally, Upadhyay et al2 compared the treat-ment effects between anterior retraction using a miniscrew and conventional retraction without use of a miniscrew in both the maxilla and mandible. The miniscrew group displayed signicantly greater retraction and reduction of the mandibular plane angle. If, during space closure, much friction is pres-ent or the space has fully closed, the line of force may well rotate the whole arch as a unit. Depending on the distance between the force vector and the CR of the full arch, it is likely that the entire maxil-lary arch may be rotated and the plane of occlusion steepened (Fig 18-6). Recent clinical studies and case reports provide ample evidence of the possibility of the reduction of vertical dimension even in adult patients.3 This is very encouraging because control of the hyperdiver-gent facial type has been considered very challeng-ing. Very few of the conventional appliances, such as high-pull headgear, have been shown to be effec-tive in controlling the vertical dimension, especially during growth.Fig 18-6 Displacement and rotation of the total arch with a mini-screw. If, during space closure, friction is present or the space is closed, the applied force should be related to the CR of the full arch, not that of the anterior segment.Fig 18-5 (a and b) If the anterior teeth tip lingually with a continuous archwire in place, the elastic deformation of the con-tinuous archwire changes the geometry be-tween the anterior and posterior segments. This can produce a side effect on the poste-rior teeth even if a TAD is used. The poste-rior teeth tip mesially.ab 18The Biomechanics of Miniscrews438Applications of TADs for Each SegmentMolar controlMolar intrusionA resultant force that passes through the CR would induce pure intrusion of a single molar along its long axis, which is the treatment goal for an extrud-ed molar. In order to compose an appropriate single force, paired forces from one miniscrew on the me-siobuccal side and another on the distopalatal side are necessary (Fig 18-7).A single force away from the CR would cause rotation. Miniscrew pairs (one at the buccal and the other at the palatal) inserted on the distal side would cause distal uprighting of the molar in addi-tion to the intrusion (Fig 18-8a). In contrast, buccal and palatal mini screws on the mesial sides would intrude and upright the mesially inclined molar (Fig 18-8b). Either way, the space on the mesial side of the extruded molar may change according to the ro-tational movement of the molar.Molar uprightingMandibular second molars tend to be inclined to-ward the mesial due to the inherent eruption path or following the extraction or noneruption of the rst molar. Depending on the severity of the angu-lation, either a single force or a couple can be used to upright the tilted molar. Conventional applianc-es using open coil springs nearly always cause un-desirable side effects such as mesial movement of the anchor segment. In order to eliminate this, a mini screw-assisted push spring from the miniscrew on the mesial side of the target tooth may help produce sufcient moment of force in case of mild tilting by using some distance (d in Fig 18-9) be-tween the line of force and the CR. If the inclination is severe, the moment of force is reduced because the distance (d) decreases. The rotation using a dis-tal single force from the miniscrew on the mesial side becomes less effective. A retromolar miniscrew and an attachment on the distal side of the crown can leave sufcient distance between the line of force and the CR even in the case of severe mesial tipping (Fig 18-10). A patient with an impacted, mesially inclined second molar was successfully treated using a single force at the distal of the crown (Fig 18-11).Fig 18-7 Force vectors for pure intrusion of a molar. Two single force vectors create a resultant force that passes through the CR.Fig 18-8 Alternatives for simultaneous intrusion and rotation of a molar. A force away from the CR would result in distal crown rotation and intrusion (a) or mesial crown rotation and intrusion (b). baPalatalBuccalBuccalPalatalMesial view Distal view 439Applications of TADs for Each SegmentFig 18-9 Molar uprighting using a push spring. (a) When using a single force, it is important to keep the distance (d) as long as possible. The CR of an unerupted tooth is different than that of a fully erupted tooth. (b to e) Molar uprighting using a ret-romolar miniscrew and miniscrew-assisted push spring in a 13-year-old girl without extraction of the third molar.Fig 18-10 Uprighting of a severely mesially inclined molar using a single force from a retromolar miniscrew. The distance (d) se-cures suf cient moment for uprighting of the molar.bad ecFig 18-11 (a to c) A 21-year-old woman was treated using a ret-romolar miniscrew to upright the mesially impacted mandibular second molar following extraction of the third molar. In spite of the severe impaction, uprighting was easily performed as a rotational movement. The rotational movement left space on the mesial side of the impacted molar, which was later closed.bcaAlternatively, a miniscrew-assisted uprighting spring can be effective in the case of severe mesio-angulation (Fig 18-12). The cantilever spring produc-es a greater moment of force than the single force used in Figs 18-9 to 18-11. Because of the inherent extrusion force, this spring is not indicated for in-clined and extruded molars. The mesioangulated, impacted molar requiring eruption may be a suit-able case. 18The Biomechanics of Miniscrews440Molar root movementRoot movement of a tooth requires higher moment-to-force ratios. Root movement of molars is particu-larly challenging for several reasons. First, bends on a continuous wire may produce a moment for rota-tional movement of a molar, but neither the mag-nitude nor the range of the action is suitable for signicant root movement. Hence, an auxiliary can-tilever or a root spring may be more suitable. Sec-ond, conventional cantilever springs inevitably pro-duce extrusive force, and the extrusive rotation may cause occlusal interference and loss of periodontal attachment. The “Sander spring“ was proposed for simultaneous intrusion and root movement of a molar.4 However, as an undesirable side effect, this spring produces a greater moment on the premolar segment (a Class V geometry). In order to eliminate these side effects, an additional anchorage device must be recruited. Third, appropriate moment-to-force ratios to in-duce proper root movement can be variable. It is not easy to pinpoint the reactivation time because the moment-to-force ratio created by convention-al root-movement appliances changes over time.5 Therefore, a practical modication may be to use a root spring with a restraining tie at the crown level. The role of the miniscrew is to provide a rm anchor for the restraining tie to hold the crown position against the posterior displacement of the crown. The miniscrew-assisted root spring combines a mini-screw and a root spring that together produce suf-cient moment for rotation and effectively suppress the extrusive force, thus leading to root movement (Fig 18-13). The magnitude of the moment is subject to change over time, but the restraining tie from the miniscrew will not allow distal displacement of the crown, leading to relatively fail-safe molar root movement. A miniscrew-assisted root spring was used in the patient shown in Fig 18-14.Incisor segment controlControlled tippingPrecision control of incisors is crucial for the correc-tion of excessive horizontal overlap in Class II pa-tients with bialveolar or maxillomandibular protru-sion, for which controlled tipping (rotation) around the root apex (as the center of rotation) is common-ly indicated. However, considering the possibility of the bite deepening, an intrusive controlled tipping would be more suitable for the majority of premo-lar extraction cases. Hence, an intrusive retraction force instead of horizontal retraction force would be necessary.Considering the position of the force vector, a line of force from the archwire hook to the interradic-ular miniscrews between the second premolar and rst molar tends to result in lingual controlled tip-ping with minor intrusion of the CR of the incisor, leading to an intrusive controlled tipping. There-Fig 18-13 Miniscrew-assisted root spring using either single or dual miniscrews. To induce active intrusion, placement of dual miniscrews is recommended. Fig 18-12 Molar uprighting using a miniscrew-assisted uprighting spring. The distance is large enough to produce an up-righting moment without any side effects on the teeth anterior to the molar. (a) The spring causes rotation around the CR, dis-placing the crown distally if the wire is free to slide distally. (b) The spring induces root movement by adding a light mesial force on the crown. Therefore, the spring in b works as a root spring rather than an up-righting spring.a b 441Applications of TADs for Each Segmentfore, the interrradicular miniscrews combined with a slightly undersized continuous archwire that al-lows phase I free tipping is readily indicated for the correction of bialveolar protrusion (Fig 18-15). How-ever, a continuous archwire must slide through the posterior brackets, and any friction can produce side effects on the posterior teeth. In addition, tipping of the incisors can cause mesial tipping of the posterior segments.Fig 18-15 An interrradicular miniscrew combined with a slightly undersized continuous archwire, which allows for phase I free tip-ping, can be indicated for the correction of bialveolar protrusion.Fig 18-14 An 18-year-old patient was treated with a miniscrew-assisted root spring to move the roots of the mandib-ular right second molar and mandibular left third molar forward to replace dis-eased molars. (a to c) Initial intraoral views showing crowding and mesially inclined mandibular right and left second molars. (d and e) A miniscrew-assisted root spring, as explained in Fig 18-12b, was used to in-duce mesial root movement of the impact-ed mandibular right second molar. On the left side, the carious mandibular second molar was extracted and the third molar protracted. (f and g) At the completion of treatment, the patient had proper molar relationships on both sides. (h and i) Com-parative panoramic radiographs before (h) and after (i) treatment, indicating the replacement of the mandibular right rst molar with the second molar and the man-dibular left second molar with the third molar. ba cd egfh i 18The Biomechanics of Miniscrews442Root movementRoot movement of incisors is indicated in Class II, di-vision 2 patients if incisor aring is not desired. Un-intentional lingual tipping of the incisors caused by an undersized wire or vertical bowing of a exible wire also necessitates the correction of incisor axes during or after retraction.As discussed previously, in order to overcome the unpredictability of the force system upon deactiva-tion of the appliance, combined use of miniscrews and a full-size continuous archwire with metal ties or even some selective torsion on the wire is necessary to compensate for an inadequate force system. Again, this can be problematic if side effects are produced on the posterior teeth from the continuous archwire.Figure 18-16 presents the force system of the seg-mental incisor root spring and a continuous archwire with additional torque on a beta-titanium archwire. Extrusion and aring of the incisors are unavoidable side effects of the conventional intra-arch appliance. In Fig 18-16a, an incisor bypass archwire prevents ex-trusion and aring of the incisors. In the continuous archwire (Fig 18-16b), the horizontal and vertical components of the force from the interradicular mini screw suppress the labial aring and extrusion of incisors. Labial apical lever arms on the archwire are not readily indicated, however, because hooks can hardly reach above the level of the CR, especial-ly when the incisors need root movement such as in Class II, division 2 cases.Translation of incisorsMaxillomandibular protrusion or Class II, division 1 malocclusions with relatively normal incisor axes re-quire lingual translation of the incisor segment fol-lowing premolar extraction. On the labial side, it is not practical to extend a lever arm up to the level of the CR to induce this translation. Therefore, ad-ditional torsion on the wire can be used to prevent phase I free tipping and to induce translation using the labial brackets and wire (Fig 18-17). Careful clini-cal monitoring of the incisor axis is essential, accord-ing to the displacement pattern needed by the inci-sors. A continuous archwire can produce side effects on the posterior teeth. The patient in Fig 18-18 required lingual root movement of the incisors. The posttreatment cepha-lometric radiograph demonstrates excellent lingual root movement without incisor extrusion. A combi-nation of bilateral TADs and lingual root torque in the archwire was used.Fig 18-16 Root movement of the incisor segment. (a) In segmental uprighting with a spring, extrusive force and aring of incisors should be prevented by a stabilizing archwire. (b) In a continuous archwire, a similar force system with unknown magnitude of moment is produced from the archwire. A single force from the miniscrew is monitored to eliminate side effects. Fig 18-17 Force system for the translation of incisors. (a) Because it is difcult to determine the appropriate location of the line of force for translation, the combined use of a lever arm and archwire torque may be required. (b) On the lingual side, sufcient extension of the lever arm is anatomically possible.bbaa 443Applications of TADs for Each Segmentbag hcd e fijkFig 18-18 A 21-year-old patient with a Class II, division 2 malocclusion treated by root movement of incisors. (a) Initial extraoral photograph showing a favorable, straight facial prole. (b to f) Initial intraoral photographs showing upright maxillary incisors and crowding in both arches. (g and h) Initial cephalometric radiograph and tracing. U1 to SN was 82.8, indicating severe linguoversion of incisors. (i to k) After leveling with a continuous archwire, intrusive root movement of incisors was performed using miniscrews between the premolar and molar and a continuous rectangular wire with selective torque on the incisor area. 18The Biomechanics of Miniscrews444Incisor intrusionA deep bite may require genuine incisor intrusion for ideal correction. The desired type of tooth move-ment is typically pure intrusion along the long axis of the central incisor, keeping the roots inside the surrounding alveolar bone. Pure intrusion also needs a lingually inclined force that passes through the estimated CR of incisors. In the mandibular arch with an accentuated curve of Spee, many patients may require intrusion of the mandibular four inci-sors; in a few patients, intrusion can be desirable for all six mandibular anterior teeth. Although four-incisor intrusion can be accomplished by a three-piece intrusion arch with dental anchorage, intrusion of six incisors is difcult without the use of TADs. According to a study using nite element analy-sis,6 interradicular miniscrews inserted distal to the canine created a single force (E and F in Fig 18-19a) that effectively resulted in pure intrusion of the mandibular incisor segments with minimal aring. The magnied (×100) displacement image of the anterior segment in Fig 18-19b shows that an intru-Fig 18-18 (cont) (l to q) Extraoral and intraoral photographs after treatment. Note that the straight prole was maintained. (r and s) Ceph-alometric superimposition showing noticeable root movement of the maxillary incisor.srlo p qm n 445Applications of TADs for Each Segmentsion force distal to the canine crown (E and F) can be expected to produce incisor intrusion by translation; more anteriorly placed intrusion forces can lead to aring of the incisors. This result is expected be-cause the CR of the anterior segment is located near the distal of the canine crown. In the same study, a force from four incisors to a TAD that was inserted between the canine and the lateral incisor always produced some incisor aring (Fig 18-19c). The more anterior the intrusive force, the greater was the amount of aring. Rather than following precise rules about TAD location and the line of action of the force, the clinician should always estimate the CR and establish a customized line of action for the intrusive force based on radiographs.Molar distalizationBecause of the large root surface area of the molars and because there is no suitable anchorage unit pos-terior to the molar, conventional wisdom held that true distalization of molars with intraoral mechanics is very challenging and quite impossible, especially if both the rst and second molars are fully erupted. However, ample clinical evidence for the feasibility of molar distalization as a single tooth or even a group of teeth has been published.7,8 The question remains whether molar distalization using mini-screws would be biomechanically appropriate when the desired movement type is largely translation of the posterior segment for the correction of antero-posterior molar relationships.BDFACEacbBD FA CEFig 18-19 Finite element study. (a) The force application points at the anterior segment (four incisors and two ca-nines), with the miniscrews positioned in various locations. In E and F, the miniscrews are positioned distal to the ca-nines. (b) A ×100 magnied displacement image of the in-cisors and canine segment in a after force application. The miniscrews placed distal to the canine (E and F) induced less aring than the other conditions in the study. (c) Finite element model and coordinates. 18The Biomechanics of Miniscrews446It is known that the intraoral molar distalizer such as the pendulum or distal jet results in forward movement of the anchorage segment at the ratio of 2:1 to 3:1, necessitating the distal relocation of the premolar and anterior segment.9 This side effect may still occur if the miniscrew is indirectly used to re inforce the anterior anchorage unit. Therefore, direct distal movement of the posterior segment using a miniscrew can eliminate the side effect and enhance the treatment efciency.Total-arch movementAs discussed previously, simultaneous displacement of the entire dental arch using interradicular mini-screws is very realistic and can be very predictable. Therefore, cases of generalized Class II or Class III malocclusion can be corrected using this protocol in-stead of sequential tooth movement. Thus, total-arch movement can be achieved in a single stage.Total-arch distalizationTotal-arch distalization can be regarded as an exten-sion of the posterior segmental distalization, only more teeth are involved. Interradicular miniscrews combined with lever arms on the main archwire should generate adequate force for total-arch dis-talization. Total-arch intrusionIn a nonextraction model, the CR is presumably lo-cated near the second premolar. Oblique force vec-tors passing near the CR are expected to lead to intrusive translation of the entire arch, which may replace the conventional intrusive mechanics for the hyperdivergent facial type.Clinically, it is difcult to accurately determine and place a TAD exactly at the CR (Fig 18-20a). One help-ful solution is to place two TADs on either side of the estimated CR. The cant of the occlusal plane is carefully monitored, and the upward and backward line of action can be varied during treatment by uti-lizing the anterior or posterior TAD (Fig 18-20b).The patient shown in Fig 18-21 had a Class I occlu-sion with a skeletal open bite. Two miniscrews were placed on one side. As shown in the nite element analysis diagram, the force resultant to the full arch was upward and backward and posterior to the CR (see Fig 18-21g). This tends to steepen the cant of the maxillary plane of occlusion and reduce the open bite. In addition, the posterior intrusive force was also responsible for signicant rotation of the mandible, which reduced the vertical dimension and increased the vertical overlap (see Fig 18-21n). All four rst premolars were extracted. Dramatic incisor retraction was possible because the TADs provided good anchorage during space closure (see Figs 18-21j and 18-21k).SummaryFrom a biomechanical standpoint, miniscrews are considered very effective solutions for many clinical-ly challenging situations. It is conceivable that the movement of the active unit has extended from a segment to an entire arch. In spite of some practical limitations such as miniscrew failure, enhancement of predictability of the clinical outcome may be one of the most remarkable advantages. Nevertheless, the clinical application of miniscrews requires a good understanding of biomechanics. This under-standing is in fact more important than what is re-quired for using typical intra-arch mechanics due to the force consistency.Fig 18-20 Schematic gure of total-arch intrusion. (a) A force away from the CR may induce rotation of the whole arch instead of intrusive translation of the whole arch. (b) An additional miniscrew may be necessary to adjust the line of action of the resultant force. Because a full arch is less sensitive to tipping, the two forces need to be apart as long as possible.abSingle miniscrewDual miniscrews 447SummaryFig 18-21 A 16-year-old girl with the hyperdivergent facial type was treated via total-arch movement with resultant reduction in the vertical dimension. (a and b) Before treatment, the extraoral photographs show a convex prole and severe lip protrusion. (c to e) Initial intraoral views showed a Class I molar relationship with anterior open bite. (f) Initial panoramic radiograph indicated a relatively short ramus height, which is common in the hyperdivergent facial type. (g) Two miniscrews were used to control the line of action. Changing either the mag-nitude of each force or the point of force application at the arch can give various points of application for the resultant force. (h to l) Dual miniscrews were placed on the buccal and palatal (gray circles).bagfc d eh i j5 m 9 m 9 m18 m 18 mk l 18The Biomechanics of Miniscrews448References1. Lee KJ, Park YC, Hwang CJ, et al. Displacement pattern of the maxillary arch depending on miniscrew position in sliding mechanics. Am J Orthod Dentofacial Orthop 2011;140:224–232.2. Upadhyay M, Yadav S, Nagaraj K, Patil S. Treatment ef-fects of mini-implants for en-masse retraction of anterior teeth in bialveolar dental protrusion patients: A random-ized controlled trial. Am J Orthod Dentofacial Orthop 2008;134:18–29.3. Bechtold TE, Kim JW, Choi TH, Park YC, Lee KJ. Distal-ization pattern of the maxillary arch depending on the number of orthodontic miniscrews. Angle Orthod 2013;83:266–273.4. Zachrisson BU, Bantleon HP. Optimal mechanics for man-dibular molar uprighting. World J Orthod 2005;6:80–87.5. Viecilli RF, Chen J, Katona TR, Roberts WE. Force system generated by an adjustable molar root movement mecha-nism. 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