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Lever arm and miniscrew implant system for distalization of maxillary molars and anterior teeth retraction
Lever arm and miniscrew implant system for distalization of maxillary molars and anterior teeth retraction
42
Seung-Min Lim and Ryoon-Ki Hong
THE LEVER ARM AND MINISCREW IMPLANT SYSTEM
INTRODUCTION This chapter will discuss the lever arm and miniscrew implant (MI) system for the treatment of Class II malocclusion through distal movement of maxillary molars and anterior teeth retraction.
DISTALIZATION OF MAXILLARY MOLARS An ideal force system for distal movement of the maxillary molars in three dimensions should have distinct effects in different planes:
■
■
■
sagittal plane: produce a bodily distal movement of molars without undesired effects of the reciprocal force on the anterior teeth vertical plane: exert intrusion forces simultaneously to distalization without changing the mandibular plane angle (i.e. avoid unintended molar extrusion) transverse plane: provide rotational control of the molars.
A
C
The lever arm and MI system fulfills these needs. Lingual lever arms can be easily fabricated and positioned in the maxillary arch to fit the available width and depth of the palate. Their use facilitates alteration of the point of force application and consequently of the moment-to-force ratio in a much easier way than using calibrated springs, since springs cannot be adjusted independently to produce differential forces and moments. For bodily translation of a tooth, two forces are applied at some distance from the center of resistance (CR) of the tooth. One force at the level of the crown and another at the root apex creates a resultant force through the CR and, thus, causes no rotational movement. The lever arm and MI system allows adjustment of the distalizing force through a buccal and a palatal force system, thus varying the overall line of force, as shown in Fig. 42.1.1 If the expansion of a posterior tooth is needed during distalization, it can be obtained simultaneously using a transpalatal arch (TPA). To prevent hanging down of the palatal cusp of the maxillary first molars during expansion, buccal root torque should be applied when using the TPA or an adequate intrusive force should be
B
D
Fig. 42.1 The lever arm and miniscrew implant (MI) system. (A) The line of action of the resultant distalizing force passes through the center of resistance (CR) of the maxillary molars, producing a bodily movement parallel to the occlusal plane. (B) The line of action of the resultant force passes through the CR of the maxillary molars in a superior direction, producing bodily distal movement along with intrusion. (C) Buccal and palatal forces together facilitate rotation control of the maxillary molars. (D) Determination of the length of the lever arm and of the position of the MIs using lateral cephalometric radiography. OP, occlusal plane; BLA, buccal lever arm; PLA, palatal lever arm.
223
224 SECTION VII: MINISCREW IMPLANTS FOR THE TREATMENT OF CLASS II MALOCCLUSION
A
B
D
E
C
F
Fig. 42.2 Indirect bonding method. (A,B) Occlusal (A) and lateral (B) views of the transfer trays and lever arms on the cast. (C,D) Intraoral occlusal (C) and lateral (D) views of the lever arm and miniscrew implant (MI) system immediately after bonding, insertion of one palatal and two buccal miniscrew implants and application of distalizing forces with elastic chains. (E,F) Intraoral occlusal (E) and lateral (F) views of the same patient after distalization of the maxillary molars.
utilized by altering the height between the midpalatal MI and the TPA.The lever arm and MI system comprises three MIs (one inserted in the palate and two inserted buccally between the roots of the maxillary first molars and second premolars on both sides), two buccally positioned lever arms, a TPA functioning as a palatal lever arm, bands on first maxillary molars (and occasionally brackets on second maxillary molars) (Fig. 42.1A–C). Forces are applied using elastic chains.
Appliance Construction The maxillary first molars are banded with double combination tubes (0.022 inch) welded on the buccal side and Burstone lingual brackets (0.032 × 0.032 inch) on the palatal side. The maxillary second molars are bonded with 0.022 inch tubes. A TPA fabricated from either TMA (0.032 × 0.032 inch) or SS wire (0.9 mm) is used as the palatal lever arm. When SS wire is used, the portion engaged in the Burstone lingual brackets is ground with a green stone bur. The buccal lever arm is SS wire (0.019 × 0.025 inch). The positions of the MI and the buccal and palatal lever arms are determined by evaluating the lateral cephalometric radiograph (Fig. 42.1D) and the maxillary cast. Figure 42.2 shows the indirect bonding method for the buccal and palatal lever arms. If there is no need for expansion of the maxillary molars, or if both first and second molars need to be intruded, the TPA can be bonded on both the first and the second molars. The length of the lever arm is set to place the applied force in the desired position with regard to the CR of the molars, which is assessed by thoroughly evaluating a lateral cephalometric radiograph (Fig. 42.1D). If the height of alveolar bone is low, the CR of the tooth moves toward the root apex; therefore, alveolar bone height should be carefully determined. The occlusal plane and facial pattern should be evaluated to decide the type of tooth movement needed, for example whether the tooth should be just distalized parallel to the occlusal plane or with simultaneous intrusion.
Position for Miniscrew Implants The depth of the palatal vault should also be taken into consideration. If it is extremely shallow, bodily movement of maxillary molars is accomplished by inserting a MI in the midpalatal suture. However, in most patients, the head of the MI is positioned vertically on the root apex level. Therefore, the vertical position of the buccal MIs should be set so the
resultant force passes through the CR of the maxillary molars. If the palatal vault is deep, the MIs are implanted more occlusally, while if it is shallow, the MIs are implanted more gingivally.
CLINICAL APPLICATION CASE 1: SEVERE ANTERIOR CROWDING AND MAXILLARY PROTRUSION A 27-year-old Korean woman presented with the chief complaint of severe anterior crowding and maxillary protrusion. She had a convex profile and Class II molar relationships (Fig. 42.3A,B). There was an arch length discrepancy of 18.5 mm and 12 mm in the maxillary and mandibular dental arches, respectively. Bilateral maxillary posterior dental constriction and slight anterior open bite were also present. Cephalometric analysis revealed Class II skeletal relationships of the maxilla and mandible and a dolichofacial growth pattern (Table 42.1). Prior to treatment, the first maxillary premolars and second molars, right mandibular first premolar and the retained root of the left first molar were extracted (Fig. 42.3D). Two MIs (length, 6 mm) were inserted between the maxillary first molars and second premolars bilaterally and both maxillary canines were retracted with elastic chains (Fig. 42.3C,D). An additional MI was inserted in the palate and, after 3 months, distalization of the maxillary molars was initiated. Because of the anterior open bite tendency and the hyperdivergent growth pattern, the extrusion of maxillary molars that usually takes place during distalization could lead to a clockwise rotation of the mandible and opening of the bite. Consequently, the vertical levels of the MI heads and lever arms on both sides were adjusted to produce an intrusive force in addition to the distalization force (Fig. 42.3E,F). Using elastic chain modules, a force of approximately 150 g was applied on each lever arm on the buccal side and a force of approximately 300 g was applied on the TPA on the palate (i.e. in total a force of approximately 300 g on each maxillary molar). In addition, a 0.9 mm SS expanded TPA was used to correct bilateral posterior maxillary dental constriction. After 9 months of treatment, the lever arms and the TPA were removed and two additional MIs were inserted on the palate between the roots of the second premolars and first molars (Fig. 42.3G,H). Elastic chains and power hooks were utilized to retract the anterior teeth and treatment was continued for an additional 12 months.
LEVER ARM AND MINISCREW IMPLANT SYSTEM
A
B
C
D
E
F
G
H
I
J
225
L
K Fig. 42.3 Case 1: distalization of maxillary molars for severe anterior crowding and maxillary protrusion. (A,B) Pretreatment. (C,D) Placement of buccal fixed appliances on the lower arch, and insertion of the palatal lever arm and of one palatal and two buccal miniscrew implants (MIs) on the maxilla. The buccal MIs facilitate canine retraction, while the palatal MI effects distalization of maxillary molars. (E,F) Insertion of the buccal lever arms on the maxillary arch. Adjustment of the vertical level of all MI heads and lever arms to apply an intrusive force simultaneous to distalization. (G,H) Insertion of two additional MIs on the palate between the roots of the second premolars and first molars to facilitate anterior teeth retraction. (I,J) Post-treatment. (K,L) Superimposition of the cephalometric tracings before (black) and after (red) treatment; overall superimposition on the SN line (K) and maxillary superimposition (L).
Table 42.1 Case 1: cephalometric measurements Measurement
Pretreatment
Post-treatment
SNA (°)
81
80
SNB (°)
73
73
ANB (°)
8
7
FMA (°)
29.5
29.5
NPo-FH (°)
73.5
73.5
U1to FH (°)
124
105
FMIA (°)
55
55.5
Overbite (mm)
−1
2
Overjet (mm)
8
2.5
Upper lip to E-line (mm)
1.5
−2
Lower lip to E-line (mm)
3
−0.5
Skeletal
Dental
Soft tissues
FMA, Frankfort-mandibular plane angle; FMIA, Frankfort-mandibular incisor angle.
The fixed appliances were removed 24 months after initiation of treatment. Ideal Class I molar and canine relationships, overbite and overjet were achieved (Fig. 42.3I,J). On the cephalometric superimposition before and after treatment, a 3.0 mm bodily distalization and 1.5 mm intrusion of the maxillary molars were evident, while the mandibular plane angle did not change (Fig. 42.3K,L).
ANTERIOR TEETH RETRACTION Temporary anchorage devices seem to be effective for intraoral anchorage reinforcement for en masse retraction of the anterior teeth. However, torque control is still a significant issue to be taken into consideration. The design of a force system for retraction of the anterior teeth is based on knowledge of the CR of the unit (the six anterior teeth) to be moved. Based on retraction with sliding mechanics, the CR of the six anterior teeth has been determined as 77% of the total root length away from the apex, which is approximately 11–12 mm apical to the incisal edge of the incisors (6.5–7.5 mm apical to labial brackets position).2,3 Factors such as wire size, the bracket slot size and the interplay between bracket slot and archwire are some of the variables affecting the biomechanical behavior of tooth movement. A three-dimensional finite element analysis of sliding mechanics showed that a lever arm placed
226 SECTION VII: MINISCREW IMPLANTS FOR THE TREATMENT OF CLASS II MALOCCLUSION
Fig. 42.4 Analysis of anterior teeth retraction. (A,B) Effects of bracket positioning and point of force application on tooth movement when bonding on the labial (A) or the lingual (B) surface of the maxillary central incisor. When using lingual appliances, the resultant force produces a larger moment, which tips the incisor more lingual than when using conventional labial appliances. (C–E) Line of action of the retraction forces with varying configurations of the lever arm or miniscrew implant (MI) position. (C) When the resultant force vector passes through the CR of the incisors; the anterior teeth will be retracted bodily and intruded (left), retracted bodily (middle) or retracted bodily and extruded (right). (D) With simultaneous lingual crown torque of the anterior teeth where the resultant force vector passes below the CR of the incisor, the anterior teeth will be retracted and intruded (left), retracted (middle) or retracted and extruded (right). (E) With simultaneous lingual root torque of the anterior teeth where the resultant force vector passes above the CR of the incisors, the anterior teeth will be retracted and intruded (left), retracted (middle) or retracted and extruded (right). CR, center of resistance; FR, retraction force; FI, intrusion force; FR+I, resultant force; LA, lever arm; M1 and M2, moments.
CR FR+I FR
FI
FI
CR
FR+I
FR
M1
M2
B
A
MI MI LA
CR
CR
CR LA
MI
LA
C
MI
D
LA
CR
MI
LA
MI
CR
CR
MI
LA
MI LA
CR
LA
CR
MI
LA
CR
E
mesial to the canine at a height of 5.5 mm apical to the bracket position produced bodily movement for anterior teeth retraction.4 As the position of the lever arm on the archwire was moved from the incisor to the premolars, the length of the lever arm had to be increased in order to produce parallel translation: the length should be 4.99 mm apical to the bracket position when the lever arm was located between the lateral incisors and the canines and 8.22 mm when located between the canines and the first premolars.5 If the interplay between the bracket slot and archwire is increased, the height of the lever arm is also increased in order to produce bodily movement, and vice versa. Therefore, the lever arm should be placed mesial to the canines in order to attain better control of the anterior teeth with sliding mechanics. Several other variables affecting the biomechanical behavior of tooth movement must be taken into consideration in clinical lingual orthodontics, for example anatomic parameters, such as the length and the shape of teeth roots, the width of the periodontal ligament, the palatal alveolar bone height, the angle of crown inclination and the physical properties of periodontal tissue. In lingual orthodontics, torque control of anterior teeth during retraction is more challenging than using labial fixed appliances because of the specific position of the brackets on the lingual surfaces of the teeth (Fig. 42.4A,B). Anterior torque control is achieved either by directly applying a moment and a force to a lingual bracket or by using lever arm mechanics to obtain the desired line of action of the force with respect to the CR of the tooth (Fig. 42.4C–E).6–8 The desired tooth movement is attained by adjusting the length of the lever arm and the point of force application. Using lingual appliances allows the lever arm system to be ideally located because appropriate space is almost always available within the width and depth dimensions of the palate. Therefore, in combination with a lever
arm, MIs can be used not only for anchorage reinforcement but also for anterior torque control in lingual orthodontics.
THE LEVER ARM AND MINISCREW IMPLANT SYSTEM To design the optimal lever arm and MI system for obtaining the desired force system during retraction with respect to the CR of the anterior segment, the point of force application and the line of action of the retraction force are planned using lateral cephalometric radiographs. Figure 42.4C–E illustrates the overall reaction that can be expected with retraction forces in various configurations. Force parallel to the occlusal plane and applied through the CR of the anterior teeth will bodily retract the anterior segment, alone or with simultaneous intrusion or extrusion (Fig. 42.4C). If the length of the lever arm is adjusted so that the line of action of the retraction force is located below the CR of the anterior teeth, there will be lingual crown torque of the anterior segment (Fig. 42.4D), while if the line of action of the retraction force is located above the CR, there will be lingual root torque of the anterior segment (Fig. 42.4E).
Appliance Construction Because the interplay between the bracket slot and archwire is a very important factor, affecting the height of the lever arm, clinicians must be aware of the real slot size and shape of the lingual brackets (Table 42.2).9 In lingual orthodontics during anterior teeth retraction with sliding mechanics using brackets with horizontal slots, there is a tendency of the archwire to come out of the lingual bracket slot. Consequently, there is a bigger chance of losing control of anterior segment torque if the bracket
LEVER ARM AND MINISCREW IMPLANT SYSTEM
slot walls are divergent. In such a case, a longer lever arm should be used and additional anterior torque should be applied to the archwire. Fujita lingual brackets are provided with horizontal and vertical slots (Fig. 42.5). The size of the main horizontal slot is 0.018 × 0.025 inch and of the vertical slot is 0.019 × 0.019 inch. An SS archwire (0.016 × 0.022 inch) is inserted into the horizontal slot as the main archwire, while Table 42.2 Bracket slot size, shape and difference between slot top and slot base Mean slot size (%) Slot base
Slot top
Slot shape
Difference from slot top to base
Ormco 7th generation
13.95
17.23
D
3.28
STB
−0.92
2.58
D
3.5
Fujita
6.08
4.33
C
−1.75
Stealth
7.61
7.63
P
0.2
In-Ovation L
3.94
5.2
D
1.26
Bracket-type prescription
227
an SS segmented archwire (0.018 × 0.018 inch or 0.019 × 0.019 inch) is inserted into the vertical slot. This segmented archwire can be used for additional anterior torque control and can prevent unintentional anterior torque loss, thus allowing shorter lever arms to be used during anterior teeth retraction.10 When the lever arm is located between the lateral incisor and the canine, its length in the vertical is 2–3 mm shorter than its actual length because of the inclination of the palatal vault. Therefore, a crimpable hook 7 mm in length is recommended to be used as lever arm for controlled tipping during anterior teeth retraction and one of 10 mm in length for bodily movement. Alternatively, a 7 mm crimpable hook fabricated from SS segmented archwire (0.019 × 0.019 inch) can be applied for additional lingual root torque. However, when a lever arm of more than 10 mm in length is needed for bodily movement, for example with low bone level or requiring lingual root movement, soldering of a 0.9 mm SS wire is recommended to prevent deformation.
CLINICAL APPLICATION CASE 2: LIP PROTRUSION AND ANTERIOR CROWDING
D, divergent; C, convergent; P, parallel.
VS
HS
0 mm Fig. 42.5 Anterior Fujita bracket. VS, vertical slot; HS, horizontal slot.
A
B
E
F
A 33-year-old Korean woman presented with the chief complaint of lip protrusion and anterior crowding. She had a convex profile and Class I malocclusion (Fig. 42.6A,B). Cephalometric analysis revealed Class II skeletal relationships of the maxilla and mandible (Table 42.3). Initially, all four first premolars were extracted and the brackets were bonded indirectly. After 5 months of treatment, both maxillary and mandibular teeth were leveled and aligned (Fig. 42.6C,D). During space closure, bodily movements of maxillary anterior teeth without anchorage loss were required. Two 9 mm MIs were inserted between the maxillary first molars and second premolars. A 7 mm crimpable hook (SS wire, 0.016 × 0.022 inch) was used as lever arm and positioned distal to the canines. An SS wire (0.019 × 0.019 inch) was also placed into the vertical
C
G
D
H
Fig. 42.6 Case 2: anterior teeth retraction for lip protrusion and anterior crowding. (A,B) Pretreatment. (C) Bonding of lingual appliances on both dental arches. (D) Insertion of the lever arm and miniscrew implant system for anterior teeth retraction on the maxilla. (E,F) Post-treatment. (G,H) Superimposition of cephalometric tracings before treatment (black) and before retraction (green) (G), and before retraction (green) and after treatment (red) (H).
228 SECTION VII: MINISCREW IMPLANTS FOR THE TREATMENT OF CLASS II MALOCCLUSION
Table 42.3 Case 3: cephalometric measurements Pretreatment
Before retraction
Post-treatment
SNA (°)
86
86
85
SNB (°)
78
78
78
ANB (°)
8
8
7
FMA (°)
32
31.5
31.5
NPo-FH (°)
87
86.5
86.5
U1 to FH (°)
107
95
93
FMIA (°)
45
56
69
Overbite (mm)
3
4.5
3.5
Overjet (mm)
4.5
5
3.5
Upper lip to E-line (mm)
5.5
4.5
0.5
Lower lip to E-line (mm)
6.5
5
1
Measurement Skeletal
Dental
Soft tissues
FMA, Frankfort-mandibular plane angle; FMIA, Frankfort-mandibular incisor angle.
slot, with 10° of additional lingual root torque. According to the evaluation of the lateral cephalometric radiograph, the upper ending of the lever arm was constructed 12 mm away from the incisal edge. After 3 further months, 6 mm MIs were inserted between the mandibular first molars and second premolars for anchorage reinforcement and treatment was continued for an additional 10 months. After a total of 18 months, treatment was completed.
After treatment, the convex profile was favorably improved, and crowding was corrected (Fig. 42.6E,F). Superimpositions of the cephalometric tracing before and after treatment showed that bodily movement of the maxillary incisors occurred without evidence of loss of anchorage of the posterior teeth (i.e. maxillary molars did not move mesially during retraction; Fig. 42.6G,H). In addition, no downward or backward rotation of the mandible was evident, and lip protrusion was improved.
REFERENCES 1. Lim SM, Hong RK. Distal movement of maxillary molars using a lever-arm and miniscrew implant system. Angle Orthod 2008;78:167–75. 2. Sia SS, Koga Y, Yoshida N. Determining the center of resistance of maxillary anterior teeth subjected to retraction forces in sliding mechanics: an in vivo study. Angle Orthod 2007;77:999–1003. 3. Sia SS, Shibazaki T, Koga Y, et al. Experimental determination of optimal force system required for control of anterior tooth movement in sliding mechanics. Am J Orthod Dentofacial Orthop 2009;135:36–41. 4. Tominaga JY, Tanaka M, Koga Y, et al. Optimal loading conditions for controlled movement of anterior teeth in sliding mechanics. Angle Orthod 2009;79:102–7. 5. Kim TS, Suh JS, Lee MK. Optimum conditions for parallel translation of maxillary anterior teeth under retraction force determined with the finite element method. Am J Orthod Dentofacial Orthop 2010;137:639–47. 6. Bantleon HP. Modified lingual lever arm technique: biomechanical considerations. In: Nanda R, editor. Biomechanics in clinical orthodontics. Philadelphia, PA: Saunders; 1997. p. 229–45. 7. Park YC, Choy KC, Lee JS, et al. Lever-arm mechanics in lingual orthodontics. J Clin Orthod 2000;34:601–5. 8. Hong RK, Heo JM, Ha YK. Lever-arm and miniscrew implant system for anterior torque control during retraction in lingual orthodontic treatment. Angle Orthod 2005;75:129–41. 9. Lim SM, Hong RK. An evaluation of slot size in lingual orthodontic brackets. Kor J Lingual Orthod 2012;1:19–23. 10. Lim SM, Hong RK. The tandem archwire technique in lingual orthodontics. J Clin Orthod 2013;47:232–40.
42
Seung-Min Lim and Ryoon-Ki Hong
THE LEVER ARM AND MINISCREW IMPLANT SYSTEM
INTRODUCTION This chapter will discuss the lever arm and miniscrew implant (MI) system for the treatment of Class II malocclusion through distal movement of maxillary molars and anterior teeth retraction.
DISTALIZATION OF MAXILLARY MOLARS An ideal force system for distal movement of the maxillary molars in three dimensions should have distinct effects in different planes:
■
■
■
sagittal plane: produce a bodily distal movement of molars without undesired effects of the reciprocal force on the anterior teeth vertical plane: exert intrusion forces simultaneously to distalization without changing the mandibular plane angle (i.e. avoid unintended molar extrusion) transverse plane: provide rotational control of the molars.
A
C
The lever arm and MI system fulfills these needs. Lingual lever arms can be easily fabricated and positioned in the maxillary arch to fit the available width and depth of the palate. Their use facilitates alteration of the point of force application and consequently of the moment-to-force ratio in a much easier way than using calibrated springs, since springs cannot be adjusted independently to produce differential forces and moments. For bodily translation of a tooth, two forces are applied at some distance from the center of resistance (CR) of the tooth. One force at the level of the crown and another at the root apex creates a resultant force through the CR and, thus, causes no rotational movement. The lever arm and MI system allows adjustment of the distalizing force through a buccal and a palatal force system, thus varying the overall line of force, as shown in Fig. 42.1.1 If the expansion of a posterior tooth is needed during distalization, it can be obtained simultaneously using a transpalatal arch (TPA). To prevent hanging down of the palatal cusp of the maxillary first molars during expansion, buccal root torque should be applied when using the TPA or an adequate intrusive force should be
B
D
Fig. 42.1 The lever arm and miniscrew implant (MI) system. (A) The line of action of the resultant distalizing force passes through the center of resistance (CR) of the maxillary molars, producing a bodily movement parallel to the occlusal plane. (B) The line of action of the resultant force passes through the CR of the maxillary molars in a superior direction, producing bodily distal movement along with intrusion. (C) Buccal and palatal forces together facilitate rotation control of the maxillary molars. (D) Determination of the length of the lever arm and of the position of the MIs using lateral cephalometric radiography. OP, occlusal plane; BLA, buccal lever arm; PLA, palatal lever arm.
223
224 SECTION VII: MINISCREW IMPLANTS FOR THE TREATMENT OF CLASS II MALOCCLUSION
A
B
D
E
C
F
Fig. 42.2 Indirect bonding method. (A,B) Occlusal (A) and lateral (B) views of the transfer trays and lever arms on the cast. (C,D) Intraoral occlusal (C) and lateral (D) views of the lever arm and miniscrew implant (MI) system immediately after bonding, insertion of one palatal and two buccal miniscrew implants and application of distalizing forces with elastic chains. (E,F) Intraoral occlusal (E) and lateral (F) views of the same patient after distalization of the maxillary molars.
utilized by altering the height between the midpalatal MI and the TPA.The lever arm and MI system comprises three MIs (one inserted in the palate and two inserted buccally between the roots of the maxillary first molars and second premolars on both sides), two buccally positioned lever arms, a TPA functioning as a palatal lever arm, bands on first maxillary molars (and occasionally brackets on second maxillary molars) (Fig. 42.1A–C). Forces are applied using elastic chains.
Appliance Construction The maxillary first molars are banded with double combination tubes (0.022 inch) welded on the buccal side and Burstone lingual brackets (0.032 × 0.032 inch) on the palatal side. The maxillary second molars are bonded with 0.022 inch tubes. A TPA fabricated from either TMA (0.032 × 0.032 inch) or SS wire (0.9 mm) is used as the palatal lever arm. When SS wire is used, the portion engaged in the Burstone lingual brackets is ground with a green stone bur. The buccal lever arm is SS wire (0.019 × 0.025 inch). The positions of the MI and the buccal and palatal lever arms are determined by evaluating the lateral cephalometric radiograph (Fig. 42.1D) and the maxillary cast. Figure 42.2 shows the indirect bonding method for the buccal and palatal lever arms. If there is no need for expansion of the maxillary molars, or if both first and second molars need to be intruded, the TPA can be bonded on both the first and the second molars. The length of the lever arm is set to place the applied force in the desired position with regard to the CR of the molars, which is assessed by thoroughly evaluating a lateral cephalometric radiograph (Fig. 42.1D). If the height of alveolar bone is low, the CR of the tooth moves toward the root apex; therefore, alveolar bone height should be carefully determined. The occlusal plane and facial pattern should be evaluated to decide the type of tooth movement needed, for example whether the tooth should be just distalized parallel to the occlusal plane or with simultaneous intrusion.
Position for Miniscrew Implants The depth of the palatal vault should also be taken into consideration. If it is extremely shallow, bodily movement of maxillary molars is accomplished by inserting a MI in the midpalatal suture. However, in most patients, the head of the MI is positioned vertically on the root apex level. Therefore, the vertical position of the buccal MIs should be set so the
resultant force passes through the CR of the maxillary molars. If the palatal vault is deep, the MIs are implanted more occlusally, while if it is shallow, the MIs are implanted more gingivally.
CLINICAL APPLICATION CASE 1: SEVERE ANTERIOR CROWDING AND MAXILLARY PROTRUSION A 27-year-old Korean woman presented with the chief complaint of severe anterior crowding and maxillary protrusion. She had a convex profile and Class II molar relationships (Fig. 42.3A,B). There was an arch length discrepancy of 18.5 mm and 12 mm in the maxillary and mandibular dental arches, respectively. Bilateral maxillary posterior dental constriction and slight anterior open bite were also present. Cephalometric analysis revealed Class II skeletal relationships of the maxilla and mandible and a dolichofacial growth pattern (Table 42.1). Prior to treatment, the first maxillary premolars and second molars, right mandibular first premolar and the retained root of the left first molar were extracted (Fig. 42.3D). Two MIs (length, 6 mm) were inserted between the maxillary first molars and second premolars bilaterally and both maxillary canines were retracted with elastic chains (Fig. 42.3C,D). An additional MI was inserted in the palate and, after 3 months, distalization of the maxillary molars was initiated. Because of the anterior open bite tendency and the hyperdivergent growth pattern, the extrusion of maxillary molars that usually takes place during distalization could lead to a clockwise rotation of the mandible and opening of the bite. Consequently, the vertical levels of the MI heads and lever arms on both sides were adjusted to produce an intrusive force in addition to the distalization force (Fig. 42.3E,F). Using elastic chain modules, a force of approximately 150 g was applied on each lever arm on the buccal side and a force of approximately 300 g was applied on the TPA on the palate (i.e. in total a force of approximately 300 g on each maxillary molar). In addition, a 0.9 mm SS expanded TPA was used to correct bilateral posterior maxillary dental constriction. After 9 months of treatment, the lever arms and the TPA were removed and two additional MIs were inserted on the palate between the roots of the second premolars and first molars (Fig. 42.3G,H). Elastic chains and power hooks were utilized to retract the anterior teeth and treatment was continued for an additional 12 months.
LEVER ARM AND MINISCREW IMPLANT SYSTEM
A
B
C
D
E
F
G
H
I
J
225
L
K Fig. 42.3 Case 1: distalization of maxillary molars for severe anterior crowding and maxillary protrusion. (A,B) Pretreatment. (C,D) Placement of buccal fixed appliances on the lower arch, and insertion of the palatal lever arm and of one palatal and two buccal miniscrew implants (MIs) on the maxilla. The buccal MIs facilitate canine retraction, while the palatal MI effects distalization of maxillary molars. (E,F) Insertion of the buccal lever arms on the maxillary arch. Adjustment of the vertical level of all MI heads and lever arms to apply an intrusive force simultaneous to distalization. (G,H) Insertion of two additional MIs on the palate between the roots of the second premolars and first molars to facilitate anterior teeth retraction. (I,J) Post-treatment. (K,L) Superimposition of the cephalometric tracings before (black) and after (red) treatment; overall superimposition on the SN line (K) and maxillary superimposition (L).
Table 42.1 Case 1: cephalometric measurements Measurement
Pretreatment
Post-treatment
SNA (°)
81
80
SNB (°)
73
73
ANB (°)
8
7
FMA (°)
29.5
29.5
NPo-FH (°)
73.5
73.5
U1to FH (°)
124
105
FMIA (°)
55
55.5
Overbite (mm)
−1
2
Overjet (mm)
8
2.5
Upper lip to E-line (mm)
1.5
−2
Lower lip to E-line (mm)
3
−0.5
Skeletal
Dental
Soft tissues
FMA, Frankfort-mandibular plane angle; FMIA, Frankfort-mandibular incisor angle.
The fixed appliances were removed 24 months after initiation of treatment. Ideal Class I molar and canine relationships, overbite and overjet were achieved (Fig. 42.3I,J). On the cephalometric superimposition before and after treatment, a 3.0 mm bodily distalization and 1.5 mm intrusion of the maxillary molars were evident, while the mandibular plane angle did not change (Fig. 42.3K,L).
ANTERIOR TEETH RETRACTION Temporary anchorage devices seem to be effective for intraoral anchorage reinforcement for en masse retraction of the anterior teeth. However, torque control is still a significant issue to be taken into consideration. The design of a force system for retraction of the anterior teeth is based on knowledge of the CR of the unit (the six anterior teeth) to be moved. Based on retraction with sliding mechanics, the CR of the six anterior teeth has been determined as 77% of the total root length away from the apex, which is approximately 11–12 mm apical to the incisal edge of the incisors (6.5–7.5 mm apical to labial brackets position).2,3 Factors such as wire size, the bracket slot size and the interplay between bracket slot and archwire are some of the variables affecting the biomechanical behavior of tooth movement. A three-dimensional finite element analysis of sliding mechanics showed that a lever arm placed
226 SECTION VII: MINISCREW IMPLANTS FOR THE TREATMENT OF CLASS II MALOCCLUSION
Fig. 42.4 Analysis of anterior teeth retraction. (A,B) Effects of bracket positioning and point of force application on tooth movement when bonding on the labial (A) or the lingual (B) surface of the maxillary central incisor. When using lingual appliances, the resultant force produces a larger moment, which tips the incisor more lingual than when using conventional labial appliances. (C–E) Line of action of the retraction forces with varying configurations of the lever arm or miniscrew implant (MI) position. (C) When the resultant force vector passes through the CR of the incisors; the anterior teeth will be retracted bodily and intruded (left), retracted bodily (middle) or retracted bodily and extruded (right). (D) With simultaneous lingual crown torque of the anterior teeth where the resultant force vector passes below the CR of the incisor, the anterior teeth will be retracted and intruded (left), retracted (middle) or retracted and extruded (right). (E) With simultaneous lingual root torque of the anterior teeth where the resultant force vector passes above the CR of the incisors, the anterior teeth will be retracted and intruded (left), retracted (middle) or retracted and extruded (right). CR, center of resistance; FR, retraction force; FI, intrusion force; FR+I, resultant force; LA, lever arm; M1 and M2, moments.
CR FR+I FR
FI
FI
CR
FR+I
FR
M1
M2
B
A
MI MI LA
CR
CR
CR LA
MI
LA
C
MI
D
LA
CR
MI
LA
MI
CR
CR
MI
LA
MI LA
CR
LA
CR
MI
LA
CR
E
mesial to the canine at a height of 5.5 mm apical to the bracket position produced bodily movement for anterior teeth retraction.4 As the position of the lever arm on the archwire was moved from the incisor to the premolars, the length of the lever arm had to be increased in order to produce parallel translation: the length should be 4.99 mm apical to the bracket position when the lever arm was located between the lateral incisors and the canines and 8.22 mm when located between the canines and the first premolars.5 If the interplay between the bracket slot and archwire is increased, the height of the lever arm is also increased in order to produce bodily movement, and vice versa. Therefore, the lever arm should be placed mesial to the canines in order to attain better control of the anterior teeth with sliding mechanics. Several other variables affecting the biomechanical behavior of tooth movement must be taken into consideration in clinical lingual orthodontics, for example anatomic parameters, such as the length and the shape of teeth roots, the width of the periodontal ligament, the palatal alveolar bone height, the angle of crown inclination and the physical properties of periodontal tissue. In lingual orthodontics, torque control of anterior teeth during retraction is more challenging than using labial fixed appliances because of the specific position of the brackets on the lingual surfaces of the teeth (Fig. 42.4A,B). Anterior torque control is achieved either by directly applying a moment and a force to a lingual bracket or by using lever arm mechanics to obtain the desired line of action of the force with respect to the CR of the tooth (Fig. 42.4C–E).6–8 The desired tooth movement is attained by adjusting the length of the lever arm and the point of force application. Using lingual appliances allows the lever arm system to be ideally located because appropriate space is almost always available within the width and depth dimensions of the palate. Therefore, in combination with a lever
arm, MIs can be used not only for anchorage reinforcement but also for anterior torque control in lingual orthodontics.
THE LEVER ARM AND MINISCREW IMPLANT SYSTEM To design the optimal lever arm and MI system for obtaining the desired force system during retraction with respect to the CR of the anterior segment, the point of force application and the line of action of the retraction force are planned using lateral cephalometric radiographs. Figure 42.4C–E illustrates the overall reaction that can be expected with retraction forces in various configurations. Force parallel to the occlusal plane and applied through the CR of the anterior teeth will bodily retract the anterior segment, alone or with simultaneous intrusion or extrusion (Fig. 42.4C). If the length of the lever arm is adjusted so that the line of action of the retraction force is located below the CR of the anterior teeth, there will be lingual crown torque of the anterior segment (Fig. 42.4D), while if the line of action of the retraction force is located above the CR, there will be lingual root torque of the anterior segment (Fig. 42.4E).
Appliance Construction Because the interplay between the bracket slot and archwire is a very important factor, affecting the height of the lever arm, clinicians must be aware of the real slot size and shape of the lingual brackets (Table 42.2).9 In lingual orthodontics during anterior teeth retraction with sliding mechanics using brackets with horizontal slots, there is a tendency of the archwire to come out of the lingual bracket slot. Consequently, there is a bigger chance of losing control of anterior segment torque if the bracket
LEVER ARM AND MINISCREW IMPLANT SYSTEM
slot walls are divergent. In such a case, a longer lever arm should be used and additional anterior torque should be applied to the archwire. Fujita lingual brackets are provided with horizontal and vertical slots (Fig. 42.5). The size of the main horizontal slot is 0.018 × 0.025 inch and of the vertical slot is 0.019 × 0.019 inch. An SS archwire (0.016 × 0.022 inch) is inserted into the horizontal slot as the main archwire, while Table 42.2 Bracket slot size, shape and difference between slot top and slot base Mean slot size (%) Slot base
Slot top
Slot shape
Difference from slot top to base
Ormco 7th generation
13.95
17.23
D
3.28
STB
−0.92
2.58
D
3.5
Fujita
6.08
4.33
C
−1.75
Stealth
7.61
7.63
P
0.2
In-Ovation L
3.94
5.2
D
1.26
Bracket-type prescription
227
an SS segmented archwire (0.018 × 0.018 inch or 0.019 × 0.019 inch) is inserted into the vertical slot. This segmented archwire can be used for additional anterior torque control and can prevent unintentional anterior torque loss, thus allowing shorter lever arms to be used during anterior teeth retraction.10 When the lever arm is located between the lateral incisor and the canine, its length in the vertical is 2–3 mm shorter than its actual length because of the inclination of the palatal vault. Therefore, a crimpable hook 7 mm in length is recommended to be used as lever arm for controlled tipping during anterior teeth retraction and one of 10 mm in length for bodily movement. Alternatively, a 7 mm crimpable hook fabricated from SS segmented archwire (0.019 × 0.019 inch) can be applied for additional lingual root torque. However, when a lever arm of more than 10 mm in length is needed for bodily movement, for example with low bone level or requiring lingual root movement, soldering of a 0.9 mm SS wire is recommended to prevent deformation.
CLINICAL APPLICATION CASE 2: LIP PROTRUSION AND ANTERIOR CROWDING
D, divergent; C, convergent; P, parallel.
VS
HS
0 mm Fig. 42.5 Anterior Fujita bracket. VS, vertical slot; HS, horizontal slot.
A
B
E
F
A 33-year-old Korean woman presented with the chief complaint of lip protrusion and anterior crowding. She had a convex profile and Class I malocclusion (Fig. 42.6A,B). Cephalometric analysis revealed Class II skeletal relationships of the maxilla and mandible (Table 42.3). Initially, all four first premolars were extracted and the brackets were bonded indirectly. After 5 months of treatment, both maxillary and mandibular teeth were leveled and aligned (Fig. 42.6C,D). During space closure, bodily movements of maxillary anterior teeth without anchorage loss were required. Two 9 mm MIs were inserted between the maxillary first molars and second premolars. A 7 mm crimpable hook (SS wire, 0.016 × 0.022 inch) was used as lever arm and positioned distal to the canines. An SS wire (0.019 × 0.019 inch) was also placed into the vertical
C
G
D
H
Fig. 42.6 Case 2: anterior teeth retraction for lip protrusion and anterior crowding. (A,B) Pretreatment. (C) Bonding of lingual appliances on both dental arches. (D) Insertion of the lever arm and miniscrew implant system for anterior teeth retraction on the maxilla. (E,F) Post-treatment. (G,H) Superimposition of cephalometric tracings before treatment (black) and before retraction (green) (G), and before retraction (green) and after treatment (red) (H).
228 SECTION VII: MINISCREW IMPLANTS FOR THE TREATMENT OF CLASS II MALOCCLUSION
Table 42.3 Case 3: cephalometric measurements Pretreatment
Before retraction
Post-treatment
SNA (°)
86
86
85
SNB (°)
78
78
78
ANB (°)
8
8
7
FMA (°)
32
31.5
31.5
NPo-FH (°)
87
86.5
86.5
U1 to FH (°)
107
95
93
FMIA (°)
45
56
69
Overbite (mm)
3
4.5
3.5
Overjet (mm)
4.5
5
3.5
Upper lip to E-line (mm)
5.5
4.5
0.5
Lower lip to E-line (mm)
6.5
5
1
Measurement Skeletal
Dental
Soft tissues
FMA, Frankfort-mandibular plane angle; FMIA, Frankfort-mandibular incisor angle.
slot, with 10° of additional lingual root torque. According to the evaluation of the lateral cephalometric radiograph, the upper ending of the lever arm was constructed 12 mm away from the incisal edge. After 3 further months, 6 mm MIs were inserted between the mandibular first molars and second premolars for anchorage reinforcement and treatment was continued for an additional 10 months. After a total of 18 months, treatment was completed.
After treatment, the convex profile was favorably improved, and crowding was corrected (Fig. 42.6E,F). Superimpositions of the cephalometric tracing before and after treatment showed that bodily movement of the maxillary incisors occurred without evidence of loss of anchorage of the posterior teeth (i.e. maxillary molars did not move mesially during retraction; Fig. 42.6G,H). In addition, no downward or backward rotation of the mandible was evident, and lip protrusion was improved.
REFERENCES 1. Lim SM, Hong RK. Distal movement of maxillary molars using a lever-arm and miniscrew implant system. Angle Orthod 2008;78:167–75. 2. Sia SS, Koga Y, Yoshida N. Determining the center of resistance of maxillary anterior teeth subjected to retraction forces in sliding mechanics: an in vivo study. Angle Orthod 2007;77:999–1003. 3. Sia SS, Shibazaki T, Koga Y, et al. Experimental determination of optimal force system required for control of anterior tooth movement in sliding mechanics. Am J Orthod Dentofacial Orthop 2009;135:36–41. 4. Tominaga JY, Tanaka M, Koga Y, et al. Optimal loading conditions for controlled movement of anterior teeth in sliding mechanics. Angle Orthod 2009;79:102–7. 5. Kim TS, Suh JS, Lee MK. Optimum conditions for parallel translation of maxillary anterior teeth under retraction force determined with the finite element method. Am J Orthod Dentofacial Orthop 2010;137:639–47. 6. Bantleon HP. Modified lingual lever arm technique: biomechanical considerations. In: Nanda R, editor. Biomechanics in clinical orthodontics. Philadelphia, PA: Saunders; 1997. p. 229–45. 7. Park YC, Choy KC, Lee JS, et al. Lever-arm mechanics in lingual orthodontics. J Clin Orthod 2000;34:601–5. 8. Hong RK, Heo JM, Ha YK. Lever-arm and miniscrew implant system for anterior torque control during retraction in lingual orthodontic treatment. Angle Orthod 2005;75:129–41. 9. Lim SM, Hong RK. An evaluation of slot size in lingual orthodontic brackets. Kor J Lingual Orthod 2012;1:19–23. 10. Lim SM, Hong RK. The tandem archwire technique in lingual orthodontics. J Clin Orthod 2013;47:232–40.