Biomechanical Considerations in the Correction of Anterior Open Bite with Maxillary Skeletal Plates
In the case shown here, skeletal plates in the zygomatic buttress allowed us to transmit ortho-dontic forces from the molars to the maxilla for posterior vertical intrusion. This article will exam-ine the orthodontic mechanics needed to control the ratio of the moment of couple (MC) to the mo-ment of force (MF) and the load-deflection ratio, working within the range, strength, and formabil-ity of the archwires to obtain light, well-controlled biological forces.
Diagnosis and Treatment Plan
A female age 12 years, 8 months, presented with a full profile, lip protrusion, a small nose and chin, an inverse smile arc, an incompetent lip seal, inadequate maxillary incisor display, and a rela-tively long face (Fig. 1). Study casts showed a Class II molar relationship, upper and lower crowd-ing, and an anterior open bite.
Cephalometric analysis (Table 1) indicated a Class II skeletal relationship (ANB = 6°) with a high FMA angle (36°), a high SN-GoGn angle (49°), and excessive total anterior facial height (130mm). The maxillary incisors were proclined (U1-SN = 115°), as were the mandibular incisors (L1-MP = 101°). Vertically, the maxillary molars were positioned inferiorly to the palatal plane (U6-PP = 26mm), with a resultant open bite and clock-wise rotation of the mandible.
Anterior open bite due to posterior vertical hyperplasia has traditionally been treated,
depending on patient age and severity, using extra-oral traction, magnets, bite plates, chin cups, func-tional appliances, or orthognathic surgery in con-junction with orthodontics.1-9 In recent years, tem-porary anchorage devices and contemporary biomechanics have afforded more effective control over tooth movements. Skeletal anchorage now makes it possible for orthodontists to intrude max-illary posterior segments without the need for orthognathic surgery.
Dr. Burrow is an Adjunct Professor, Department of Orthodontics, University of North Carolina, Chapel Hill, NC, and in the private practice of orthodontics at 2711 Randolph Road, Charlotte, NC 28207; e-mail: [email protected].
Biomechanical Considerations in the Correction of Anterior Open Bite
Fig. 1 12-year-old female patient with Class II molar relationship, up-per and lower crowding, anterior open bite, incompetent lip seal, and inverse smile arc before treatment.
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3. Anteroposterior Class II dentoskeletal relationship (ANB = 6°) Proclined maxillary and mandibular incisors 6mm overjet4. Alignment and symmetry Moderate upper and lower crowding Bolton 12 analysis: 1.6mm mandibular excess5. Esthetics Inverse smile arc Lip and mentalis strain upon closure Weak chin Excessive incisor display Full, convex profile
Two treatment options were presented to ad-dress these problems. In the surgical-orthodontic
A prioritized problem list was prepared as follows.1. Pathological concerns TMJ issues: popping, clicking, frequent head-aches Left condyle short and flattened2. Vertical a. Skeletal Excessive total anterior facial height Excessive lower facial height FMA = 36° Occlusal plane-SN = 24° SN-GoGN = 49° Anterior open bite b. Dental U6-PP = 26mm
TABLE 1CEPHALOMETRIC DATA
Pre- Post- Two Years Norm treatment Treatment Post-Treatment
Biomechanical Considerations in the Correction of Anterior Open Bite
controlling side effects during intrusion, six bands can make placement challenging. In this case, the bands were soldered together with lingual bars, and a TPA was fabricated to join the two poste-rior segments (Fig. 2). The TPA was kept 5-6mm away from the palate, so that it would not contact the palatal soft tissue as the posterior teeth were intruded.
The patient was referred to an oral surgeon for placement of Stryker single-headed titanium miniplates** in the zygomatic buttress on both sides (Fig. 3). Soft-tissue dissection to expose the lateral walls of the maxilla and zygoma was ac-complished through small incisions in the vesti-bule. The plates were adapted, oriented, and sta-bilized with rigid fixation screws to the base of the buttress, with the anchors emerging through the soft tissue adjacent to the molars at the junction of the fixed gingiva and mucosa. The tissue was re-approximated with resorbable stitches.
Fig. 2 Upper second premolars and molars band-ed and soldered together with lingual bars; left and right segments connected by transpalatal arch (TPA) placed 6mm away from soft tissue to allow for posterior intrusion. Sectional archwires placed in anterior and posterior segments.
plan, the maxillary arch would be leveled segmen-tally, and a space for the surgical incision would be opened between the canines and second pre-molars. A three-piece segmental Le Fort I oste-otomy would be used to superiorly reposition the posterior segments while bringing the anterior segment slightly down, thus improving the incisor display. The mandible would rotate counterclock-wise to close the open bite.
The parents instead chose a nonsurgical op-tion involving orthodontic intrusion of the maxil-lary posterior segments, with anchorage from tem-porary skeletal plates at the base of the zygomatic arch. The treatment plan was as follows.Vertical: Sectional mechanics would involve level-ing archwires from upper right canine to left ca-nine and posterior sectional wires from upper sec-ond premolar to second molar. A transpalatal arch (TPA) would be used to control vertical side effects during posterior intrusion. It would also help con-trol the transverse dimension, because the vertical forces from the posterior segments would be di-rected facial to the center of resistance of the seg-ment, thus tending to tip the segment facially.Anteroposterior: After extraction of all four first premolars, the maxillary incisors would be re-tracted more than the mandibular incisors. To procline the incisors, the MC/MF ratio would be designed to produce uprighting, and a low load-deflection ratio would be used to ensure light, consistent forces.Alignment and symmetry: Part of the premolar extraction spaces would be used to alleviate the crowding.Esthetics: Retracting the incisors would improve the profile and smile arc, increase the amount of incisor display, and reduce the mentalis strain upon closure.
Orthodontic System
After extraction of the four first premolars, the upper and lower incisors and lower second premolars were bonded with .022" brackets.* The upper second premolars, first molars, and second molars were banded. Although a rigid appliance cemented to the posterior segments is ideal for
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of the materials and the mechanics must be well defined. Any change in materials will require a compensatory change in the system to produce the desired results. Because the principal properties of the elastic components (stiffness, range, and strength) do not vary in direct proportion to one another, a change in one can have a considerable
An intrusion force was applied to the poste-rior sectional archwires by means of elastic thread from the skeletal plate. Four months later, some 3mm of posterior intrusion had been achieved (Fig. 4A).
Segmental posterior intrusion mechanics were continued in the maxillary arch while the lower canines were retracted using 150g nickel titanium retraction springs. Upon completion of the canine retraction, the lower incisors were bracketed (Fig. 4B). After the maxillary posterior segments had been intruded, the arch was leveled and an .018" stainless steel working archwire was placed; the upper canines were retracted indi-vidually using 150g superelastic retraction springs (Fig. 5).
Biomechanical Considerations
To predict and control the responses of an orthodontic system, both the physical properties
Fig. 4 A. After four months of upper posterior intrusion, lower canines retracted with 150g nickel titanium retraction springs. B. Lower incisors bracketed after lower-canine retraction.
Fig. 3 Single-headed titanium miniplates placed in zygomatic buttress on both sides, with elastic thread used to apply intrusive force to posterior teeth.
Fig. 5 .018" stainless steel working archwire placed in conjunction with 150g superelastic re-traction springs to retract upper canines.
A B
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effect on another. The length of a wire segment will influence the maximum elastic load and the load-deflection ratio in several ways, depending on the design of the mechanical system. Conse-quently, it is important to plan the materials, brack-ets, loops, segments, and strategically positioned bends to systematically control the load-deflection ratio, forces, MC/MF ratio, range, and strength and thus control positioning of the teeth.
For this patient, a closing loop (CL) was de-signed to retract the maxillary incisors (Fig. 6). The components of an ideal CL—including the inter-
bracket distance, leg length, wire size, loop shape, and type of metal—will be properly programmed to control the load-deflection and MC/MF ratios.10 A CL should accommodate a relatively large acti-vation without causing patient discomfort. An off-center anterior gable bend (crown-facial/root- lingual) can be placed to control the MC/MF ratio and produce translation (MC/MF = 1), controlled tipping (MC/MF < 1), or torque (MC/MF > 1), depending on the situation. An off-center posterior gable bend (crown-distal/root-mesial) can be used to enhance posterior anchorage.
In this case, we needed a light, continuous force with an initial activation of 250g/mm. By-passing the canine and premolar, increasing the leg length, and increasing the wire size of the CL helped increase the range and reduce the load-deflection ratio. Because we wanted to retract and upright the maxillary incisors with controlled tip-ping (MC/MF < 1), we did not need an off-center anterior gable bend. Nor did we need an off-center posterior gable bend, since the skeletal plate could be used for additional anchorage if required. A 3mm activation of the CL produced a load of 250g, or 62g per tooth (Fig. 7). As the CL deactivated, the force was reduced by about 80g/mm, indicat-ing a low load-deflection ratio.
After 19 months of treatment, once incisor retraction had been completed, a new TPA was attached only to the maxillary first molars, and all remaining spaces were closed. A light elastic thread was used continuously to control the pos-terior vertical dimension (Fig. 8). Brackets and bands were removed after 30 months of treatment.
Fig. 6 Closing loop bypasses canine and premo-lar to increase range and reduce load-deflection ratio. Off-center gable bends (not used in this case) can be placed as needed to help control force systems.
Fig. 7 Closing loop activated in mouth.
Posterior gable bend if needed
Posterior gable bend if needed
Anterior gable bend if needed
Anterior gable bend if needed
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firmed retraction and uprighting of the upper inci-sors and intrusion of the upper molars.
Removable upper and lower retainers were fitted. The maxillary retainer, with acrylic cover-age of the posterior teeth, had hooks for connecting elastics to the skeletal plate (Fig. 10). After six months of full-time wear, retainer wear was re-duced to eight hours per night.
Two-year post-treatment records continued to demonstrate proper occlusion and improved dentofacial proportions, although the lateral ceph-alogram showed a slight increase in overbite and overjet (Fig. 11A). Superimposition of the post-treatment and two-year post-treatment cephalo-metric tracings indicated that the upper molars had extruded somewhat and that the upper and lower incisors were slightly proclined (Fig. 11B, Table 1). Superimposition of the pretreatment and two-year post-treatment tracings confirmed retraction and uprighting of the upper and lower incisors and intrusion of the upper molars (Fig. 11C).
Discussion
The etiology of an anterior open bite may involve mechanical interferences as the teeth are
Treatment Results
The patient’s anterior open bite was correct-ed by intrusion of the maxillary posterior segment; the incisor proclination was improved by incisor retraction, which also improved the profile and smile arc, increased the amount of incisor display, and reduced the mentalis strain upon closure (Fig. 9A). After treatment, the maxillary and mandib-ular midlines were coincident with each other and with the facial midline, and the canine and molar relationships were Class I on both sides. The final panoramic radiograph showed adequate root paral-lelism with no root resorption; the skeletal plates had remained stable in their positions above and between the upper first and second molars.
Superimposition of the pre- and post-treat-ment tracings confirmed retraction and uprighting of the upper and lower incisors (Fig. 9B). The up-per lip to E plane was reduced from 6mm to 2mm and the lower lip to E plane from 9mm to 4mm (Table 1), substantially improving the convex pro-file. Superimposition on SN indicated upper-molar intrusion and retraction of the upper incisors. Su-perimposition on the mandible showed uprighting of the lower incisors and vertical development of the molars; superimposition on the maxilla con-
Fig. 8 After retraction phase, TPA replaced and light intrusive force continued with elastic thread.
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Fig. 9 A. Patient after 30 months of treatment. B. Superimposition of pre- and post-treatment cephalomet-ric tracings, showing slight growth during treatment.
A
BA
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Fig. 11 A. Two-year post-treatment records, showing stable results with slight increase in overbite and overjet. B. Superimposition of post-treatment and two-year post-treatment cephalometric tracings, show-ing little skeletal growth but slight extrusion of upper molars, consequent clockwise mandibular rotation, and proclination of upper and lower incisors. C. Superimposition of pretreatment and two-year post-treat-ment cephalometric tracings.
Fig. 10 Retainer with posterior acrylic coverage and elastic hook for application of intrusive force from skel-etal plate.
A
B C
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erupting,11-13 alveolar growth discrepancies,14-16 or genetic factors. An open bite may be dental, skel-etal, or a combination of the two.
Various treatment modalities have been used to treat skeletal open bites caused by posterior vertical hyperplasia. Bell recommended a Le Fort I osteotomy,17 which was shown by Proffit and col-leagues to have only a 10% chance of slight long-term relapse.18 Magnets and springs have also been used to intrude the posterior segments,19-24 but Lopez-Gavito and colleagues found significant relapse in 33% of the anterior open bites corrected nonsurgically.25
In the case shown here, we used posterior skeletal plates as anchorage for segmental intru-sion of the maxillary posterior segments, in con-junction with well-designed biomechanics. During intrusion of the posterior segments, the mandible rotated counterclockwise, which not only helped correct the open bite but reduced the anterior facial height. Retraction of the upper and lower incisors also improved the vertical dimension.
The Stryker single-headed skeletal plate re-quired us to thread an additional orthodontic wire through the head to direct the vertical forces (Fig. 12A). This can be cumbersome and even unstable when the segment tips. We now use a Stryker T-shaped skeletal plate** that allows us to better control tipping, since vertical forces can be placed from numerous positions (Fig. 12B).
Superimposition of the pretreatment and post-treatment cephalometric tracings showed that this patient grew during the 30 months of treat-
ment; the upper molars were intruded, the upper and lower incisors were retracted and uprighted, and the mandible rotated counterclockwise. The L6-MP measurement increased, indicating that a mandibular skeletal plate or miniscrew could have helped control the vertical position of the lower molars. In fact, a recent study reported that “skel-etal open bites had more molar eruption than con-trol groups and Class II’s had more eruption of mandibular molars.”26
Superimposition of the post-treatment and two-year post-treatment tracings revealed some relapse. Although there was little growth during this period, the upper molars were slightly ex-truded and the upper and lower incisors slightly proclined, resulting in an increase in overjet and overbite. The patient wore elastics from the re-tainer to the skeletal plates full-time for only six months, after which retention was reduced to nighttime only and she quit wearing the elastics. In hindsight, continuing the elastic wear at night might have better controlled the vertical dimen-sion. In addition, if a mandibular skeletal plate or miniscrew had been placed during orthodontic treatment, the minor relapse would have been eas-ier to control. Retainer wires could also have been bonded from the maxillary skeletal plate to the upper first molars to prevent the extrusion. In any case, this molar relapse was dental and not growth-related.
Fig. 12 A. Stryker single-headed skeletal plate as used in this case. Vertical control is more difficult when segment tips, since separate wire must be threaded through head of plate to direct vertical forces. B. Stryker T-shaped skeletal plate enhances vertical control.
risk factors for an anterior open bite in the mixed dentition, Am. J. Orthod. 128:517-519, 2005.
12. Bowden, B.D.: A longitudinal study of the effects of digit and dummy-sucking, Am. J. Orthod. 52:887-901, 1966.
13. Subtelny, J.D. and Subtelny, J.D.: Oral habits: Studies in form function, and therapy, Angle Orthod. 43:849-883, 1973.
14. Schudy, F.F.: Vertical growth versus anteroposterior growth as related to function and treatment, Angle Orthod. 34:75-93, 1964.
15. Subtelny, J.D. and Sakuda, M.: Open bite: Diagnosis and treat-ment, Am. J. Orthod. 50:337-358, 1964.
16. Worms, F.W.; Meskin, L.H.; and Isaacson, R.J.: Open-bite, Am. J. Orthod. 59:589-595, 1971.
17. Bell, W.H.: Le Fort I osteotomy for the correction of maxillary deformities, J. Oral Surg. 33:412-426, 1975.
18. Proffit, W.R.; Bailey, L.J.; Phillips, C.; and Turvey, T.A.: Long term stability of surgical open-bite correction LeFort I osteo-tomy, Angle Orthod. 70:112-117, 2000.
19. Dellinger, E.L.: A clinical assessment of the active vertical corrector, a nonsurgical alternative for skeletal open bite treat-ment, Am. J. Orthod. 89:428-436, 1986.
20. Kalra, V. and Burstone, C.J.: Effects of a fixed magnetic appli-ance on the dentofacial complex, Am. J. Orthod. 95:467-478, 1989.
21. Killiaridis, S.; Egermark, I.; and Thilander, B.: Anterior open bite treatment with magnets, Eur. J. Orthod. 12:447-457, 1990.
22. Işcan, H.N.; Akkaya, S.; and Koralp, E.: The effects of the spring-loaded posterior bite-block on the maxillo-facial mor-phology, Eur. J. Orthod. 14:54-60, 1992.
23. Kuster, R. and Ingervall, B.: The effect of treatment of skeletal open bite with two types of bite-blocks, Eur. J. Orthod. 14:489-499, 1992.
24. Işcan, H.N. and Sarisoy, L.: Comparison of the effects of pas-sive posterior bite-blocks with different construction bites on the craniofacial and dentoalveolar structures, Am. J. Orthod. 112:171-178, 1997.
25. Lopez-Gavito, G.; Wallen, T.R.; Little, R.M.; and Joondeph, D.R.: Anterior open bite malocclusions: A longitudinal 10 year-post-retention evaluation of orthodontically treated pa-tients, Am. J. Orthod. 87:175-186, 1985.
26. Arriola-Guillen, L.E. and Flores-Mir, C.: Molar height and incisor inclination in adults with Class II and Class III skeletal open bite malocclusions, Am. J. Orthod. 145:325-332, 2014.
Conclusion
The present case demonstrates that if proper diagnostic records are obtained and orthodontic biomechanics are well designed, a stable result can be achieved in a patient with anterior open bite using skeletal plates for anchorage of posterior vertical intrusion and consequent counterclock-wise rotation of the mandible.
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