139

Mechanics of Class II malocclusion compensation with miniscrew implant-supported anchorage

Madhur Upadhyay, Sumit Yadav and Ravindra Nanda

26 

INTRODUCTION

Class II division 1 malocclusions are described as having labially inclined maxillary incisors, increased overjet and a vertical overbite, which might vary from a deep overbite to an open bite. In contrast, Class II division 2 malocclusions show excessive lingual inclination of the maxillary central incisors overlapped on the labial by the maxillary lateral incisors. They always show a deep overbite with minimal overjet. There are two broad approaches for Class II correction: alteration of the skeletal base by surgi-cal or orthopedic means or dental compensation, which involves masking the underlying skeletal discrepancy (if any) by dental movements.

This chapter discusses compensation mechanics using miniscrew implant (MI)-supported anchorage. Earlier chapters in this book have covered the general principles of MIs as anchorage and the biomechanics involved. Broadly speaking, the following typical dental movements are sought:

■ distal movement of the maxillary teeth, either stepwise with molar first followed by rest of the teeth or en masse

■ retraction of the maxillary anterior teeth into a premolar extraction space while maintaining the Class II molar relationship

■ combination of retraction of the maxillary teeth and forward movement of the lower teeth.

Fig. 26.1 outlines the possible steps required, depending on the individual patient. The following will be discussed in this chapter:

■ retraction of the maxillary anterior teeth while maintaining a Class II molar relationship

■ distalization of the posterior teeth■ molar protraction/mesialization.

RETRACTION OF THE MAXILLARY ANTERIOR TEETH

Treatment for the correction of Class II malocclusions in non-growing patients usually involves selective removal of permanent teeth, with sub-sequent dental camouflage to mask the skeletal discrepancy and provide a good facial balance. Extractions can involve two maxillary premolars or two maxillary and two mandibular premolars. The extraction of only two maxillary premolars and anterior teeth retraction is generally indicated when there is no crowding or cephalometric discrepancy in the mandibular arch. When retracting anterior teeth in a full cusp Class II malocclusion, anchorage control assumes profound importance because maintaining the posterior segment in place becomes very critical. A loss in molar anchor-age can not only compromise correction of the anteroposterior discrepancy but also affect the overall vertical dimension of the face.1

The preferred location for MI placement is between the roots of the second premolars and first molars close to the mucogingival junction. An SS archwire (0.017 × 0.025 inch) and a force of 150–200 g are considered as optimum conditions for efficient retraction of the maxillary anterior teeth.2,3 The biomechanics involved during en masse retraction of the anterior teeth with MI-assisted anchorage is illustrated in Fig. 26.2. In the second stage after space closure, continuation of the force from the springs, as is sometimes done for en masse distalization, can cause some intrusion and distalization of the entire arch. This is primarily because of increased friction between the archwire and the bracket. Therefore, the thicker the archwire, the greater the distalizing effect on the entire arch will be.

DISTALIZATION OF THE POSTERIOR TEETH

Sagittal movement of the dentition without extractions is often difficult and time consuming. Issues of unwanted tooth movements during distali-zation, which have to be corrected in a subsequent stage (“round tripping,” see Chapter 2), can be avoided if MIs are used for skeletal anchorage.

MI-based anchorage can be lingually from the palate, buccally from the alveolar bone (between the roots), or from the zygomatic buttress.

MOLAR DISTALIZATION

Fig. 26.3 shows a simple buccal approach to distalize molars individually. The treatment plan demanded the creation of space by molar distalization before leveling and aligning the rest of the dentition. A push-coil spring from the MI inserted between the roots of the first premolar and canine linked to the molar with an SS wire (0.016 × 0.022 inch) bent passively from the MI to the auxiliary tube of the molar. Uprighting of the molar required the center of rotation to be closer to the root apex, leading in principle to greater tip-back of the crown and less root movement. This force system is illustrated in Fig. 26.3C–E.Fig. 26.1  Treatment objectives for compensating a dental Class II malocclusion. 

Class II(dental compensation)

BuccalocclusionOverjet

MaintainAlter to

Class I orClass II

Retraction ofmaxillary

anterior teeth

Proclinemandibular

anterior teeth

Protractmandibular teethto Class I relation

Maxillary teeth

Protract toClass IIrelation

Distalize to Class Irelation

140  SECTION VII: MINISCREW IMPLANTS FOR THE TREATMENT OF CLASS II MALOCCLUSION

Fig. 26.2  Biomechanics of en masse retraction of the anterior teeth with miniscrew implant anchorage. The force (F) exerted by the bilateral coil springs has two distinct components: a larger and predominantly retractive force (r) and a smaller intrusive force (i). There is also a clockwise moment (M) on the anterior segment as the total force passes below the estimated center of resistance (CR) of the anterior teeth. (A) Initial stage to close the extraction space. Significant M on the anterior segment causes tipping of the anterior teeth. (B) After space closure, the canine is in contact with the second premolar. The moment is reduced as the force from the spring is applied closer to the CR of the entire arch. Note the increase  in the angulation of the total force relative to the occlusal plane. 

M i

F

B

F

r

i

MCR

A

Fig. 26.3  Molar distalization for a mesially tipped maxillary left first molar and to make space for eruption of an impacted maxillary  left second premolar. (A) Immediately after miniscrew implant insertion and force application through a coil spring. (B) After molar distalization. (C) The force (Fa) applied from the MI through the push-coil pushes the molar distally, producing a moment (Mf) because Fa is not applied at the center of resistance (CR), causing the molar to tip around a point apical  to the CR (approximately). (D) Once molar distalization is complete and a Class I relationship is obtained, the molar is included in the rest of the arch by placing continuous archwires, resulting in a couple (Mc) at the molar tube. (E) A couple creates a moment around the CR; therefore, although there is a moment to distalize the root, the same moment is also acting to mesialize the crown of the molar. This makes it imperative to have a passive distalizing force (Fp) on the molar too, which will ensure only root movement. 

Mc

Fp

E

Fp

Mc

D

Fa

Mf

C

A B

MECHANICS OF CLASS II MALOCCLUSION COMPENSATION WITH MINISCREW IMPLANT-SUPPORTEd ANCHORAgE  141

orthodontic mechanotherapy, in addition to preventing extrusion of the maxillary and mandibular posterior teeth, it is equally important that the maxillary anterior teeth are intruded as they are retracted. This maximizes the upward rotation of the mandible as the incisal stop is moved further up. Without intrusion and control of the axial inclination of the maxillary anterior teeth, the patient is likely to have a longer face and a more downward and backward rotated mandible, exaggerating the Class II appearance.

LIMITATIONS OF DISTALIZATION

One of the limitations encountered in maxillary molar distalization is associated with the lack of space in the maxillary tuberosity area to move teeth distally. Extraction of maxillary third molars can provide sufficient space in the tuberosity area for teeth to move within the alveolar bone.

A further limitation can arise if the MI supporting the distalization can interfere with adjacent roots. To prevent such interference, MIs should be angulated superiorly, to give clearance between the MI and the root apex in coronal view; alternatively, roots can be tipped apart to create sufficient space before insertion of the MI.

MOLAR PROTRACTION/MESIALIZATION

Protraction of mandibular molars to correct a Class II molar relationship is biomechanically one of the most challenging scenarios. The mandibular molars are more difficult to move mesially because the mandible is made of thick cortical bone connected by coarse trabecular bone, and the molar roots are extremely wide buccolingually.10 Conventional techniques not only cause mesial tipping of the molars but also lingual tipping of the anterior teeth, resulting in an increased overjet and worsening of the Class II profile (Fig. 26.5A). The large root surfaces of molars make their move-ment uncertain, while simultaneously they can cause unwanted tooth movements, such as lingual tipping of the incisors. Placement of a dental implant in the retromolar area for absolute anchorage has allowed effective closure of the space from a missing mandibular first molar.11,12 The dental implants were strong enough to withstand the reactive forces and thus produced effective mesial movement of the mandibular molars without any lingual tipping of the incisors.13,14

Molar protraction usually involves pure forward bodily translation. If the line of force does not pass through the CR of the molar, there will be a tendency for the molar to tip forward during protraction. This may cause binding of the molar tube on to the archwire, resulting in bowing of the archform in the premolar area, which may lead to cessation of protraction or unwanted side effects on adjacent teeth (Fig. 26.5B). There are several methods to avoid these side effects during molar protraction with MIs:

■ using thick SS archwire that almost fills the bracket slot: this reduces play between the molar tube and the archwire (Fig. 26.6A,D)

■ using a rigid SS power arm or lever arm on the molar: the line of force from the MI passes through the CR of the molar (Fig. 26.6B)

■ using regular sliding mechanics by engaging an elastic chain from the molar tube to the MI directly: after some protraction an uprighting spring has to be used to upright the molar (i.e. mesialize the root) and it is important to tie the molar to the MI with a ligature wire during uprighting to prevent distal movement of the crown (Fig. 26.6C,D)

■ using a palatal MI (see Fig. 30.2): force can be applied at the CR of the molars, ensuring bodily movement with minimal side effects.

EN MASSE DISTALIZATION

Distalization of the maxillary arch by more than 2–3 mm cannot be achieved using an inter-radicular MI for anchorage. Placement of a MI, or miniplate, buccally above the mucogingival junction in the zygomatic bone is a feasible option but the surgery (particularly for miniplates) can be more extensive and there are more oral hygiene issues. Placement of MIs or implants in the palate is also an option (see Chapter 7). The mid-palatal suture area seems a particularly good placement site. Some clini-cians favor the palatal area behind the incisive foramen while others prefer the paramedian region of the palate as there is more bone support.4–8 One of the primary advantages of using palatal implants is the application of force closer to the center of resistance (CR) of the posterior teeth to achieve bodily translation with minimal tipping. In addition, there is no flaring or proclination of the anterior teeth and no mesial movement of the molar during retraction of the anterior teeth.

Distalization with MIs helps in efficient control of the vertical dimen-sion by preventing the extrusion of the molars, thereby maintaining the mandibular plane angle (Fig. 26.4). A clockwise rotation of the mandibular plane can worsen the Class II malocclusion and make relapse more likely.

The vertical component of the total force can result in binding (or increase of friction) of the archwire to the brackets or tubes, thereby pre-venting sliding and resulting in the transmission of the force to the entire archwire and consequently greater amounts of distalization and intrusion. Small vertical changes at the posterior teeth can produce profound changes in the anterior area: 1 mm of intrusion at the posterior teeth can produce 3 mm of upward and forward movement at point gnathion and probably more at the chin.9 Such a movement can be a critical factor for the correc-tion of Class II relationships, particularly in high-angle relations. The occlusogingival position of the MI, or the use of power arms or crimpable hooks, can also be critical factors in defining the vertical component of the total force. With MIs placed higher up in the vestibule, the vertical component of the total force is significantly increased.

Patients with moderate to severe vertical facial patterns do not always have an anterior open bite, because often the incisors have been supra-erupted, compensating for the posterior vertical excess. Therefore, during

Fig. 26.4  En masse distalization of the entire arch using miniscrew implants (MIs) placed in the buccal alveolar bone. A single force (red) is applied bilaterally from the anterior segment to the MI. Horizontal and vertical components (in green) of the force tend to distalize and intrude the teeth. Stiff SS archwires will provide more effective distalization with fewer side effects than flexible archwires. The position of the MI will limit the amount of distalization as it could obstruct the roots. 

F

142  SECTION VII: MINISCREW IMPLANTS FOR THE TREATMENT OF CLASS II MALOCCLUSION

REFERENCES

1. Upadhyay M, Yadav S, Nanda R. Vertical dimension control during en masse retraction with mini-implant anchorage. Am J Orthod Dentofacial Orthop 2010;138:96–108.

2. Upadhyay M, Yadav S, Nagaraj K, et al. Treatment effects of mini-implants for en-masse retraction of anterior teeth in bialveolar dental protrusion patients: a rand-omized controlled trial. Am J Orthod Dentofacial Orthop 2008;134:18–29.

3. Upadhyay M, Yadav S, Nagaraj K, et al. Dentoskeletal and soft tissue effects of mini-implants in Class II, division 1 patients. Angle Orthod 2009;79:240–7.

4. Kim HJ, Yun HS, Park HD, et al. Soft-tissue and cortical-bone thickness at orthodontic implant sites. Am J Orthod Dentofacial Orthop 2006;130:177–82.

5. Bernhart T, Vollgruber A, Gahleitner A, et al. Alternative to the median region of the palate for placement of an orthodontic implant. Clin Oral Implants Res 2000;11: 595–601.

6. Miyawaki S, Koyama I, Inoue M, et al. Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage. Am J Orthod Dento-facial Orthop 2003;124:373–8.

7. Gracco A, Luca L, Cozzani M, et al. Assessment of palatal bone thickness in adults with cone beam computerised tomography. Aust Orthod J 2007;23:109–13.

Fig. 26.5  Conventional approach for molar protraction. (A) Although the anchorage of the anterior teeth is greater than the molar, the moments produced (blue) by an elastic chain (green) from the molar to the anterior segment. can cause rapid tipping of the anterior teeth. (B) If this is continued, the archwire can be distorted (deflected or bent). This can lead to stagnant tooth movement, poor control over the anterior teeth and the molar, and bite deepening. 

F

B

F

A

Fig. 26.6  Molar protraction without side effects from archwire distortion. (A) A stiff archwire avoids unwanted bending of the archwire during protraction, thereby effectively creating a countermoment (red) that prevents mesial tipping of the molar (blue). (B) An appropriately positioned power arm from the molar ensures bodily movement, as the force application (red) passes though the center of resistance of the molar. (C) Protraction divided into distinct phases; in the first the molar is tipped forward and subsequently, an uprighting moment (red) can be applied through an uprighting spring. The crown of the molar is prevented from tipping back by tying it to the MI with a ligature wire (black). (D and E) Clinical example of molar protraction as shown in (C). 

F

B

F

A C

D E

8. King KS, Lam EW, Faulkner MG, et al. Vertical bone volume in the paramedian palate of adolescents: a computed tomography study. Am J Orthod Dentofacial Orthop 2007;132:783–8.

9. Kuhn RJ. Control of anterior vertical dimension and proper selection of extraoral anchorage. Angle Orthod 1968;38:340–9.

10. Roberts WE. Bone physiology, metabolism, and biomechanics in orthodontic practice. In: Graber TM, Vanarsdall RL Jr, editors. Orthodontics: current principles and tech-niques. St. Louis, MO: Mosby; 1994. p. 193–257.

11. Nagaraj K, Upadhyay M, Yadav S. Titanium screw anchorage for protraction of man-dibular second molars into first molar extraction sites. Am J Orthod Dentofacial Orthop 2008;134:583–91.

12. Upadhyay M, Yadav S. Mini-implants for retraction, intrusion and protraction in a Class II, division 1 patient. J Orthod 2007;34:158–67.

13. Roberts WE, Marshall KJ, Mozsary PG. Rigid endosseous implant utilized as anchor-age to protract molars and close an atrophic extraction site. Angle Orthod 1990;60:135–52.

14. Roberts WE, Nelson CL, Goodacre CJ. Rigid implant anchorage to close a mandibular first molar extraction site. J Clin Orthod 1994;28:693–704.