Product News In June 2021 Issue

Temporary implants may also be used as "posts" for the application of intramaxillary and/or intermaxillary elastics to support space closure, improve a Class II relationship, or correct a deep overbite (Figs. 3,19A, BC,20).

Although lingual appliances represent another esthetic alternative, a transition from preliminary treatment with full lingual brackets to Invisalign^* would be highly unlikely due to the costs and discomfort involved. Nevertheless, it seems reasonable in an extraction case to take advantage of miniscrew anchorage for a substantial amount of retraction and space closure with partial fixed lingual mechanics prior to the introduction of clear aligners (Fig. 20). In addition, there are ongoing investigations of the effectiveness of devices such as AcceleDent^**61,62 ](Fig. 21ABC,DE), Propel^***, and OrthoPulse^† in accelerating the rate of tooth movement during clear-aligner therapy.

Torquing moments of couple at the incisors create moments of force at the distal aspects of the anchoring first molars--a mesial force transmitted throughout the dental arch. Although the same biomechanical situation occurs with traditional brackets, it can be counterbalanced by using crown tipbacks or perfectly placed and tied-back omega stops in the archwire, or by other means of anchorage support. If the wire is not tied to the molars, the incisors will flare. Consequently, forces transmitted through the wire will produce a force at the distal aspect of the molar, perpendicular to the first-molar trifurcation, and a moment of force will rotate (tip) the molar forward.

If the force of occlusion is coupled through plastic held between the posterior teeth. There is only limited control of intrusion, however, so that the mesial aspects of the molars tend to intrude as the crowns tip forward. When anterior lingual root torque is applied with clear aligners, this unintended lack of occlusion of the upper molars'nterproximal marginal ridges must be counteracted with a balancing distal crown rotation (tip) of the molars (Fig. 22B). That may involve programming a mesial extrusion of the molars (mesial tip of the first molar roots), perhaps in conjunction with prominent horizontal rectangular attachments on the molars.

In 1996, as the basis for an Objective Grading System, the ABO determined the most common mistakes made by candidates who had failed the Phase III examination.⁶⁵ Improper buccolingual inclination in the posterior dentition was the most frequently noted error.^65-67 According to Marshall and colleagues, "For proper occlusion, there should be no significant difference between the heights of the buccal and lingual cusps of molars and premolars."⁶⁸ Yang-Powers and colleagues reported that "orthodontists are deficient in placing adequate torque in the buccal segments",⁶⁹ and Sondhi added that "the degree of torque in the maxillary second molars is the most important factor in reducing interferences".⁷⁰

The combination of inadequate buccal root torque for the maxillary posterior teeth and excessive lingual crown tip for the mandibular posterior teeth appears to "roll out" the upper molars while "rolling in" the lower molars. Although this is a common concern with preadjusted appliances, ^66,67,71 the same problem may occur in clear-aligner treatment, since the applied biomechanical system is often expansive in "rounding out" the archforms. If the prescribing orthodontist and setup technicians do not address these concerns in the treatment plan, the results may be an accentuated curve of Wilson, prominent palatal cusps, improper and incomplete buccal cusp interdigitation, and inappropriate posterior overjet⁶ (Figs. 13, 15). Compounding this situation is the so-called "passive posterior intrusion" inherent in the longterm wear of two plastic trays, along with the previously noted intrusive effects on the first molars during the application of anterior torque. Add in the effects of en masse retraction, distalization, or extractions, and the development of posterior open bites becomes almost inevitable.

Interdisciplinary Treatment

Clear aligners have also entered the realm of multidisciplinary treatment involving orthodontic patients. For example, a 29-year-old female presented with a mutilated Class II occlusion, an anterior open bite, significant overjet, a canted occlusal plane, a short face, and a retrusive mandible (Fig. 24A).

Clinical examination revealed numerous restorations; the upper first premolars, lower left canine, and lower right lateral incisor were missing. The interdisciplinary treatment plan included crown lengthening; extraction of the third molars; a preliminary phase of fixed appliances to level and align the dentition, including vertical control supported by miniscrews^73-76; temporary replacement of existing crowns and bridges with resin crowns; Invisalign treatment after interproximal reduction; chin-augmentation surgery; dental bleaching; and final replacement of crowns and bridges.

After the crown lengthening and three months of fixed-appliance treatment (Fig. 24B), a ClinCheck projection indicated the amount of en masse anterior retraction and midline correction planned for the aligner phase (Fig. 24C). Interproximal reduction was then carried out prior to the initiation of Invisalign therapy. Using 21 sets of trays, the anterior open bite and overjet were improved and the teeth were well aligned, closely matching the ClinCheck projections (Fig. 24D).

Still, the overjet, upper gingival margins, and upper midline required further refinement. For the next three months, unilateral Class II elastics were hooked from notches in the aligners mesial to the upper canines to a bonded button on the lower left molar for midline correction.

After 25 months of orthodontic treatment, the patient's occlusion and smile evidenced significant esthetic improvement (Fig. 24E). Superimpositions of cephalometric tracings demonstrated retraction and extrusion of the upper anterior teeth, intrusion and retraction of the lower anterior teeth, intrusion of the upper molars, and mesial movement of the lower molars, along with a slight retraction of the lips and mandibular autorotation (Fig. 24F). 

The patient was then referred to a plastic surgeon for chin augmentation to increase her chin projection and lower anterior facial height. Four months later, cosmetic restorations were completed (Fig. 24G).