Prospective Multicenter Investigation of Invisalign Treatment with the Mandibular-Advancement Feature: An Interim Report

The incidence of Class II mal­occlusion in North America is estimated to be 19.56%.¹ Various methods have been devised for correction of this malocclusion, which is often characterized by mandibular retrognathia and excessive overjet.² Functional appliances that reposition the mandible forward—including the Twin Block,^** bio­nator, Mandibular Anterior Repositioning Appliance^*** (MARA), and Herbst^† appliance—can be used to correct Class II relationships in growing patients. Clinical studies have demonstrated the effectiveness of such appliances, as indicated by reduced overjet, improved molar and canine relationships, and orthopedic changes.^3,4

Most Class II functional appliances reposition the mandible in one step, so that the patient’s jaw is in the desired final position (or an over­corrected position) throughout treatment. An alternative to this method is incremental repositioning, which has been shown to be effective with the Twin Block.⁵ A key advantage of incremental advancement is the enhancement of patient comfort and muscular adaptation, especially at the start of therapy.

A recent systematic review by Cacciatore and colleagues stated, “Functional appliances produced a significant improvement of the maxillo-mandibular relationship at almost all time points, although the improvement of the mandibular projection was negligible or not significant. The quality of evidence was ‘very low’ for most of the outcomes at both primary time points.”⁶ The authors concluded that “functional appliances may be effective in correcting skeletal Class II malocclusion in the long-term, however the quality of the evidence was very low, and the clinical significance was limited.”

Conversely, in another systematic review of mandibular changes produced by Class II functional appliances, Cozza and colleagues found that two-thirds of the 22 analyzed studies reported a clinically significant supplementary elongation of the mandible as compared with the untreated group.⁷ Therefore, current evidence suggests that functional appliances are effective at correcting Class II malocclusions in growing patients, owing to a combination of dentoalveolar and skeletal changes.

The sample included patients age 11-19 in the fully erupted dentition (not including second pre­molars and second and third molars) with bilateral Class II malocclusions of at least 3mm, as measured from the first-permanent-molar relationship. Patients who had undergone orthodontic treatment within 12 months prior to the study or any previous treatment with sagittal orthodontic appliances were excluded, as were those with interproximal spaces of more than 3mm between two adjacent teeth, active caries or periodontal disease, TMD symptoms, partially erupted first permanent molars, or known allergies to latex or plastic, and those who were pregnant or became pregnant during the study. Any subject was free to withdraw from the study without a stated reason. In addition, the investigator had the discretion to exclude noncompliant subjects at any time.

A total of 60 subjects were enrolled; 55% were male and 45% were female, with a mean age of 12.75 years at the start of treatment (13.11 for males and 12.15 for females). The mean time from the initiation of treatment (T1) to the end of the MA phase (T2) was 12.6 months, and the mean time from T1 to the end of the post-MA phase (T3)—representing the mean total treatment time—was 25.6 months.

Case-report forms detailing treatment progress were collected from the study investigators at T1, T2, and T3; progress markers included millimetric measurements of overbite, overjet, and the molar relationship from the mesiobuccal cusp tip of the upper first molar to the buccal groove on the lower first molar. A quality-of-life survey focusing on the subject’s ease of use and discomfort was also administered at each time point.

Results

Mean cephalometric measurements for the 47 subjects completing T2 are shown in Table 1, and for 25 subjects completing T3 in Table 2.

There were statistically significant changes from T1 to T3 in SNB (+1.13°), ANB (–1.67°), Wits appraisal (–3.25mm), convexity (–3.62°), mandibular length (CoGn = +3.59mm), overbite (–1.83mm), and overjet (–3.36mm). At both T2 and T3, there were no statistically significant changes in MP-SN, FMA, interincisal angle, or IMPA.

Linear dental measurements indicated a mean Class II molar discrepancy at the beginning of treatment (T1) of 4.08mm on the right and 3.83mm on the left, and a mean improvement in the molar relationship from T1 to T3 of 3.53mm on the right and 3.21mm on the left (Table 3).

In the quality-of-life surveys, the average pain reported was minor (Table 4). At the end of T2, 89% of the subjects said they experienced discomfort between “sometimes” and “never.”

Discussion

The current consensus is that the primary mechanism of action of functional appliances in achieving Class II correction is dentoalveolar, with a smaller component of skeletal change.^5,8 The preliminary results of our clinical study support this opinion. While subjects generally experienced a correction from a Class II to a Class I relationship, skeletal changes were consistent with the concept of a lesser component of “growth modification” than of dentoalveolar changes within the overall occlusal correction.