Aparelho de Protração Mandibular: diferenças entre adolescentes e adultos

Material

The sample size of each group was calculated based on an alpha significance level of 0.05 and a beta of 0.2 to achieve 80% of power to detect a mean difference of 0.5^o in ANB angle change between the groups, with 0.5^o of estimated standard deviation. ¹⁷ The sample size calculation showed that 9 patients in each group were needed.

The sample comprising this retrospective study consisted of the pretreatment and posttreatment lateral cephalograms of 39 patients presenting with Class II malocclusion treated at two different orthodontic private practices. Patients presented mild to moderate Class II malocclusion. All subjects were in the permanent dentition up to first molars erupted when MPA was installed. No exclusion criteria related to occlusal result was adopted.

The sample was divided into two groups:

» Group 1 comprised 23 subjects (10 male; 13 female), at a mean pretreatment age of 11.75 ± 1.13 years, treated with the Mandibular Protraction Appliance and fixed appliances (Fig 1), for a mean treatment time of 3.32 ± 1.20 years, at two orthodontic private practices. Class II division 1 was present in 21 subjects and division 2, in 2 patients. Nineteen had bilateral Class II and 4 showed Class II subdivision malocclusion.

» Group 2 comprised 16 subjects (7 male; 9 female), at a mean pretreatment age of 22.41 ± 4.79 years, also treated with the Mandibular Protraction Appliance and fixed appliances (Fig 1), for a mean treatment time of 4.24 ± 2.44 years at two orthodontic private practices. Twelve subjects presented Class II division 1 malocclusion and 4 had Class II division 2. Class II subdivision was seen in 5 subjects and bilateral Class II was present in 11 patients.

Figure 1

The mean MPA treatment time was 9 months in both groups. After removal of the MPA, active retention was provided with Class II elastics to maintain the anteroposterior relationship correction. Gradual decrease in Class II elastics use was conducted in each case, as stability of the anteroposterior relationship was observed.

Methods

Lateral cephalograms were obtained at the pre (T₁) and posttreatment (T₂) stages. The anatomic tracing and the location of cephalometric landmarks were manually carried out by a single investigator. These radiographs were digitized in grayscale at 300 dpi, in a scanner (Numonics AccuGrid XNT, model A30TL.F (Austin,Texas/USA) and imported into Dentofacial Planner 7.02 software (Toronto, Ontário, Canada); the landmarks were digitized and measurements were performed. The magnification factor of the radiographic images, which was between 6% and 9.8%, was corrected by the software.

Cephalometric maxillary and mandibular dentoalveolar variables used are shown in Figures 2 and 3.

Figure 2

Figure 3

Error study

Within a month interval from the first measurement, 15 randomly selected radiographs were re-measured by the same examiner. The random error was calculated according to Dahlberg’s formula (Se ² = Σd ² /2n), ¹⁸ in which Se ² is the error variance and d is the difference between two determinations of the same variable. The systematic errors were evaluated with dependent t tests for p< 0.05. ¹⁹

Statistical analysis

Sex distribution, type of Class II malocclusion (division 1 or 2; bilateral or unilateral) and number of patients treated by the different clinicians in the groups were compared with Chi-square tests. Application of t tests requires normal distribution, which was verified with Kolmogorov-Smirnov tests. Results were not significant for all variables. Therefore, intergroup comparison of the pretreatment age, treatment time, pretreatment (T₁) and posttreatment (T₂) variables and the treatment changes (T₂-T₁) were performed with t tests.

All statistical analyses were performed with Statistica software (Statistica for Windows 6.0; Statsoft, Tulsa, Okla), and results were considered significant at p< 0.05.

Sample selection and methodology

The lack of a control group that would allow to separate growth changes effects from the MPA appliance effects, especially in Group 1, is a limitation of the present study. However, previous studies also were published with the absence of a control group, without prejudice of the results. ⁹

The groups were compatible regarding sex distribution, malocclusion types (division 1 or 2, and bilateral or unilateral), and also regarding the orthodontists who conducted the treatments, which can influence the results (Table 1). Only the pre and posttreatment ages were significantly different between the groups, as expected, since the study compares groups with different age ranges (Tables 2 and 4).

Maxillary skeletal component

Adults presented significantly greater maxillary retrusion (A-Nperp) at the pretreatment stage (Table 2). The effective length of the maxilla significantly increased in the adolescent group, which was expected because these patients were still in the growing stage of craniofacial development ^{20 - 22} (Co-A; Table 4). Nevertheless, the therapy with the MPA did not seem to have influenced maxillary anterior displacement because the adult group continued to present significantly greater maxillary retrusion as compared to the adolescent group at the posttreatment stage (Table 3). Similar results have been previously observed. ^{9 , 23}

Mandibular skeletal component

The adult group presented greater mandibular body (Go-Gn) and effective mandibular length (Co-Gn), as well as mandibular ramus height (Co-Go) at the pretreatment stage, and the adolescents had greater increase in these structures during the treatment period (Tables 2 and 4). The MPA may have contributed to some of this growth increase.^20-22 On the other hand, growth changes in the adult group was negligible so that at the posttreatment stage the sizes of these mandibular structures were similar in the groups (Table 3).

Maxillomandibular relationship

At the pretreatment stage, the adolescent group presented a significantly greater apical base Class II discrepancy and profile convexity than the adult group (Table 2). The adolescents still presented a significantly greater Class II apical base discrepancy and convex facial profile than the adults at the posttreatment stage, although milder than at the pretreatment stage (Tables 3 and 4). Longitudinal comparisons indicate that growth trends are essentially similar between Class II division 1 and normal subjects in the various dentofacial parameters compared. The differences in mandibular length and position are more evident in the early stages of development than at the later stages. This may indicate the possibility of a “catch up” period in mandibular growth in Class II division 1 subjects at the later stages of development. ²⁴

Vertical component

Both groups were very similar in the pretreatment stage regarding the growth pattern, with only the adult patients presenting a significantly greater posterior face height than the adolescents and therefore a more horizontal growth tendency (Table 2). However, as the adolescents had significantly greater increase in the lower anterior and posterior face heights, there were no significant differences in any variable of the vertical components at the posttreatment stage (Tables 3 and 4). Most likely these greater increases were consequent to growth in the adolescents because the adult group also experienced slight increases in these variables. These results would be expected, since they have been shown before. ²⁵

Maxillary dentoalveolar component

As it would be expected, in the pretreatment stage the adults had significantly greater maxillary molar dentoalveolar height and mesial positioning than the adolescents, since growth had already been fully expressed (Table 2). ²⁶ During treatment, there was significantly greater palatal tipping of the maxillary incisors in the adolescents, most likely due to the appliance effects, and also greater maxillary molar vertical dentoalveolar development and mesialization, most probably due to growth. ^{22 , 26} With these changes, the maxillary incisors ended up still with greater palatal tipping in the adolescents than in the adults and the maxillary molar dentoalveolar height became similar in the groups, in the posttreatment stage (Table 3).

Mandibular dentoalveolar component

Similar to the maxillary molars, in the pretreatment stage the adults had significantly greater mandibular molars dentoalveolar height and mesial positioning than the adolescents, because growth had already been fully expressed (Table 2). ^{22 , 27} During treatment, the adolescents had greater dentoalveolar development of the mandibular incisors and molars, probably due to the greater vertical development of the alveolar processes on growing patients, in comparison to adults (Table 4). ²² Although the other dentoalveolar changes were not significantly different between the groups, the adult patients had a significantly greater proclination of the mandibular incisors at the posttreatment stage (Table 3). Probably this was the result of a cumulative effect of a greater non-significant mandibular incisor proclination at the pretreatment stage and during treatment that ultimately produced a significantly greater proclination at the posttreatment stage. Greater dentoalveolar changes may be expected in adult patients under treatment with fixed functional appliances because the skeletal changes are minimal. ^{7 , 9} Besides, the adult patients had greater initial mandibular crowding which could have contributed to some incisor flaring during the initial stages of leveling and alignment. ^{3 , 28}

Dental relationships

The overjet, overbite and molar relationship were similar in the groups at the pretreatment stage as were the treatment changes (Tables 2 and 4). Only the overbite was significantly smaller in the adult group at the posttreatment stage. Again this result may be the cumulative effect of a non-significantly greater overbite in the adolescent group in the pretreatment stage, associated to a non-significantly greater reduction in overbite in the adult group.

Soft tissue component

The upper and lower lips were significantly more protruded in the adolescents than in the adults at the pretreatment stage (Table 2), in accordance with Pecora et al. ²⁹ During treatment the upper lip of the adolescents had significantly greater retraction than in the adult patients (Table 4). However, despite the greater retraction, the upper lip continued to exhibit greater protrusion in the adolescents, in the posttreatment period (Table 3).