The relationship between dental and basal anatomy of mandible in class I and class III malocclusion

Introduction. The purpose of this study was to investigate the mandibular arch form in Class I and Class III malocclusion at the level of clinically relevant application points of the orthodontic bracket (FA points) and at points representative of the basal arch form as defined by the WALA points. Through this study, we tested the consistency in arch forms derived from WALA points and their usefulness as a reliable reference for determining dental arch form. We investigated if there is any difference in mandibular arch forms between Class I and Class III patients. Possible confounding factors such as age, sex, and ethnicity were evaluated. Methods. Three-dimensional graphic representations of mandibular casts from 37 Class I patients and 37 Class III patients were created using a laser scanning system. Anatomic reference points were subjectively identified and used to represent the dental arch form (FA points) and the arch form of the basal bone (WALA points). Independent samples t-test was used to compare the mandibular intercanine and intermolar widths and depths of the two groups. Interactions between factors such as gender, age, ethnicity and malocclusion were determined with ANOVA. Finally, correlation statistics were used to assess the correlation between FA and WALA point distances at the canine and molar areas for both groups. Result. The mean (SD) FA canine width for Class I was 28.36 (1.71) and for Class III was 30.20 (1.83), resulting in a p-value of < 0.001. The mean (SD) FA molar width for Class I was 51.65 (2.77) and for Class III was 54.15 (2.53), resulting in a p-value of < 0.001. Within the same classification of occlusion, the male mandibular arch form was significantly wider than the females'. The mean (SD) FA molar width for Class I females was 50.71 (2.76) and for males was 52.75 (2.40), resulting in a p-value of 0.02. The children's dental arch depth was significantly larger than that of adults. The mean (SD) FA molar depth for Class I children was 27.17 (1.56) and for Class I adult was 25.12 (2.13), resulting in a p-value of < 0.001. No significant difference was found between arch parameters of different ethnic groups studied. A highly significant correlation (R=0.8 and p < 0.001) between basal and dental arch form was found at the canine and molar area in both the Class I and III groups. Conclusions. The mandibular arch form of Class III patients is significantly wider than that of Class I patients in the dental and basal region. Gender and age could contribute to the difference observed in arch width and arch depth between groups while ethnicity was found to be not contributory. The use of WALA points to predict the clinical arch form for an individual patient is possible.