The purpose of this study was to compare the arch width of the hyperdivergent group with that of the neutral group in Class III malocclusion based on the vertical patterns and to compare the arch width of Class III neutral group with that of normal occlusion group based on sagittal patterns.
The subjects consisted of 118 pairs of study casts, divided into three groups ; 37 Class III hyperdivergent group(18 males and 19 females, SN-Mn plane angle>39.5°), 40 Class III neutral group(20 males and 20 females, SN-Mn plane angle: 32
2.5) and 41 Class I normal occlusion group(20 males and 21 females). The intercanine, interpremolar, and intern lar width of the maxillary and man dibular stud- casts were measured, then the ratios of dental width to basal width and mandibular width to maxillary width were obtained. Basal arch width and dental arch width were measured to obtain the pure basal arch relation in transverse plane as ruled out the transverse dental compensation.
The results were as follows
1. There were no significant differences in any ratios between Class III hyperdivergent group and Class III neutral group as different vertical pattern.
2. As the ratios of dental arch width to basal arch width between normal occlusion group and Class III neutral group were compared, the maxillary teeth flared buccally to the basal bone, and the mandibular teeth tilted lingually to the basal bone in Class III neutral group.
3. The ratios of mandibular arch width to maxillary arch width in basal arch level were significantly different in all regi
ons. Maxillary basal arch width of Class III neutral group was narrower than that of normal occlusion group.
4. The ratios of mandibular arch width to maxillary arch width in teeth level were not significantly different between
normal occlusion group and Class III neutral group.
In spite of discrepancies of maxillary and mandibular basal arch width, the dental arch width of Class III malocclusion group compensated very well. At the presurgical orthodontic treatment in clinic, it would not be desirable to decompensate for compensated dental arch width too much, for obtaining an appropriate arch compatibility and good results for orthognathic surgery.
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