[The influence of three-dimensional maxillary protraction simulation on dental and basal arch width coordination in early permanent dentition skeletal Class Ⅲ malocclusion].

PURPOSE: To evaluate the effects of three-dimensional maxillary protraction simulation using cone-beam CT (CBCT) on the width of the upper and lower jaws in early permanent dentition patients with skeletal Class Ⅲ malocclusion.

METHODS

Twenty skeletal Class Ⅲ malocclusion and 20 skeletal Class I patients according to the inclusion criteria from the Department of Orthodontics，Shanghai Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine between January 2016 and January 2021,were selected in this study. CBCT images were taken in all patients before treatment, and Mimics 20.0 software was used to measure the buccal-lingual inclination of the first molars and the width of the dental and basal arch of two groups of patients. Three-dimensional segmentation and maxillary forward movement simulation were performed to evaluate the dental and basal arch width after the sagittal relationship between the maxilla and mandible were changed. SPSS 21.0 software package was used for statistical analysis of the data.

RESULTS

The maxillary molars inclined buccally and the mandibular molars inclined lingually in Class Ⅲ patients(P<0.01); there was no significant difference in the width of the mandibular basal arch between Class Ⅲ and Class I , but the width of the maxillary base arch in Class Ⅲ was significantly smaller than that in Class I (P<0.01), the difference between the maxillary and mandibular base arch widths of Class Ⅲ was significantly smaller than that of class I(P<0.01); however, there was no significant differences in the width of the maxillary and mandibular dental arches between the two groups. During the maxillary protraction simulation in Class Ⅲ patients, the width of the maxillary dental arch of Class Ⅲ was significantly larger than that of the mandible when the maxilla was moved 3 mm forward(P<0.05). The width of maxillary basal arch and the difference between upper and lower basal arch width were still smaller than that of Class I when moving 2 mm(P<0.05); but when the forward movement was 3 mm, there were no significant difference in maxillary basal arch width and maxillomandibular basal coordination between Class Ⅲ and Class I (P>0.05).

CONCLUSIONS

Skeletal Class Ⅲ patients in early permanent dentition showed some insufficient development of maxillary basal arch width and buccal inclination of maxillary molars. During maxillary protraction treatment, the width of dental and basal arch change significantly with the amount of maxillary forward movement and mandibular rotation. The sagittal change of the upper and lower jaws is one of the most important factors to decide whether the maxillary expansion is needed. If the change exceeds 3mm, maxillary expansion might not be necessary.