A Cephalometric Study Of High-angle And Low-angle Class Ⅱdivision 1 Cases Treated By MBT

Objective: The purpose of this study was to compare the differences of craniofacial structures of class â…¡division 1 cases of high-angle and low-angle facial skeletal types and to observe their skeletal and dentoalalveolar changes after MBT straight wire appliance treatment. Methods: 60 subjects of class â…¡division 1 cases including 30 high-angle (8 males and 22 females),and 30 low-angle (9 males and 21 females) were selected .Forty-eight hard tissue items of cephlometric radiographs were analyzed. All the items between the two groups before treatment were compared statistically. Measurement changes of each group before and after treatment were also analyzed statistically. Results: 1.Differences between high-angle and low-angle groups before treatment: The angles of SNA,SNB,C.V.,and The distances of Go-Po, Ar-Go,,PO-NB and xchchd in high-angle group were smaller than those in low-angle group(p<0.001). The angles of Y-angle Ar-Go,-Me,SUM,SN-PP , OP-MP and the distances of U1-PP, L1-MP (mm) , U1-Apo , L1-Apo , Li-MP in high-angle group were greater than those in low-angle group(p<0.001) . The items of S-Go , FHI (S-Go/N-Me) and N-Me,ANS-Me ,ANS-Me/N-Me were smaller in low-angle group(p<0.05). Although the means of N-S-Ba, S-Ar-Go,, Ar-Go,-Me, SN-FH , U1-L1, U6-PP, L6-MP, N-ANS were different between the two groups, no statistical differences were found. 2. Changes before and after treatment in high-angle group: No statistical difference were found for items of Y-angle, Ar-Go,-Me , SN-FH, SN-PP, SN-MP, PP-FH, FH-MP , L1-MP(mm) , Li-MP, N-S-Ba, N-S-Ar and S-Ar-Go, . After treatment the linear measurements of Go-Po, Ar-Go,, PO-NB, U1-L1, U1-PP, U6-PP, L6-MP and Ui-PP were increased , while those of U1-SN ,L1-MP(deg), U1-Apo and L1-Apo were reduced after treatment (P<0.001) . As to ANS-ME/ N-Me, no statistic change was found. The distance of S-Go was increased after treatment (p<0.001). 3. After treatment in low-angle group: Those items of Y-angle, A-Ptm , Go-Po , Ar-Go, , PO-NB , SN-OP, SN-MP,OP-FH , PP-OP, PP-MP, L1-MP(deg) ,U1-L1,U1-PP, U6-PP,L6-MP and Ui-PP were increased (p<0.05). The distance of L1-MP(mm) were reduced , while those measurement of N-S-Ba ,N-S-Ar , S-Ar-Go, , SN-PP ,FH-PP and FH-MP did not show statistical difference . Although thetotal facial hight (N-Me), low-facial hight (ANS-Me) and the ratio of ANS-Me/N-Me increased significantly , the upper facial hight did not show significant difference. Conclusion: 1 .The craniofacial structure of Class â…¡division 1 cases of high-angle and low-angle skeletal types of early permanent dentition demomstrated significant difference. In the high-angle group, the length of mandibular body and ramus are shorter. The posterior face hight is shorter, and the smaller posterior face hight resulted in unproportional anterior-posterior face hight ratio. In the low-angle group, the anterior face hight is short which lead to the proportion of anterior-posterior face hight in disharmony . 2. After MBT treatment the upper and lower molars and the upper anterior teeth extruded while the lower anterior teeth intruded among the two groups,these may be the reason for clockwise rotation of occlusion plane .Although the mandibular plane of low-angle group rotated clockwisely ,it showed no significant change in high-angle group . These different changes are favorable for the treatment of the two skeletal types.