OPENING ROTATIONS OF THE MANDIBLE DURING AND AFTER TREATMENT

In this retrospective cephalometric study of mandibular rotational cha nge, we studied 60 cases. All patients were treated with full fixed ap pliances and exhibited an opening rotation of at least 1.5 degrees dur ing treatment, as measured on the basis of the angle of the Y-axis to the sella-nasion line (SN). Our objective was to determine whether thi s opening rotation was sustained during retention. Paired t tests were used to test the hypothesis that the treatment change or opening rota tion was stable and that these patients did not return to their origin al mandible-to-cranium relationship in the posttreatment period. Stepw ise regression analysis was used to determine which (if any) changes i n the independent variables during treatment could predict the subsequ ent behavior of the angle of the mandibular plane to the SN and the an gle of the Y-axis to the SN during retention. During treatment, the me an increase in the angle of the Y-axis to the SN was 2.43 degrees. Aft er an average posttreatment period of 54 months, this angle was reduce d on average by only 0.73 degrees. Stepwise linear-regression analysis indicated that none of the treatment changes seen in the independent variables strongly predicted the ensuing closing rotation seen during retention. The correlation coefficient between the Y-axis angle and th e mandibular-plane angle during treatment was 0.67. Mandibular opening rotations as a consequence of orthodontic treatment do not invariably return to the pretreatment value, and their negative effects-although sometimes small-cannot be discounted. Because the preponderant eviden ce of a closing rotation occurs in the terminal pubertal growth stages , the net effect may be even more significant.