Condylar displacement and changes in condylar angulation after sagittal splitting ramus osteotomy

To evaluate mandibular condylar displacement and changes in angulation of the condylar longitudinal axis to the transverse line running through both auditory canals after sagittal splitting ramus osteotomy, 15 dried human skulls were tested.In the first five test specimens (M-DP group), the mandibles were split with the vertical external cortical bone splitting line situated medially to the mandibular second molar. In the next five specimens (D-DP group), the vertical external splitting line was placed at the distal part of the second molar. In the other five specimens (OB group), the splitting line was made from the distal part of the second molar to the mandibular angle.Among these three groups, the effects of the position of the external cortical bone splitting line on the position of the condyle and the change in condylar angulation were also compared. In addition, the effects of the magnitude of mandibular setback and the methods for repositioning of external segments (flexible type and firm type) were examined. The amount of condylar displacement was detected three-dimensionally using two Saphon visitrainers. The change in condylar angulation was measured with a protractor.The following results were obtained:1) The magnitude of mandibular setback did not influence condylar position or angulation.2) In the M-DP group, the magnitudes of condylar displacement and the change in condylar angulation were larger than those in the D-DP and OB groups. These results indicated that it is desirable to place the external cortical bone splitting line at the posterior portion of the second molar.3) The angulation of the condylar axis changed as the external cortical bone splitting line moved. The M-DP group changed by 7-8°, the D-DP group by 6-7° and the OB group by 4-5°.4) The angulation of the condylar axis was not affected by the method for external segment repositioning. However, the amount of condylar displacement with firm repositioning was larger, especially in the lateral direction, than that with flexible repositioning. This fact indicated that the repositioning device should be flexible. Furthermore, since this flexible type appliance, sufficient to maintain the antero-posterior position of the condyle, could not completely prevent lateral movement of the condyle, a new technique to reposition the proximal segment should to be established.