RELATIONSHIP BETWEEN BONE AND MUSCLES OF MASTICATION IN HEMIFACIAL MICROSOMIA

The relationship between the bone and muscles of mastication in hemifa cial microsomia was studied using three-dimensional volumetric compute d tomography scans and image processing techniques. High resolution he ad computed tomography scans were obtained from 31 patients with unila teral hemifacial microsomia and eight normal patients. Using three-dim ensional volume renderings of bone, mandibular deformities in patients with hemifacial microsomia were classified using the Pruzansky system . For each patient, specific craniofacial bones (temporal bone, maxill a mandible) and the muscles of mastication (masseter, temporalis and l ateral and medial pterygoid) were segmented bilaterally from the image volume for independent display and volume measurement. Volumes were e xpressed as the ratio of the affected: unaffected sides. For the masse ter and temporalis, the relationship between muscular hypoplasia and o sseous hypoplasia in its origin and insertion was studied by plot ring affected:unaffected bone volume as a function of affected:unaffected muscle volume for each muscle, bone of origin, bone of insertion tripl et. The volumes of the pterygoid muscles were compared with hemimandib ular volumes. The precision of object segmentations was examined by re petitive definition tasks, whereas the accuracy of volume measurement was tested by scanning custom-made phantom objects and comparing digit al to physical object volume measurements. Volume measurements perform ed using these techniques were both accurate and precise. In hemifacia l microsomia, the extent of hypoplasia of specific muscles of masticat ion predicted the extent of dysplasia in their osseous origin and inse rtion. However, the reverse was not true. The extent of hypoplasia of the facial bones did not necessarily predict the extent of hypoplasia in the attached muscles of mastication. Pruzansky grade of the mandibl e described the degree of mandibular hypoplasia on the affected side, but was inconsistent in its prediction of volume decrease of the other facial bones.