A computer-based assessment of structural and displacement asymmetries of the mandible

Abstract

The purpose of this article was to assess and quantify the different components that can lead to mandibular asymmetry in a person during or at the end of growth and to investigate the extent to which improvement can be obtained in the different situations. Three boys and 17 girls, 8 to 21 years of age, with facial asymmetry and chin deviation were selected. Posteroanterior cephalometric radiographs were taken in intercuspal position. On each radiograph, three mandibular points (menton, gonion or antegonion, and articular point) were selected to define a mandibular area. A vertical axis of reference was also determined. Computer-aided design was employed to develop two systems (A and B) to assess a symmetry degree of the mandible. With system A, the left mandibular area was rotated around the vertical axis of reference, and the degree of the superimposition between the left and right areas was plotted. With system B, the left area was rotated around an axis that ran through menton and was perpendicular to a line connecting the two articular points of the mandible. Also, in this case, the degree of superimposition between the two areas was plotted. With system A, the degree of superimposition was a function of mandibular position and mandibular symmetry. With system B, it was a function of mandibular symmetry only. Thus through comparative examination of the data, assessment of displacement asymmetry and structural asymmetry was possible. In two patients, only displacement asymmetry was present, whereas 14 patients showed various features of structural asymmetry. The patients were treated with orthopedic splints to keep the mandible in a position of symmetry. Orthodontic treatment followed so that the intercuspal position would finally coincide with the position of symmetry. Computer-aided design analysis was performed again after a mean observation period of 41.1 months. In 11 patients an improvement in symmetry was observed with both systems. The patients in whom no improvement of structural asymmetry was observed showed a relatively high degree of symmetry at the beginning or had a considerably higher age than the mean age of the whole group. The clinical implications of the data are discussed. (A M J O RTHOD D ENTOFAC O RTHOP 1991;100:19-34.)