MAGNETIC-RESONANCE-IMAGING OF THE MASSETER MUSCLE - A PRELIMINARY GENETIC-STUDY IN MONOZYGOTIC AND DIZYGOTIC TWINS

Magnetic resonance images of the head were taken in five monozygotic a nd seven dizygotic twins in order to calculate bilaterally masseter mu scle cross-sectional areas and total volume. Comparing correlation coe fficients between cotwins, genetic influences could be expected for ma ximal cross-sectional area but not for volume measurements. Model fitt ing revealed that additive genetic factors explained 93.9% of the vari ance for the left and 82.4% for the right maximal masseter muscle cros s-sections. It is anticipated that while the number of masseter muscle fibers is under strong genetic control, the length of individual fibe rs can be influenced by specific environmental factors. In the second part of the investigation, cephalometric measurements from lateral hea dplates were compared with these masseter muscle values in 10 twin pai rs. Only three of the 15 angular and five (three vertical, one transve rsal, and one sagittal measurement) of the 20 linear measurements were significantly correlated with masseter muscle values.