A comparison of human jaw muscle cross-sectional area and volume in long- and short-face subjects, using MRI.

OBJECTIVE: In humans, the vertical craniofacial dimensions vary significantly with the size of the jaw muscles, which are regarded as important controlling factors of craniofacial growth. The functional relevance of the maximum cross-sectional area (CSA), indicating maximum muscle strength, is questionable since peak forces are generated only a fraction of the day. Alternatively, muscle volume (indicating the generated loads) might be a more meaningful functional variable. Therefore, the aim of this study was to investigate if jaw muscle volume is stronger related with vertical craniofacial dimensions than with jaw muscle CSA.
DESIGN: Thirty-one adult healthy subjects with varying vertical craniofacial morphology participated in this study. Axial magnetic resonance imaging (MRI) scans were used for segmentation of the masseter (Mas) and medial pterygoid muscles (MPM). This enabled measurements of the muscle CSA and volume. Cephalometric analysis was performed using lateral radiographs. With factor analysis, the number of cephalometric variables was reduced into two factors that represented the anterior face height and the posterior face height (PFH), respectively. Subsequently, mutual relationships between these factors and muscular variables were assessed using a multiple regression analysis.
RESULTS: It was found that vertical craniofacial dimensions were significantly better (up to 12%) related with muscle volume rather than with CSA. Furthermore, it was shown that especially the PFH factor was significantly correlated with the Mas and MPM.
CONCLUSION: Vertical craniofacial dimensions are stronger related with jaw muscle volume than with CSA. Tentatively, it can be assumed that the generated muscle loads, rather than maximum forces, influence vertical craniofacial growth.