Analysis of Cortical Bone Distribution in the Human Mandible

Objective: During mastication, the mandible experiences complex patterns of stresses and strains.  Experimental studies have documented that variation in masticatory function (i.e., hard- vs. soft-diets) results in predictable differences in mandibular cortical bone properties in response to differences in strain magnitudes.  It is unclear, however, how the results of these studies relate to masticatory function and mandibular cortical bone properties in humans.  We therefore tested the null hypothesis that there is no correlation between masticatory function and mandibular cortical bone properties in a human sample.

Method: We assessed mandibular cortical bone properties at the symphysis and the corpus (at M1/M2 septum) using CT scans of n=20 males.  We calculated cortical bone area, subperiosteal area, symphyseal orientation (theta), second moments of area (Imax and Imin) and polar moments of inertia (J). Masticatory force was calculated using in vivo masticatory muscle cross-sectional areas (for masseter, medial pterygoid and temporalis) and muscle moment arm lengths (measured from the TMJ to the muscle line of action).  We analyzed the relationship between cortical bone properties and masticatory function using Pearson’s correlation coefficient.

Result: With respect to the symphysis, masticatory force was negatively correlated with Imax (r=-.453; P= 0.045) and positively correlated with Imin (r=0.540; P=0.014).  Thus, individuals with increased muscle force exhibit both an increase in resistance to wishboning strains and a decrease in dorsoventral symphsyseal bending. Regarding the corpus, masticatory force was significantly correlated with the ratio Imin/Imax (r=0.620; P=0.004). This suggests that greater muscle force is associated with relatively wider corpus dimensions.

Conclusion: Correlations between masticatory function and mandibular cortical bone properties suggest that while aspects of the mandible appear to respond to masticatory strains, other aspects (i.e., symphyseal Imax) may be responding to other facial growth dynamics.