Masseter muscle thickness and mechanical advantage in relation to vertical craniofacial morphology in children.

OBJECTIVE To investigate the relationship between vertical craniofacial morphology and masseter muscle thickness and mechanical advantage in children. MATERIAL AND METHODS The sample comprised 72 children (36 F, 36 M), 8.5-9.5 years of age, with various malocclusions and no previous orthodontic treatment. The thickness of the masseter was measured bilaterally by means of ultrasonography, and the recordings were performed both in relaxation and under contraction. Mechanical advantage was measured on the lateral cephalograms as the ratio between the masseter moment and the bite force moment arms. Two linear ratios and three angular measurements were used to describe vertical craniofacial morphology. RESULTS The mean masseter thickness was greater in the male group (p<0.05) in both relaxed and contracted conditions. There were no significant sex differences for the mechanical advantage or for the measurements of vertical craniofacial morphology. In females, there is a positive association between masseter muscle thickness and its mechanical advantage. Multiple regression analysis showed a positive association between posterior to anterior facial height ratio in both genders and a negative association between masseter thickness and the intermaxillary angle in females. CONCLUSIONS There is a significant association between posterior to anterior facial height and the masseter muscle in children. The importance of the masseter muscle is more evident in the vertical facial morphology of females.