RESUMEN
PURPOSE: To study the relationship between ultrasonographic thickness and EMG activity of the masseter muscle in subjects with different vertical craniofacial morphology. METHODS: Thirty female students were separated into two groups (14 cases with high-angle, 16 cases with low-angle) based on SN-MP angle, FH-MP angle, and FHI. The thickness of the masseter muscle under relaxed conditions and during maximal clenching was assessed by ultrasonography. EMG activity of the masseter muscle under relaxed conditions and during maximal clenching was recorded with bipolar surface electrodes.All measurements were analyzed with SPSS 11.0 software package. Differences between groups were tested for statistical significance using Student's t test. The relationship between masseter muscle thickness and its EMG activity was estimated by Pearson's correlation coefficient. RESULTS: The thickness of the masseter muscle in the low-angle individuals was significantly greater than that in the high-angle individuals under relaxed conditions (P=0.009) and during maximal clenching (P=0.000). Although there was no significant difference in resting EMG activity between the two groups, the EMG activity of masseter muscle in the low-angle individuals was also significantly higher than that in the high-angle individuals during maximal clenching(P=0.022). Relaxed thickness of masseter muscle was significantly correlated with its mean maximum EMG activity in the low-angle group (r=0.61, P=0.003) and moderately correlated with that in the high-angle group (r=0.38, P=0.023). Similar correlation was found between contracted thickness of masseter muscle and the mean maximum EMG activity, being significantly correlated in the low-angle group (r=0.73, P=0.002) and moderately correlated in the high-angle group(r=0.53, P=0.006). CONCLUSIONS: The present findings support the concept that subjects with different vertical craniofacial morphology have different form and function of masseter muscle. The ultrasonographic thickness and EMG activity of masseter muscle in the low-angle individuals are both greater than those in the high-angle individuals.
