The present study investigated the pattern of muscle activity during swallowing in patients with mandibular prognathism.
The subjects comprised 12 mandibular prognathism patients (5 males, 7 females, average age of 20.8 years old), and 12 volunteers (2 males, 10 females, average age of 23.8 years old) with individual normal occlusion and no abnormalities of deglutition function as a control.
Electromyographic (EMG) activity was recorded from the masseter and suprahyoid muscles bilaterally, mentalis on the midline and orbicularis oris and infrahyoid muscles on the right side.
Each subject was asked to perform the following: (1) maximum clenching for 5 seconds at 10-second intervals, (2) maximum jaw opening for 5 seconds at 10-second intervals, and (3) swallow 4 ml of half-solid nutrient made of 0.8% agar powder. The first two tasks were repeated four times and the last one was repeated five times.
Peak amplitudes of masseter and suprahyoid EMGs during maximum clenching and jaw opening were first obtained. For analysis of swallowing, mean values of duration of EMG, mean values of amplitude, and root mean square (RMS) of EMGs of orbicularis oris, mentalis and infrahyoid muscles, and those of %peak (swallow activity/peak activity) amplitude of masseter and suprahyoid EMGs were calculated and compared between the patient group and the normal group. Morphological characteristics of the patients were also compared between the groups using lateral cephalograms.
The duration of activity in all muscles recorded except the infrahyoid was significantly longer in the patient group than in the control. Peak amplitude and RMS of mentalis muscles were significantly larger in the control than in the patient group. %peak amplitude of masseter and suprahyoid muscles were both significantly larger in the patient group than in the control. On the contrary, there were no significant differences in all the variables of infrahyoid muscles.
The present study successfully showed the pattern of orofacial muscle activity in patients with mandibular prognathism in terms of peak amplitude and duration. Although the activities of orofacial muscles recorded in this study are reflexively controlled at the lower brainstem during swallowing, these are also known to be voluntary muscles and therefore they are likely to adapt the activity patterns to their morphologic characteristics.