Jaw muscle function and wishboning of the mandible during mastication in macaques and baboons.

An analysis of in vivo bone strain indicates that the mandibular symphysis of macaques experiences lateral transverse bending or "wishboning" during the power stroke of mastication, and this loading regime results in relatively intense concentrations of stress along the lingual aspect of the symphysis (Hylander 1984, 1985). It has been hypothesized that peak wishboning of the macaque mandible, which probably occurs at the very end of the power stroke, that is, after the initial occurrence of maximum intercuspation, is associated with the late peak activity of the balancing-side deep masseter muscle coupled with the rapid decline in the activity of the balancing-side medial pterygoid and superficial masseter muscles (Hylander et al., 1987). The main purpose of this study is to do a detailed analysis of the electromyographic (EMG) activity of the deep and superficial masseter and medial pterygoid muscles so as to provide a better understanding of the external forces associated with wishboning. This was done by recording and analyzing EMG activity from the superficial masseter, deep masseter, and medial pterygoid muscles in macaques and baboons. EMG activity was recorded from bipolar fine-wire electrodes, and the data were quantified and analyzed using digital techniques. The EMG data clearly support our original observation that the balancing-side posterior deep masseter exhibits peak EMG activity relatively late in the power stroke at a time when activity is rapidly decreasing in the balancing-side medial pterygoid and superficial masseter muscles. Moreover, peak activity of the balancing-side deep masseter occurs at a time when the activity of the working-side deep and superficial masseter and medial pterygoid muscles are also decreasing. Thus, these data are consistent with the hypothesis that wishboning of the mandible is influenced significantly by the late occurrence of force from the balancing-side deep masseter. The EMG data also indicate that residual force from the relaxing superficial masseter may contribute significantly to wishboning. Finally, patterns of wishboning are not fully accounted for by our EMG analysis of the deep and superficial masseter and medial pterygoid muscles. This is probably because the lateral ptergyoids, which can either counter increased wishboning or actually cause reverse wishboning, were not included in our EMG analysis. The EMG data demonstrate that jaw-closing muscle recruitment patterns for macaques and baboons differ from those of humans. Nevertheless, in spite of these differences, it appears, based on previously published EMG data, that the human symphysis may also experience wishboning.(ABSTRACT TRUNCATED AT 400 WORDS)