The Effect of Acute and Chronic Orofacial Pain on Jaw Muscle Activity and Jaw Movement during Chewing

Abstract

The general aims of the present study were to test whether these previous theories can explain (a) possible effects of different intensities of experimental pain on jaw motor activity, (b) possible long-term effects of a past history of clinical pain on jaw muscle activity, and (c) possible associations between psychological variables and jaw motor activity. The first group was healthy young subjects who received experimental pain in the masseter muscle through injection of a 5% hypertonic saline solution and the second group had a history of neuropathic orofacial pain (NOP) and were matched with pain free controls. Electromyographic (EMG) activity and jaw movement was recorded with an advanced jaw tracking system during chewing. In the experimental pain study there were no significant (p>0.05) effects of mild and significant (p<0.05) effect of moderate experimental pain intensity on velocity or amplitude of jaw opening or closing in chewing and that moderate experimental pain was associated with a significant increase in velocity of chewing as compare to control and mild experimental orofacial pain groups.. There were also no significant effects noted of experimental pain on jaw muscle EMG activity except for right anterior temporalis under mild pain during jaw closing phase. Significant associations were noted between some psychological variables (e.g. catastrophizing) and EMG activity levels for right anterior temporalis, and right and left masseter. The lack of association between pain and jaw motor effects, together with the presence of significant associations with some psychological variables, is consistent with neither the Vicious Cycle Theory nor the Pain Adaptation Model. Rather these findings lend support to more complex models (IPAM) that necessitate the involvement of higher centres in the pain-motor interaction. In the neuropathic orofacial pain study, although individuals were not in pain during the experimental recording session, there were some significant differences noted between NOP history group and the control group in the activity of the jaw muscles during chewing. There were some associations also identified between jaw muscle activity and psychological variables during chewing. The presence of some changes in jaw muscle activity between the NOP history and control groups, together with the association with some psychological variables is consistent with neither the Vicious Cycle Theory nor the Pain Adaptation Model.