Spread and Referral of Experimental Pain in Different Jaw Muscles

Aims: To test the hypothesis that there would be no differences in perceived pain intensity and spread and referral of pain evoked by injection of a similar amount of hypertonic saline into 6 different jaw-muscle sites in healthy female subjects. Methods: A total of 15 healthy women participated in 3 experimental sessions separated by 1 week. In a randomized sequence, the deep layers of the masseter, superficial layers of the masseter, anterior temporalis, lateral ptery-goid, medial pterygoid, and anterior digastric muscles were injected with 5.8% hypertonic saline (0.2 mL). The subjects rated the per-ceived intensity of pain on an electronic 0- to 10-cm visual analog scale (VAS). The distribution of pain was drawn by the subjects on anatomical maps of the face, and a Danish version of the McGill Pain Questionnaire (MPQ) was filled out. Results: All injections were associated with moderate to strong pain intensity (mean peak value: 5.6 to 6.4 cm) with no significant differences between muscle sites (analysis of variance [ANOVA]: P = .520). Pain rating indices derived from the MPQ did not suggest significant differences between muscle sites (ANOVA: P = .898). However, the area of perceived pain differed significantly between muscle sites (ANOVA: P = .038) with the greatest area following the injection into the anterior temporalis muscle (Tukey: P ＜ .05). On direct inspection, the pain maps appeared quite similar, but a new analysis technique based on a center-of-gravity method revealed significantly different coordinates and length of vectors (ANOVA: P ＜ .001) with longer vectors associated with the pain areas in the anterior temporalis muscle compared with the other muscle sites (Tukey: P ＜ .05). All muscles were frequently associated with referral of pain to intraoral structures (40% to 87%), but only pain in the anterior digastric muscle was referred to the tip of the tongue (53%). Conclusion: The data suggest no major differences in pain sensitivity between the examined jaw-muscle sites, but pain in the anterior temporalis muscle spreads to a larger area independent of pain intensity. There are subtle but detectable differences in the location and referral of pain patterns between jaw muscles. This will be helpful in the dif-ferential diagnosis of myofascial temporomandibular disorder pain.

jaw-muscle physiology; pain assessment; referred pain; temporomandibular disorder

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