Sensory and motor changes of the human jaw muscles during induced orthodontic pain.

The aim of this study was to evaluate the short-term effects of orthodontic pain on the pressure pain threshold (PPT) of the masseter and anterior temporalis muscles, and on their electromyographic (EMG) activity during clenching and chewing. Orthodontic pain was induced in 14 healthy subjects (mean age 26.6, SEM 1.1) by placing orthodontic separators. The subjects were randomly assigned to an experimental and to a control session in a double-blind crossover study. PPT was significantly reduced (Student's t-test; P < 0.001) after experimental sessions for both the masseter and the anterior temporalis muscles, whereas no significant differences were found during control sessions (P > 0.05). EMG activity during clenching and chewing was significantly reduced (0.001 < or = P < 0.05) after experimental sessions for both masseter and anterior temporalis muscles, whereas no significant differences were found during control sessions (P > 0.05). The decrease of PPT found in this study can be related to the occurrence of muscle pain and headache reported by patients during orthodontic or other dental treatment. The decrease of EMG activity of the jaw muscles associated with orthodontic pain is consistent with the pain adaptation model and should be considered as a potential factor for loss of occlusal anchorage during orthodontic treatment.