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 tem poralis muscles, and on their electromyographic (EMG) activity during clenc hing and chewing. Orthodontic pain was induced in 14 healthy subjects (mean age 26.6, SEM 1.1) by placing orthodontic separators. The subjects were ra ndomly assigned to an experimental and to a control session in a double-bli nd crossover study. PPT was significantly reduced (Student's t-test; P < 0.001) after experimen tal sessions for both the masseter and the anterior temporalis muscles, whe reas no significant differences were found during control sessions (P > 0.0 5). EMG activity during clenching and chewing was significantly reduced (0. 001 less than or equal to P < 0.05) after experimental sessions for both ma sseter 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 associate d with orthodontic pain is consistent with the pain adaptation model and sh ould be considered as a potential factor for loss of occlusal anchorage dur ing orthodontic treatment.