Immediate Effect of Ultrasound on Bilateral Masseter Myalgia: A Randomized Dose-Response Clinical Trial

(1) To determine the dose-response relationship of therapeutic ultrasound for TMD-related pain in the masseter muscle among four doses comprised of two intensities (0.4 W/cm² and 0.8 W/cm²) and two duty cycles (50% and 100%); and (2) to determine if therapeutic ultrasound applied to the masseter muscle would elicit a segmental effect on the ipsilateral temporalis muscle.

A total of 28 adult women with bilateral myalgia were randomly allocated to one of the four intervention doses. Therapeutic ultrasound was applied on each side of the masseter sequentially for 5 minutes. The following outcomes were measured before and immediately after each intervention: self-reported pain score, pressure pain thresholds for the masseter and temporalis muscles, and intraoral temperature adjacent to the treated masseter.

Self-reported pain scores showed neither significant main effects nor significant interaction among the intensity or duty cycle doses (all P > .05). The change in the pressure pain threshold of the masseter showed a significant interaction (P = .02) attributed to the 0.4 W/cm² and 100% duty cycle dose. Intraoral temperature was significantly increased and associated with the duty cycle (P = .01). A significant segmental effect of the pressure pain threshold of the temporalis was found for intensity (P = .01).

There was an increase in the pressure pain threshold of the painful masticatory muscles and an increase in intraoral temperature adjacent to the treated area immediately after the use of ultrasound at 0.4 W/cm² with a 100% duty cycle.