Immediate Effect of Ultrasound Therapy on Bilateral Masseter Myalgia: Randomized Single Blinded Investigational Trial

Introduction: Temporomandibular disorders (TMD) are a group of conditions affecting the masticatory system primarily associated with symptoms of pain and decreased functional capacity. Therapeutic ultrasound is a method of physical therapy that has shown clinically to provide immediate relief of pain in patients with different musculoskeletal disorders. Aims: First, to determine a dose response relationship between two different intensities (0.4 W/cm2 and 0.8 W/cm2) and between two sets of duty cycles (50% and 100%). The second aim was to determine if therapeutic ultrasound applied to the masseter muscle is capable of eliciting segmental effects on the temporalis muscle. Methods: A total of 28 adult females with bilateral myalgia according to the Diagnostic Criteria for TMD (DC/TMD) were included in this investigational trial. Each subject was randomly allocated to one of the four different ultrasound intervention doses with different settings, and was randomly assigned to a starting side. Therapeutic ultrasound was applied on the masseter for five minutes on each side of the face sequentially. The following outcomes were measured before and after each intervention: Self-reported pain scale, pressure pain threshold (PPT) for the masseter and the temporalis muscles and intraoral muscle temperature for the masseter muscle. Two-way ANOVA was used to compare the outcome changes (dependent variables) with the two device settings (independent variables). Results: Self-reported pain scale showed neither significant interaction among the doses, nor significant main effects (all p>0.05). Pressure pain threshold of the masseter muscle had no interaction with the settings, but showed a significant main effect of intensity settings upon the use of 0.4 W/cm2 among the doses, p= .028. Intraoral temperature had no significant interaction with the settings, however the increase in intraoral temperature was related to the significant main effect of the duty cycle, p= .004. Segmental effects represented in changes in the pressure pain threshold of the temporalis muscle showed no significant interaction, but the results showed a significant main contribution with the intensity setting, p= .024. Conclusion: The use of ultrasound was a successful method for an immediate increase in PPT and temperature of painful masticatory muscles. Treatment directed on painful masseter muscles with the use of an intensity of 0.4 W/cm 2 with a 100% continuous wave increased the pressure pain threshold of the masseter as well as the temporalis muscle.