Investigating the Reliability of Ultrasonography Measurements of Masticatory Muscles in subjects with and without Temporomandibular Disorders: An Intra-rater Reliability Study

Introduction: Ultrasonography is a common tool for assessing muscle morphology. This study aims to investigate the intra-rater reliability of ultrasonography measurements of the masseter and lateral pterygoid muscles (LPM) in subjects with and without temporomandibular disorder (TMD) and to compare the measurements between the two groups. Materials and Methods: A total of 30 individuals (15 subjects with TMD and 15 subjects without TMD) participated in the study. Imaging of the masseter muscle at rest and contraction, in addition to LPM, was performed using sonography in 2 sessions with a 2-day interval and the thickness of these muscles was measured. To assess the reliability, the intraclass correlation coefficient (ICC(3, 3)), standard error of measurement, minimal detectable change, and limits of agreement were used. Additionally, for the between-group comparison of measurements, the independent sample t-test was used. The Cohen d was used to determine the size of the differences. Results: ICC(3, 3) for intra-rater reliability of masseter thickness in rest and clenched positions and LPM were 0.89, 0.80, and 0.86 in the asymptomatic group, and 0.88, 0.80, and 0.90 in the TMD group, respectively. The values of absolute reliability parameters, such as standard error of measurement and minimal detectable change were in a small range. Additionally, compared to the asymptomatic group, the TMD group had thicker masseter muscle in the rest position and the effect size revealed a large difference between the groups. No significant differences were found between the groups for the thickness of the masseter muscle in contraction position and LPM. Conclusion: The results of the intra-reliability study revealed that ultrasonography is a reliable method to measure the thickness of the masticatory muscles in TMD subjects and asymptomatic subjects. Additionally, TMDs can change the thickness of the masticatory muscles.