A Study on Reproducible Locations for Evaluating Masseter Muscle Function with Ultrasonography

Purpose: This study aimed to identify reproducible locations for evaluating masseter muscle function by measuring its thickness using ultrasonography (US).The study focused on comparing two measurement locations: the thickest part of the masseter muscle during ultrasonographic scanning (TMUS) and the most prominent part during clenching (PMC).Methods: Forty healthy adults (20 males and 20 females) participated in the study.US images were obtained from both sides of the masseter muscle under resting and clenching conditions.Measurements were taken at the TMUS and PMC locations, and the clenching-to-resting (C/R) ratio was calculated.Intra-and inter-rater reliability were assessed using intraclass correlation coefficients (ICCs), and the agreement between the two locations was further analyzed using Bland-Altman (BA) plots. Results:The measurements at both TMUS and PMC showed high intra-and inter-rater agreement, with no significant difference in measurements between the two locations.However, the PMC location demonstrated slightly higher ICC values (0.94) compared to TMUS (0.91).The C/R ratio for PMC showed higher consistency (0.89) compared to TMUS (0.65).BA plots indicated that the agreement between TMUS and PMC was slightly better during clenching than at rest, with smaller mean differences in clenching (-0.06 mm) than resting (-0.13 mm).Additionally, the number of measurements outside the upper and lower limits was lower during clenching (10) than at rest (13). Conclusions:Both TMUS and PMC locations demonstrated reliable measurements, but the PMC location showed slightly better consistency across different muscle states.The findings suggest that PMC provides a more reproducible and standardized approach for masseter muscle assessment, making it a better choice for both clinical practice and research in evaluating masticatory function.