Ultrasound protocols for measuring masseter muscle

Objective

Temporomandibular disorders are frequently treated using botulinum neurotoxin, which may relieve pain by limiting masseter muscle function and reducing size. Masseter size is generally measured by direct visualization/palpation. Muscle size can be objectively and noninvasively imaged using 2-dimensional (2-D) ultrasound, but protocols have not been standardized. This study aims to validate 2-D ultrasound protocols for measuring masseter muscle.

Study Design

Ten adult volunteers were recruited. Each volunteer was scanned (right/left masseters) in relaxed/clenched positions. A single muscle examination consisted of a 2-,D a 3-dimensional (3-D), and then a second 2-D scan. SCAN 2-D scanning protocol included 3 short-axis (superior/middle/inferior) and 3 long-axis (anterior/middle/posterior) views. Each subject was scanned twice, roughly 15 minutes apart. The 3-D scanning protocol consisted of 30-second sweeps acquired over the muscle. Shallow/deep muscle borders were manually traced on all images. Average muscle thickness and thickness at the image center were calculated using custom software. Agreement between scans was assessed using the Bland-Altman plot. We compared 2-D scan measurements with the 3-D reconstructed measurements.

Results

(1) High intra-rater (intraclass correlation coefficient = 0.99) and inter-rater (intraclass correlation coefficient = 0.98) reliability was found. (2) Muscle thickness measured at the image center is similar to the average muscle thickness. (3) Most repeatable scan location is at the longitudinal middle in both relaxed/clenched states. (4) Scanning muscle at both states are repeatable, with relaxed scans yielding slightly greater repeatability. (5) Difference in repeatability when the operator stood at the same and opposite side of the muscle scanned is negligible. (6) Location of the maximum thickness increase in the clenched state varied between subjects. (7) Measurements from 2-D scans are correlated to 3-D volume measurements, R-squared = 0.86 (clenched), 0.95 (relaxed).

Conclusions

Our 2-D scanning protocol is reproducible regardless of muscle contraction state or the operator position. The result of this study is that our protocol allows better observation of muscle size/thickness changes, which can lead to better treatment analysis after neurotoxin or trigger point injection in the clinic.