Ultrasonographic cross-sectional characteristics of muscles of the head and neck

Objective. Computed tomography and magnetic resonance imaging are the common techniques for evaluating cross-sectional areas and volumes of human jaw muscles. Because computed tomography has the disadvantage of showing cumulative biological effects and because MRI poses a problem in terms of clinical availability and cost, the purpose of this study was to determine whether ultrasonography could be used to measure local linear cross-sectional dimensions of muscles of the head and neck. Study design. in 46 patients with signs and symptoms of temporomandibular disorders, the anterior temporalis, anterior masseter, deep masseter, anterior digastric, posterior digastric, and sternocleidomastoid muscles were measured bilaterally by ultrasonography to assess linear local cross-sectional dimensions. Measurements were made in 2 sessions with a time interval of at least 5 minutes. Data were analyzed for reliability and variability through use of the intraclass correlation coefficient (ICC) and the repeatability coefficient (RC). To assess local muscle asymmetry patterns, the absolute asymmetry index was used, with the mean maximum muscle diameters of the respective right and left sides calculated from 3 consecutive measurements. Results. Satisfactory Visualization of muscles was obtained in 93.8% of 1104 imaging procedures. For the ultrasound measurements there was a significant difference in local cross-sectional dimensions between the first and second sessions for the anterior temporalis muscle only (P <.01). Acceptable intrarater reliabilities were obtained for the deep masseter (ICC = 0.91), anterior digastric (ICC = 0.91), and sternocleidomastoid (ICC = 0.86) muscles, whereas evaluation of the posterior digastric (ICC = 0.74), anterior masseter (ICC = 0.72), and anterior temporalis (ICC = 0.70) muscles was associated with moderate reliability Variability of repealed measurements was found to be acceptable for the anterior temporalis (RC = 0.32 mm) and posterior digastric (RC = 0.48 mm) muscles. Analysis of muscle site-related local cross-sectional dimensions showed a significant difference between the right and left sides for the deep masseter muscle only (P <.05). The study population investigated revealed mean asymmetry indices ranging from 5.3% for the anterior digastric muscle to 8.7% for the deep masseter muscle. Conclusions. Ultrasonography may prove to be a reliable diagnostic technique for the evaluation of cross-sectional dimensions and areas of muscles of the head and neck.