Cross-sectional characteristics of the masseter muscle: an ultrasonographic study.

Cross-sections of the masseter muscle may now be measured non-invasively using ultrasonography. The purpose of the present study was to determine (1) whether the scanning level may affect cross-sectional measurements and (2) whether measurements made at identical levels may be reproducible. The study included 42 asymptomatic volunteers. Unilateral ultrasonographic investigation was performed with a linear (B-scan) 7.5 Mhz small-part transducer to register cross-sections of the masseter muscle on five different levels. Scans were made on the relaxed and contracted muscle. Measurements were made in two sessions with a time interval of at least 5 min. Statistical analysis consisted of univariate analysis of variance for repeated measurements. Data were analysed for reproducibility by using the method error and measurement error. For the ultrsonographic measurements the main effects of the variables 'session' (P=0.0001), 'level' (P=0.0001), and 'condition' (P=0.0001) were significant. Analysis of simple session-within-level effects revealed a significant difference between the repeated measurements for the most upper level (P=0.022), the upper level (P=0.012), and the most lower level (P=0.0001) of the relaxed muscle. An additional significant effect was found for the most lower level of the contracted muscle (P=0.015). Cross-sections evaluated at the middle (method error=0.31 mm; measurement error=2.0%) and lower level (method error=0.32 mm; measurement error=2.4%) of the contracted muscle were the most reproducible. The conclusion is that ultrasonography is a reproducible method for measuring cross-sections at the middle and lower level of the contracted masseter muscle.