Ultrasonography of masseter muscle size in normal young adults

The present study was planned to determine the relationship between linear dimensions of human masseter muscle cross-section and cross-sectional area (CSA), and to assess symmetry between the two sides in normal young adults. Cross-sectional images of the masseter muscle were measured bilaterally by real-time ultrasound imaging in 39 healthy dentate subjects, 19 males and 20 females, aged 21-47. From stored images, CSA and two linear measurements of muscle cross-section were obtained (the shortest and the longest distance through the muscle group). Correlation and regression analyses were performed to examine the relationship between CSA and the linear dimensions (both individually and with the linear dimensions multiplied). Symmetry of CSA between the two sides of the face was examined using the paired t-test. The significance of correlation coefficients (r) and the difference between the slopes of the regression lines were also examined. Masseter CSA was larger in males than in females. All correlation values between CSA and linear measurements were significant but muscle CSA was most accurately predicted when the linear measurements were multiplied (r = 0.97; P < 0.001). Although the correlation in this regard was high, the linear dimensions consistently overestimated the actual CSA by approximately 25%. Males showed more symmetry of CSA than females. The range of values for symmetry of masseter CSA was too large to assess abnormal asymmetry in patients with unilateral symptoms.