To evaluate the test-retest reliability of supraspinatus cross-sectional area measurement by ultrasonography.

Both shoulders of 11 normal subjects (22 shoulders in total) were included in this study. The supraspinatus muscle was examined with the arm alongside the body in the coronal oblique and sagittal oblique planes. The occupational ratio of the supraspinatus fossa was measured. To calculate the occupational ratio, the Y view of MRI was reproduced with sonography by locating the suprascapular notch in the coronal oblique plane (in the plane of the scapula) and then rotating the transducer 90° to that plane. The cross-sectional area was measured using the tracing and ellipse tool. The second measurement was performed 7 days after the initial measurement.

The Pearson correlation coefficient and intraclass correlation coefficient between the first and the second occupational ratio measurements were 0.43 and 0.44, respectively, for the tracing method, and 0.53 and 0.47, respectively, for the ellipsoidal method. The difference between the first and second occupational ratio measurement was 4.1±3.9% (0.1-13.2%) for the tracing method, and 4.5±3.4% (0.01-10.5%) for the ellipsoidal method. The maximum difference was 13.2%. The occupational ratio was 86.2±5.3% (70.6-95.8%) for the tracing method and 85.0±5.2% (69.3-96.1%) for the ellipsoidal method.

Supraspinatus occupational ratio by sonography is a low to moderately reliable intrarater method. However, the maximum difference was not significant. The main reason for its low to moderate reliability was the narrow value range. Therefore, the study method should be re-evaluated in stroke patients and in patients with rotator cuff disease. Knowledge of the anatomy is a prerequisite to attain intrarater reliability.