Evaluation of the Interrater Reliability of Sonographic Measurements of Muscle Thickness of 38 Piriformis Muscles in 19 Patients with Piriformis Syndrome.

BACKGROUND The piriformis muscle is a flat superficial muscle of the deep gluteal muscles that externally rotates the hip. Ultrasound is widely used to identify the piriformis muscle, especially for guidance of the needle during injections; however, its diagnostic use has recently gained popularity. The operator-dependent nature of ultrasound requires demonstration of reliability between operators. This study aimed to evaluate interrater reliability of sonographic measurements of muscle thickness of 38 piriformis muscles in 19 patients with piriformis syndrome. MATERIAL AND METHODS An ultrasound transducer was placed transversely on the sacral spinous process and moved caudo-laterally until the piriformis muscle was visualized under the gluteus maximus while patients were lying in prone position. The thickness of piriformis muscle was measured with a 2 to 5-MHz broadband curvilinear transducer in 3 regions (thickest regions of muscle over the ilium, near the greater trochanter, and near the sacrum). The interrater reliability of measurements of 2 examiners who were blinded to each other's measurements was assessed by intraclass correlation coefficient. RESULTS In total, 114 samples from 38 piriformis muscles of 19 patients with a diagnosis of piriformis syndrome were evaluated by 2 raters in this study. The median (interquartile range) patient age was 41 (15) years. Intraclass correlation coefficient value for overall thickness measurements of piriformis muscle was 0.836. Intraclass correlation coefficient values for 3 different regions were over the ilium, near the greater trochanter, and near the sacrum were 0.777, 0.883, and 0.811, respectively. CONCLUSIONS Ultrasound measurement of piriformis muscle thickness has good interrater reliability.