A novel sonographic method of measuring patellar tendon length.

Obtaining accurate and readily repeatable measurements is a prerequisite for using measures of soft tissue structures both clinically and in the research setting. Few studies have evaluated the interrater reliability of ultrasound measurements of tendons. The objective of this study was to determine the accuracy and reliability of a new method of sonographic measurement of patellar tendon length using direct dissection as the gold standard. Four cadaveric knees were sonographically evaluated by two independent investigators. Two custom designed straps with nylon strapping and stainless steel wire were used to firmly mark position on the leg and create an acoustic shadow on the ultrasound image. Anatomic landmarks were the distal patellar pole and the bony ridge on the anterior proximal tibia. After sonographic evaluation, the knee was dissected to expose the patellar tendon, which was measured using digital calipers. Intraclass correlation coefficients (ICC) were used to determine reliability of measurements between observers, where ICC >0.75 was considered good and >0.9 was considered excellent. Validity was measured using a Bland-Altman plot, which measures bias between measurement methods as well as variability of scatter. Three sonographic measurements were made by each investigator on each tendon. The length of each of the four tendons based on the mean values of sonographic measurements was 53.8 mm, 53.4 mm, 49.4 mm and 46.8 mm. The length based on visual inspection of the dissected tissue was 54.6 mm, 52.8 mm, 49.8 mm and 46.9 mm. The calculated ICC between raters was 0.96. On the Bland-Altman plot, the bias, or mean difference between sonographic and visual measures, was 0.17 mm, with a standard deviation of 0.71. The 95% limit of agreement was -1.55 to 1.22 mm. Measurement of patellar tendon length with ultrasound using adjustable surface markers and calipers is highly accurate and has good interrater reliability.