Intra- and inter-observer repeatability of radiographic measurements for previously slipped capital femoral epiphysis at skeletal maturity.

BACKGROUND Recent studies suggest that even a mild slip of the femoral capital epiphysis may lead to later degenerative changes when undiagnosed. However, little is written on the accuracy of radiographic measurements used to diagnose a slip at skeletal maturity. PURPOSE To assess the accuracy of radiographic measurements commonly used for assessment of previously slipped capital femoral epiphysis (SCFE) at skeletal maturity. MATERIAL AND METHODS All children born at our hospital during 1989 (n = 4006) were invited to participate in a follow-up hip trial at age 18-19 years. Erect pelvic anteroposterior and supine frog leg radiographs were obtained in a standardized fashion. For the purpose of this study, we selected a subset of 100 radiographs. To balance the data-set, we added another 28 radiographs from skeletal mature patients diagnosed and operated for a SCFE. Two observers independently measured Southwick's head-shaft angle, Murray's tilt-index, and the femoral head-neck angle. Intra- and inter-observer variation was assessed using the mean difference, with its 95% limits of agreement. RESULTS A high percentage of the images (40%), particularly for the measurement of the Southwick's head-shaft angle, were judged immeasurable by at least one observer. Mean head-shaft angle was 11.0° (SD = 17.0), head-neck angle was 8.0° (SD = 12.0), and Murray's tilt-index was 1.18 (SD = 0.4). For head-shaft angle, the mean difference between measurements (Observer 2) was 0.8° (SD = 2.7°, 95% limits of agreement -4.5° to 6.1°), while the corresponding figure for the Murray's tilt-index was 0.02 (SD = 0.08, 95% limits of agreement -0.18 to 0.14), and for the head-neck angle 0.9° (SD = 4.0, 95% limits of agreement of -6.9° to 8.7°). Slightly higher variance was seen for Observer 1 and between the two observers. CONCLUSION Common radiographic measurements for the assessment of a previously slipped capital femoral epiphysis are relatively inaccurate in skeletal mature adolescents, in particular between observers (inter-observer), but also for the same observer (intra-observer). Our results underscore the importance of thorough standardization for both image and measurement technique when used in a clinical setting.