A few reports have shown that the reliability of the Japanese Orthopaedic Association stage classification of hip osteoarthritis was not high. The objective of this study was to assess the reliability of the stage classification of coxarthrosis, and to evaluate whether a modification of the classification improves reliability.
We retrospectively investigated 200 hips in 100 patients with hip pain. We collected radiographs of their hip joints with the patients in both a supine and a standing position. Four orthopaedic surgeons evaluated the stage of coxarthrosis employing the JOA classification. The percentage of agreement of examiners and the value of the kappa statistic were calculated. Furthermore, to improve the reliability of classification, we modified the classification based on previous reports. Partial narrowing of the joint space and disappearance of the joint space were defined as maintained if it was 2 mm or more, and as the width of the loss of joint space that was 10 mm or more respectively. Using this classification, the same examiners assessed the stage on the same radiographs again three months after the previous evaluation.
The percentages of agreement were 28.5% and 27% and the interobserver value of the kappa statistic was 0.45 and 0.44 in supine and standing position respectively. After modification of the classification, the percentages of agreement were 36.5% and 44% and the interobserver value of the kappa statistic was 0.48 and 0.56 in supine and standing positions respectively, and the intraobserver value of the kappa statistic was 0.55. The modification significantly improved the reliability only in the percentage of the agreement for the standing position.
This study showed that the reliability of the JOA stage classification of coxarthrosis was not as high as previous reports have showed. Modification of the classification improved interobserver reliability.