Developmental dysplasia of the hip (DDH) leads to multiple treatment challenges during adulthood. Surgical treatment is mainly based on radiographic evaluation of the anatomical alterations. Several classification systems have been described in the published English scientific literature, but the French Cochin classification has not been used very much. Its primary advantage lies in its ability to intricately describe the DDH alterations with a large number of grades. We hypothesized that the inter- and intra-observer reproducibility of the SOFCOT-modified Cochin classification system was equal to that of the Crowe and Hartofilakidis classifications.

Five French orthopaedic surgeons who were DDH experts classified 94 A/P pelvis radiographs (179 hips) using the Crowe (Cr), Hartofilikadis (Ha) and modified Cochin (Co) systems. This evaluation was repeated a second time one month later. The intra-observer reproducibility was determined with weighted Kappa and concordance coefficients. The inter-observer reproducibility was performed by calculating the multirater Kappa coefficient on each of the two data series.

For the intra-observer reliability, the average weighed concordance coefficients (95% CI) were 88.62–94.52 for Cr, 89.43–93.80 for Ha and 92.14–95.71 for Co. The average weighed Kappa coefficients (95% CI) were 0.70–0.85 for Cr, 0.67–0.82 for Ha and 0.75–0.83 for Co. For the inter-observer reliability, the Kappa for each assessment round was 0.57 and 0.48 for Cr, 0.43 and 0.44 for Ha, and 0.43 and 0.37 for Co.

The intra- and inter-observer reliability for the modified Cochin classification system is the same as the one for the Crowe and Hartofilakidis classifications. The theoretical advantage of this classification system should be confirmed by comparing the findings with intra-operative anatomical observations.

IV.