Method

Fifteen consultant Paediatric Orthopaedic surgeons, members of the BOSS Collaborative from different UK centres, were asked to rate a series of 100 healed AP radiographs of hips affected by Perthes' Disease from the Liverpool Perthes' Disease Register using the Stulberg' s grading. Two independent observers categorised roundness error using Digitimizer™ software. Kappa scores were used to determine the inter-observer concordance amongst the 15 observers for Stulberg classification. Lin concordance was used to determine roundness error assessment. The relationship between the two outcomes was explored statistically and graphically; considering the mean Stulberg grade recorded by the 15 observers as a continuous outcome.