Ottawa classification for symptomatic acetabular dysplasia: assessment of interobserver and intraobserver reliability

Aims The aim of the current study was to assess the reliability of the Ottawa classification for symptomatic acetabular dysplasia. Methods In all, 134 consecutive hips that underwent periacetabular osteotomy were categorized using a validated software (Hip2Norm) into four categories of normal, lateral/global, anterior, or posterior. A total of 74 cases were selected for reliability analysis, and these included 44 dysplastic and 30 normal hips. A group of six blinded fellowship-trained raters, provided with the classification system, looked at these radiographs at two separate timepoints to classify the hips using standard radiological measurements. Thereafter, a consensus meeting was held where a modified flow diagram was devised, before a third reading by four raters using a separate set of 74 radiographs took place. Results Intrarater results per surgeon between Time 1 and Time 2 showed substantial to almost perfect agreement among the raters (Kappa = 0.416 to 0.873). With respect to inter-rater reliability, at Time 1 and Time 2 there was substantial agreement overall between all surgeons (Time 1 Kappa = 0.619; Time 2 Kappa = 0.623). Posterior and anterior rating categories had moderate and fair agreement at Time 1 (posterior Kappa = 0.557; anterior Kappa = 0.438) and Time 2 (posterior Kappa = 0.506; anterior Kappa = 0.250), respectively. At Time 3, overall reliability (Kappa = 0.687) and posterior and anterior reliability (posterior Kappa = 0.579; anterior Kappa = 0.521) improved from Time 1 and Time 2. Conclusion The Ottawa classification system provides a reliable way to identify three categories of acetabular dysplasia that are well-aligned with surgical management. The term 'borderline dysplasia' should no longer be used.