A New Classification System for the Adult Dysplastic Hip Requiring Total Hip Arthroplasty: A Reliability Study

Current classification systems used for developmental dysplasia of the hip (DDH) in adult patients about to undergo total hip arthroplasty use a combined assessment of the acetabulum and femur which ignores femoral geometry and problems related to reconstruction of leg length. Currently accepted systems are those of Crowe and Hartofilakidis; it is our opinion that they do not predict surgical difficulties encountered at total hip arthroplasty. We describe a new classification system for adult DDH that divides the acetabulum and femur into separate components. The acetabular classification comprises: AI: Dysplastic acetabulum; AII: The acetabulum associated with a low femoral dislocation; AIII: The post-surgical acetabulum, with (AIIIa) or without retained metalwork (AIIIb). The femoral classification consists of: FI: Dysplastic femur but contained within true or low acetabulum; FII: The high femur; FIII: Post-surgical femur, again with or without metalwork (FIIIa and FIIIb). 50 pre-operative radiographs of hips with DDH about to undergo total hip replacement were assessed by orthopaedic consultants, registrars and medical students. They were classified using the new system, Crowe and Hartofilakidis systems. Interobserver and intraobserever reliability was assessed using Fleiss’ kappa coefficient with combined (acetabulum and femur) kappa scores for the new system of 0.69 for interobserver and 0.67 for intraobserver reliability. This equated to ‘substantial agreement’ according to Landis and Koch and the new system showed at least comparable levels of reliability to the Hartofilakidis and Crowe systems. We have demonstrated reproducibility of our new classification system for DDH in the adult population and believe it could provide useful information when planning arthroplasty and be used to predict technical difficulties and outcome.