Recognition of minor adult hip dysplasia: which anatomical indices are important?

The rise in popularity of hip arthroscopy has led to a renewed interest in mild hip dysplasia. There is a lack of clarity in the literature regarding both the diagnosis and management of such patients. The aim of this study was to analyse the relative importance of and the inter-relationship between the classically described anatomical indices of dysplasia.One hundred and fifty hips with varying degrees of hip dysplasia were studied. The following were measured: centre-edge (CE), Sharp's and Tönnis angles, acetabular head index (AHI), and acetabular index of depth to width (AIDW). Spearman's correlation coefficient was calculated.Using the CE angle 82 hips were classified as normal and 68 dysplastic. Of the 82 patients with a normal CE angle, 20-39% were dysplastic on at least one other variable. The CE angle did not have a significant correlation to other variables. The remaining four variables showed inter-correlations between 0.26 and 0.54. Overall the Tönnis angle showed the strongest correlation with the other variables. In the patient group with CE angles 21o to 25o (minor dysplasia) 72% of hips had Tönnis angles greater than 10o and 28% had angles greater than or equal to 15o indicating the great variability in the level of dysplasia within this group.In patients with mild dysplasia we have shown that measurement of a single anatomical variable may lead to under-diagnosis. We recommend the measurement the CE angle combined with at least one other variable and suggest the use of the Tönnis angle.