How many different types of femora are there in primary hip osteoarthritis? An active shape modeling study

PURPOSE
To assess the variation in proximal femoral canal shape and its association with geometric
and demographic parameters in primary hip OA.
METHODS
In a retrospective cohort study, the joint geometry of the proximal femur (femoral offset (FO); neck-shaft-angle (NSA); femoral anteversion (FA)) was assessed on radiographs and corresponding CT scans of 345 consecutive patients with primary hip OA. Point-based active shape modelling (ASM) was performed to assess the variation in shape of the proximal femoral canal. To identify natural groupings of patients, hierarchical cluster analysis of the
shape modes was used. Differences in geometric measures of the proximal femur and demographic parameters between the clusters were evaluated.
RESULTS
ASM identified 10 independent shape modes accounting for over 96% of the variation in proximal femoral canal shape within the dataset. Greatest variation in proximal femoral canal shape occurred in the metaphysis and the medial cortex. Cluster analysis revealed 10 specific shape clusters. Significant differences in geometric and demographic parameters
between the clusters were observed.
CONCLUSIONS
ASM and subsequent cluster analysis have the potential to identify specific morphological patterns of the proximal femur despite the overall variability in proximal femoral anatomy. The study identified 10 distinct patterns of proximal femoral canal shape in end-stage hip OA which allow a comprehensive classification of variation in proximal femoral shape and its association with joint geometry. The present data may improve future designs of cementless femoral components that will optimise stem fit and simultaneously allow individual restoration of physiological biomechanics of the hip.