The determination of variation in proximal femoral geometry and shape in primary hip osteoarthritis using active shape modelling
Keywords: arthroplasty
shape modelling
hip
osteoarthritis

The success of cementless femoral reconstruction in contemporary total hip arthroplasty is dependent on both an adequate endosteal stem fit and the precise restoration of physiologic joint mechanics. With regard to the rising popularity of cementless fixation, modularity and short stem designs, accurate pre-operative templating has become essential as small inaccuracies in the assessment of proximal femoral shape and geometry may compromise the clinical outcome. In addition, the current knowledge of variation in proximal femoral geometry and canal shape in arthritic hips is limited, and the association of geometry and canal shape is poorly understood. Therefore, the broad aims of this thesis were (1) to improve pre-operative planning in hip arthroplasty and (2) to provide information for future femoral component design accounting for individual patient anatomy.
Based on a large (n=345) retrospective cohort of patients with primary end-stage hip osteoarthritis the differences in the assessment of femoral offset (FO) on corresponding AP pelvis radiographs, AP hip radiographs and CT scans were examined. FO was used as a surrogate parameter to represent proximal femoral geometry. Active Shape Modelling (ASM) was performed to assess differences in the radiographic appearance of the proximal femoral canal on corresponding modes of imaging, and to determine the variation and underlying patterns in proximal femoral anatomy.
It was demonstrated that - in contrast to AP pelvis radiographs - measurements on standardised AP hip radiographs provide a more accurate and reliable assessment of FO and canal shape of the proximal femur, comparable to 3D measurements performed on CT. A linear relationship between FO measurements on corresponding modes of imaging was observed which allowed gender specific regression equations to be established in order to predict 3D FO from AP pelvis radiographs. ASM revealed an association between endosteal canal shape and joint geometry. Ten distinct shape patterns of the proximal femoral canal were identified and each pattern was associated with a specific set of geometric parameters. Variations in canal shape shape were independent of femoral head size, BMI, gender or age.
In conclusion, orthopaedic surgeons need to be aware that the assessment of proximal femoral geometry and canal shape may be misleading on AP pelvis radiographs. Standardised AP hip radiographs are a practical, cost-effective and readily available method which allows accurate and reliable assessment of joint geometry and canal shape without the routine performance of CT. When AP hip radiographs are not available for templating, the provided regression equations can be used to compensate for the adverse effects of positioning inconsistencies and beam divergence in the assessment of FO on AP pelvis radiographs. The present findings have the potential to aid surgeons in selecting an appropriate implant design and size, and assist in planning the intra-operative position of femoral components.
The findings further provide a comprehensive classification of proximal femoral geometry and shape in patients with primary hip OA. The identification of distinct shape patterns has implications for future cementless femoral component designs that aim for optimal metaphyseal stem fit and concomitant restoration of individual patient anatomy.