No proportional relationship between shape and size of the femoral canal and the external proximal femur morphology in elderly patients

Introduction: The proximal femur morphology changes with age, which may complicate the compatibility of contemporary cementless stem designs in very elderly patients. This study investigated the internal and external proximal femur morphology, correlated canal dimensions with external dimensions, and examined whether age-associated changes in the femoral canal and external morphology are related in subjects aged 80 years and older. Methods: Three-dimensional models of human femora were reconstructed from computed tomographic (CT) scans of 90 very elderly subjects (mean 84 years, range 80–105 years). Morphological parameters describing the location of the femoral head center (FHC) (i.e. neck-shaft angle [NSA], mediolateral offset [ML-offset], and distance between lesser trochanter (LT) and FHC [LT-FHC]) and parameters describing the canal morphology (i.e. the cortices, canal dimensions, and canal flare index [CFI]) were measured. Regression and correlation analyses were performed in order to assess the relation between internal and external morphology. Results: No significant associations regarding dimensions nor geometry between internal and external femur morphology could be detected. Canal dimensions were not able to predict the external dimensions more accurately than the deviation between the individual value and the mean value for the total cohort. Conclusions: Based on these findings, proportional sizing of the cementless femoral component is not necessarily endorsed in very elderly patients, and age-associated changes of the femoral canal and external morphology do not appear to be related. However, further research is needed to evaluate the ability of contemporary non-modular cementless stems to anatomically reconstruct the proximal femur in very elderly patients specifically.