The morphology of the proximal femur in the very elderly

Abstract

Introduction. The population of the very elderly (i.e. ≥80 years of age) is growing rapidly due to demographic changes, an increased life expectancy, in particular. As a consequence, a 170% increase in the number of performed total hip arthroplasties (THAs) is projected for the next decades. Previous studies described the age-related changes in the morphology of the proximal femur, however, almost exclusively in subjects <80 years of age and using 2D X-rays. In THA the emphasis lies on reconstruction of the center of rotation (CoR) in order to preserve leg length, leg rotation and muscle tension (more precise: lever arm). The present study focuses on the reconstruction of the center of rotation (CoR) in the very elderly, with emphasis on the neck-shaft angle (NSA), the femoral neck anteversion angle (FNAA) and the mediolateral offset (ML offset). Furthermore, the effect of patient demographics such as gender, length and weight on the internal and external morphology of the proximal femur, the relationships between dimensions and angles on the proximal femur, and the effect of the internal morphology of the femoral canal on parameters describing the external geometry of the proximal femur were investigated. Methods. Ninety very elderly subjects (avg. 84.2 ± 3.6 years, range 80-105 years, 50M/40F) got a high-resolution CT-scan of their right femur (voxel size 1x1x1mm). Cortical bone models were derived from these scans using a specialized threshold algorithm in Materialise Mimics v10 and were further processed in Inus Technology Rapidform 2006. In order to classify the proximal femoral canal shape in stovepipe, normal or champagne flute the canal flare index (CFI) was measured according to Noble et al. Results. Compared to previous studies, the mean NSA (125.1 ± 5.5) and FNAA (10.0 ± 5.6) values found in the present study did not show notable differences. Based on the coronal CFI, the studied femora can be divided into stovepipe (23.3%) and non-stovepipe (76.7%). This percentage of stovepipe shaped proximal femoral canals is high compared to literature about younger populations. There are no differences in the external geometry between femora with a stovepipe or a non-stovepipe shaped canal. Furthermore, the FHD and the CoR-LTabsolute proved to be positively correlated (r=0.449) in the current study, the CoR-LTabsolute/FHD-ratio showed to be 1.17. The distance between the lesser trochanter and the center of rotation was both absolute (59.54 ± 5.76mm vs. 54.85 ± 6.31mm) and vertical (44.37 ± 6.77mm vs. 38.95 ± 7.41mm) larger in males than in females. Discussion. Based on the current study, no alternations in the external geometry of the current hip stem have to be made for an optimal elderly hip stem. However, due to advanced osteoporosis, this population of very elderly subjects contained more stovepipe shaped proximal canals based on their CFI, than younger populations. This has implications for hip stem design in the elderly. Namely, the proximal canal in which the hip stem is fixated is widened, while the external morphology remains the same. A hip stem with a thicker part for fixating in the shaft and the same external features as the currently used hip stem seems thus optimal in the very elderly (≥80yrs). Furthermore, a new equation for determining the head height, based on the diameter of the femoral head is suggested, which may be useful in hemiarthroplasty.