Influence of size and CCD-angle of a short stem hip arthroplasty on strain patterns of the proximal femur - an experimental study.

PURPOSE The number of primary total hip arthroplasties (THA) is steadily increasing. Over the last decade numerous so-called short stem hip arthroplasties were introduced on the market. The aim of these implants with a predominantly metaphyseal anchorage is to reduce stress shielding and thereby the risk of aseptic loosening. One of the short stem arthroplasties with predominant metaphyseal fixation is the METHA® short stem (Aesculap, Tuttlingen, Germany). In order to reconstruct the biomechanics the METHA stem is available in different sizes with different centrum-collum-diaphysis-angles (CCD-angle). In this study, we want to address the research question of how the size of the implant and different CCD-angles influence the strain patterns of the proximal femur. METHODS Three different stem sizes (size 2, 3 and 4 - CCD-angle 130°) and three stems with different CCD-angles (size 3 - 120°, 130° and 135° CCD-angle) were successively implanted in a synthetic femur. Eight strain gauges monitored the corresponding strain patterns of the proximal femur. RESULTS Independent of stem size and CCD-angle only small changes in the strains were recorded around the distal part of the METHA stem when compared to the intact femur. However, all stems increased the strains in the region of the calcar. This was most pronounced by smaller CCD-angles and major sizes. CONCLUSION The stem size and CCD-angle primarily influence the region of the calcar. Greater sizes and smaller CCD-angles lead to increased strains at the calcar. The other regions are hardly influenced by the stem size and CCDangle of the femoral component.