Although total hip arthroplasty is a very successful operation, complications such as: dislocation, aseptic loosening, and periprosthetic fracture do occur. These aspects have been studied in large populations for traditional stem designs, but not for more recent short stems. The design rationale of short stems is to preserve bone stock, without compromising stability. However, due to their smaller bone contact area, high peak stresses and areas of stress shielding could appear in the proximal femur, especially in the presence of atypical bone geometries. In order to evaluate this aspect, we quantified the stress distribution in atypical proximal femurs implanted with a commercially available calcar guided short stem. Geometrical shape variations in neck-shaft angle (NSA), neck-length (NL) and anteversion (AV), were determined three-dimensionally in the Mimics Innovation Suite (Materialise N.V., Leuven, Belgium) from a CT dataset of 96 segmented femurs. For each shape variation, the femurs that had the two lowest, two average and two highest values were included (18 femurs). Using scripting functionality in Mimics, CAD design files of the calcar guided Optimys short stem (Mathys, Bettlach, Switzerland) were automatically sized and aligned to restore the anatomical hip rotation center. Stem size and position were manually corrected by an orthopedic surgeon before finite element (FE) models were constructed using a non-manifold assembly approach (Figure 1). Material properties were estimated from the CT dataset and loads representing walking and stair climbing were applied [1]. Stress-shielding was evaluated by the change in average strain energy density pre- and post-operatively in three different regions (calcar, midstem, tip) each being subdivided in four quarters (medial, lateral, anterior, posterior) (Figure 2).Introduction
Methods
The bowing of the femur defines a curvature plane to which the proximal and distal femoral anatomic landmarks have a predictable interrelationship. This plane can be a helpful adjunct for computer navigation to define the pre-operative, non-diseased anatomy of the femur and more particularly the rotational alignment of the femoral component in total knee arthroplasty (TKA). There is very limited knowledge with regards to the sagittal curvature -or bowing- of the femur. It was our aim (1) to determine the most accurate assessment technique to define the femoral bowing, (2) to define the relationships of the curvature plane relative to proximal and distal anatomic landmarks and (3) to assess the position of femoral components of a TKA relative to the femoral bowing.Summary sentence
Background and aims
