Abstract
Introduction: Surgeons in the UK and Europe generally use a thinner cement mantle than their counterparts in the USA for the femoral component in total hip replacement (THR). The aim of this study was to compare the performance of different thicknesses of cement mantle using finite element analysis. The measures by which comparison might be made include cement cracking, subsidence, migration and stress shielding. In this study, we use a linear-elastic model of the implanted femur to give a prediction of the stresses in the cement mantle and in the femoral cortex. These measures give an indication of the relative rates of cement cracking and loss of bone stock due to stress shielding. To assess the reliability of our model in representing patients with different bone densities, we use a range of cancellous bone stiffnesses.
Method: Two cadaveric femora from the same donor were sized, reamed and implanted with identical plastic replica femoral components following standard surgical technique for the Stanmore Hip system. One was prepared using UK rasps, over-reaming by ~2mm, the other using US rasps, over-reaming by ~5mm. Serial CT-scans were used to create three-dimensional geometric models of the implanted femora. Two finite element meshes were hand-built in MSC. Marc finite element software, incorporating cortical and cancellous bone, bone cement and prosthesis. Each model consisted of 10,000 eight-noded brick elements, with a fully bonded stem-cement interface. The thick and thin cement mantles had thicknesses of 2.5mm and 1.0mm respectively, in regions where thickness is affected by rasp size. Models were identical in the distal medullary canal. Cortical bone was modelled as transversely isotropic, with longitudinal and transverse moduli of 17.0 and 11.5 GPa. Bone cement was given a modulus of 2.7 GPa. Loading conditions were chosen to represent the heel-strike phase of gait. In order to assess the impact of variability in patient bone density, cancellous bone modulus was varied between 0.06 and 2.90 GPa.
Results: Equivalent stress was examined on the external surface of the cortex and the internal surface of the cement mantle. The lowest cortical bone stresses were proximal and the highest cement stresses around the distal tip of the prosthesis. In the proximal cortex, higher equivalent stresses were observed medially and laterally with a thick cement mantle. Distally, lower cement stresses were observed in the thick cement mantle. With the highest cancellous modulus, there was little difference between the two models. As this modulus was reduced, stress differences between the models became more apparent. For all cancellous bone moduli, peak distal cement stresses were lower and minimum proximal calcar stresses higher in the thick cement mantle.
Discussion: Proximal stress shielding was greatest in the calcar, in agreement with clinical findings. The thicker cement mantle led to less stress shielding in this region. Cement stresses, highest around the distal tip of the prosthesis, were larger in the thin cement mantle. This suggests a higher rate of both cracking and bone resorption in thin cement mantles. Although observed over a range of cancellous bone stiffness, this finding applies particularly to patients with low bone density.
Correspondence should be addressed to Dr Carlos Wigderowitz, Honorary Secretary of BORS, Division of Surgery & Oncology, Section of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School Tort Centre, Dundee, DD1 9SY.
