Effects of prosthesis stem lengths and tapers on stress distribution in cemented hip arthroplasty

Abstract

Stress shielding and bone remodeling effects are critical issues in considering the biomechanics of femur that has undergone total hip replacement (THR). Stress shielding occurs when local stress distribution in the presence of the prosthesis is lower than that observed with intact femur. In this study, the stress distributions in intact and THR femur are established using finite element method. The THR femur model consists of a cemented hip Ti-6Al-4V prosthesis implanted inside the femoral canal. Major muscle loads and contact forces are simulated for walking (toe-off phase) and stair-climbing conditions that represents 800N of bodyweight. The effects of Charnley’s prosthesis stem lengths and tapers on the resulting stress and strain distributions are investigated. For the stem length cases, results show that tensile stress dominates in the lateral plane while compressive stress in the medial plane of the femur. In the iso-strain condition, greater part of the load to the THR femur is shifted to the stiffer Ti-6Al-4V alloy prosthesis. The stresses in the surface of the cortical bone are relatively low in the central region of the THR femur. The largest magnitude of maximum principal stresses are 24 and 34 MPa for walking and stairclimbing load cases, respectively, for THR femur while the corresponding stress levels for intact femur are 22 and 29 MPa, respectively. For the stem taper cases, the magnitude of Tresca stress for the THR femur in stair-climbing load case remains higher in the region of 85 MPa while the walking load case induces around 40 MPa. The stress range in the straight and single taper stem prosthesis is lower than 260 MPa, while localized Tresca stress is in the order of the yield strength of Ti-6Al-4V alloy for double and triple taper stem design.