Adaptive bone remodeling and biomechanical design considerations for noncemented total hip arthroplasty

Clinical problems with noncemented total hip arthroplasty (THA) stems, directly or indirectly related to load transfer, include mid-thigh pain due to relative (micro) motions or excessive endosteal interface stresses, subsidence and loosening due to inadequate primary stability and fit, and proximal femoral bone atrophy due to stress shielding. In this article, the load-transfer mechanisms associated with noncemented THA stems and their resulting stress patterns are discussed in relation to design features, bonding characteristics, and materials choice. Nonlinear finite-element models and computer simulation programs for strain-adaptive bone remodeling have been used for this study. Canal-filling, fully bonded metal stems have been found likely to cause proximal bone atrophy, possibly leading to long-term failure of the implant/bone composite. The use of flexible (isoelastic) materials and/or press-fit fixation reduces stress shielding, but also reduces the potential for interface stability. The stem material, the stem shape, and the coating geometry interact in relation to the load-transfer mechanism, and it is suggested that optimal combinations of these characteristics can be determined through the computer simulation methods presented.