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Stress-shielding effect continues to be one of the major factors that cause failure in cementless total hip replacement. How to avoid the stress-shielding effect has become a important issue for orthopedic surgeons and prosthesis design engineers. Therefore, the goal of this study is to use finite element method to investigate the stress-shielding effects of different porous-coating extents on a maximum-canal-fill custom prosthesis. Four different extents of porous coating (un-coated, 1/3 coated, 2/3 coated, fully- coated) with the assumption of fully bone ingrowth to the porous-coated areas were considered. The stress distributions for the four implanted models and the intact femur model were compared and the stress-shielding effects were evaluated. Results showed that for the less coated prosthesis, less stress-shielding effect in the proximal femur, and more stress concentration in the distal region near the tip of the prosthesis were found. But for the un-coated prosthesis, micro-motion and subsidence in the bone-implant interface is likely to occur, so it is not used in practice. On the other hand, for the fully-coated prosthesis, stress concentration will occur in the bone region near the tip of the prosthesis. Therefore, partially coated prostheses are more desirable than the un-coated and the fully-coated prostheses.
In this study, an automatic modular mesh generation procedure was developed to generate the finite element meshes for the cortical and cancellous bone regions in the femur, and the implant respectively. These modular meshes can be merged together to get the implanted femur model. The highly irregular geometry and the non-homogeneous bone materials were obtained directly from the computed tomography image data. By using this modular mesh generation procedure, it can provide a more accurate and faster procedure in the analysis of bone-implant systems.
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