EFFECTS OF IMPLANT MATERIAL PROPERTIES AND IMPLANT-BONE BONDING ON STRESS SHIELDING IN CEMENTLESS TOTAL HIP-ARTHROPLASTY

Bone loss around total hip replacement femoral components is one of the problems threatening the long-term fixation of these implants. In particular uncemented implants are threatened by this problem, because they are usually larger and as a consequence stiffer than cemented implants. It is generally believed that this phenomenon has a mechanical basis and that it results from reduced stresses in the bone (stress shielding). In this present study the mechanical consequences of different femoral stem materials were investigated using the finite element method. The effects of prosthesis-bone bonding characteristics were also investigated. A number of cases were analysed using the finite element method, where the coating conditions (fully, or partly) were changed for stems of different materials. The results indicate that for a fully coated (bonded) prosthesis, a doubling of the proximal bone stress magnitude may be obtained by employing the lower modulus implant. In the case of the partly coated stem, the bone stresses are similar for both stem materials and much more physiological except for the most proximal region. We conclude that for full implant-bone bonding it is advantageous to use the lower modulus material to reduce the stress shielding effect. More physiological stresses can be expected for partly coated stems independent of the stem modulus, however at the likely expense of much greater relative micromotion between the implant and bone.