The effects of interfacial conditions and stem length on potential failure mechanisms in the uncemented resurfaced femur.

It is hypothesized that changes in stem length and implant-bone interfacial conditions would affect the mechanical environment within the uncemented resurfaced femur, thereby influencing potential short- and long-term failure mechanisms. This study is aimed at investigating the influence of changes in implant-bone interfacial conditions and stem length on eventual failure, using 3D FE models integrated with bone remodeling simulations. Musculoskeletal forces corresponding to normal walking and stair climbing were used as applied loading conditions. Sliding micromotions of 26-72 microm at the implant-bone interfaces for both the stem designs suggest bone ingrowth on the coated surface of the implant was likely. The initial risk of femoral neck fracture was less for the uncemented designs as compared to the cemented designs, irrespective of interfacial conditions and variation in stem length. For the uncemented variety, shortening the stem length provided only slight advantages (5%) with regard to strain shielding and bone remodeling. However, bone resorption was considerably higher when fully bonded interfaces were simulated. It may, therefore, be concluded that cementless fixation seems to be a viable alternative to cemented fixation, provided sufficient initial fixation and secondary stability through bone ingrowth into the coated surface of the implant can be achieved.