Finite Element Analysis of Bone Remodelling Around an Uncemented Resurfaced Femoral Head

Hip resurfacing is a conservative hip joint replacement surgery that, in contrast to a traditional total hip replacement preserves the bone stock in the proximal femur. Traditionally, the femoral component of the hip resurfacing implant is fixed to the bone with a PMMA bone cement. More recently, however, cementless resurfacing designs have been undergoing clinical trials to alleviate the disadvantages of bone cement. Many early design iterations of these implants fail on account of femoral neck narrowing - a suspected consequence of stress shielding in the femoral head and neck. This study investigates the biomechanical cause of this failures with an experimentally validated adaptive finite element model of a proximal human femur. The model was fitted with a commercially available cementless resurfacing implant in order to investigate the effects of stress shielding in the femur through simulation of the bone remodelling in key regions. Three separate clinically relevant cases are investigated: complete fixation of the implant and peg, friction based contact across the implant surface and the peg, and a hybrid arrangement where the implant surface is fixed but the peg experiences frictional contact. Cases in which the inner surface of the implant is fixed to the bony surface demonstrate severe resorption of the bone tissue in the proximal femoral head; particularly in the regions above the implant peg. Fixing the peg has a positive impact in the distal region below the peg, but increases the degree of stress shielding in the upper aspect of the femoral neck and other regions away from the stem. The three tests all show significant narrowing of the cortex above the implant stem. Changing the contact properties has a large influence on the degree and nature of the stress shielding in the femoral head and neck. For the implant design, this translates to careful selection of regions that are to be coated to promote biological fixation through bone in-growth.