The effect of primary stability on load transfer and bone remodelling within the uncemented resurfaced femur

One of the major causes of aseptic loosening in an uncemented implant is the lack of any attachment between the implant and the bone. The implant's stability depends on a combination of primary stability (mechanical stability) and secondary stability (biological stability). The primary stability may affect the implant-bone interface condition and thus influence the load transfer and mechanical stimuli for bone remodelling in the resurfaced femur. This paper reports the results of a study into the affect of primary stability on load transfer and bone adaptation for an uncemented resurfaced femur. Three-dimensional finite element models were used to simulate the intact and resurfaced femurs and the bone remodelling. As a first step towards assessing the immediate post-operative condition, a debonded interfacial contact condition with varying levels of the friction coefficient (0.4, 0.5, and 0.6) was simulated at the implant-bone interface. Then, using a threshold value of micromotion of 50 microm, the implant-bone interfacial condition was varied along the implant-bone boundary to mechanically represent non-osseointegrated or osseointegrated regions of the interface. The considered applied loading conditions included normal walking and stair climbing. Resurfacing leads to strain shielding in the femoral head (20-75 per cent strain reductions). In immediate post-operative conditions, there was no occurrence of elevated strains in the cancellous bone around the proximal femoral neck-component junction resulting in a lower risk of neck fracture. Predominantly, the micromotions were observed to remain below 50 microm at the implant-bone interface, which represents 97-99 per cent of the interfacial surface area. The predicted micromotions at the implant-bone interface strongly suggest the likelihood of bone ingrowth onto the coated surface of the implant, thereby enhancing implant fixation. For the osseointegrated implant-bone interface, the effect of strain shielding was observed in a considerably greater bone volume in the femoral head as compared to the initial debonded interfacial condition. A 50-80 per cent peri-prosthetic bone density reduction was predicted as compared to the value of the intact femur, indicating bone resorption within the superior resurfaced head. Although primary fixation of the resurfacing component may be achieved, the presence of high strain shielding and peri-prosthetic bone resorption are a major concern.