Anabolic effects of bisphosphonates on peri-implant bone stock

The long-term durability of total joint replacements is critically dependent on adequate peri-implant bone stock, which can be compromised by wear debris-mediated osteolysis. This study investigated the effects of bisphosphonates on enhancing peri-implant bone in the presence of clinically relevant ultra-high molecular weight polyethylene (UHMWPE) wear debris. Fiber-mesh coated titanium-alloy plugs were implanted bilaterally in the femoral condyles of 36 New Zealand white rabbits. Implants in the left femora were covered with submicron UHMWPE particles during surgery. Rabbits were administered either no drug, subcutaneous alendronate weekly (1.0 mg/kg/week) or a single dose of intravenous zoledronate (0.015 mg/kg). A total of 6/12 rabbits in each group were sacrificed at 6 weeks and the remainder at 12 weeks postoperatively. Peri-implant bone stock was analyzed radiographically and histomorphometrically. Radiographically, both bisphosphonates significantly increased periprosthetic cortical thickness at 6 weeks (p<0.0001; alendronate: +18%; zoledronate: +11%) and at 12 weeks (p=0.001; alendronate: +17%; zoledronate:+19%). Histomorphometrically, alendronate and zoledronate raised peri-implant bone volume (BV/TV) up to 2-fold after 6 weeks without added wear debris and more than 3-fold when wear debris was present. Furthermore a 6-week bisphosphonate treatment increased osteoid thickness in the absence of wear debris (alendronate: +132%, p=0.007; zoledronate: +67%, p=0.51) and in the presence of wear debris (alendronate: +134%, p=0.023; zoledronate: +138%, p=0.016). In summary, alendronate and zoledronate treatment increased periprosthetic bone stock in a rabbit femoral model, particularly in the presence of UHMWPE wear debris. These new findings suggest that bisphosphonates may more than compensate for the well-documented negative effects of wear debris on peri-implant bone stock. The combined antiresorptive and osteoanabolic effects of bisphosphonates on periprosthetic bone stock may have an important role for critically improving the biological fixation and ultimate durability of total joint arthroplasty.