Implant loosening is associated with inflammatory bone loss induced from ultra-high molecular weight polyethylene debris. We hypothesized that the targeted delivery of bisphosphonates, which are commonly used antiresorptive agents, would improve periprosthetic bone quality and osseous integration of an intramedullary implant even in the presence of ultra-high molecular weight polyethylene particles of a size and distribution comparable with wear particles in vivo. In an experimental rat femur model, our findings of higher bone mass and better implant fixation in the study group of hydroxyapatite-zoledronate-coated implants compared with the control group with hydroxyapatite-coated implants without zoledronate support our hypothesis.
Zoledronate has been found to be a potent inhibitor of osteoclastogenesis13
. It also has a direct effect on bone mineralization by enhancing osteoblastogenesis14,15
. A concern when bisphosphonates are used as a coating material is the possible inhibition of osteoblastic activity and osseointegration since bisphosphonates may reach a high local concentration. Our in vitro study data showed that this undesirable effect is actually concentration-dependent and avoidable. While zoledronate effectively induced cell death in osteoclast precursor cells at low concentrations, concentrations of >50 μM caused substantial apoptosis of osteoblasts as well. For this study, a 50-μM concentration was found to be optimal for inducing apoptosis in osteoclasts and macrophages, while minimizing osteoblast cell death8,16
. Our in vitro study data also showed that zoledronate did not hinder the proliferation and maturation of human mesenchymal stem cells. This study showed that the number of alkaline phosphatase-positive cells, a marker for mature osteoblasts, did not exhibit any zoledronate dose dependency.
There have been recent attempts to improve implant fixation by the application of bisphosphonate both systemically17
and locally by bonding it to hydroxyapatite-coated implants8
. Bisphosphonates are known to bind avidly to hydroxyapatite18,19
. In a previous study, implants dipped in a low concentration of zoledronate had an increase in pullout force of up to 42% compared with implants without zoledronate8
. However,those studies considered only the initial integration between the bone and the implant. In the in vivo situation, implant loosening is frequently associated with osteolysis, even after achieving initial integration, because of wear debris particles that initiate an inflammatory response from the host over time and cause a degradation of osseointegration. Bisphosphonates have been shown to reduce particle-induced osteolysis and implant loosening in animal models20,21
and in humans22
by preventing osteoclastogenesis6,7,23
For this study, dual x-ray absorptiometry imaging was performed and encompassed the entire femur. Both zoledronate treatment and time were found to significantly increase the bone content and density. However, these effects were not as pronounced as those measured by microcomputed tomographic imaging because the dual x-ray absorptiometry scan measured an average for the entire femur, whereas our post-processing analysis of the microcomputed tomographic scan measured just bone immediately adjacent to the implant. No difference between the left and right femora was found with regard to bone area, bone mineral content, or bone mineral density, which implies that the material type of the nail, either stainless steel or ultra-high molecular weight polyethylene, had no effect on the response of the bone or on the dual x-ray absorptiometry imaging modality.
The area of peri-implant trabecular bone for the hydroxyapatite-zoledronate treatment group was more than double that of the hydroxyapatite control group as determined from the microcomputed tomography measurements. This could be the result of either increased new bone formation or reduced osteolysis in the zoledronate-treated group. However, previously established experimental models, such as the mouse calvarial osteolysis model24
, have suggested that the ultra-high molecular weight polyethylene particles inserted into the femoral canal cause osteolysis. Hence, the increased bone area is most likely due to reduced osteolysis in the study group. This suggests that zoledronate reduced the osteolysis induced by ultra-high molecular weight polyethylene particles in this model. After six months, the area of periprosthetic trabecular bone was still 86% higher for the treated group compared with that of the six-week group, suggesting that zoledronate may have a long-term effect, possibly by a slow release from hydroxyapatite. Although microcomputed tomography is not as accurate as the histomorphometric method in measuring mineralized area, it is a better tool for measuring the mineral density and mineral content of newly formed tissue than is histomorphometry. In addition, microcomputed tomography is simple and a good tool for obtaining an overall value of mineralized area at the implant interfaces.
Pullout testing demonstrated that zoledronate improves the strength of implant fixation, which has been correlated to periprosthetic bone mass. The hydroxyapatite-zoledronate group required significantly higher peak pullout force and pullout energy compared with the control group. We can conclude that the topical addition of zoledronate enhanced osseointegration of the implant in the presence of ultra-high molecular weight polyethylene particles.
It is also interesting to note that, in our study, the peak pullout force was significantly greater in the six-month femora than in the six-week femora. Other investigators have found no significant difference in pullout force for study periods of greater than six weeks because osseous integration of the implant to the host bone was complete by six weeks25
. Consistent with the above observation, the control group in our study did not show any significant difference in pullout force between the six-week femora and the six-month femora. Conversely, there was an increase in the peak pullout force from six weeks to six months in the hydroxyapatite-zoledronate-treated femora. This may imply that simply soaking hydroxyapatite-coated implants in a zoledronate solution has a long-term beneficial effect. However, later loosening may occur because of possible zoledronate-related impaired bone-remodeling at the implant surface, and long-term studies will be needed to assess this effect. The large variance in the energy-to-pullout measurements may be explained by the large variation in the amount and pattern of bone remaining on the nails, suggesting variable osseous ingrowth.
The primary limitation of this study was the lack of a continuous supply of ultra-high molecular weight polyethylene particles into the femoral canal, which would have more closely mimicked the clinical setting. In our model, it is likely that the number of ultra-high molecular weight polyethylene particles in the femoral canal decreased over time. We do not know whether the beneficial effects of adding zoledronate to hydroxyapatite-coated implants would have been reduced had there been a continuous supply of ultra-high molecular weight polyethylene particles. Another limitation of the study is the use of the rat femur model itself since its physiologic environment and bone-remodeling potential may be different from those of humans. A previous study has described a turnover rate of trabecular bone in twelve-week-old rats similar to the rate in adult human iliac crest trabecular bone26
. However, these results were dependent on the age of the rat, with younger rats having faster bone-remodeling and, furthermore, the pathologic state of inflammatory osteolysis was not assessed. Given these limitations in our current knowledge, clinical trials are necessary to evaluate whether a similar reduction in the rate of osteolysis is achieved in humans. Another potential limitation is the use of hydroxyapatite-coated ultra-high molecular weight polyethylene nails. These nails have different biomechanical properties compared with the hydroxyapatite-coated stainless steel nails. The ultra-high molecular weight polyethylene nails may bend more, load the surrounding bone differently, and affect the amount and type of bone formation around the implant. However, the analysis with dual x-ray absorptiometry, in which the femora with both types of nail were compared, found no significant difference in bone density between them. The ultra-high molecular weight polyethylene nails may also bind hydroxyapatite differently—the hydroxyapatite could loosen from the nails leaving behind a smooth surface not conducive to osseous integration. However, given that the mechanical results (with metal nails) and the microcomputed tomographic results (with ultra-high molecular weight polyethylene nails) showed no significant differences, it is unlikely that this is a major problem.
In summary, site-specific delivery of bisphosphonate by means of hydroxyapatite-coated implants resulted in higher bone mass and better biomechanical performance, even in the presence of ultra-high molecular weight polyethylene particles. Our data suggest that hydroxyapatite alone, as a coating on a stainless steel nail, has limited capacity in maintaining periprosthetic bone mass in the presence of ultra-high molecular weight polyethylene particles. Although the results of the present study support the concept that hydroxyapatite-bisphosphonate improves osseointegration, studies of larger animals and clinical trials will be necessary to verify the beneficial effects of biological modification of the implant-host bone interface.