Bone augmentation around and within porous implants by local bisphosphonate elution

The bisphosphonate zoledronic acid chemically and physically was bound to hydroxyapatite-coated porous tantalum implants. The zoledronic acid elution characteristics in saline were determined as a function of time and the in vivo effects of elution were quantified at 12 weeks in a canine ulnar implant model. Intramedullary implants surgically were implanted bilaterally into the ulnae of a control group of five dogs and a zoledronic acid-dosed (0.05 mg zoledronic acid) group of four dogs. Computerized image analysis of undecalcified histologic sections was used to quantify the amount of peri-implant bone within the intramedullary canal, the percentage of available pore space filled with new bone, and the number and size of the individual bone islands within the implant pores. The data were analyzed using a hierarchical analysis of variance with 95% confidence intervals. The peri-implant bone occupied a mean of 13.8% of the canal space in controls and 32.2% of the canal space in zoledronic acid-dosed dogs, a relative difference of 134% (2.34-fold) that was significant. The mean extent of bone ingrowth was 12.5% for the control implants and 19.8% for the zoledronic acid-dosed dogs, a relative difference of 58% that was statistically significant. Individual islands of new bone formation with the implant pores were similar in number in both implant groups but were 71% larger on average in the ZA-dosed group. We are the first authors to show that local elution of a bisphosphonate can cause substantial bone augmentation around and within porous orthopaedic implants. The concept represents a potential tool for restoration of bone stock and enhancement of implant fixation in primary and revision cementless joint arthroplasty surgeries in the face of compromised or deficient bone.