['Aseptic loosening of orthopaedic implants secondary to wear debris-induced osteolysis is a serious problem. Osteoprotegerin (OPG) is a natural decoy protein that inhibits osteoclast activation and bone resorption. This study investigated whether gene therapy using a recombinant adeno-associated viral vector that expresses OPG can inhibit wear debris-induced osteolysis.', 'A recombinant adeno-associated virus (rAAV) vector co-expressing OPG (rAAV-OPG-IRES-EGFP) was generated. A control vector expressing b-galactosidase (rAAV-LacZ) was also prepared. In vitro validation experiments were performed to determine rAAV-OPG-IRES-EGFP transduction efficiency, OPG expression level and function in bone wafer, and osteoclastic activity. The effect of rAAV-OPG-IRES-EGFP in vivo gene therapy on wear debris-induced osteolysis was then evaluated in a mouse calvarial model in which a single intramuscular injection of the vector was administered prior to the introduction of the wear debris. The effects of the rAAV-OPG-IRES-EGFP gene therapy on wear debris-induced osteoclastogenesis and bone resorption were determined by histomorphometry on day 10.', 'In vitro experiments revealed that 100% of human embryonic kidney 293 cells were transduced at a multiplicity of infection of 1000 with both rAAV-OPG-IRES-EGFP and rAAV-LacZ. At a rAAV-OPG-IRES-EGFP multiplicity of infection of 1000, an OPG concentration of 135 ng/mL of culture media was achieved after four days. Using a bone-wafer assay for osteoclast activity, we found that treatment with rAAV-OPG-IRES-EGFP reduced resorption sevenfold compared with parathyroid hormone-stimulated controls and elevenfold compared with rAAV-LacZ controls. Furthermore, a seventeenfold decrease in RANKL and macrophage colony-stimulating factor-induced splenocyte osteoclastogenesis was observed in co-cultures containing rAAV-OPG-IRES-EGFP-infected fibroblasts. In vivo administration of rAAV-OPG-IRES-EGFP resulted in detectable transduction of myocytes at the injection site and a significant increase in expression of serum OPG levels by the second day (p < 0.05). Maximal concentrations were obtained on day 6 and then leveled off throughout the observation period. In contrast, serum OPG could not be detected in the sham-treated, uninfected titanium-stimulated, or rAAV-LacZ-infected mice. In the control mice, titanium implantation resulted in a threefold increase in the mean number of osteoclasts adjacent to the sagittal suture as well as a twofold increase in the mean area of soft tissue in the sagittal suture compared with the sham-treated mice. In contrast, osteoclast numbers remained at basal levels, and the area of soft tissue in the sagittal suture was markedly reduced in titanium-implanted animals that received rAAV-OPG-IRES-EGFP treatment, demonstrating a complete inhibition of osteolysis in response to titanium particles.'] Does recombinant adeno-associated virus-mediated osteoprotegerin gene therapy inhibit wear debris-induced osteolysis?