Bone graft incorporation around titanium-alloy- and hydroxyapatite-coated implants in dogs

To evaluate cancellous allogenic bone graft incorporation into porous-coated implants, the fixation of titanium alloy-(Ti) and hydroxyapatite-(HA) coated implants with and without bone graft was compared. An unloaded model with unilateral carragheenin-induced osteopenia of the knee was used in 12 mature dogs. Ti- and HA-coated cylinders were implanted in the distal femoral condyles and centralized in 2-mm overreamed drill holes. Allogenic, fresh-frozen (-80 degrees) cancellous bone graft was packed around the implants in six dogs. In a matched group of six other dogs, the implants were left in overreamed canals without bone graft. After six weeks the interface shear strength of grafted Ti-coated implants had significantly increased compared to the nongrafted Ti implants. However, HA coating used without bone graft was capable of enhancing the bone-implant interface shear strength to nearly the same degree. The fixation of grafted Ti- and HA-coated implants was equal. No significant difference in implant fixation was found between osteopenic and control bone. Histomorphometric evaluation of mineralizing surfaces in direct contact with the implant confirmed the results from the push-out test. Bone-implant fixation when using allogeneic fresh-frozen cancellous bone graft in osteopenic and control bone was enhanced by hydroxyapatite coating but the HA coating alone appeared to offer almost the same improvement in anchorage in 2-mm defects. Loss of bone stock around loose prosthetic implants often requires bone grafting. However, because of anatomic constraints in joint prosthetic surgery, a complete filling of defects with bone graft is difficult, and areas of gaps between bone and implant will remain. Provided mechanical stability of the prosthesis, the results reported here suggest that these areas will probably be filled early with new mineralizing bone if the prosthesis is coated with a thin layer of hydroxyapatite.