Fixation of titanium and hydroxyapatitecoated implants in arthritic osteopenic bone

Retrieval studies of porous-coated prostheses have demonstrated deficient bony ingrowth in high percentages. Possible reasons for this are lack of initial mechanical stability and the presence of osteopenia. The authors studied ingrowth of osteopenic bone into titanium alloy (Ti) porous-coated implants with and without hydroxyapatite (HA) coating in an experimental dog model. Unilateral osteopenia of the knee with a 20% reduced bone density as judged by computed tomography (CT) scanning (P<.001) was induced in 12 mature dogs by weekly intraarticular injections of Carragheenin into the right knee for 12 weeks, with the left knee serving as control. Ti porous-coated cylinders were inserted in press-fit bilaterally in the lateral femoral condyles in six dogs. HA-coated titanium plugs were implanted similarly in another sex-, age-, and weight-matched group of six dogs. Bony ingrowth after 4 weeks was significantly reduced for Ti implants in osteopenic bone compared to control bone, but HA-coated implants were covered by equal amounts of bone tissue. Bone-implant shear strength of Ti implants also was reduced in osteopenic bone compared to control bone. In control bone, the anchorage of Ti implants was stronger than HA-coated implants, whereas the fixation of Ti and HA-coated implants was equal in the osteopenic bone. The results demonstrate that the bony fixation of Ti porous-coated implants is weakened by the presence of experimentally induced osteopenia. However, the fixation of HA-coated implants was not affected by the osteopenic condition in the surrounding bone. The fixation of Ti and HA-coated implants was equal in osteopenic bone, whereas the fixation of Ti porous-coated implants was superior to that of HA-coated implants in control bone.