THE INFLUENCE OF A HYDROXYAPATITE AND TRICALCIUM-PHOSPHATE COATING ON BONE-GROWTH INTO TITANIUM FIBER-METAL IMPLANTS

A study was done in rabbits to determine the effect of a hydroxyapatite and tricalcium-phosphate. coating on bone growth into titanium fiber-metal im plants. Titanium fiber rods with a solid titanium core were implanted bilaterally into the distal aspect of the femora of fifty-five New Zealand White rabbits. One rod was uncoated and the other rod was surface-coated with hydroxyapatite and tricalcium phosphate by the plasma-spray technique. Thirty-five rabbits were labeled sequentially with fluorochromes; killed at one, two, three, four, six, twelve, or twenty-four weeks after the operation; and studied histologically and histomorphometrically. The implants in the remaining twenty rabbits were subjected to pull-out testing to determine the shear strength at the implant-bone interface at three, six, twelve, and twenty-four weeks after the operation. Histomorphometry revealed significant effects of the hydroxyapatite and tricalcium-phosphate coating. When whole-group means (which included all time-points) were compared, it was found that 44 per cent of the perimeter of the hydroxyapatite and tricalcium-phosphate-coated implants was covered with bonecompared with 12 per cent of the perimeter of the uncoated implants. The percentage of the internal surface of the implant that was covered with bone was also significantly higher in the hydroxyapatite and tricalcium-phosphate-coated implants: 27 per cent of the internal surface of the coated implants was covered compared with 8 per cent in the uncoated implants. The amount of bone in the pores of the implants was also higher in the hydroxyapatite and tricalcium-phosphate-coated implants: 12 per cent of the available pore space in the hydroxyapatite and tricalcium-phosphate-coated implants was filled with bone compared with 4 per cent in the uncoated implants. Scanning electron microscopy of the implants, done in backscatter mode, demonstrated apposition of new bone directly on the hydroxyapatite and tricalcium-phosphate coating, with variable degrees (amounts) of hydroxyapatite and tricalcium-phosphate resorption and new-bone replacement over time. Bone was never directly apposed to uncoated titanium fiber-metal. The pull-out strength of the hydroxyapatite and tricalcium-phosphate-coated implants was consistently greater than that of the uncoated implants, at all time-periods. CLINICAL RELEVANCE: The growth of bone into porous surfaces is compromised by conditions encountered in the revision of failed total joint prostheses, by osteoporosis, or by abnormal osseous anatomy. This study suggests that hydroxyapatite and tricalcium-phosphate-coated titanium fiber-metal intramedullary implants have more rapid and voluminous bone ingrowth than do uncoated implants. The use of this surface to enhance fixation of the component in unfavorable clinical circumstances (where the ingrowth of bone is compromised) may help to improve the functional outcome of total joint replacements.