THE RELEVANCE OF HYDROXYAPATITE AND SPONGIOUS TITANIUM COATINGS IN FIXATION OF CEMENTLESS STEMS - AN EXPERIMENTAL COMPARATIVE-STUDY IN RAT FEMUR EMPLOYING HISTOLOGICAL AND MICROANGIOGRAPHIC TECHNIQUES

Pure titanium rods plasma-spray coated with hydroxyapatite (HA) or por ous titanium (Ti) of controlled roughness were implanted bilaterally i n the distal femur of Sprague-Dawley rats to compare the extent of bon e growth on the two types of coating. The relevance of other factors, like mechanical stability and biological adaptation of the bone to the insertion of a foreign body implant, were investigated in femora whic h were over-reamed (absence of primary fit) or reamed without insertio n of the rod. Continuous tetracycline labeling for the first 30 days a nd for the last 2 weeks in the 90-day group was performed; histologica l/histometric, fluorescence and microangiographic studies were carried out on serial sections of the implanted and control femora. In the gr oup of stable implants, HA-coated rods showed 90% integration versus 5 3% With Ti-coated implants (P < 0.001); in over-reamed implants neithe r surface bone growth nor endosteal fixation occurred, and both types of rods were surrounded by a thick layer of connective tissue. The stu dy documented early adhesion of osteoblasts and direct deposition of b one matrix on the substrate, while on spongious titanium osteogenesis was observed only in proximity to the surface. Remodeling of the react ive, primary bone to mature, lamellar bone took the form of a capsule surrounding the implants and radial bridges connecting the latter to t he endosteal surface. The number, height and thickness of these bridge s appeared to be the factors determining implant stability, rather tha n the extent of the bony capsule on the perimeter of the implant. Inte gration was a function not only of mechanical conditions and surface g eometry, but also of the biological response of the whole bone to chan ges in the vascularization pattern. The reported phenomena can be seen more easily in experimental models involving small rodents because of their fast bone turnover and revascularization. but it is expected th at they take place, even at a lower speed, in clinical situations like cementless stems of total hip replacement.