Tissue ingrowth into titanium and hydroxyapatite-coated implants during stable and unstable mechanical conditions

Lack of initial mechanical stability of cementless prostheses may be responsible for fibrous tissue fixation of prosthetic components to bone. To study the influence of micromovements on bony ingrowth into titanium alloy (Ti) and hydroxyapatite (HA)-coated implants, a loaded unstable device producing movements of 500 microns during each gait cycle was developed. Mechanically stable implants served as controls. The implants were inserted into the weight-bearing regions of all four femoral condyles in each of seven mature dogs. Histological analysis after 4 weeks of implantation showed a fibrous tissue membrane surrounding both Ti and HA-coated implants subjected to micromovements, whereas variable amounts of bony ingrowth were obtained in mechanically stable implants. The pushout test showed that the shear strength of unstable Ti and HA implants was significantly reduced as compared with the corresponding mechanically stable implants (p less than 0.01). However, shear strength values of unstable HA-coated implants were significantly greater than those of unstable Ti implants (p less than 0.01) and comparable to those of stable Ti implants. The greatest shear strength was obtained with stable HA-coated implants, which was threefold stronger as compared with the stable Ti implants (p less than 0.001). Quantitative determination of bony ingrowth agreed with the mechanical test except for the stronger anchorage of unstable HA implants as compared with unstable Ti implants, where no difference in bony ingrowth was found. Unstable HA-coated implants were surrounded by a fibrous membrane containing islands of fibrocartilage with higher collagen concentration, whereas fibrous connective tissue with lower collagen concentration was predominant around unstable Ti implants. In conclusion, micromovements between bone and implant inhibited bony ingrowth and led to the development of a fibrous membrane. The presence of fibrocartilage and a higher collagen concentration in the fibrous membrane may be responsible for the increased shear strength of unstable HA implants. Mechanically stable implants with HA coating had the strongest anchorage and the greatest amount of bony ingrowth.