Role of different loading conditions on resorption of hydroxyapatite coating evaluated by histomorphometric and stereological methods

The role of different loading conditions on resorption of plasma-sprayed hydroxyapatite coating was investigated in an experimental study. Resorption of hydroxyapatite was quantified by histomorphometric and stereological methods on backscattered scanning electron images. Hydroxyapatite-coated titanium implants were inserted unilaterally into the medial femoral condyle of the knee in 14 mature dogs. Initially, all implants were subjected to controlled micromotion of 150 mu m. After 4 weeks, the dogs were randomly assigned either to have the implant surgically immobilized to prevent further micromovement or to have a sham operation. Sixteen weeks after the first operation, the implants were analyzed. Six noninserted implants served as controls. The surface area and volume of the hydroxyapatite coating were reduced on the immobilized implants by 53 and 67% (p < 0.05), respectively, and were further significantly reduced on the continuously loaded implants by 83 and 87%, respectively, compared with the control implants. The hydroxyapatite coating was significantly thinner on immobilized (15 mu m) and continuously loaded (15 mu m) implants as compared with control implants (23 mu m), but no difference between the inserted implants was found. Areas not covered with hydroxyapatite had 29 and 24% bone coverage on the immobilized and continuously loaded implants (not significant). Resorption of hydroxyapatite coating did occur in vivo. Continuous loading of the implants accelerated resorption significantly compared with immobilization of the implants. It is suggested that completely resorbed hydroxyapatite was partly replaced by bone in direct contact with the metal implant surface.