An animal study was performed to assess whether hydroxyapatite could induce bone ingrowth to close gaps at the bone – implant junction in a weight-bearing model.

14 titanium alloy plates, 2 x 0,5 X 0,1 cm, sprayed on one flat side with HA and left as bare metal on the other, concave surface, were inserted into the distal metaphysis of the femur of 14New Zealand adult rabbits, perpen-diculary to the diaphyseal long axis . The sections of the femur were performed through the plate and were observed by S.E.M.

During the first week, we found medullary hyper-plasia and differentiation of osteogenic cells producing osteoid. By the 2nd-3rd week, immature bone formed from the endosteum towards the plate, leading to the filling of the gap with the HA – coated surface of the plate. Between the 4th and the 8th week the remodeling process occurred with formation of lamellar bone. At 12 weeks bone – ingrowth appeared to be greater in areas of proximity to the endosteal wall, especially when the plate was oriented in such a way that acute angles with the endosteum were present. Ha –coatings were degraded by a process of creeping substitution. The gap between bone and non-coated surface was not filled at the end of the experiment. At six months the crystalline part of the coating appeared as separated grains (for dissolution of the amorphous phase). This degradation is responsible for the release of calcium-phosphate debris.

The present study shows great evidence of HA osteo-conductivity. The physico-chemical similarity between artificial HA and mineralized bone probably explains that the mechanism of HA resorption is similar to that of bone resorption. The remodeling cycles of resorption and new formation give as a final result the deposition of bone tissue onto the metallic alloy.