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.

Aims: Histologically evaluate, at various intervals of time, osteochondral autografts transplanted in 10 adult goats. Methods: A full-thickness chondral defect of 1 cm in diameter was created on weight bearing surface of medial femoral condyle of goats. Multiple osteochondral grafts were harvested from the lateral trochlea using commercial arthroscopic tools (OATS); plugs were then inserted into prepared recipient site or the medial condyle without using a þxation device. The animals were killed at 3, 6, 9 and 12 months after surgery. Histologic examination using H and E staining was performed. The specimens were observed with light microscopy and polarized light microscopy. Results: At macroscopic evaluation, chondral lesions were completely reþlled by chondral tissue; the edges of the graft did not show complete incorporation with the host articular cartilage. Histology at 3 months showed the four layers of the articular cartilage in the middle of the implant; peripherically, we observed clusters of cells with many hypertrophic chondrocytes. Fibrocartilagineous tissue was present between the transplanted cartilage and the surrounding tissue. A well appearing tidemark was observed at all the follow-up. We also observed a change of staining of the intercellular matrix in the periphery. The donor sites were covered by þbrocartilagineous tissue. Conclusions: Multiple osteochondral autografts allow good clinical and functional results in chondral lesion treatment, but, at the second-look arthroscopy we always observed a þbrous layer between the implant and the host cartilage. The results of our experimental study shows that, at 12 months, the graft is viable, but there is a þbrocartilagineous layer between graft and the surrounding cartilage.