Changes in subchondral bone in cartilage resurfacing--an experimental study in sheep using different types of osteochondral grafts

OBJECTIVE: This article addresses the subchondral bone integrity in cartilage resurfacing by comparing fresh, untreated auto-, xeno-, and photooxidized osteochondral allo- and xenografts. Photooxidation was expected to improve mechanical stability of the osteochondral grafts through an improved linkage of the collagen fibers within the bone matrix. DESIGN: Untreated auto- and xenografts and with photooxidation pretreated allo- and xenografts were surgically implanted in femoral condyles of sheep (n=40). After 2, 6, 12 and 18 months results were evaluated histologically using non-decalcified bone embedded in acrylic resin. Qualitative evaluation was performed with emphasis on bone matrix, biomechanical stability of graft anchorage, formation of cystic lesions, and bone resorption and formation. Quantitative evaluation of the total subchondral bone area was conducted histomorphometrically. Statistical analysis (factorial ANOVA test) was used to compare differences between groups with respect to the percentage of bone matrix and fibrous tissue per section. RESULTS: Subchondral bone resorption was fastest in untreated, fresh autografts, followed by photooxidized allografts, untreated, fresh xenografts and last pretreated photooxidized xenografts. Cystic lesions were seen in all types of grafts, but were most pronounced at 6 months in autografts and least in photooxidized grafts. Cyst-like lesions had subsided substantially in the untreated auto- and photooxidized xenografts, if no graft dislocation occurred during the healing period. Mononuclear cell infiltration and an increase in the presence of multinuclear cells were observed at 2 months, mostly in untreated autografts, followed by photooxidized allo- and untreated xenografts. They were much higher in numbers compared to photooxidized grafts, at least in the early specimens at 2 months. Graft stability was linked to the rate of bone resorption. CONCLUSION: Substantial resorption of the subchondral bone, involving the development of cyst-like lesions, lead to dislocation and finally to cartilage matrix degradation of the grafts. The process of photooxidation decreased the speed of bone resorption in osteochondral grafts and, thus, improved graft stability and cartilage survival. These results suggest that the remodeling of the subchondral bone of the host and the graft within the first 6 months is an important factor in graft stability and overall results of cartilage resurfacing