Observations of subchondral plate advancement during osteochondral repair: a histomorphometric and mechanical study in the rabbit femoral condyle.

OBJECTIVE The subchondral plate and its reconstitution has been an under-researched aspect of articular cartilage repair. The extent to which the subchondral plate is restored by natural healing remains controversial. This study aimed to quantify advancement of subchondral bone during repair of an osteochondral defect, and to examine the effect of subchondral bone height on the quality of articular surface repair. DESIGN Osteochondral defects, 3mm diameter by 3mm deep, were made by controlled drilling through the articular surface into the subchondral bone in femoral condyles of 33 rabbits. The repair response was examined at 8, 16 and 32 weeks (n=14, 12 and 7, respectively) post surgery. The specimens were subjected to mechanical testing, radiography, histology and histomorphometrology using an image analysis system. RESULTS At 8 weeks, the level of reparative subchondral bone was 0.79+/-0.36 mm below the native tidemark. By 16 weeks, reformed subchondral plate was irregular, showing that 76.5% of the plate had extended beyond the native tidemark (0.13+/-0.05 mm) whilst 16.9% of the plate remained below (0.19+/-0.15 mm). The repaired surface non-osseous layer became thinner than the adjacent cartilage (0.23+/-0.08 vs 0.38+/-0.11 mm, P<0.05). This persisted up to 32 weeks. The repaired surface layers showed disappearance of safranin-O staining, increased separation splits at the boundary, and eventual degradation. General histological scores were similar across 8, 16 and 32 weeks although the scores of defect filling and restoration of osteochondral junction were decreased from 8 to 16 weeks. Mechanically, repaired defects had lower contact pressure and greater indentation than the normal controls at all time (P<0.05). Indentations of the cartilage adjacent to the defects were also greater than the normal at 8 and 32 weeks (P<0.05). CONCLUSION The reparative subchondral bone advanced beyond the level of the native subchondral plate by 16 weeks in osteochondral defects of the rabbit femoral condyles. The presence of an advanced and irregular subchondral plate was associated with degradation of repaired articular surface. Abnormal subchondral plate is likely one of the major factors in influencing the long-term outcome of articular cartilage repair.