Repair of steps and gaps in articular fracture models

To examine the healing of surface defects in articular fractures, gaps measuring 0.5 mm wide and 2 mm deep were created in 0.5-mm coronal step-offs on the medial femoral condyles of 16 rabbits, and identical gaps without step-offs were created in another set of 13 rabbits. Evaluation of repair was done 6, 12, and 24 weeks postoperatively. Histologically, subchondral bone restoration of gaps in step-offs was complete by 24 weeks, whereas restoration was incomplete in most gap-only lesions. Bone density measured by peripheral quantitative computed tomography was normal in gaps in step-offs by 24 weeks, but values were less than in controls in the gap-only group. A moderate degree of degeneration was detected only at the high sides of step-off + gap lesions. The joint surface was restored by fibrous cartilage that showed gradually improving maturity in all defects, however, real integration with adjacent original cartilage did not occur. Immunohistologic examination showed decreasing collagen Type I and increasing Type II staining intensity in repair tissue of both types of lesions. These observations suggest that minor surface gaps and steps of articular fractures may regenerate without inducing severe early joint degeneration. However, certain repair features make the long-term outcome of these defects uncertain.