Cartilage changes caused by a coronal surface stepoff in a rabbit model.

Coronal stepoffs of 0.5 mm (equal to the cartilage height) were created on the medial femoral condyles of adult, skeletally mature rabbits as a model for articular surface incongruity. After 3, 6, 12, and 24 weeks, tissue was analyzed histologically using hematoxylin and eosin and Safranin O staining, autoradiographs were made of the femoral condyles, and immunohistologic analysis was done for 3-B-3(-) and 7-D-4 chondroitin sulfate epitopes. An overlapping flap from the high toward the low side and an increase of the cartilage height on the low side of the defect were observed as permanent features of adaptation throughout the entire followup. Significant degeneration was not seen around the lesion or in the tibial cartilage opposing a stepoff defect. Autoradiography showed a three-phase response to the lesion: an early increase in radiolabeled sulfate (35SO4) uptake, a sharp decline of 35SO4 uptake, and finally a late recovery of the autoradiographic signal indicating partial recovery of proteoglycan synthetic activity. After an early increase, immunohistologic analysis for 3-B-3(-) showed a subsiding tendency by 24 weeks, and the staining with 7-D-4 remained elevated uniformly in the vicinity of the lesion. A rabbit femoral stepoff defect with an offset of 0.5 mm may remodel and not lead to degeneration within the first 6 months after injury in a stable joint.