Healing of articular cartilage in intra-articular fractures in rabbits.

Intra-articular fractures were created in the distal end of the femur in adult rabbits. Fractures were then reduced in three different fashions: incomplete reduction, adequate reduction without compression of the fregments, and ideal reduction, which consisted of reduction with strong compression of the fracture fragments. Animals were killed at intervals ranging from seven weeks to one year, and the cartilage fractures were studied by light, transmission, and scanning electron microscopy, using proteoglycan stains. Cartilage fractures that were inadequately reduced or were adequately reduced without compression healed by fibrocartilage only. Fractures that were reduced with compression across the fragments healed with a tissue which, by light and electron microscopy, appeared to be hyaline cartilage, which showed the collagen and proteoglycan orientation and relationships of normal hyaline cartilage. The healing of these defects appears to be the result of a proliferation of cells emanating from the region of the tidemark, and perhaps of migration of chondrocytes from the surface into the lesion. We postulate that compression of the cartilage surfaces either creates a physical environment that allows certain chondrocytes to heal the defect with hyaline cartilage, or, by coapting the surfaces, prevents ingrowth of granulation tissue from the subchondral bone that might interfere with repair by hyaline cartilage. This study suggests a hitherto unappreciated capacity for hyaline cartilage to repair structural defects.