Degradation of cartilage in contact with soft tissue.

Observations have been made on histological changes in femoral-head articular cartilage transplanted into soft tissue in rats under a variety of conditions. Articular cartilage was implanted either whole or minced. Implants were made into deflated subcutaneous air pouches with or without an inflammatory irritant and into normal subcutaneous tissue. Cartilage-matrix loss occurred in four patterns: marginal zone encroachment, loss of metachromatic staining without loss of tissue mass, surface invasion and lacunar enlargement and coalescence. The presence of an inflammatory reaction had no discernible effect on loss of cartilage matrix. Changes at the marginal zone preceded changes at the centre. Loss of metachromatic staining without loss of tissue mass occurred in areas of chondrocyte death. Surface invasion was not affected by the viability of the underlying chondrocytes. Surface invasion occurred earliest in cartilage transplanted into subcutaneous tissue, and latest in cartilage implanted into air pouches inflamed with carrageenan. The onset of surface invasion appeared to be dependent on soft-tissue adhesion to the cartilage surface. Mincing of cartilage was associated with an increase in surface invasion in the deeper cartilage zone with large lacunae but not in the superficial zone with small lacunae. Occasional cut surfaces showed outgrowth of new cartilage matrix. These observations on artefactual modes of cartilage degradation may assist in analysing the critical steps in cartilage degradation in arthritic joints.