Transplantation of articular cartilage following a step-cooling cryopreservation protocol.

Using a step-cooling cryopreservation protocol that held the tissue 60 min at -4 degrees C, 30 min at -8 degrees C, and 10 min at -40 degrees C before plunging into liquid nitrogen, we were able to get a substantial improvement in the magnitude and pattern of chondrocyte recovery following cryopreservation, achieving postthaw recoveries of 62 +/- 13%. These results are consistent with the hypothesis that ice growth within articular cartilage is planar, but they provide no direct support for that hypothesis. Transplanting (step-cooled) cryopreserved osteochondral allografts into adult Suffolk/Romanoff crossbred sheep for periods of 3 months and 1 year further tested the efficacy of the cryopreservation protocol. Unfortunately, the cryoinjury sustained by the chondrocytes during cryopreservation, although apparently nonlethal immediately after thawing in many cases, was not innocuous in the long term. The presence of large clusters of chondrocytes at 1 year after transplantation illustrates that cryoinjury not detectable with a membrane integrity assay can still have far-reaching effects on transplanted tissue.