Further work on the cryopreservation of articular cartilage with particular reference to the liquidus tracking (LT) method.

The cryopreservation of articular cartilage with survival of living cells has been a difficult problem. We have provided evidence that this is due to the formation of ice crystals in the chondrons. We have developed a method in which the concentration of the cryoprotectant dimethyl sulphoxide (Me(2)SO) is increased progressively, in steps, as cooling proceeds so that ice is never allowed to form, but the very high concentrations of Me(2)SO required at low temperatures are reached only at those low temperatures. In this paper, we describe some new experiments with discs of ovine articular cartilage similar to those used in our previous studies and we show that continuous stirring throughout the process resulted in a significant increase in the rate of (35)S sulphate incorporation into glycosoaminoglycans (GAGs), now reaching 87% of the corresponding fresh control values. We confirmed that the method is also effective for human knee joint cartilage, which gave 70% of fresh control ability to synthesise GAGs; continuous stirring was also used in this experiment. We then extended the method to ovine knee joint osteochondral dowels and showed that, again with continuous stirring, the method produced tissue concentrations of Me(2)SO that were sufficient to prevent freezing in dowels too, and to permit cell function at 60% of control. The most important mechanical property (instantaneous compressive modulus) was unaffected by the process. Finally, we experimented with some technical variations to facilitate clinical use-a more rapid process for warming and removal of Me(2)SO was developed and a method of short-term storage before or after cryopreservation was developed. Finally, pilot experiments were carried out to provide proof of principle for a closed, continuous flow method in which both temperature and Me(2)SO concentration were computer-controlled.