MECHANISM RESEARCH OF CRYOANALGESIA

The sciatic nerves of rabbits were frozen at different temperatures (- 20 degrees C, - 60 degrees C, - 100 degrees C, - 140 degrees C, and -1 80 degrees C). The morphology and function of the frozen nerves were e xamined with light microscopy (hematoxylin and eosinophilin stain and a histochemical thiocholine method) and electron microscopy. The funct ion of the nerve after freezing was assessed using short latency somat osensory evoked potentials, sensory conduction velocity, and electromy ogram at various intervals after freezing. There were no changes in mo rphology or function of nerves cryolesioned al - 20 degrees C. The ner ve fibers cryolesioned at - 60 degrees C showed signs of freezing dege neration and lost their conductive function although, these nerves all recovered. Approximately half of nerve fibers cryolesioned at -100 de grees C showed Wallerian degeneration, and although the time to remyel ination was delayed, nerve regeneration was still complete. At - 140 d egrees C and - 180 degrees C the nerve fibers showed immediate necrosi s, with destruction of basal membranes and proliferation of collagen f ibers. The results explained the mechanism of cryoanalgesia. Our study demonstrates that cryo-temperatures lower than -140 degrees C will ca use permanent alterations in nerve morphology and function, whereas wa rmer temperatures do not result in permanent nerve damage and are ther efore not likely to provide long-term analgesia to patients.