Cryoanalgesia: the response to alterations in freeze cycle and temperature.

One hundred and forty male rats received a cryoinjury to a segment of the exposed sciatic nerve. A variety of temperatures and freeze patterns was used and the recovery of function was monitored. Where obvious nerve freezing occurred, the cryoinjury caused complete loss of function in the limb. The return of motor activity was first noted at 25 days (mean) but complete return of motor activity and of pain sensation was noted after 38 days (mean). The findings were consistent with an axonal growth rate of 1 mm per day. The time taken for regeneration was independent of both the duration of freezing and the application of a repeat freeze cycle. The temperature attained by the nerve was, however, important. Where it remained greater than -20 degrees C the result were unpredictable, but below this temperature the interruption was prolonged and uninfluenced by greater reductions in temperature.