Repeat Intradiscal Drug Injection: an experimental study

We investigated the changes in pathology and intradiscal pressure after repeat intradiscal drug injection in the mature rabbit, Chymopahain (500pku/ml) or 10% hypertonic saline was injected into five lumbar intervertebral discs of 9 rabbits. The injection volume was 0.02ml/disc, and a repeat injection was performed 4 weeks after the first injection. Rabbits were sacrificed at 1 week, 4 weeks, and 8 weeks after the repeat injection. They were classified into 3 groups; the HH group received hypertonic saline twice, the HC group was first injected with hypertonic saline and with chymopapain at the second injection, and the CC group received chymopapain twice. The intradiscal pressure changed depending on the degree of disc degeneration and on any regeneration. When hypertonic saline was injected first (HH and HC groups), the degeneration in the disc was slight compared to when chymopapain was injected first (CC group), and the intradiscal pressure was further decreased by the second injection of hypertonic saline (HH group). In comparison, when chymopapain was injected first (CC group), marked degeneration and disc narrowing were observed with a marked decrease in pressure compared to when hypertonic saline was injected first (HH and HC groups). Following a second injection of chymopapain (CC group) there was further degeneration but there was no additional decrease in pressure. Disc regeneration was observed in the HH group, but not in the HC or CC groups.