Analysis of peridural scar formation and its prevention after lumbar laminotomy and discectomy in dogs

Peridural fibrosis after lumbar laminectomy and discectomy has been implicated (not proven) as one of the factors that contribute to continuing or recurrent radicular and/or low back pain. This animal experimental study was designed to unequivocally show the stages in the development of scar tissue and to what extent, if any, scar tissue development is influenced by interposing fat grafts and Na hyaluronate of different molecular weights. A four-level unilateral lumbar laminotomy, anular fenestration, and nucleotomy was performed in 11 dogs. In each dog, levels were selected at random: one to serve as an empty control and three to insert the following: a fat graft, a viscous (1.9%) solution of Na hyaluronate, and a 1% high molecular weight solution of Na hyaluronate solution. The animals were killed at 1 day, 2 days, and 1, 2, 4, and 12 weeks postoperatively. Immediately after the dogs were killed, the lumbar spines were frozen in situ with dry ice, the lumbar spines were harvested and sectioned with a cryomicrotome. Close-up photographs taken at submillimeter intervals at each level were digitized and postprocessed with a computer. In the early postoperative period a hematoma was found in the pathway of the surgical dissection. During a 2-4 week period, this hematoma was replaced by a thick, white fibrotic tissue mass. Fibrosis was markedly less pronounced at the hyaluronate levels, especially the high molecular weight subset. Two-way statistical analysis of variance without replications revealed significantly less scar formation at the 0.05 level in the hyaluronate vs. the control segments. Dunnett's test, comparing each group individually with the control, revealed no difference between the fat groups and the control subjects. There was a significant difference between 1.9% Na hyaluronate and control. Viscous hyaluronate solution with its semifluid properties coats the nerve roots and dura anteriorly and posteriorly and reduces peridural fibrosis in the critical anterior region where adhesions form between the nerve root and anulus fibrosus.