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Histologic Changes in the Intervertebral Disc After Intradiscal Injections of Methylprednisolone Acetate in Rabbits

Study Design Histologic changes in the intervertebral disc were evaluated by light and electron microscopy after an intradiscal injection of methylprednisolone acetate. Objective To evaluate the histologic changes that occur in the disc after an intradiscal injection of corticosteroids. Summary of Background Data Several orthopedic surgeons have managed lumbar disc herniation with intradiscal steroid injection. Few studies have addressed the histologic changes after this treatment. Methods Thirty rabbits were surgically treated. Rabbits were divided into two groups. Group A: 25 rabbits underwent intradiscal steroid injections. Methylprednisolone acetate was injected into the L3-L4 disc, whereas methylprednisolone sodium succinate was injected into the L4-L5 disc. Physiologic saline was injected into the L5-L6 disc as a control. Groups of five rabbits were killed 1 day, 1, 4, 12, and 24 weeks after the injection and subjected to microscopic evaluation. Group B: Another five rabbits underwent an injection of polyethylene glycol 4000, a vehicle for methylprednisolone acetate, into the L3-L4 and L4-L5 discs. All five rabbits were killed 24 weeks after the injection and subjected to microscopic evaluation. Results Group A: Light microscopic evaluation showed tissue in the nucleus pulposus, and the inner layer of the anulus fibrosus had degenerated in the methylprednisolone acetate group killed 24 weeks after the injection. Matrix vesicles, an indication of primary tissue calcification, were observed by electron microscopy. In the methylprednisolone sodium succinate group and saline group, however, no histologic changes were observed. Group B: Degeneration of the nucleus pulposus and primary tissue calcification were observed in the discs of all rabbits. Conclusions Methylprednisolone acetate and its vehicle polyethylene glycol cause degeneration and primary calcification in discs.