Effects of Irradiation on Posterior Spinal Fusions A Rabbit Model

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Abstract
The biological and biomechanical effects of irradiation on posterior bone graft healing in the lumbar spine of rabbits were investigated. Twenty-seven New Zealand white rabbits were divided into four groups. Each rabbit underwent a posterior lumbar spine fusion with autogenous iliac crest bone graft, and three of the experimental groups received perioperative irradiation. This study evaluated the histologic and biomechanical effects of perioperative irradiation on posterior spinal fusions using a rabbit model. Treatment of metastatic disease of the spine depends on the neurologic status of the patient, stability of the spine, location of the tumor, and the tissue of origin. Some patients require surgical decompression and stabilization. The response of a posterior spinal bone graft to irradiation has not been studied previously. Group 1 (n = 7), the control group, did not receive irradiation. Group 2 (n = 6) received preoperative irradiation. Group 3 (n = 7) received immediate (day 3) postoperative irradiation, and Group 4 (n = 7) received delayed (day 21) postoperative irradiation. The radiation protocol consisted of 480 centigrade/fraction for 5 consecutive days. At 3 months, the rabbits were euthanatized. Nondestructive biomechanical testing was performed, followed by histologic evaluation of the fusion mass. Compared with the control group, Group 3 (immediate postoperative irradiation) specimens were less stiff in extension (P = .0001), flexion (P = .0006), compression (P = .018), and left lateral bending (P = .018). The preoperatively irradiated spines (Group 2) were less stiff in extension (P = .0008) and in compression (P = .035) than controls. The control group and the delayed irradiation group had the highest histologic scores and more mature fusion mass. The immediate postoperative irradiation group had the worst results, with consistent fibrous union of the graft. Healing of a posterior spinal fusion is influenced by the timing of radiation therapy. Adjuvant radiation therapy for patients with spinal neoplasm requiring a posterior fusion should, if possible, be delayed for 3 to 6 weeks postoperatively to maximize the probability of successful arthrodesis.

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