THE BIOLOGICAL AND BIOMECHANICAL EFFECTS OF IRRADIATION ON ANTERIOR SPINAL BONE-GRAFTS IN A CANINE MODEL

We evaluated the effects of irradiation on the healing of anterior vertebral strut grafts with use of a canine model. Through a left thoracotomy, a partial corpectomy of the seventh thoracic vertebra and autogenous iliac strut-grafting from the sixth to the eighth thoracic levels were performed in twenty-two adult beagles. Four groups were established: Group I (control) received no irradiation, Group II received preoperative irradiation, Group III received postoperative irradiation that began on the third postoperative day, and Group IV received postoperative irradiation that began on the twenty-first postoperative day. The irradiation protocol was five treatments of 500 centigray three times a week for a total of 2500 centigray. Fluorochromes were administered at regular intervals postoperatively. The beagles were killed three months postoperatively, and non-destructive biomechanical testing was done to evaluate the stiffness of the construct. The quality of healing at the junctions of the graft with the sixth and eighth thoracic vertebrae, the degree of revascularization of the graft, and the amount of new-bone formation were evaluated histologically. Statistical evaluation of the biomechanical data revealed no significant difference in the stiffness of the construct between Groups I, II, and IV. The specimens from Group III were significantly less stiff than those from Group I (the control group) in torsion (p = 0.03) and left lateral bending (p = 0.04) and than those from Group II in flexion (p = 0.02) and left lateral bending (p = 0.005). Grafts that had been irradiated immediately after implantation (Group III) had the lowest histological scores for quality of healing, degree of revascularization, and new-bone formation (p < 0.05). CLINICAL RELEVANCE: Patients who have a spinal tumor that will be treated with an anterior corpectomy and arthrodesis with anterior bone strut-grafting often need perioperative irradiation. The timing of the irradiation has important effects on the quality of healing of the vertebral strut graft. Preoperative irradiation may not adversely affect healing of these anterior interbody strut grafts, but immediate postoperative irradiation has severe adverse effects. A delay in the postoperative irradiation of the spine may be indicated to promote successful fusion of the anterior interbody graft.