Effects of irradiation on posterior spinal fusions. A rabbit model.

STUDY DESIGN: 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. OBJECTIVES: This study evaluated the histologic and biomechanical effects of perioperative irradiation on posterior spinal fusions using a rabbit model. SUMMARY OF BACKGROUND DATA: 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. METHODS: 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. RESULTS: 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. CONCLUSION: 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.

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).(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of a hydroxyapatite and tricalcium-phosphate coating on bone growth into titanium fiber-metal implants.

A study was done in rabbits to determine the effect of a hydroxyapatite and tricalcium-phosphate coating on bone growth into titanium fiber-metal implants. Titanium fiber rods with a solid titanium core were implanted bilaterally into the distal aspect of the femora of fifty-five New Zealand White rabbits. One rod was uncoated and the other rod was surface-coated with hydroxyapatite and tricalcium phosphate by the plasma-spray technique. Thirty-five rabbits were labeled sequentially with fluorochromes; killed at one, two, three, four, six, twelve, or twenty-four weeks after the operation; and studied histologically and histomorphometrically. The implants in the remaining twenty rabbits were subjected to pull-out testing to determine the shear strength at the implant-bone interface at three, six, twelve, and twenty-four weeks after the operation. Histomorphometry revealed significant effects of the hydroxyapatite and tricalcium-phosphate coating. When whole-group means (which included all time-points) were compared, it was found that 44 per cent of the perimeter of the hydroxyapatite and tricalcium-phosphate-coated implants was covered with bone compared with 12 per cent of the perimeter of the uncoated implants. The percentage of the internal surface of the implant that was covered with bone was also significantly higher in the hydroxyapatite and tricalcium-phosphate-coated implants: 27 per cent of the internal surface of the coated implants was covered compared with 8 per cent in the uncoated implants. The amount of bone in the pores of the implants was also higher in the hydroxyapatite and tricalcium-phosphate-coated implants: 12 per cent of the available pore space in the hydroxyapatite and tricalcium-phosphate-coated implants was filled with bone compared with 4 per cent in the uncoated implants. Scanning electron microscopy of the implants, done in backscatter mode, demonstrated apposition of new bone directly on the hydroxyapatite and tricalcium-phosphate coating, with variable degrees (amounts) of hydroxyapatite and tricalcium-phosphate resorption and new-bone replacement over time. Bone was never directly apposed to uncoated titanium fiber-metal. The pull-out strength of the hydroxyapatite and tricalcium-phosphate-coated implants was consistently greater than that of the uncoated implants, at all time-periods.

The effects of vascularity and cyclosporin A on the mechanical properties of canine fibular autografts.

We studied the biomechanical behavior of orthotopic canine autografts as influenced by vascularized supply and the administration of cyclosporin A at three months and six months post-surgery. The model was the proximal 8 cm of the fibula in young adult dogs. In vascularized grafts, blood supply was re-established by microvascular re-anastomosis. Experimental controls were sham-operated and unoperated bones. Mid-graft test sections were subjected to loading-to-failure in torsion to determine the strength and stiffness. In both three- and six-month groups, vascularized grafts were significantly stronger and stiffer than contralateral nonvascularized grafts. Vascularized grafts were not significantly different from sham-operated bones. A 30-day regimen of cyclosporin A was found to have no measurable effect on mechanical properties for any individual treatment group. The results indicate that re-established blood supply can be a major factor in maintaining the mechanical integrity in large-segment cortical autografts.

Temporal analysis of vascularized and nonvascularized rib grafts in canine spine surgery.

The authors have previously reported work demonstrating the superiority of vascularized vs. nonvascularized rib grafts, which were inlaid to bridge three vertebral bodies studied at 3 months postoperatively. They questioned whether the mechanical and biologic properties of the nonvascular grafts would improve over time to approach the performance of the vascularized grafts by 6 months. They found that, with increased time, the vascularized grafts continued to improve, showing hypertrophy, maintenance of cortical integrity, and bonding to the recipient vertebral bodies. The nonvascularized grafts, however, showed porosity, fragmentation, and replacement by spongiosa. Mechanical properties showed increased performance in the vascularized vs. nonvascularized grafts, and no improvement was seen between the nonvascularized grafts from 3 to 6 months.

The superiority of vascularized compared to nonvascularized rib grafts in spine surgery shown by biological and physical methods.

Eleven canine experiments were performed to demonstrate the advantages of immediate vascular preservation in ribs used to fill a surgically created osseous gap bridging three vertebral bodies. In the five vascular rib grafts, mean blood flow was retained and measured at .101/ml/min/ml bone tissue. At autopsy 3 months post-operatively, vascularized grafts were more robust and hypertrophic. Mechanical testing in six stiffness modes confirmed the advantage of vascularized grafts. The experimental results indicate that vascularized grafts retain greater viability and better mechanical properties during graft incorporation. Providing a vascularized construct may be helpful in reconstructing large vertebral osseous gaps.