In vivo evaluation of calcium sulfate as a bone graft substitute for lumbar spinal fusion.

Background Context: Posterolateral fusions of the lumbar spine have nonunion rates as high as 35%. The availability of autologous bone to promote fusion is limited, particularly for multilevel fusions. Bone substitutes have been proposed to augment or replace autologous bone for spinal surgery. Calcium sulfate offers high porosity, osteoconductivity, and high resorption rate. This material has been used successfully for treatment of long bone defects but has not been investigated as a bone graft substitute for spinal fusions.

Purpose: To determine whether the use of calcium sulfate granules in conjunction with an implantable electrical stimulator is a safe and effective means of attaining spinal fusion.

Study Design/setting: A rabbit lumbar fusion model was used to assess a calcium sulfate bone graft substitute in combination with electrical stimulation for spinal fusion.

Methods: Thirty-six adult New Zealand White female rabbits were divided into three groups. Each group underwent a single-level (L5-L6) fusion, receiving 3.0 cc calcium sulfate granules with bone marrow aspirate from the iliac crest. Group 1 had no electrical stimulator applied. Groups 2 and 3 received a 40-microA (Group 2) or a 100-microA (Group 3) implantable electrical stimulator. The animals were sacrificed at 8 weeks, and the rabbit spines were subjected to radiographic assessment, manual palpation, and mechanical testing.

Results: Two rabbits died postoperatively. The radiographic assessment revealed no fusions occurred at the adjacent nonoperated control levels (L4-L5). There were no fusions observed within Group 1, containing the calcium sulfate and bone marrow aspirate alone. The sites with the implantable stimulators showed a dose-dependent increase in fusion stiffness. However, no fusion mass in Group 2 or 3 was graded as bilaterally complete.

Conclusion: This study found that calcium sulfate as a bone graft substitute was unsuccessful in promoting spine fusion in a rabbit model. There was radiographic evidence of rapid resorption of the calcium sulfate within 4 weeks after surgery. The use of electrical stimulation created a dose-dependent increase in mechanical competence of the bony mass. However, the addition of direct current (DC) current did not significantly alter fusion rates with calcium sulfate used as the bone graft substitute in this model.