In vivo evaluation of coralline hydroxyapatite and direct current electrical stimulation in lumbar spinal fusion.

STUDY DESIGN: An animal model of posterolateral intertransverse process lumbar spinal fusion using autologous bone, coralline hydroxyapatite, and/or direct current electrical stimulation. OBJECTIVES: To evaluate the effect of an osteoconductive bone graft substitute and direct-current electrical stimulation on the rate of pseudarthrosis in a rabbit spinal fusion model. SUMMARY OF BACKGROUND DATA: Conventional techniques for the surgical treatment of degenerative conditions in the lumbar spine have a substantial failure rate and associated morbidity. Bone graft substitutes and electrical stimulation are alternative techniques to enhance fusion rates and limit the morbidity associated with posterolateral intertransverse process fusion using autologous iliac crest bone graft. METHODS: Fifty-three adult female New Zealand White rabbits underwent single-level lumbar posterolateral intertransverse process fusion. Animals were assigned to one of four groups using either autologous bone (Group I), coralline hydroxyapatite with autologous bone marrow aspirate (Group II), coralline hydroxyapatite with a 40-microA implantable direct current electrical stimulator and bone marrow aspirate (Group III), or coralline hydroxyapatite with a 100-microA implantable direct current electrical stimulator and bone marrow aspirate (Group IV). Animals were killed at 8 weeks, and fused motion segments were subjected to manual palpation, mechanical testing, and radiographic and histologic analysis to assess the fusion mass. RESULTS: Successful fusion was achieved in 57% (8/14) of animals in Group I, 25% (3/12) in Group II, 50% (6/12) in Group III, and 87% (13/15) in Group IV. Mean stiffness and ultimate load to failure were significantly higher in Group IV than in all other groups (P < 0.05). Histologic analysis demonstrated a qualitative increase in fusion mass in Group IV versus all other groups. CONCLUSIONS: Direct-current electrical stimulation increased fusion rates in a dose-dependent manner in a rabbit spinal fusion model. Coralline hydroxyapatite is an osteoconductive bone graft substitute, and thus requires an osteoinductive stimulus to ensure reliable fusion rates. Furthermore, coralline hydroxyapatite and direct current electrical stimulation can be used together to increase fusion rates in a rabbit spinal fusion model while avoiding the morbidity associated with harvesting iliac crest bone.