ABSTRACT
STUDY DESIGN: A rabbit model of posterolateral intertransverse process spine arthrodesis was used. OBJECTIVE: To assess the osteogenic potential of hydroxyapatite (HA) sticks soaked with marrow and resultant strength in spinal fusion and effects of various biologic agents, in particular fibronectin (FN) on the strength. SUMMARY OF BACKGROUND DATA: Mesenchymal stem cells (MSCs) contained in marrow can differentiate into cells constituting bone, cartilage, muscles and fat. Unlike autogenous bone grafts, prosthetic bone grafts combined with marrow have failed to attain adequate strength of new bone even if they are used with marrow. METHODS: Porous HA sticks were used as a vehicle for new bone growth in spinal fusion. In experiment 1, rabbits received 2 HA sticks each with or without iliac marrow into the dorsal subcutaneous muscle. The sticks were explanted 6 weeks after surgery and tested mechanically and histologically. In experiment 2, 6 groups of animals underwent spinal fusions with 6 different bone grafts, either autogenous or prosthetic, and the local osteoinductive activity and mechanical strength of the fusion were evaluated. RESULTS: In experiment 1, the HA sticks implanted with marrow showed histologically more active osteogenesis and were mechanically stronger than those implanted alone. The increase in mechanical strength was significantly higher in the former group. In experiment 2, the spinal fusion with autogenous iliac bone was significantly stronger and more elastic than those with autogenous local bone and HA sticks with or without marrow (P < 0.05). The spinal fusion with HA sticks soaked with FN and marrow was significantly stronger than that with HA sticks soaked with marrow. The local osteogenesis after the fusion with HA sticks soaked with FN and marrow was more active than that with HA sticks plus marrow and HA stick plus FN. CONCLUSION: The impregnation of marrow into HA sticks produced histologically demonstrable active osteogenesis, possibly due to MSCs incorporated into the sticks. In the group receiving HA sticks soaked with FNand marrow, both osteogenesis and strength were increased to levels comparable to those achieved by spinal fusion with autogenous iliac bone grafts. It can be expected that the additional use of FN, which increases incorporation of MSCs into HA, will improve osteogenesis and hence the strength of the fusion.