Comparison of Healos/bone marrow to INFUSE(rhBMP-2/ACS) with a collagen-ceramic sponge bulking agent as graft substitutes for lumbar spine fusion

Study Design. A rabbit lumbar intertransverse process arthrodesis model was used to evaluate the efficacy of two different bone substitute materials: 1) collagen-hydroxyapatite sponge (Healos bone void filler) combined with heparinized bone marrow; and 2) recombinant human bone morphogenetic protein-2 delivered in a collagen sponge ( INFUSE Bone Graft) wrapped around an additional collagen-ceramic sponge (Mastergraft Matrix) as a bulking agent. Objectives. To compare the relative efficacy of two different bone graft substitutes to achieve posterolateral lumbar spine fusion in rabbits. Summary of Background Data. Autogenous bone graft is considered the gold standard graft material for spine fusion. Complications with its use, however, may occur in as many as 30% of patients. A variety of bone substitutes have been used for spine fusion, but there are few direct comparison experiments to determine the relative efficacy of any two alternatives. Methods. Adult New Zealand white rabbits ( n = 24) were divided into two groups and underwent bilateral posterolateral intertransverse process spine arthrodesis at L5 - L6. The fusions were augmented by different bone substitute materials as follows: Group 1 ( n = 12) received 3 mL of collagen-hydroxyapatite sponge ( Healos bone void filler) (10 x 30 x 5 mm, two per side) with 3.0 mL of heparinized bone marrow on each side of the spine. ( ratio 1: 1); Group 2 ( n = 12) received 1.5 mL of rhBMP-2 (0.43 mg/mL solution) on a Type 1 collagen sponge ( INFUSE Bone Graft) wrapped around an additional 1.5 mL collagen-ceramic (15%HA/85% TCP) sponge ( Mastergraft Matrix) as a bulking agent to provide 3 mL of graft on each side of the spine. Bone marrow was aspirated from posterior iliac crest, and 1 mL of bone marrow was sent to count number of nucleated cells. The rabbits were killed after 8 weeks; the spines were evaluated by manual palpation, radiographs ( plain radiograph and CT scan), tensile mechanical testing, and nondecalcified histology. Results. The bone marrow had average of total nucleated cell count 9 x 10(7) cells. All rabbits (100%) in Group 2 (INFUSE/Mastergraft Matrix) achieved solid spinal fusions by manual palpation and radiographs, whereas solid spinal fusion was not achieved by manual palpation and radiographs in any of the rabbits treated with Healos combined with heparinized bone marrow ( Group 1). The plain radiograph and CT scans of Group 1 showed some minimal new bone formation near the transverse processes, but none of these rabbits formed a continuous fusion mass. In contrast, all of plain radiographs and CT scans in Group 2 showed continuous fusion mass and complete graft incorporation between transverse processes bilaterally. Biomechanically, the relative strength and relative stiffness values of L5 - L6 ( fusion segment) in Group 2 were statistically significant greater than L5 - L6 in Group 1 ( P < 0.001). Histologic sections confirmed the palpation and radiographic results. Conclusion. From the manual palpation, radiographic and biomechanical assessment of fusion, the results in this study showed that INFUSE (rhBMP-2/collagen sponge) consistently produced spine fusion when wrapped around a collagen-ceramic sponge bulking agent (Mastergraft Matrix). Meanwhile, Healos was ineffective as a bone graft material when combined with heparinized autogenous bone marrow.