PURPOSE: To investigate the microstructural differences and responsible mechanisms in early bone formation in anterior lumbar interbody fusion (ALIF) in the spine using rhBMP-2 (INFUSE), equine bone protein extract (COLLOSS E) or autograft. METHODS: Twelve Danish female landrace pigs underwent a 3-level ALIF procedure at L3-6. PEEK interbody cages packed with rhBMP-2, COLLOSS E, or autograft were inserted. The animals were divided into two groups of six, and observed for four and eight weeks postoperatively. MicroCT was performed for evaluation of microstructure of the bone within the cage. A mathematical finite element model was developed to investigate the aqueous behavior within the cages when exposed to external compressive forces. RESULTS: At 4 weeks postoperative bone surface volume fraction (BS/TV) using rhBMP-2 was higher than with use of COLLOSS E and autograft, while trabecular thickness (Tb.Th.) was lower using rhBMP-2 at this time-point. At eight weeks BS/TV and trabecular number (Tb.N.) were still higher using rhBMP-2 than autograft and COLLOSS E. Connectivity density was significantly higher using rhBMP-2 than using autograft or COLLOSS E at both time-points. Between four- and eight-week time-points BV/TV and Tb.Th. rose while Tb.N. declined using rhBMP-2. The degree of anisotropy and the calculated amount of trabeculae with main direction along the spinal axis, were higher at four weeks using COLLOSS E. rhBMP-2 had the highest amount of trabeculae directed along the spinal axis at eight weeks. A change in main direction between four and eight weeks was observed for both autograft and rhBMP-2. The numerical results from the finite element model verify that significantly different flow pattern emerges as the boundary conditions are altered. At four weeks there was an evident correlation between trabecular orientation and flow pattern using rhBMP-2. CONCLUSION: This study reveals large differences in microstructure in the early osteogenesis and explains important mechanisms of early bone formation using rhBMP-2, COLLOSS E or autograft treatment. These differences might explain some of the unfortunate events reported such as edema, swelling, and excessive bone formation using different bone graft substitutes in spinal fusion procedures.
PURPOSE: To investigate the microstructural differences and responsible mechanisms in early bone formation in anterior lumbar interbody fusion (ALIF) in the spine using rhBMP-2 (INFUSE), equine bone protein extract (COLLOSS E) or autograft. METHODS: Twelve Danish female landrace pigs underwent a 3-level ALIF procedure at L3-6. PEEK interbody cages packed with rhBMP-2, COLLOSS E, or autograft were inserted. The animals were divided into two groups of six, and observed for four and eight weeks postoperatively. MicroCT was performed for evaluation of microstructure of the bone within the cage. A mathematical finite element model was developed to investigate the aqueous behavior within the cages when exposed to external compressive forces. RESULTS: At 4 weeks postoperative bone surface volume fraction (BS/TV) using rhBMP-2 was higher than with use of COLLOSS E and autograft, while trabecular thickness (Tb.Th.) was lower using rhBMP-2 at this time-point. At eight weeks BS/TV and trabecular number (Tb.N.) were still higher using rhBMP-2 than autograft and COLLOSS E. Connectivity density was significantly higher using rhBMP-2 than using autograft or COLLOSS E at both time-points. Between four- and eight-week time-points BV/TV and Tb.Th. rose while Tb.N. declined using rhBMP-2. The degree of anisotropy and the calculated amount of trabeculae with main direction along the spinal axis, were higher at four weeks using COLLOSS E. rhBMP-2 had the highest amount of trabeculae directed along the spinal axis at eight weeks. A change in main direction between four and eight weeks was observed for both autograft and rhBMP-2. The numerical results from the finite element model verify that significantly different flow pattern emerges as the boundary conditions are altered. At four weeks there was an evident correlation between trabecular orientation and flow pattern using rhBMP-2. CONCLUSION: This study reveals large differences in microstructure in the early osteogenesis and explains important mechanisms of early bone formation using rhBMP-2, COLLOSS E or autograft treatment. These differences might explain some of the unfortunate events reported such as edema, swelling, and excessive bone formation using different bone graft substitutes in spinal fusion procedures.