MECHANICS OF INTERBODY SPINAL-FUSION - ANALYSIS OF CRITICAL BONE-GRAFT AREA

Bone graft subsidence is a serious complication of interbody spinal fu sion. In this study, 66 mechanical tests were performed on 35 thoracic vertebral bodies to investigate the in situ mechanics of interbody sp inal fusion. The relationships among trabecular bone density, bone str ength, and size of bone graft area were analyzed. All vertebral bodies were scanned by quantitative computer tomography (QCT) to determine t heir bone density before mechanical testing. The decorticated trabecul ar beds of the vertebral bodies, void of all posterior elements, were loaded in a manner similar to that which occurs after surgical interbo dy fusion. That is, rectangular blocks of polymethylmethacrylate, repr esenting bone grafts, were used to transfer controlled compressive loa ds to the decorticated vertebral trabecular surface. Both destructive and nondestructive tests were performed. The relationship between QCT bone density and trabecular bone strength was related by a power funct ion, and, on average, the bone density and trabecular bone strength we re 0.137g/cm3 and 3.97 MPa, respectively. Eighty percent of the verteb ral bodies with graft covering 25% of the total end plate area or less failed at loads less than 600 N, while 88% of the vertebral bodies wi th 30% or greater covered were able to carry a load greater than 600 N . The results suggest that the intrinsic behavior of trabecular bone l oaded within the vertebral body is little different from the behavior of the whole vertebral body, that QCT bone density is indicative of bo ne strength, and that interbody graft area should be significantly gre ater than 30% of the total end plate area to provide a margin of safet y. Additionally, the relationship between QCT bone density and bone st rength permits generation of a family of curves to predict critical mi nimum graft area based on QCT measurements and anticipated physiologic al loads or scaled to patient weight.