Byung-Ho Jin, et al. condition C specimens (317 ±165 N). A one-way ANOVA with a Newman-Keuls analysis indicated that the condition C specimens had significantly greater yield loads compared to those subjected to conditions A and B (p < 0.05). No significant dif- ference was noted between conditions A and B (p > 0.05). A Pearson correlation indicated that there was no statistically significant correlation between yield load and BMD (p > 0.05). The yield load of condition A plus that of condition B was approxi- mately equal to that of condition C, with condi- tion A contributing about one-third and condition B contributing two-thirds of the overall load- bearing capacity. DISCUSSION Many factors affect graft subsidence after sur- gery. These include the closeness of fit of the bone graft in the vertebral body mortise, the surface area of contact between the bone graft and ver- tebral body, and the quality of the contact sur- faces.12 Historically, the literature has emphasized the importance of the contact surface between the graft and the vertebral endplate.1,12,14,15 The ventral vertebral body cortex can provide a significant advantage as a buttress. It is capable of bearing axial loads much more effectively than softer can- cellous bone.16-19 The centrum of the vertebral endplate is significantly thinner and weaker than its periphery, thus increasing the risk of subsi- dence and fracture through the central portion of the endplate.15,20-22 In 1998, Wang et al. concluded that a greater construct strength is achieved when the cortical portion of a graft is positioned in line with the ventral cortical surface.13 Theoretically, an even more favorable situation exists when the bone graft has nearly the same diameter as the vertebral body, since the bone graft can thus contact the entire cortical margin of the vertebral body in the region of the endplate. However, in clinical situations, fashioning a bone graft with a diameter similar to that of the vertebral body can be challenging. There are also several experimental studies that support the importance of the contribution of the cortical shell to the vertebral strength. Faulkner et al. estimated that the cortical shell Yonsei Med J Vol. 46, No. 3, 2005 contributed 12% of the total vertebral strength in healthy individuals and 56% in osteoporotic ones.20 Burr et al. concluded from a parametric finite element study that the shell carries ap- proximately 50% of the vertebral force in a young, healthy spine, and approximately 90% in a spine with advanced osteoporosis.23 Rockoff et al. concluded that the cortical shell accounts for 45-75% of vertebral strength, and that a greater portion of axial loads are transferred via the cortical shell in older individuals compared to young ones.24 Yoganadan et al. also concluded that the cortical shell accounts for approximately 40% of vertebral strength.25 Based on this information, the use of the ver- tebral body margins to buttress an axial load takes advantage of the strength of the cortical shell and the boundary effect. The boundary effect is herein defined as the enhanced buttressing of an axial load provided by supporting the load at the edge of an inhomogeneous vertebral body that is denser at its periphery.1 The fin components of DOC VCSS were designed to enhance the axial load-bearing ability of the construct based on this concept. The fins need only pass beyond the cortical edges of the vertebral body in the region of the endplate to provide an advantage for axial load bearing. The condition C specimens (fins and screws) yielded the greatest stiffness, whereas the condi- tion A specimens (fins alone) showed the least stiffness. Statistically, the condition A specimens showed significantly lower stiffnesses compared to those subjected to conditions B and C. However, no statistically significant difference was found between condition B and condition C. This finding suggests that construct stiffness is predominantly affected by screw insertion itself more than by the fin components. In the model used herein, the fins were simply resting against the superior endplate, and did not penetrate through the endplate. Moreover, there was no bone graft placed against the fin. This is one of the limitations of the model employed, because the fin was originally designed to be positioned between the cortical margin of the endplate and the bone graft. If a bone graft is used, and the fins are positioned properly between the bone graft and the endplate, the results might be altered. Although no statistically