Stress shielding in box and cylinder cervical interbody fusion cage designs

Study Design. A numerical analysis of stress shielding of the bone graft in box and cylinder interbody fusion cages was performed. Objectives. To evaluate the stress shielding characteristics of box and cylinder interbody fusion cages for the cervical spine with regard to their rigidity and contiguous pore size. Summary of Background Data. Cage design has been shown to influence loading of the augmented bone graft tissue. In addition, a large contiguous pore design is believed to be important to avoid stress shielding effects. Methods. A two-dimensional axisymmetric, biphasic finite-element model of the cage incorporating the bone graft and the adjacent vertebral bodies was developed. Analysis was performed in two parts. First, the vertebrae were loaded by an axial compressive force, and second, the effect of vertebral penetration by the interbody cage was simulated. Results. Straining of bone graft in the box cage was generally lower than that of the cylinder cage. The strains in the cylinder cage were seen to be more uniformly distributed, whereas in the box cage straining was concentrated in the graft under the endplates. Vertebral penetration by the cylinder cage resulted in significant straining of the bone graft (28% strain), whereas lower strains were determined in the box cage ( a maximum of 17% strain). Conclusions. The central pore in the box design does not seem as effective as the fully open cylinder cage in transferring loads to the augmented graft tissue. Early penetration of the adjacent vertebrae by the cylinder cage may provide early postoperative stability and load the graft tissue, thereby imparting the necessary signals for fusion.