Effect of Dynamic Stabilization Device Stiffness on Disc Loading under Compression

An anatomically realistic three-dimensional (3D) finite element (FE) model of L4–L5 motion segment was created. The intact model was further modified to simulate two different disc degeneration grades, slight and moderate, at the L4–L5 segment. The implanted models were accordingly developed by incorporating a pair of 6mm diameter straight longitudinal rods position bilaterally into the degenerated models. In order to study the effect of implant stiffness on disc loading under compression, the stiffness of the longitudinal rod was varied between 1 and 83,000 N/mm in discrete steps. The intact, degenerated and implanted models were then exercised by applying 3,000N compressive force on the superior surface of L4 vertebra to investigate the load transmission characteristics of different implants. The results showed that the side effect of a degenerated disc is obvious for the lumbar segment load bearing capacity. The stiffness of the segment was increased due to the stiffer disc, which is caused by the severity of disc degeneration. The implant stiffness of lower than 2,000N/m has greater effect on disc loading and segment behavior. For an implanted model, the predicted annulus stress is close to the intact model when the implant stiffness is around 2000N/m. In addition, the effect of stiffness became less significant beyond this value. For dynamic stabilization purpose, the stiffness implant is suggested to be lower than 2000N/m