Abstract
The effects of intervertebral joint stiffness as well as implant stiffness on the load sharing characteristics across the stabilized motion segment were investigated using the finite element technique. Intervertebral joint stiffness variations were modeled by using various Young's modulus ( Edisc ) values of the ground substance that had replaced the nucleus pulposus. Variations in the implant stiffness were made by simulating the use of three different pedicle screw-plate instrumentation constructs: 1) bilateral fixation using stainless steel (ST) plates; 2) unilateral fixation using one stainless steel plate; and 3) bilateral fixation using Plexiglas (PL) plates. For the case of constant fixation device stiffness, the axial force across the VSP plate changed in a step-wise manner with the changes in the "disc" stiffness. The axial force carried by the plates was about 10% of the applied compression for Edisc ≥ 1000 MPa. The plates carried about 40% of the applied compression in the intact and degenerated disc ( Edisc = 4.2 and 8.2 MPa) models, as compared to 100% load by the plates for the totally denucleated case. For the case of constant disc stiffness ( Edisc = 8.4 MPa), ST and PL plates were predicted to take over 38% and 18% of the applied compression load, respectively, while for the uniaxial fixation this value was 24%. The load borne by the plates decreased with a decrease in the stiffness of the device. Larger average stresses were predicted in all spinal components when a less rigid VSP system was used, although still lower than the intact stress values. By contrast, the stresses in the device components were predicted to decrease by a considerable amount due to the use of PL plates. These results clearly demonstrate that both the spinal segment and fixation devices are important load supporting components. However, further studies are required to determine an optimum range of stiffness of the unstable spine-graft-implant construct for fast and solid fusion to occur.
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