Impact response of the intervertebral disc in a finite-element model

Study Design. A three-dimensional nonlinear poroelastic finite-element mode l of a vertebra disc was used to analyze the biomechanical effects of impac t loading on the spinal segment. Objectives, To predict changes in biomechanical parameters such as intradis cal pressure, dynamic stiffness, stresses in the endplate region, and the s hock-absorbing mechanism of the spine under different impact duration/loadi ng rates, and to investigate the relation between the rate of loading and t he fracture potential of the vertebral body. Summary of Background Data. It is not practical to discern the role of impa ct duration using experimental protocols. Analytical studies are better sui ted to this purpose. However, previous poroelastic finite-element models of the motion segments have dealt mostly with creep phenomena. Methods. A three-dimensional, L3-L4 motion-segment, finite-element model wa s modified to incorporate the poroelastic properties of the disc, endplate, and cancellous core, and thus simulate the shock-absorbing phenomena. The results were analyzed under variable impact durations for a constant maximu m compressive impact load of 3 kN. Results. For a shorter impact duration and a given F-max, relatively high c ancellous core pressure was generated as compared with a case of long impac t duration, although the amount of impulse was increased. In contrast, rela tively constant pore pressures were generated in the nucleus regardless of the impact duration. The changes in spinal segment stiffness as a function of impact duration indicated that for a shorter duration of impact, high dy namic stiffness increases the stability of the spinal segment against the i mpact load. However, the corresponding increase in stresses within the vert ebral body and endplate may produce fractures. Conclusions. The finite-element technique was used to address the role of i mpact duration in producing trauma to the spinal motion segment. Within the limitations of the model, the results suggest that fractures are likely to occur under shorter impact duration conditions. Depending on the strength of the region, a fracture may be initiated in the endplate region or the po sterior wall of the cortical shell. The nucleus pressure is independent of the impact duration and depends only on the magnitude of the impact force.