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

STUDY

DESIGN

A three-dimensional nonlinear poroelastic finite-element model of a vertebra disc was used to analyze the biomechanical effects of impact loading on the spinal segment.

OBJECTIVES

To predict changes in biomechanical parameters such as intradiscal pressure, dynamic stiffness, stresses in the endplate region, and the shock-absorbing mechanism of the spine under different impact duration/loading rates, and to investigate the relation between the rate of loading and the fracture potential of the vertebral body.

SUMMARY OF BACKGROUND DATA: It is not practical to discern the role of impact duration using experimental protocols. Analytical studies are better suited 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 was 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 maximum compressive impact load of 3 kN.

RESULTS

For a shorter impact duration and a given F(max), relatively high cancellous core pressure was generated as compared with a case of long impact duration, although the amount of impulse was increased. In contrast, relatively 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 dynamic stiffness increases the stability of the spinal segment against the impact load. However, the corresponding increase in stresses within the vertebral body and endplate may produce fractures.

CONCLUSIONS

The finite-element technique was used to address the role of impact 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 posterior wall of the cortical shell. The nucleus pressure is independent of the impact duration and depends only on the magnitude of the impact force.