Do bending, twisting, and diurnal fluid changes in the disc affect the propensity to prolapse? A viscoelastic finite element model

Study Design. A finite element model of a lumbar motion segment was constructed. Objectives. The model was directed toward understanding the effect of compression, bending and twisting, and diurnal fluid changes in the disc on the propensity to disc prolapse. Tensile stresses in the anulus fibers were computed and used to determine the successive steps required to create a fissure in the disc. Summary of Background Data. Disc prolapse is more likely under combined loading involving compression and bending and twisting. Changes in fluid content in the disc also affect the mechanical behavior of the disc. Methods. The three-dimensional model accounted for the viscoelastic material properties of the anulus fibers and ligaments. Diurnal fluid exchange was simulated by changing the fluid content in the nucleus of the disc. Combined with bending and twisting, a compressive load was applied at different loading rates. Results. The maximum tensile stress in the anulus fibers always occurred in the fibers at the inner posterior anulus at the junction of the disc and the endplate. Of the three models tested, the ''weakest'' (or the first to fail) was the saturated disc subjected to compression and bending and twisting. As the loading rate increased, anulus fiber failure was initiated at a lower value of compressive load. An increasing compressive load applied to a flexed, or fissure propagation, starting at the posterior failure, or fissure propagation, starting at the posterior inner anulus at the junction of the disc and the endplate. Conclusions. The results from this study suggest that there are several key factors involved in the initiation and propagation of anulus failure: axial compressive load, bending and twisting, and disc saturation. If one of these is lacking, anulus failure is harder to achieve.