Numerical Prediction of the Mechanical Failure of the Intervertebral Disc under Complex Loading Conditions

Abstract

Finite element modeling has been widely used to simulate the mechanical behavior of the intervertebral disc. Previous models have been generally limited to the prediction of the disc behavior under simple loading conditions, thus neglecting its response to complex loads, which may induce its failure. The aim of this study was to generate a finite element model of the ovine lumbar intervertebral disc, in which the annulus was characterized by an anisotropic hyperelastic formulation, and to use it to define which mechanical condition was unsafe for the disc. Based on published in vitro results, numerical analyses under combined flexion, lateral bending, and axial rotation with a magnitude double that of the physiological ones were performed. The simulations showed that flexion was the most unsafe load and an axial tensile stress greater than 10 MPa can cause disc failure. The numerical model here presented can be used to predict the failure of the disc under all loading conditions, which may support indications about the degree of safety of specific motions and daily activities, such as weight lifting.

Keywords: anisotropic hyperelastic; annulus fibrosus; finite element analysis; herniation; intervertebral disc; ovine model.

Figure 1

(a) Section of the annulus fibrosus in which the stress was analyzed; (b) division in subsections of the sections represented in (a).

Figure 2

Tensile axial stress generated in the annulus fibrosus in the loading cases listed in Table 2. The stress is expressed in MPa. Areas with negative stresses are shown in gray.

Figure 3

Tensile circumferential stress generated in the annulus fibrosus in the loading cases listed in Table 2. The stress is expressed in MPa. Areas with negative stresses are shown in gray.

Figure 4

Average tensile axial stress generated by the loading cases listed in Table 1 in POST-LAT1 section. The red and the green lines indicated the thresholds identified as “high risk” and “low risk” of failure, respectively.

Figure 5

Average tensile circumferential (Circum.) stress generated by the loading cases listed in Table 1 in POST-LAT1 section. The red and the green lines indicated the thresholds identified as “high risk” and “low risk” of failure, respectively.

Figure 6

Average tensile stress generated by the loading cases listed in Table 1 at the interface between the annulus and the caudal endplate in the POST LAT-2 section. The red and the green lines indicated the thresholds identified as “high risk” and “low risk” of failure, respectively.

Figure 7

Axial stress generated in the POST section by combined loads. AC means axial compression, FL means flexion, AR means axial torsionrotation, LB means lateral bending.

Figure 8

Circumferential (Circum.) stress generated in the POST-LAT1 section by combined loads. AC means axial compression, FL means flexion, AR means axial torsionrotation, LB means lateral bending.

Figure 9

Axial stress generated in the POST section by pure loads. AC means axial compression, FL means flexion, AR means axial torsionrotation, LB means lateral bending.

Figure 10

Circumferential (Circum.) stress generated in the POST section by pure loads. AC means axial compression, FL means flexion, AR means axial torsionrotation, LB means lateral bending.