Introduction: Lumbar disc herniation is a common cause of lower back pain, resulting in significant socioeconomic burdens. Unfortunately, herniation etiology is not well understood, partially due to challenges in provoking herniation in vitro [1]. Previous studies suggested that flexion increased herniation risk. Thus, the objective of this study was to use a finite element modeling approach to evaluate herniation risk under torque-driven flexion, which represents most in vitro setups, and the more physiologically representative muscle-driven flexion.

Methods: Triphasic bovine caudal disc motion segment finite element models were created based on our recently validated multiscale structure-based model, with model parameters defined from our previous work [2]. Models were developed to represent torque-driven flexion with the instantaneous center of rotation (ICR) located on the disc, and muscle-driven flexion with the ICR located anterior of the disc; ten cases were investigated (Figure 1A). All models were loaded in three steps: 1) free swelling in 0.15 M phosphate-buffered solution until equilibrium, 2) 0.5 MPa axial compression, and 3) 5° flexion. Model predictions of herniation were based on AF mechanics data in the literature. Specifically, effective strain and fiber stretch were analyzed in the posterolateral AF. Bulk disc deformation and intradiscal stress-strain distributions were also evaluated.

Results: The average effective strain in the posterolateral inner and outer AF was 0.25 and 0.26 before flexion, with no elements exceeding the failure threshold. Torque-driven flexion did not affect the effective strain. By contrast, as the ICR moved anteriorly with muscle-driven flexion, the effective strain in the posterolateral AF increased, and failure was predicted for Cases H – J (Figure 1B – top). Overall, failure was predicted in the outer AF before the inner AF based on the effective strain criterion.

Average fiber stretch in the posterolateral inner and outer AF was 1.11 and 1.07 before flexion and increased pseudo-linearly with ICR distance; no elements exceeded the failure threshold pre-flexion. For the inner AF, average fiber stretch was 1.21 in Case D, resulting in >60% of the elements exceeding the failure threshold. For the outer AF, the average fiber stretch did not reach the threshold until Case H, where ~50% of the elements failed (Figure 1B – bottom). Overall, failure was predicted in the inner AF before the outer AF based on the average fiber stretch criterion.

Discussion: Simulating flexion under a range of ICR locations helps understand intradiscal mechanics under various physiological activities (Figure 1C). Torque- and muscle-driven flexion resulted in distinct disc mechanics, resulting in disparate predictions for AF failure, a widely acknowledged precursor for disc herniation. Particularly, torque-driven flexion did not induce herniation, agreeing with the limited success reported in the literature in provoking herniation through in vitro experiments. However, muscle-driven flexion greatly increased the likelihood of in vitro herniation through the posterolateral AF, agreeing with the AF failure location observed for herniations. In conclusion, this study provided a computational framework for designing in vitro testing protocols that can advance the assessment of disc failure behavior.