# Combined Flexion, Torsion and Compression Drive Distinct Intervertebral Disc Failure Mechanisms Under Asymmetric, High‐Cycle Loading

## TL;DR

This study shows how different spinal movements cause specific types of disc damage, suggesting new ways to treat disc degeneration.

## Contribution

The study identifies distinct failure mechanisms in intervertebral discs under asymmetric, high-cycle loading using a novel bioreactor system.

## Key findings

- Symmetrical loading preserved disc structure and cell viability, while asymmetrical loading caused fissures and delamination in the outer annulus fibrosus.
- Asymmetrical and frequent loading promoted cell death in the nucleus pulposus and structural damage in the annulus fibrosus.
- Region-specific responses suggest independent failure mechanisms contributing to disc degeneration.

## Abstract

Recent advancements in next‐generation bioreactors have substantially improved the simulation of complex, detrimental spinal mechanics in ex vivo intervertebral disc models. This study investigated intervertebral disc responses to combined flexion, torsion, and static compression. A range of loading frequencies, magnitudes, and patterns was applied to identify conditions that contribute to disc degeneration under complex motion.

Twelve bovine coccygeal intervertebral discs (mean age 9 months) were subjected to three distinct loading regimes, with four samples per condition. Static compression of 0.1 MPa was combined with: (1) symmetrical 3° flexion/extension and 2° torsion, (2) symmetrical 6° flexion/extension and 4° torsion, and (3) asymmetrical 6° flexion and 4° torsion. Loading frequencies and durations ranged from 0.2 Hz for 1 h in symmetrical loading to 1 Hz for 2 h in asymmetrical loading over a 14‐day period. Structural integrity, cell viability, tissue composition, and molecular responses were evaluated using histology, biochemical assays, and gene expression analysis.

Lower‐cycle symmetrical flexion/extension and torsion, regardless of magnitude, preserved disc structure and maintained a high cell viability (88% ± 14%) across all disc regions. Higher cycle numbers and asymmetrical loading induced significant fissures in the outer annulus fibrosus (AF) on the tensed side (p < 0.01) and delamination on the compressed side. This structural damage occurred in AF regions with high cell viability (81% ± 17%), whereas significantly reduced cell viability was observed in the inner AF (30% ± 33%) and nucleus pulposus (28% ± 35%).

Under conditions of asymmetrical and more frequent loading, complex motion involving flexion, torsion, and compression led to structural damage in the outer disc regions and promoted cell death in inner regions. These region‐specific responses suggest the independent development of distinct failure mechanisms contributing to disc degeneration. They also underscore the importance of developing targeted strategies that address both structural integrity and cellular resilience in degeneration models and therapeutic interventions.

Using advanced multiaxial bioreactor systems, this study investigated the effects of flexion, torsion, and static compression on whole intervertebral discs. High cycle asymmetric loading resulted in cell death in the nucleus pulposus and structural changes in the annulus fibrosus. This suggests independent development of distinct failure mechanisms contributing to disc degeneration.