Intradiscal pressurisation predicts intervertebral disc herniation: insights from a low-cost, open-source loading rig

Ex vivo biomechanical research is essential for understanding disc herniation and developing spinal treatments. However, reproducing physiologically relevant multi-axis loading remains challenging, as commercial systems are costly or mechanically restrictive. Therefore, we developed a low-cost, open-source rig that integrates with standard hydraulic testing machines and adds flexion-extension up to 45 Nm. The rig was characterised under uniaxial, cyclic, and three degree of freedom loading to confirm accurate application of loads. The capability to apply prolonged cyclic loading, and combined flexion-compression to failure, for inducing disc herniation was evaluated using bovine tail discs. Specimens (n = 18) were instrumented with a pressure probe and initially compressed (0-100 N) to quantify intradiscal pressurisation capacity prior to cyclic loading (n = 9, 100,000 cycles) or flexion-compression to failure (n = 9). Rig characterisation confirmed precise load application: under 8 Nm flexion, off-axis moments were < 0.5 Nm, with RMS errors ≤ 2% for axial compression and rotation, and ≤ 7% for lateral bending. Loads were applied accurately up to 2 Hz, with reduced accuracy at 10 Hz. The rig successfully applied prolonged cyclic loading and ultimate flexion-compression to failure, reliably inducing herniation in bovine tail discs. The pressure-force slope strongly discriminated herniation status (AUC = 0.92, p < 0.001). Logistic regression estimated a 50% herniation probability at 0.0019 MPa N⁻¹, with herniation occurring in 92% of discs above this value versus 17% below. The open-source rig enables reproducible, physiologically relevant multi-axis loading for future herniation and spinal implant device testing. Intradiscal pressurisation was a predictor of herniation in bovine tail discs.