Introduction: Repetitive lifting (fatigue) and rapid application of high compressive loads (sudden overload) cause intervertebral disc injuries and posterolateral herniation, potentially impinging nerve roots1,2,3,4. Studies have shown that fatigue loading involving isolated and combined bending and twisting motions can alter six degree of freedom (6DOF) mechanical properties, leading to herniation4,5. However, the 6DOF stability between these fatigue postures and resulting failure mechanics have not been studied. This study aimed to compare the 6DOF mechanical properties and sudden overload failure between three combinations of lifting postures after fatigue.
Methods: Twenty-four sheep lumbar segments (L4-L5) were dissected, keeping facet joint capsules and anterior and posterior longitudinal ligaments intact. Before testing, specimens were randomly assigned to one of three combined loading postures: 13° flexion, 2° left axial rotation (F+R); 10° left lateral bending, 2° left axial rotation (LB+R); 13° flexion, 10° left lateral bending (F+LB). Each specimen underwent testing in a hexapod robot in the following order: Overnight hydration in a phosphate buffered saline bath, pre-fatigue 6DOF characterisation, 10,000 cycles of combined fatigue for each posture at 1 Hz with 1.7 MPa of compressive load applied, post-fatigue 6DOF characterisation and failure by rapid axial compression to 3 mm at 400 mm/min in each respective posture. A repeated measures ANOVA was performed to identify significant differences in 6DOF stiffness after fatigue within each posture. A one-way ANOVA compared failure stress and stiffness between postures (p<0.05). Bonferroni tests determined post-hoc significant differences between postures. Failure mode was determined via visual inspection.
Results: One specimen per group was removed due to technical errors during testing. Significant decreases in 6DOF stiffness after fatigue, for all postures, were found in right lateral shear (p<0.035), left/right axial rotation (p<0.045) and flexion (p<0.012). Posterior shear stiffness decreased significantly after F+R (p<0.001) and F+LB (p=0.031) fatigue. Extension significantly decreased only after F+LB fatigue (p=0.040). No significant change was measured in anterior shear, right lateral bending and left lateral bending after fatigue for all postures (p>0.193). During sudden overload, there were no significant overall differences in failure stress between the three postures (p>0.142). However, the overall effect of posture was significant for stiffness during overload failure (p=0.045). Post-hoc tests revealed that LB+R specimens were marginally significantly stiffer than F+LB (p=0.05). Visually, the postures including flexion had a higher occurrence of posterior or posterolateral herniation.
Discussion: Fatigue in combinations of postures showed significantly different mechanical effects on the tested spinal segments. During combined flexion and lateral bending postures, the herniations occurred on the contralateral side to bending. Reduced ability to resist shearing deformation, from 6DOF testing, may lead to annulus delamination from large interlamellar shearing strains, leaving the disc more susceptible to herniation during combined lateral bending and flexion. This research contributes to a deeper understanding of the complexity of herniation and its underlying mechanisms.