Influence of Complex Loading Conditions on Intervertebral Disc Failure

STUDY DESIGN: High resolution imaging investigation of the failure of ovine lumbar intervertebral discs under complex loading. OBJECTIVE: To investigate how different loading combinations influence the mechanism and extent of intervertebral disc failure. SUMMARY OF BACKGROUND DATA: Even though there has been extensive research on how an intervertebral disc fails under various conditions, failure mechanisms remain unclear. In addition, the influence of different loading directions on the mode and extent of failure under complex loading was never systematically investigated. METHODS: Thirty ovine lumbar spinal segments were loaded in a newly developed, dynamic, 6-degree-of-freedom (6-DOF) disc loading simulator under five combinations of the following loading parameters: 0°-13° flexion, 0°-10° lateral bending, 0°-4° axial rotation, 0-800 N axial compression. A total of 1000 cycles at 2 Hz were done. After testing, imaging of the discs was performed in an ultra-high field magnetic resonance imaging (11.7 T) scanner and with a micro-computed tomography scanner. RESULTS: A total of 13 large endplate junction failures (EPJFs) occurred, of which all but one maintained an intact cartilaginous endplate. Ten out of 13 EPJFs occurred caudally. Four solely annulus failures occurred affecting only the outer posterior annulus. A herniation was not observed. The maximum moments measured in any group (median) were 52.5 N · m flexion, 16.5 N · m lateral bending, and 14.0 N · m axial rotation. CONCLUSION: Complex loading protocols could lead to EPJFs (76%) and annulus failures (24%) in vitro. The combination of flexion, lateral bending, axial rotation, and axial compression bears the highest risk for caudal EPJF. Flexion without lateral bending and vice versa has the lowest risk for failure. Both axial compression and axial rotation seem to have a smaller influence than flexion and lateral bending. It seems that a herniation requires an additional failure of the cartilaginous endplate, likely initiated by further axial compressive load. LEVEL OF EVIDENCE: 4.