Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compressive force

Objective. To determine whether repeated motion with low magnitude joint forces, and flexion/extension moments consistently produce herniation in a non-degenerated, controlled porcine spine motion segment. Design. Combined loading (flexion/extension motions and compressive forces) was applied to in vitro porcine functional spinal units. Biomechanical and radiographic characteristics were documented. Background. While most studies performed in vitro have examined uniaxial or fixed position loading to older specimens, there have been few studies that have examined whether ‘healthyʹ intervertebral discs can be injured by low magnitude repeated combined loading. Methods. Porcine cervical spine motion segments (C3–C4) were mounted in a custom jig which applied axial compressive loads with pure flexion/extension moments. Dynamic testing was conducted to a maximum of 86 400 bending cycles at a rate of 1 Hz with simultaneous torques, angular rotations, axial deformations recorded for the duration of the test. Results. Herniation (posterior and posterior-lateral regions of the annulus) occurred with relatively modest joint compression but with highly repetitive flexion/extension moments. Increased magnitudes of axial compressive force resulted in more frequent and more severe disc injuries. Conclusions. The results support the notion that intervertebral disc herniation may be more linked to repeated flexion extension motions than applied joint compression, at least with younger, non-degenerated specimens.