An Investigation of the Role of Dynamic Axial Torque on the Disc Herniation Mechanism
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Background: Disc herniations are common and have been demonstrated as one potential source of low back pain. To date epidemiological studies have found associations between lifting, lifting and twisting and twisting with increased risk in the development of disc herniations (Greenough and Fraser, 1994, Kelsey et al., 1984, Mundt et al., 1993). Subsequent, in vitro investigations were able to produce disc herniations through repeatitive flexion extension motions on cervical porcine functional spinal units (Callaghan and McGill, 2001). However, in vitro investigations on axial torque have drawn mixed conclusions and controversy remains on the role it plays with respect to disc herniations (Farfan et al., 1970, Adams et al., 1981). Therefore, the work in this thesis was to investigate the role of dynamic axial torque on the disc herniation mechanism. Methods: Porcine cervical spines were used as they are a good approximation to the human lumbar spine (Yingling et al., 1999). The study design involved repetitive flexion extension motions of the spinal units either preceded or followed by dynamic axial torque. During axial torque the spinal units were loaded to 17.5 Nm (standard deviation = 0.5 Nm) of dynamic axial torque for either 2000 or 4000 testing cycles. These spinal units were compared to spinal units that were loaded in repetitive flexion extension motions only and axial torque only. The spinal units were tested in a servohydraulic dynamic testing machine, combined with a custom jigs which allowed loading in flexion/extension, axial torque and compression. Plane film radiographs with contrast in the nucleus were obtained at regular intervals during and following the mechanical testing. Final dissection determined the disc injury patterns. Results and Discussion: Examination of the sectioned intervertebral discs indicated axial torque in combination with repetitive flexion extension motions, regardless of order, encouraged radial delamination. While, repetitive flexion extension motion alone encouraged posterior or posterolateral herniation patterns. Axial torque alone was unable to initiate a disc herniation. There was an increase in both rotation and stiffness of the intervertebral disc in response to repeated axial torque. There were no differences in rotation and stiffness between the groups. Both x-ray images and computed tomography scans were equally as good at identifying posterior or posterolateral herniations but were not good at detecting radial delamination.