The role of axial torque in disc herniation

BackgroundEpidemiological studies have found associations between lifting, lifting and twisting and twisting alone with increased incidence of disc herniation. This study investigated the role of repeated dynamic axial torque/twist combined with repeated flexion on the disc herniation mechanism.MethodsPorcine cervical spines were tested in one of the following four testing protocols: flexion–extension only; axial torque/twist only; flexion–extension followed by axial torque/twist; or axial torque/twist followed by flexion–extension. Plane film radiographs and computed tomography with contrast in the nucleus were obtained at regular intervals during and following the mechanical testing process together with final dissection to determine the disc injury patterns.FindingsAxial torque/twist in combination with repetitive flexion extension motion, regardless of order, encouraged radial delamination within the annulus (67.5% of specimens). Alternatively, repetitive flexion motion alone encouraged posterior or posterolateral nucleus tracking through the annulus. Axial torque/twist alone was unable to initiate a disc herniation. Both X-ray images with contrast and computed tomography were not good at detecting radial delamination observed during dissection.InterpretationThe clinical relevance is that twisting may cause more radial delamination while repeated flexion causes more posterior tracking of the nucleus giving guidance for both prevention and rehabilitation decisions. In addition, X-ray images with contrast are not effective at detecting the radial delamination which was exacerbated by combined loading in flexion extension and axial torque/twist.