INTRODUCTION: The role of torsion in the aetiology of lumbar disc herniation is unclear. While torsional movements in vivo are small, and are thus unlikely to damage lumbar discs when applied as the sole rotation, torsion may be damaging when combined with flexion. Using mechanical testing followed by micro‐CT and microscopy, the current study aimed to determine whether torsion, when combined with flexion, assists the disc herniation process.
METHODS: 17 ovine lumbar spines, age 2‐5 years, were dissected into L12, L34, and L56 motion segments (posterior elements removed). A hollow injection screw was inserted longitudinally through the inferior vertebra of each segment such that its tip contacted the centre of the nucleus. After being potted in steel rings using dental plaster, each segment was positioned in a posture of 7o flexion either with (n=30) or without (n=21) 2o of applied torsion. While maintained in their designated posture, a viscous radio‐opaque gel was injected into the nucleus of each segment using a gradual ramp‐and‐hold loading regieme until failure occurred. Following testing, motion segments were inspected using micro‐CT and oblique illumination microscopy in tandem.
RESULTS: Motion segments tested with torsion suffered disc failure at a significantly lower applied nuclear pressure (p=0.01). While fewer motion segments suffered disc failure in the flexion plus torsion group (32% vs. 65%), an equal proportion in each group (˜22%) failured via the formation of a central or mediolateral posterior radial tear involving a systematic annulus‐endplate‐annulus disruption pattern.
DISCUSSION: Torsion markedly reduces the nuclear pressure that some discs can support. Given the morphological similarities between the disc ruptures observed in this study and those seen clinically, small levels of torsion, when combined with flexion, appear to be detrimental the structural integrity of some currently undefined cohort of lumbar intervertebral discs.