The Effect of Axial Torsion on the Mechanical Properties of the Annulus Fibrosus

Study Design. In-vitro study of the tissue mechanics of annulus fibrosus. Objective. To determine the effect of axial torsion on the mechanical properties of the inter-and intralamellar matrices. Summary of Background Data. Axial torsion, when combined with repetitive flexion, has been associated with an increased risk of intervertebral disc herniation. However, the mechanisms behind this relationship are poorly understood. Methods. Bovine intervertebral discs (IVDs) from the caudal region were exposed to a combination of either 08 or 128 of static axial torsion and 0N or 1000N of compression for 2 hours in an attempt to created micro-damage to the IVD. Following the loading protocol, one multilayered sample and two single layer samples were dissected from the annulus fibrosus to undergo tensile testing of the inter-and intralamellar matrices. Histological staining was also performed. Results. The strength of the interlamellar matrix was not affected by axial torsion or compression, suggesting that torsion did not damage the interlamellar matrix. However, intralamellar matrix strength of samples exposed to axial torsion, regardless of compressive loading magnitude, was 48% lower than those from samples that were not exposed to torsion (P<0.001). Similarly, intralamellar matrix stiffness of samples exposed to axial torsion was 42% lower than from samples that were not exposed to torsion (P = 0.010). Additionally, histological analysis demonstrated more disruption within individual lamellae of the samples exposed to axial torsion compared with samples that were not. Conclusion. This study suggests that axial torsion damages the components of the intralamellar matrix as a result of the strain it puts on the matrix, thus making the intervertebral disc more susceptible to herniation.