Master of Kinesiology (MKin)
Faculty of Science
Dr. Diane Gregory
Introduction: Intervertebral disc (IVD) herniation is a common injury to the IVD and a frequent source of low back pain. IVD herniation occurs when the nucleus pulposus migrates through both the inter- and intralamellar matrices of the annulus fibrosus (AF). There are a number of mechanical risk factors associated with herniation, including repetitive flexion and twist. In vivo, twist combined with repetitive flexion has been associated with increased experience of low back pain and herniation. Additionally, in vitro, when repetitive flexion is combined with twist, IVD herniation occurs more easily than repetitive flexion alone. However, the mechanisms behind this relationship are not well understood. Therefore, the purpose of the current study was to determine the effect of twist on the mechanical properties of the inter- and intralamellar matrices.
Methods: Thirty-six bovine IVDs from the caudal region were exposed to a combination of either 0° or 12° of static twist and 0N or 1000N of compression for two hours. Following mechanical loading, three samples were dissected from each disc. One sample, containing at least two adjacent layers of the AF, was mechanically delaminated to measure interlamellar matrix strength. The variables of interest from this test were adhesion peel strength and peel strength variability. The other two samples were single layers of the AF which were mechanically tested in tension until failure. The single layer samples were pulled perpendicular to the orientation of the collagen fibres in order to measure intralamellar matrix strength. The variables of interest from the single layer tensile test were the stress and strain at the end of the toe region, Young’s modulus, and the yield point. Finally, one IVD per condition underwent histological analysis in order to visually assess any damage within the AF.
Results: There were significant differences between the twisted and untwisted samples on the mechanical properties of the intralamellar matrix. Specifically, lower stress at the end of the toe region (p=0.006), lower Young’s moduli (p=0.010), and lower yield points (p<0.001) were observed in the twisted samples compared to the untwisted samples. Additionally, histological analysis demonstrated more disruption within individual lamellae of the twisted samples compared to the untwisted samples. However, there were no significant differences between the twisted and untwisted samples on the mechanical properties of the interlamellar matrix. Additionally, there was no effect of compression on the inter- or intralamellar matrices.
Discussion and Conclusion: Twist, regardless of compression exposure, negatively affected the strength of the intralamellar matrix. This may explain why, when twist is combined with repetitive flexion, the occurrence of IVD herniation is accelerated compared to repetitive flexion alone. A mechanism of IVD herniation is migration through the AF layers via clefts within the intralamellar matrix, therefore, if the intralamellar matrix is weakened, such as from twist, then this progression will occur more easily and thus herniation will occur more easily. These findings suggest that twist may put the IVD at an increased risk for injury as a result of damage to the intralamellar matrix.
Harvey-Burgess, Maxine, "The Effect of Twist on the Mechanical Properties of the Intervertebral Disc" (2017). Theses and Dissertations (Comprehensive). 1958.