Poster Presentation 51st International Society for the Study of the Lumbar Spine Annual Meeting 2025

Introduction: Anterior shear loading occurs primarily when the trunk is in flexion and can result in deformation of the intervening discs. If deformation is excessive enough, damage can occur. The structures of the spine aid in supporting the disc when under shear leading; however, animal research has shown that static prolonged shear force applied to the lumbar spine can lead to disc degeneration [1]. The objective of this study was to quantify the mechanical damage to the annulus fibrosus following exposure to static anterior shear load using a porcine model.

Methods: The C2/C3/C4 functional spinal units (FSUs) were excised from 12 previously frozen porcine cervical spines. The porcine cervical spine has been shown to have anatomical and biomechanical similarities to the human lumbar spine [2,3]. The dens and posterior bony elements of the vertebrae were removed to isolate shear force effects on the disc (Figure 1A). Specimens were randomly divided into two groups: six underwent 100 N of static anterior shear force on C3/C4 for 1 hour; six controls were loaded at 0 N for 1 hour.
To apply the anterior shear load, a pin was inserted through the C4 vertebra in an anterior-to-posterior direction which was secured to a material testing system to prevent rotational motion. The C2/C3 FSU was clamped to prevent movement, while the C3/C4 disc and C4 vertebra remained free to move. Loading was applied to the C4 vertebra such that the result was anterior shear of C3 with respect to C4 (Figure 1A and 1B). To minimize dehydration, specimens were wrapped in PBS-soaked gauze. Following shear loading, two annulus samples were dissected from the disc for mechanical testing. The first sample contained approximately four adjacent lamellae and was used to quantify tensile properties in the circumferential direction of the annulus while the second sample was dissected into a peel test configuration and was used to quantify the mechanical properties of the interlamellar matrix.

Results: One hour of static anterior shear did not significantly affect the annular tensile properties in the circumferential direction. However, the shear loading did significantly damage the interlamellar matrix, reducing both mean peel stiffness by 52% (p=0.02) and lamellar adhesive strength by 46% (p=0.02) compared to controls (Figure 1C).

Discussion: The reduced adhesive stiffness and strength suggest localized damage to the interlamellar matrix caused by static anterior shear loading. The shear force likely causes individual lamellae to move relative to one another, leading to damage and localized failure in the interlamellar matrix. This would, in turn, reduce the annulus’s resistance to delamination and result in a compromised disc. This study provides insight into the mechanisms of disc damage induced by shear loading.