A New Slant on Shear Loading: Uncovering Its Effect on the Intervertebral Disc.

Prolonged anterior shear loading may contribute to disc degeneration by damaging the annulus fibrosus. To address this, annular mechanical properties were quantified following static shear loading using a porcine model. Twelve porcine cervical motion segments were dissected, with posterior bony elements removed to isolate shear to the intervertebral disc. Specimens were randomized into two conditions: (1) shear-loaded (100 N static anterior shear applied to C3/C4, n = 6) or (2) control (0 N, n = 6). Shear force was applied via a pin through C4, secured to a testing system to prevent rotation, while C3 was clamped such that anterior shear of C3 with respect to C4 resulted. Following 1 h of loading, two anterior annulus samples were extracted per specimen. The first sample underwent circumferential tensile testing, while the other was prepared for a peel test to assess interlamellar adhesion. Tensile properties in the circumferential direction remained unchanged after shear loading. However, interlamellar adhesive stiffness decreased by 52% (p = 0.02), and adhesive strength dropped by 46% (p = 0.02) in shear-loaded specimens compared to controls. Shear loading weakened the interlamellar matrix, reducing resistance to delamination and compromising disc integrity. These findings suggest that prolonged shear loading may contribute to early-stage disc damage.