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

This technical brief examined how prolonged anterior shear loading alters the mechanical integrity of the annulus fibrosus in a porcine cervical spine model. Twelve C3/C4 motion segments were assigned to either a shear-loaded group, which received 100 N static anterior shear for one hour, or an unloaded control group. Following shear loading, two anterior annulus samples were harvested from each specimen: one underwent circumferential tensile testing to assess fiber-dominated mechanics, and the second underwent peel testing to quantify interlamellar adhesion. Circumferential tensile stiffness, strength, and toe-region behavior were unchanged after shear exposure. In contrast, interlamellar adhesive stiffness decreased by 52% and adhesive strength by 46% in shear-loaded samples, indicating substantial weakening of the delamination interface. These findings demonstrate that even moderate-duration shear can selectively degrade the interlamellar matrix without affecting circumferential tensile properties, suggesting a mechanistic pathway through which shear forces may initiate early disc damage and destabilization.