Anterior-Posterior Shear Stiffness of C6/C7 Functional Spinal Units: Effects of Mechanical and Intrinsic Factors

Determine the effect of mechanical and intrinsic (demographic and anthropometric) variables on anteroposterior shear stiffness in cervical functional spinal units (FSUs), and provide response corridors for computer model validation.

Thirty-nine human FSUs were loaded non-destructively in statically rotationally unconstrained pure shear up to 200N, in anterior and posterior directions, at rates of 1, 10, and 100 mm/s. Due to the inherent non-linear behavior, the resulting stiffness response was analyzed as bilinear (lower Phase I and higher Phase II). The effect of mechanical (direction and rate) and intrinsic (age, sex, anteroposterior endplate dimension, disc and facet joint degeneration, bone volume fraction) factors on stiffness were evaluated using a linear mixed models approach.

In Phase I, specimens were 67% stiffer in anterior shear than posterior (p < 0.001), and 25% less stiff at 100 mm/s than at 1 and 10 mm/s (p < 0.001) across both directions. In Phase II, specimens were 42% stiffer in anterior shear than posterior (p < 0.001) and displacement rate was positively correlated with stiffness (p < 0.001). Males were 8% stiffer than females on average (p = 0.001), although a significant interaction between sex and direction (p = 0.008) showed that this was more prominent in the anterior direction, where males were 12% stiffer. Disc degeneration was positively correlated with stiffness (p = 0.003).

The unique data measured in this study provides improved understanding of lower cervical spine shear response and provide critical data for validating computational models of the spine.