Mapping of Intervertebral Disc Annulus Fibrosus Compressive Properties is Sensitive to Specimen Boundary Conditions.

Predicting the mechanical behavior of the intervertebral disc (IVD) in health and in disease requires accurate spatial mapping of its compressive mechanical properties. Previous studies confirmed that residual strains in the annulus fibrosus (AF) of the IVD, which result from non-uniform extracellular matrix deposition in response to in-vivo loads, vary by anatomical region (anterior, posterior, and lateral) and zone (inner, middle, and outer). We hypothesized that as the AF is composed of a nonlinear, anisotropic, viscoelastic material, the state of residual strain in the transverse plane would influence the apparent values of axial compressive properties. To test this hypothesis, axial creep indentation tests were performed, using a 1.6mm spherical probe, at nine different anatomical locations on bovine caudal AFs in both the intact (residual strain present) and strain relieved states. The results showed a shift towards increased spatial homogeneity in all measured parameters, particularly instantaneous strain. This shift was not observed in control AFs, which were tested twice in the intact state. Our results confirm that time-dependent axial compressive properties of the AF are sensitive to the state of residual strain in the transverse plane, to a degree that is likely to affect whole disc behavior.