Shear mechanical properties of human lumbar annulus fibrosus

Function, failure, and remodeling of the intervertebral disc are all relate d to the stress and strain fields in the tissue and may be calculated by fi nite element models with accurate material properties, realistic geometry, and appropriate boundary conditions. There is no comprehensive study in the literature investigating the shear material properties of the annulus fibr osus. This study obtained shear material properties of the annulus fibrosus and tested the hypothesis that these properties are affected by the amplit ude and frequency of shearing, applied compressive stress, and degenerative state of the tissue. Cylindrical specimens with an axial orientation from seven nondegenerated and six degenerated discs were tested in torsional she ar under dynamic and static conditions. Frequency sweep experiments over a physiological range of frequencies (0.1-100 rad/sec) at a shear strain ampl itude of 0.05 rad were performed under three different axial compressive st resses (17.5, 25, and 35 kPa). At the largest compressive stress, shear str ain sweep experiments (strain amplitude range: 0.005-0.15 rad at a frequenc y of 5 rad/sec) and transient stress-relaxation tests (shear strain range: 0.02-0.15 rad) were performed. The annulus fibrosus material was less stiff and more dissipative at larger shear strain amplitudes, stiffer at higher frequencies of oscillation, and stiffer and less dissipative at larger axia l compressive stresses. The dynamic shear modulus, \ G*\, had values rangin g from 100 to 400 kPa, depending on the experimental condition and degenera tive level. The shear behavior was also predominantly elastic, with values for the tangent of the phase angle (tan delta) ranging from 0.1 to 0.7. The annulus material also became stiffer and more dissipative with degenerativ e grade; however, this was not statistically significant. The results indic ated that nonlinearities, compression/shear coupling, intrinsic viscoelasti city, and, to a lesser degree, degeneration all affect the sheer material b ehavior of the annulus fibrosus, with important implications for load-carri age mechanisms in the intervertebral disc. These material complexities shou ld be considered when choosing material constants for finite element models .