An Examination of the Influence of Strain Rate on Subfailure Mechanical Properties of the Annulus Fibrosus

Disk herniation is often considered a cumulative injury in that repetitive stress on the posterior annulus can result in the nucleus pulposus penetrating the annulus fibrosus and eventually extruding posteriorly. Further, it has been documented that the nucleus pulposus works its way through the annulus through clefts, which form as a result of repetitive tensile strain. The annulus fibrosus is viscoelastic in nature and therefore could express different mechanical responses to applied strain at varying rates. Other viscoelastic tissues, including tendons and ligaments, have shown altered mechanical responses to different rates of applied strain, but the response of the annulus to varying rates of strain is largely unknown. The present study examined the mechanical properties of 20 two-layered samples of porcine annulus fibrosus tissue at three distinct rates of applied 20% biaxial strain (20% strain over 20 s (slow), over 10 s (medium), and over 5 s (fast)); these three rates are considered applicable to nontraumatic loading. No differences in the stiffness or maximum stress in each of the two directions of applied strain were observed between the three strain rates. Specifically, the average (standard deviation) moduli calculated at the fast, medium, and slow rates, respectively, in the axial direction were 7.42 MPa (6.06), 7.77 MPa (6.61), and 7.63 MPa (6.67) and 8.22 MPa (8.4), 8.63 MPa (9.00), and 8.49 MPa (8.69) in the circumferential direction. The maximum stress values reached during the fast, medium, and slow rates, respectively, in the axial direction were 0.40 (0.36) MPa, 0.40 (0.36) MPa, and 0.39 (0.35) MPa and 0.45 (0.47) MPa, 0.44 (0.46) MPa, and 0.43 (0.46) MPa in the circumferential direction. At submaximal strain magnitudes over a range of nontraumatic rates likely to result in clefts in the annulus and potentially leading to disk herniation, any strain rate dependence is not significant.