Residual strains in the intervertebral disc annulus fibrosus suggest complex tissue remodeling in response to in-vivo loading

The annulus fibrosus (AF) of the intervertebral disc (IVD) serves the dual roles of containing hydrostatic pressure from the inner nucleus pulposus (NP) and allowing flexible connection between adjacent vertebral bodies. Previous work has indicated that in the unloaded state, the AF is under a state of residual circumferential strain that, on average, is comparable to that which is believed to reduce peak stresses in other pressure containing organs. The complex in-vivo loading of the IVD, however, led us to hypothesize that variations with anatomical region should also exist. Residual strains were measured by imaging bovine caudal IVDs at both macro and micro scales in both the intact state (under residual strain) and opened into anterior, posterior, and lateral quadrants (residual strains relieved). Calculation of macro scale residual strains using changes in lamellar arc length and thickness confirmed circumferential tension (anterior: 0.63±2.1%, lateral: 8.3±1.5%, posterior: 4.4±2.1%) and radial compression (anterior: 12.4±5.8%, lateral: 11.120±2.8%, posterior: 4.8±4.2%) around the outer zone of the AF. The inner zone, however, had residual circumferential strains ranging from 28.7±3.4% compression in the anterior region to 3.4±3% tension in the posterior region, with radial strains of 9.7±5.5% tension and 0.2±4.4% compression respectively. This pattern of residual circumferential strain was corroborated at the microscale by comparing the crimp period of collagen fiber bundles in the intact and open states. The results of this study point toward a complex pattern of residual strains in the AF, which develop in response to stresses from both NP pressurization and bending movements.