Mechanics of the annulus fibrosus lamellae under physiologic loading conditions: do interlamellar connections matter?

Abstract

During healthy function, the spine provides the body with stability, strength, and flexibility. Unfortunately, spinal injuries such as annular tears are prevalent in human spines after age 10 (Boos et al. 2002), and at some point in their lives, about 75% of individuals experience low back pain (Andersson 1999). There are many hypotheses related to the origin of pain, but it is often attributed to injury and/or degeneration of the intervertebral disc (IVD) in the lower, lumbar spine (Andersson 1999). While the vertebral bodies are rigid structures, the IVD is a flexible, composite structure of two main components; the nucleus pulposus (NP) and the annulus fibrosus (AF), which is a fibrous structure that surrounds the NP with largely concentric layers containing highly aligned collagen fibers. There are connections that traverse between layers (C. A. Pezowicz, Robertson, and Broom 2006). The organization and composition of the lamellae allow the IVD and thus the spine to exhibit multi-axial motion including flexion, extension, and lateral bending, common to many activities of daily living.The purpose of this dissertation was to assess the influence of the interlamellar connection through pre-failure and failure mechanics of discrete AF lamellae by creating a physiologically relevant test method to deform single and multiple AF lamellae and evaluate the kinetic response using a validated structural model. Vertebral kinematics were quantified from human <italic>in vivo</italic> flexion. Average intervertebral strains were found to be symmetric during the flexion sequence but intervertebral angles were not, suggesting a physiologic decoupling of the two.A structural model was validated for use to characterize AF lamellae. Through parameter sensitivity analysis and calculating confidence intervals of the fitted parameters, it was found that the fitted parameters were more robust when using both surface displacements and grip forces.Single and multiple AF lamellae were characterized using the biaxial protocol generated from the analysis of vertebral kinematics. Single lamella samples produced significant in-plane shear force and moments, while multiple lamellae samples did not, after accounting for the number of lamellae. This suggests isolated single lamellae experience complex loading in biaxial tension but the AF as a whole reduces this response. Parameters fitted from the structural model were not statistically different between single and multiple lamellae samples. This work suggests the interlamellar connection is mechanically significant in shear rather than a planar biaxial context. AF lamellae in shear were found to withstand significant displacement prior to failure as well as carry a non-zero load during the sliding phase. This response suggests a preventative feature within the AF region to resist and mitigate damage due to axial rotation. Although the model used was unable to characterize the shear stress of the experimental data in its present form, further improvements to the model such as more anatomically accurate interlamellar layer may improve the capabilities of the model.The work accomplished in this dissertation forms a base for further assessment of discrete AF lamella(e) and interlamellar connections. Using porcine tissue, experiments performed within Chapters 4 and 5 should be continued to increase the sample size and strengthen possible trends seen within this work. With these tools, these experiments should also be performed with a larger sample size using healthy human cadaver tissue. It would also be interesting to use these tools to assess human cadaveric tissue from the degenerative spectrum. The shear testing showed the interlamellar connection to be mechanically significant in that context, but the test configuration as well as the simplistic modeling did not elucidate whether this mechanical significance originates from a fibrous connection or a matrix material. Further testing and modeling should work towards determining the connection to attribute the mechanical significance.