Mechanics, Recovery, and Injury Risk of the Intervertebral Disc in Repeated Loading

Abstract

Low back pain (LBP) is the leading cause of disability worldwide and is often linked to occupational tasks that involve complex loading conditions such as combined flexion and compression. Despite its prevalence, the etiology of LBP is unknown in about 90% of patients. While the intervertebral disc (IVD) and vertebral endplates are recognized as potential sources of pain, their early mechanical and structural responses to repeated loading remain poorly understood. IVD hydration plays a critical role in disc biomechanics, nutrient transport, and injury susceptibility. Therefore, studying how creep loading and recovery influence disc structure and hydration is essential for identifying precursors to injury and pain. Using porcine lumbar functional spinal units (FSUs), this dissertation explores how cyclic combined loading, tissue hydration, and rest influence mechanical behavior and structure of the lumbar spine. Coupling mechanical testing with high-resolution X-ray computed tomography (microCT) enables a clearer understanding of how disc structure relates to lumbar mechanical behavior. This work establishes the importance of cyclic loading by showing that lumbar spine behavior and development of endplate fracture under cyclic combined loading differ from quasistatic combined loading conditions. This finding emphasizes the critical role of repeated loading in spinal degeneration and injury. MicroCT of frozen specimens is found to provide visualization of IVD substructures, including the nucleus pulposus (NP), annulus fibrosus (AF), and endplates (EPs). The resolution of microCT is much higher than clinical diagnostic imaging which enables detection of small changes in IVD structure prior to overt injury. The high-resolution visualization of intact IVD soft-tissue architecture in microCT of frozen specimens offers a practical and accessible technique for studying IVD morphology. Coupling this imaging approach with mechanical testing of FSUs under creep loading and recovery provides a way to relate loading magnitude and duration to structural and hydration changes in the IVD. A semi-autonomous segmentation method was developed to quantify changes in the NP due to creep loading and recovery, indicative of fluid flow in the disc. This coupled approach reveals that NP volume recovers in a stress-dependent manner, with higher stresses showing limited recovery of NP volume. This integrated approach offers new insight into disc hydration dynamics and helps elucidate the early changes in the IVD that may precede chronic LBP and mechanical injury. The viscoelastic behavior of the IVD under cyclic combined creep loading and subsequent recovery was captured using a quasilinear viscoelastic (QLV) framework. Differences in time constants highlight the difference in behavior between creep loading and recovery. The developed QLV model provides an approachable framework for simulating long-term spinal creep and recovery. An injury risk analysis for porcine endplate fracture was reinterpreted using strain as the injury metric to evaluate how recovery modulates cumulative injury potential. Recasting injury risk in terms of strain rather than stress and duration further illustrates how incomplete recovery can cause cumulative strain to reach levels associated with the onset of endplate fracture. Expressing injury risk in terms of strain enables direct coupling with recovery behavior, providing a tool for modeling chronic lumbar injury. Overall, the approaches developed in this dissertation provide new methods to study IVD changes that may be critical to understanding the progression of LBP. These studies advance understanding of early structural and hydration alterations in the lumbar FSU that may contribute to pain and injury prior to the emergence of clinically identifiable biomarkers. These insights support the development of evidence-based work-rest protocols, improve the fidelity of computational spinal models, and advance the coordinated use of high-resolution imaging and mechanical testing to capture structure-function relationships in the lumbar spine.