Mechanical Consequence of Induced Intervertebral Disc Degeneration in the SPARC-Null Mouse

Intervertebral disc (IVD) degeneration is associated with low back pain (LBP) and accompanied by mechanical changes to the spine. Secreted protein acidic and rich in cysteine (SPARC) is a protein that contributes to the functioning and maintenance of the extracellular matrix. SPARC-null mice display accelerated IVD degeneration and pain-associated behaviors. This study examined if SPARC-null mice also display altered spine mechanics as compared to wild-type (WT) mice. Lumbar spines from SPARC-null (n = 36) and WT (n = 18) mice aged 14–25 months were subjected to cyclic axial tension and compression to determine neutral zone (NZ) length and stiffness. Three separate mechanical tests were completed for each spine to determine the effect of the number of IVDs tested in series (one versus two versus three IVDs). SPARC-null spine NZs were both stiffer (p < 0.001) and smaller in length (p < 0.001) than WT spines. There was an effect of the number of IVDs tested in series for NZ length but not NZ stiffness when collapsed across condition (SPARC-null and WT). Correlation analysis revealed a weak negative correlation (r = −0.24) between age and NZ length in SPARC-null mice and a weak positive correlation (r = 0.30) between age and NZ stiffness in WT mice. In conclusion, SPARC-null mice had stiffer and smaller NZs than WT mice, regardless of the number of IVDs in series being tested. The increased stiffness of these IVDs likely influences mobility at these spinal joints thereby potentially contributing to low back pain.

Core Strengthening

Core strengthening remains a vital component of maintaining spine mechanics and assists in the prevention of injury. A complex group of muscles make up the core and function together to instill spine stability and assist with segmental motion. Understanding the key muscles and how they function is essential to developing a core strengthening program for patients with spinal pathology. This chapter summarizes the anatomy and biomechanics of the core involved in maintaining spine stability. It discusses electromyography and imaging data to help understand how these muscles function during specific exercises. Examples of core strengthening exercises are provided.

Stepwise Validated Finite Element Model of the Human Lumbar Spine

Understanding the kinematics of the lumbosacral spine and the individual functional spinal units (FSU) is essential in assessing spine mechanics and implant performance. The lumbosacral spine and the FSU are comprised of bones and complex soft tissues such as intervertebral discs (IVD) and ligaments. Prior studies have focused on the behavior of isolated structures, but the contribution of each structure to the overall kinematics of the spine needs to be further understood. In this study, the behavior of various structural conditions was determined by experimentally dissecting each ligament in a stepwise fasion until only the IVD remained, and applying loading conditions to the FSU. The FE model was validated through optimization to match the in vitro load-deflection characteristics and contact mechanics for the various structural configurations.

Combined Rigid-Deformable Modeling of Lumbar Spine Mechanics

Finite element (FE) models of the spine have been used to assess natural and pathological spine mechanics and evaluate performance of various fusion and posterior stabilization devices [1–3]; however, analysis times may be prohibitive for clinical and design phase assessments. Muscle-actuated, rigid body models have also been developed and used to estimate spinal loading conditions during simulated activities [4]. Although rigid body dynamics platforms typically require less computational time, they are unable to evaluate internal stresses and strains in deformable structures. This study proposes to develop a combined rigid – deformable surrogate spine model where the behavior of the intervertebral disc, facet cartilage and ligaments are replicated by simulated mechanical constraint at desired levels. The explicit FE platform is able to accommodate the spectrum of representations, including fully deformable, fully rigid body, implanted, or any combination. Accordingly, the objective of the current study was to assess the ability of a combined rigid-deformable spine model to accurately reproduce the behavior of the fully deformable representation in the natural state with improved computational efficiency. Specifically, this study compared results for a lumbar (L1-L5) spine under follower load and moment conditions for representations ranging from fully deformable to fully rigid. The combined rigid-deformable model includes the deformable disc, facet cartilage contact, ligament representations at L4-L5, while the other levels are modeled using a simplified mechanical constraint.

Vertebroplasty and Kyphoplasty Can Restore Normal Spine Mechanics following Osteoporotic Vertebral Fracture

Osteoporotic vertebral fractures often lead to pain and disability. They can be successfully treated, and possibly prevented, by injecting cement into the vertebral body, a procedure known as vertebroplasty. Kyphoplasty is similar, except that an inflatable balloon is used to restore vertebral body height before cement is injected. These techniques are growing rapidly in popularity, and a great deal of recent research, reviewed in this paper, has examined their ability to restore normal mechanical function to fractured vertebrae. Fracture reduces the height and stiffness of a vertebral body, causing the spine to assume a kyphotic deformity, and transferring load bearing to the neural arch. Vertebroplasty and kyphoplasty are equally able to restore vertebral stiffness, and restore load sharing towards normal values, although kyphoplasty is better at restoring vertebral body height. Future research should optimise these techniques to individual patients in order to maximise their beneficial effects, while minimising the problems of cement leakage and adjacent level fracture.