The degeneration of the intervertebral disc (IVD) is a widespread condition that can either be pathological or age-related. It strongly affects the IVD tissue composition, and its most important consequence is the decrease in the IVD proteoglycans (PGs) content. This causes a decrease in the disc swelling pressure, which is essential for the disc load-bearing capabilities. In the most severe cases of degeneration, invasive surgery may be required (i.e., spinal fusion). Computational models of the lumbar spine have been used by several authors in the literature for the investigation of the lumbar spine biomechanics, especially in pathological or post-surgical conditions. Multibody models are one of the computational tools available for this type of analysis, but the multibody models currently available do not include the IVD swelling properties in the representation of the intervertebral discs. The objectives of this dissertation were to develop a multibody model of the lumbar spine which included the IVD swelling properties, and to apply such a model to the investigation of simulated relevant clinical cases.
The IVD was considered as a triphasic mixture of a solid phase, a water phase and an ions phase. A new multibody swelling pressure model was developed starting from a commercially available repository model. The newly developed model was validated with in vivo data on spinal loads and kinematics, and then applied to the investigation of the lumbar spine biomechanics in cases of interest.
The application of the new model to disc degeneration showed that the structures surrounding the lumbar spine (muscle, ligaments) may have an important role in maintaining the spine stability in case of degenerative discs. Multifidus (MF) muscle atrophy resulted particularly important for the stability of the lowermost L5- S1 level.
When simulating a spinal fusion procedure, the model predicted dramatic effects on kinematics and loads on adjacent segments in case of L5-S1 fusion, and a milder impact on the lumbar spine biomechanics in case of L4-L5 fusion. Simulating the combination of spinal fusion and MF damage, which is a likely consequence of open spinal surgery, confirmed that MF muscles may have a central role in the stabilization of the L5-S1 level.
The investigation on the lumbar spine kinetics in 0-gravity environment suggested that the absence of gravity may generate a spinal kinematics and loading patterns considerably different from the ones correspondent to the same motions on Earth. This abnormal biomechanics may be linked to the low back issues frequently experienced by astronauts.
The model developed in this dissertation represents a new tool for the analysis of the lumbar spine biomechanics. The investigations presented in this work provide additional insights into the lumbar spine biomechanics in various clinical cases.