Biomechanical analysis of dynamic stabilization under the effects of patient conditions [electronic resource]
Keywords: Mechanical Engineering

While coronary heart diseases are considered the most deadly medical conditions in terms of overall deaths per year, orthopedic maladies are the costliest. One of the most prevalent orthopedic conditions is low back pain, which stems from degenerative tissue changes within the spinal column and associated spinal cord and peripheral nerve injuries. Treatment successes in conservative and surgical treatment are mixed, although the rates of spine surgery have dramatically increased over the past decade. Surgical treatment is considered a last resort, and of all the surgical approaches, spinal fusion is the most common for the treatment of low back pain. Since spinal fusion has been in use in the clinic for several decades now, results of long-term retrospective clinical reviews are now becoming available. Some of these studies have shown that spinal fusion may induce secondary injuries such as adjacent tissue regeneration, which may require additional surgical treatment. To overcome some of these complications, posterior dynamic stabilization has been introduced as an alternative to fusion surgery. Posterior dynamic stabilization is still considered mostly experimental and the majority of spine companies with ambitions in this field have not yet settled on particular design goals and implant concepts. A significant hurdle for entry to the market for such devices is the lack of understanding of what the ideal function of the device is, and how the implants should interact with the spinal column. The objective of this thesis was to evaluate the functionality of posterior dynamic stabilization depending on patient conditions such as mobility and body mass and to suggest efficient rigidities of the dynamic device when considering patients' characteristics. This study is divided into three specific aims. The purpose of the first aim was to investigate the significant influence of a patient's spinal kinematics on dynamic stabilization. The patients were divided by segmental range of motion (L3-L4) into two groups (hyper-mobility and hypo-mobility) and finite element (FE) models were generated for these respective groups. This study showed that patient characteristics such as mobility produced different spinal kinematics after dynamic stabilization and demonstrated that the effectiveness of dynamic stabilization was increased when the mechanical properties of the device were changed in response to patient characteristics. The purpose of the second aim was to evaluate the stabilization devices in relation to patients' body mass and spinal mobility, testing the effects of dynamic stabilization devices of varying levels of rigidity. Based on analyzed results of the spinal mobility at the diseased level (L4-L5), the hyper-mobility patients were divided into three groups, based on severity. Depending on the body mass in the hyper-mobility patients, the patients were divided into three groups. The findings of the study demonstrated the significant influence of patients' body mass and mobility on spinal kinematics after dynamic stabilization. The purpose of the third aim was to investigate the effect of implant rigidity on spinal kinematics utilizing a cadaveric tissue model. This in-vitro study was designed to validate the biomechanical influence of physiological loading after dynamic stabilization. The results of this in-vitro study showed that patients' characteristics change spinal segmental motion and different implant rigidities of the dynamic stabilization device also produce varying spinal kinematics depending on patients' conditions. Through these in-vitro tests, this thesis readdresses the importance of considering patient characteristics in the design of appropriate devices for spinal stabilization, and to select the right implants for the right patient population.