Design Of Thoracic Expandable Pedicle Screw And Biomechanical And Trajectory's Histological Evaluation In Osteoporotic Spine In Sheep

Pedicle screw instrumentation is one of the most commonly used and rapidly growing forms of stabilization for spinal fusion. Transpedicular fixation, however, can be very challenging in the osteoporotic (OP) spine as mechanical stability of the pedicle screws is determined by the bone mineral density (BMD). In addition, poor rigidity of the bone-screw contact can lead to loosening of the implant in osteoporotic patients. With rapid advances in the field of spinal surgery and the aging of the population, an increasing proportion of elderly patients undergo surgical treatment for their spinal disorders. To address these issues, we had designed expandable pedicle screws (EPS). Biomechanical studies have demonstrated that the use of EPS significantly improved the fixation strength compared with conventional pedicle screws. Clinical results have indicated that these screws are particularly useful in the OP lumbar spine. However, this kind of screw cannot be used in the OP thoracic spine, and no systemic study has been reported concerning the augmentation of thoracic pedicle screw.Objective: The aim of this study was to design a thoracic expandable pedicle screw (TEPS), to evaluate its stability and feasibility and to offer a novel strategy for the augmentation of pedicle screw in the OP thoracic spine. Methods: 1. The TEPS was developed according to the anatomy of the thoracic spine. The Axial pull-out test, the cyclic bending resistance test and the three-point bending test were performed to compare the properties of stabilization of the single TEPS with a standard pedicle screw (SINOS). Static and dynamic mechanical tests of the TEPS assemblies and the SINOS assemblies were employed based on the guidelines of the American Society for Testing and Materials (ASTM) F1717-09 using polyethylene blocks as the test vertebral bodies. 2. The OP sheep model was induced by an ovariectomy and continual injections of methylprednisolone, and was confirmed by dual energy X-ray absorptiometry (DEXA). TEPSs and SINOSs were inserted on the vertebral bodies in OP sheep. After four months, the axial pull-out test was performed to compare the holding strength of these pedicle screws. A high-resolution micro-computed tomography (Micro-CT) was performed for three-dimensional (3D) image reconstruction of the screws and the bone. The same length of regions of interest (ROI) surrounding the anterior part of the TEPS and the SINOS were reconstructed and analyzed in the same threshold. A thorough microscopic analysis was performed on sections of these specimens. The safety and reliability of removal of the TEPS was evaluated by the measurement of trajectory using Micro-CT imaging and a 3D reconstruction.Results: 1. Biomechanical studies in vitro:â‘ The axial pull-out tests showed that the TEPS can increase pull-out strength by 15.44% and 48.83%, compared with the SINOS, in these two polyurethane test blocks of different densities.â‘¡In the cyclic bending resistance tests, the TEPS withstood a greater number of cycles or loads with less displacement before loosening.â‘¢In three-point bending test, there was no significant difference in the maximum load between the TEPS and the SINOS.â‘£Furthermore, compressive bending, tensile bending and dynamic compressive bending fatigue tests demonstrated that the TEPS assembly can provide biomechanical stability of the thoracic pedicle screw fixation. 2. Experiments after the four-month insertion on the vertebral bodies in the OP sheep:â‘ All the OP sheep were euthanized and the lumbar spines including TEPSs and SINOSs were harvested. The TEPSs provided a significant increase in maximum axial pullout strength over the SINOSs in the four-month group and this increase was also significantly higher than that of them in an immediate group.â‘¡A Micro-CT image reconstruction showed that these 3D parameters were significantly better in the expandable portion of the TEPSs than those in the anterior portion of the SINOSs. The persistent pressure provided by the expanded fins could act as regulators of bone remodeling, leading to increased interfacial BMD. Histological, newly formed bone tissues grew into the groove of the fins. These bone tissues constructed two compartments and increased the contact area of the osteointegration in the screw-bone interface. As shown by the unique 3D structure, compared with SINOS, the bone and the screw grew into each other, which could further improve the pull-out strength of the TEPS.â‘¢Though the diameter of the expanded TEPS was indeed larger than the original diameter, expansion was less than that measured by in vitro studies, due to compression of the surrounding trabecular bone. During extraction from the vertebral body to the pedicle, the expanded fins of the TEPS gradually shrunk in diameter. The diameter of the EPS trajectory in the pedicle was 4.72mm, which was a 4.9% increase from the original diameter. Furthermore, the limited expansion indicated that the EPS could be backed-out safely and reliably.Conclusions: 1. TEPS could provide a significant stabilization in the osteoporotic spine because of its superior biomechanical properties. 2. The mechanical properties of TEPS and the TEPS assembly demonstrated TEPS assembly can provide biomechanical stability of a thoracic pedicle screw fixation. 3. The TEPS could be backed-out safely without fracturing the pedicle. Therefore, the TEPS assembly could be a novel approach for increasing the pedicle screw fixation in the osteoporotic thoracic spine.