Empirical Study On Stability Mechanism Of Expensive Pedicle Screw In Osteoporosis.

Pedicle screws with plates or rods have been used to stabilize motion segments, correct spinal deformations and provide segmental fixation to promote graft incorporation in patients with degenerative, traumatic or neoplastic disorders of the spine. But pedicles screw fixation can be challenging in osteoporotic spine as mechanical stability of the pedicles screw-bone contract is affected by Bone mineral density (BMD). Poor rigidity of the bone-screw contract can lead to loosening of implant in osteoporotic patients. Analysis of pull-out strength of pedicles screw in lumbar spine has been performed in the past with a view to optimize the screw size, the screw insertion depth or direction and the screw design. Besides, to improve the strength of the screw-bone interface in osteoporosis (OP), mechanical tests have been performed with augmentation using polymethylmethacrylate (PMMA), Hydroxyapatite stick (HA), calciumphosphate cement (CPC), et al. These screw modification and augmentation strategies have been limited by possible complications and problems such as increased risks of pedicle fracture with resultant neural injury for larger screws, or anterior body penetration with ensuing vascular or visceral injury for longer screws, or potential problems associated with a non-absorbable foreign body in the spinal canal and with uncertain long-term rigidity on the course of substitution for those absorbable cements.To address these issues, we had designed expansive pedicle screws (EPS). EPS can improve bone fixation by increasing the screw tip diameter, allowing for greater bone contact without increasing screw diameter or length. Biomechanical studies have demonstrated that the use of an expansive screw design significantly improved the fixation strength compared with conventional pedicle screws. However, no systemic evaluation has been reported concerning the properties of screw-bone interface and the stable mechanism of EPS, especially in a living OP body.Objective To investigate the properties of the screw-bone interface and the stable mechanism of EPS in osteoporotic sheep, using three-dimensional (3D) imaging and reconstruction by micro-CT and histological analysis of microtome sections by microscope.Methods 1) Eves were ovariectomized bilaterally. Before ovariectomy(OVX)and 12 months after OVX, lumbar BMD was measured by dual energy X-ray absorptiometry(DEXA)to ensure the establishment of OP sheep model for this study.2) After EPS insertion in each femoral condyles of 6 OP sheep, 3 sheep were bred for 3 months, while another 3 sheep 6 months. Femoral condyles with EPS were 3D imaged and reconstructed by micro-CT. Histology was evaluated thereafter.Results 1) The trabecular microstructure was denser at the screw-bone interface than in the distant parts in expansive section, especially within spiral marking. In the non-expansive section, however, there was no significant difference between the interface and the distant parts. The regions of interest (ROI) adjacent to EPS were reconstructed and analyzed by micro-CT using the same thresholds. The 3D parameters generated, including tissue mineral density (TMD), bone volume fraction (BVF, BV/TV), bone surface/bone volume (BS/BV) ratio, trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), in expensive sections were better than those in non-expensive sections. Each pair of these parameters was compared using Mann-Whitney analyses, and were considered statistically significant(P<0.05). 2) Histologically, newly formed bony trabeculae crawled along the expansive fissures and into the center of EPS. The newly-formed bones, as well as the bone at the bone-screw interface, closely contacted the EPS without connective tissue layer, it concluded that EPS had excellent biocompatibility, and they constructed four compartments and wrapped up the pins tightly.Conclusions 1) The fins'continuous pressure to the bony trabeculae of the expensive section ensured the EPS's mechanical stabilization at earlier stage. 2) The special 3D structure, which bone contained pin as well as pin contained bone, kept on biological fixation and stiffness of EPS enduringly. EPS is one of the best chooses in OP patients for its special properties of mechanical stabilization and biological fixation, if segmental fixation of spine is needed.