The Study Of Improving Pedicle Screw

The bonding strength of pedicle screw is significantly decreased in the osteoporosis (OP), which results in screw loosening, migration or back-out. In order to enhance the stability of the screw and limit the complications, many methods have been used. Resent years, some researchers have used expandable pedicle screw to increase the stability of screw. Due to the mechanical expansion, the bone-screw contact and friction are increased, which improves the stability of the screw. In our previous studies we designed a novel expandable pedicle screw (EPS), and it was proved that the EPS can significantly improve the stability of the screw in both experiments and clinical practices.However, the elastic modulus of screw is much higher than that of bone tissue, and the modulus mismatch will be more obvious in the OP, which causes"stress shielding effect", peri-screw bone resorption, and finally screw loosening. Moreover, the material of screw has no bioactivity, so that only mechanical intergration not including biochemical bonding can be formed at the screw-bone interface, which decreases the bonding strength of the screw-bone interface and increases the possibility of screw loosening.Objective: 1. to study the impact of low elastic modulus on the fixation of pedicle screw; 2. to study the impact of micro-arc oxidation on the fixation of pedicle screw.Methods: 1. After the pedicle screw (PS), expandable pedicle screw (EPS) and low elastic modulus (Ti24Nb4Zr7.9Sn, 42GPa) expandable pedicle screw (L-EPS) were implanted into the OP biomechanical tests block through the prepared pilot holes, the X-ray and axial pull-out tests were performed; 2. The EPS and L-EPS were inserted into the OP sheep spine, after 12 weeks implantation, the axial pull-out tests, Micro-CT analysis and histological observation were performed; 3. The features of micro-arc oxidation (MAO) treated surface were detected, the osteoblasts (OB) were seeded onto the MAO treated disks and the adhesion, prolification and differentiation abilities of the OB were observed, moreover, the bone formation genes were also detected; 4. The MAO treated L-EPS (MAO-L-EPS) and L-EPS were put into the OP sheep spine, after 12 weeks implantation, the axial pull-out tests, Micro-CT analysis and histological observation were performed.Results: 1.In OP biomechanical tests block, the maximum pull-out strength (Fmax) and energy absorbed to failure (E) in EPS and L-EPS groups were significantly higher than those in PS group (P < 0.05), but there was no significant difference in both Fmax and E between EPS and L-EPS groups (P > 0.05); 2. In OP sheep spine, the Fmax and E in L-EPS were significantly higher than those in EPS group (P < 0.05), Micro-CT analysis and histological observation showed that the density and structure of trabecular bone in L-EPS group were superior to those in EPS group (P < 0.05). 3. After the MAO treated, a homogeneous, polyporous oxided coating with Ca and P elements was formed onto the surface of materials. The adhesion, prolification and differentiation abilities of the OB were significantly improved after seeded onto the MAO treated group compared to untreated group (P < 0.05). 4. In OP sheep spine, the Fmax and E in MAO-L-EPS were significantly higher than those in L-EPS group (P < 0.05). A large of newly formed trabecular bone was detected in MAO-L-EPS group, which was better in both quantity and quality than those in L-EPS group (P < 0.05), moreover the better screw-bone interface was found in MAO-L-EPS group compared to L-EPS group.Conclusions:1.Compare with PS, the L-EPS and EPS can significantly improve the stability of the screw.2.With low elastic modulus, the L-EPS reduces the"stress shielding effect"caused by mismatch of the modulus between screw and bone tissue and makes the stress transferred into the peri-screw bone tissue. This encourages the bone formation, reduces the peri-screw bone resorption, enhances the stability of screw and longens the screw survival time.3.After MAO treated, a special oxide coating is formed which is benefit for OB to adhere, proliferate and differentiate.4.After MAO treated, the bioactivity of MAO-L-EPS is significantly improved, which makes the screw-bone bonding changed from single mechanical intergration to mechanical and biochemically combined intergration. This combined intergration will further improve the stability of the screw.5.From PS to EPS, EPS to L-EPS, and L-EPS to MAO-L-EPS, we have successfully established a novel pedicle screw which maybe use in the clinical practice owning to its superior mechanical properties and superficial bioactivity.