Comparison of interfaces of different pedicle screws with micro-CT technique in lumbar vertebrae with osteoporosis of sheep

Objective To compare the changes in interfaces of expandable pedicle screw (EPS) and polymethylmethacrylateenhanced pedicle screw (PMMA-PS) after being used in osteoporotic sheep lumbar vertebrae with micro-CT technique. Methods Six lumbar vertebrae (L1-L6) in each sheep were randomly divided into three different screw-insertion groups (two vertebrae with four pedicles in each group) after reproduction of osteoporosis in sheep. After making the pilot hole using the same method, CPS was inserted through the pilot hole into vertebral body in CPS group, while PMMA (1.0ml) was injected into the pilot hole prior to the insertion of CPS in PMMA-PS group, and EPS was inserted through pedicle into vertebral body in EPS group. All the sheep were sacrificed, and lumbar vertebrae (L1-L6) were harvested respectively at the 6- and 12-week postoperatively. The micro-CT three dimensional reconstruction and histomorphometric analysis were performed to evaluate the interfacial conditions. Results It was clearly demonstrated that interface was formed where the bone trabeculae was directly in contact with the screw to form "screw-bone" interface in both CPS and EPS groups both 6 weeks and 12 weeks after the operation. The screw was fully surrounded by PMMA and formed "screw-PMMA-bone" interface in PMMA-PS group. The anterior part of EPS expanded in vertebral body to form a clawlike structure, pressing against the surrounding bone trabeculae, thus significantly improved the local bone quality (amount and density of bone trabeculae). From 6 weeks to 12 weeks after the operation, there was no visual difference in bone quality around the screw in both CPS and PMMA-PS groups. There was no degradation and absorption of PMMA, and it led to form the second non-biological interface in PMMA-PS group. Nevertheless, bone quality around expanding part of EPS at 12-week post-operation was significantly improved compared with that at 6-week post-operation, thus forming a good "screw-bone" interface. Conclusions EPS can improve the bone quality around the anterior part of screw through expanding compression in early stage in lumbar vertebrae of osteoporotic sheep. With a continuous compression stress in anterior part of EPS, it may result in a significant improvement in the local bone quality in late stage by forming a good biological interface, thus providing a good osseous foundation for long-term stability. DOI: 10.11855/j.issn.0577-7402.2015.07.0