Bone Abstracts

Objective: It was proved that expansive pedicle screw (EPS) could significantly enhance immediate screw stability through mechanical expanding and compressing. However, there were little researches on growth of bone tissue around screw and dynamic screw stability in vivo in osteoporotic vertebrae. This study was designed to evaluate growth and microstructure of bone tissue around screw and fixation strength of screw in osteoporotic sheep lumbar vertebrae.

Method: Osteoporotic sheep model was established through bilateral ovariectomy and methylprednisolone injection. Conventional pedicle screw (CPS) and EPS were randomly inserted through pedicle into vertebral body. Sheep were sacrificed at both 6-week and 12-week post-operation. Biomechanical tests were performed to evaluate screw stability and micro-CT analysis and histological observation were performed to evaluate microstructure and growth of bone tissue around screw.

Results: The bone trabeculae around expanding part of EPS were more and denser than those around CPS,and the microstructural parameters of bone trabeculae around EPS were significantly improved compared with those around CPS at both 6-week and 12-week. From 6-week to 12-week, more and more bone trabeculae surrounded screw, crept into interspaces between two fins and connected with each other forming mature structure of cavitas medullaris around screw. At 12-week mature bone tissue took place of fibrous tissue at 6-week and formed biological interface improving quality of bone tissue around anterior part of EPS. Those parameters of EPS at 12-week were significantly improved compared those at 6-week. At both 6-week and 12-week, stabilities of EPS were significantly higher than that of CPS. The stability of EPS at 12-week was significantly higher than that at 6-week.

Conclusions: EPS showed excellent stability and biological interface compared with CPS in vivo in osteoporosis. With continuous expanding and compressing of EPS, microstructure of bone tissue around EPS and stability showed further significant improvement in long-term.