Biomechanical evaluation of an expansion pedicle screw(EPS) in calf vertebrae

Objective:To measure the strength of bone-screw interface of the newly-designed expansion pedicle screws(EPS) in calf lumbar spine,and to evaluate the biomechanical effectiveness of EPS.Method:100 fresh calf lumbar vertebrae were randomly classified into group A,B,C.In comparing with USS,Tenor and CDH screws,the turn-back testing,pull-out testing and revision testing were performed in group A,B,C respectively.The maximum turning-back torque(Tmax) and axial pullout strength(Fmax) were recorded.Fatigue testing was performed on ultra-high molecular weight polyethylene blocks served as vertebral elements in a corpectomy model.Result:The Tmax and Fmax of EPS screws was significantly greater than USS,Tenor and CDH screws.In revision testing,the Fmax of the EPS were higher than the three control screws significantly (P0.05).No screws were broken or bent at the end of fatigue testing.Conclusion:Expansion pedicle screws can significantly improve the bone fixation and the pullout strength compared to USS,Tenor and CDH screws.The fatigue characteristic of EPS is similar to CDH,USS and Tenor screws.