Biomechanical effect of the revision function of expansive pedicle screws

Aim: To measure and compare the maximum axial pullout strength revised of newly-designed expansive pedicle screw(EPS) with USS, CDH pedicle screws in revising calf lumber spine, and to evaluate the revision function of EPS. Methods: Twenty fresh calf lumbar vertebrae harvested from 4 calf lumber spine were divided into 2 groups by means of random number table: group A and group B with 10 vertebral bodies in each group. CDH screws (6.5 mm × 40 mm) were implanted in both pedicles of every vertebral body in the two groups. Pull-out test was performed to make the loose screw prolapse model. For each pedicle pair, the salvage of one CDH screw was performed using a EPS screw in each group at random; In addition, the salvage of the other CDH screw was performed using a USS screw in group A, and the salvage of the other CDH screw was performed using a CDH screw in group B(the diameters of all the above three screws were 7.0 mm × 40 mm). Pull-out test was performed on the revised screws again, and then the maximum axial pullout strength was recorded. Cyclic loading fatigue test for internal fixation devices of CDH, USS, Tenor and EPS was performed at 5 Hz and 200 - 250 N for 1.5 million cycles. Results: The maximum axial pullout strengths of the rescue CDH, USS and EPS screws were (1 206.8 ± 597.4), (1 522.2 ± 392.1) and (2 161.5 ± 555.4) N respectively. The maximum axial pullout strength of EPS screws were all larger than those of CDH and USS screws significantly (P < 0.05). No screws were broken or bent at the end of fatigue tests. Conclusion: EPS screws offer better anchoring in revision surgery than most of the non-expansive pedicle screws do.