Comparison of the Pull-out Strength Between a Novel Micro-dynamic Pedicle Screw and a Traditional Pedicle Screw by Using Pull Out Test and Micro-CT

Objective To investigate the effectiveness of the micro-dynamic pedicle screw for the prevention of loosening between the screw and bone surface in lumbar spinal fusion.For this evaluation,the peek pull-out force(PPF)after fatigue resistance testing at different cycles and semidiameter of screw insertion area of micro-CT image was compared between the micro-dynamic and traditional pedicle screws.Methods Forty lumbar vertebrae received a traditional pedicle screw on one side and a micro-dynamic pedicle screw on the other side as follows:15 vertebrae underwent instant pull-out testing(group A,traditional vs.group B,micro-dynamic);15 vertebrae underwent pull-out test after 5000-cyclic fatigue loading(group C,traditional vs.group D,micro-dynamic);and 10 vertebrae underwent pull-out test after 10000-cyclic fatigue loading(group E,traditional vs.group F,micro-dynamic).All vertebrae that after fatigue loading were scanned by using micro-ct to check the semidiameter of screw insertion area.The PPF,normalized PPF(PPFn),bone mineral density(BMD)and diameter of screw insertion area were compared between all groups.Results The BMD showed a weak significant correlation with PPF(r=0.252,P=0.024).The PPF in groups A and B(P=0.485),and groups C and D(P=0.184)did not show significant difference.However,the PPF in group F was significantly greater than that measured in group E(P=0.005).The PPFn of groups A,C,and E significantly decreased as the number of cycles increased(P=0.015).Meanwhile,the PPFn of groups B,D,and F remained consistent regardless of the number of cycles(P=0.367). The semidiameter of traditional pedicle screw insertion area was significantly larger than that of micro-dynamic pedicle screw insertion area(P<0.001).Conclusions This study compared the performance of a micro-dynamic pedicle screw versus that of a traditional pedicle screw using pull-out testing and micro-CT.The results showed that the micro-dynamic pedicle screw provides similar resistance in lowcycle fatigue testing and favorable resistance in high-cycle fatigue testing versus the traditional pedicle screw.The micro-dynamic pedicle screw provides stronger fixation stability versus the traditional pedicle screw,especially in high-cyclic fatigue loading condition versus the traditional pedicle screws.