Stability Evaluation Of A New Semirigid Pedicle Screw Device

PrefacePedicle screw systems allow for fixation of all three columns. Transpedicular instrumentation has gained popularity because of the ability to apply rigid, short, segmental fixation to stabilize a destabilized spine. In recent years, many pedicle screw systems were used clinically. These devices, however, were considered very rigid. Consequently, these rigid spinal implants might, unfortunately, result in fusion mass stress shielding or stress concentration, increase stresses of the adjacent segments and fatigue failure of implants. Therefore, it is ideal to develop a system that permits controlled motion and increases load sharing without sacrificing construct stability. According to the principle, we design a new semirigid pedicle screw device (NSRD) which can enhance load sharing without sacrificing construct stability. The purpose of this study was to investigate the range of motion of the NSRD in stabilizing a destabilized thoracolumbar spinal segment.Methods一,Source of the materialNew titanium alloy was gained from Institute of Metal Research Chinese Acadamy of Science.二,Characteristics of the new semirigid pedicle screw device1,The device consist of:①polyaxial pedicle screws(6.0mm×45mm);②rods (5.5mm);③interlocking bars.2,The theory of semirigid fixation:①the slight movement between screw head and stem;②hyperelasticity and low stiffness of the new titanium alloy 三,Processing methodsThe new titanium alloy semirigid pedicle screw device were made by accurate method.四,Biomechanical evaluationSix fresh frozen calf spines (T11-L1) were used in the study. Extraneous soft tissues were removed with careful preservation of all spinal ligaments and bone. Both ends of the specimens were embedded in polymethylmethacrylate, and four markers were attached to each of the T12 and L1.A loading frame was attached to the T11 vertebra. Loading was applied to the top of the construct at T11 to produce pure moments in six directions: flexion, extension, right and left lateral bending, and fight and left axial rotation. A maximum moment of 10Nm was achieved. One camera was used to track the three-dimensional segmental motions from T11 to T12, and T12 to L1.Each specimen was biomechanically evaluated under five conditions: intact spine, destabilized spine, fixed with NRD, NSRD, and RD.ResultsIn every motion, the ROM of the destabilized spine at T12-L1 was significantly increased in comparison with the intact spine (P＜0.01). Three devices provided significant stability for the destabilized spines(P＜0.01). Although, the ROM of the fixed spine with RD at T11-T12 was also significantly increased in comparison with the intact spine under flexion, extension, and fight axial rotation (P＜0.05). The ROM of the flexible stabilized spine with NSRD approximated the ROM of the intact spine at each motion of T11-T12 level.DiscussionRigid pedicle screw systems can provide greater initial stability, increase fusion rates, decrease requirements for external immobilization, and make patients earlier return to work. Conversely, there are reports of instrumenitation-associated complications, and concerns about the stress-shielding properties of the rigid pedicle screw systems, the risk of device-related osteopenia, the decreased biomechanical properties of the spinal ligaments, the increasing mechanical stress at the adjacent segments, and the risk of facet and disc degeneration.Increased load transfer through the fusion mass should lead to a more favorable environment for fusion. This can be achieved in several different ways, including the development and use of semirigid devices. The stability afforded by the "semirigid" device across the "injured" segment, however, may decrease, as compared with its equivalent "rigid" device. A decrease in stability may lead to a decreased fusion rate as motion increases across the injured level(s), form pseudartllrosis, and make instrumentation failure. It would be ideal to develop a system that permits increased load sharing, and prevents instrumenitation-associated complications without sacrificing the rigidity of the construct, as compared with a fully rigid equivalent system.we develop the new semirigid pedicle screw device which can increase load sharing, reduce adverse effects of rigid pedicle screw devices without sacrificing construct stability. The results of biomechanical test demonstrated that the device could restore stability to levels similar to those using the fully rigid devices, and the semirigid device did not augment the ROM of adjacent segments.ConclusionIn every motion, three devices provided significant stability of similar magnitudes. The NSRD could not augment the ROM of adjacent segments. The NSRD offers a design that may enhance load sharing and decrease stress concentration without sacrificing construct stability.