The Designs And Basic Studies About A Novel Percutaneous Pedicle Screw System

Part I The Designs and Fabrications of Novel Percutaneous Pedicle Screw SystemObjective:To design a novel percutaneous pedicle screw fixation systems for treatment of lumbar degenerative diseases and thoracolumbar compression fractures. Methods:With assist of Beijing International Orthopeadic Research Center(BIORC), we accomplished the designs and manufacture of novel percutaneous pedicle screw fixation systems. BIORC is in charge of sketching, sampling and processing based on the design concepts which were proposed by Prof. Zhou and me. Then relevant basic tests on samples were performed and constructive amendments were put forward to fix the defects found in experiment test, and finally perfect the new percutaneous pedicle screw system. Results: From October2007to July2008, primary designs and sample of novel percutaneous pedicle screw system were accomplished. From November2008to September2009, further improvement of novel percutaneous pedicle screw system was accomplished. The complete operating systems and pedicle screw kit was prepared and available for clinical research. Conclusion:The percutaneous pedicle screw fixation system provide a novel design concepts, excellent operational tools place and a multi-functional characteristics.Part II The Study of Clinical Feasibility about Novel Percutaneous Pedicle Screw SystemObjective:To evaluate the feasibility and safety of clinical operation for novel ercutaneous pedicle screw system. Methods:Surgical operations were simulated with novel ercutaneous pedicle screw system on formalin-immersed cadaver specimens at Department of Anatomy of Third Military Medical University. Both sides of specimens were divided randomly into two groups (group A and group B) before surgery. Group A was fixated with novel percutaneous pedicle screw system, and group B with Sextant system served as the control group. In order to assess whether novel percutaneous pedicle screw system suitable for all lumbar segments, Segmental fixation methods for each specimen were determined using random allocation, following as:①.M1:L1-L2, L4-L5;②.M2:L2-L3, L5-S1; ③.M3:L1-L3, L4-S1. Note operation time and assess the good rate of modified pedicle screw. Results:Mean operation time was (38.2±6.20) min at group A and (56.4±12.8) min at group B with significant difference (P<0.05). Image results showed that the good rate was91.7%at group A and95.8%at group B. There was no significant difference (P>0.05). Conclusion:operation time by novel percutaneous pedicle screw system was significantly reduced comparing with that by Sextant system and good rate of modified pedicle screw was satisfactory. Novel percutaneous pedicle screw system had shown good feasibility and safety in clinical operation.Part Ⅲ The Distraction Function Measurements of Novel Percutaneous Pedicle Screw SystemObjective:To evaluate distraction function of novel percutaneous pedicle screw system, which provides the experimental evidence for treatment of thoracolumbar fractures in late-stage clinical application. Methods:15fresh calf (3-6m) lumbar specimens (L1-L5) were collected and stored at-20℃refrigerator. Before the test, the specimens thaw at room temperature, resected muscles and soft tissue around the vertebral body with ligament tissues and intervertebral disc retained. The L3vertebral body was pretreated referring to modified Gepstein method, and then specimen was fixated in material testing machine before loads were applied. Fracture specimens meeting the requirements were randomly divided into three groups (monoaxial screw fixation group, modified screw fixation group and the polyaxial screw fixation group). After screw-rod system was installed by cross-injured vertebral fixation method, and then made the markers in the L2and L4vertebral body for subsequent measuring. Distraction device was installed in the screw-rod system and test construct was fixated at platform of material testing machine that imposed different loads to distraction device. The longitudinal distance values (mm) between the markers in the anterior or posterior vertebral bodied were measured using electronic caliper under different loads. Then calculate the distraction displacement values (mm) corresponding to different loads respectively. The distraction effects of three kinds of screw-rod system were compared in the anterior or posterior vertebral bodied. Results: Novel percutaneous pedicle screw operating system had a good distraction efficiency on the three kinds of screw-rod systems, but there were differences in the distraction effects in anterior or posterior part of vertebral body. Modified and monoaxial pedicle screws both offer satisfactory distraction effects in vertebral anterior column with no significant difference (P<0.05). Modified and monoaxial pedicle screws could also distract vertebral middle and posterior column to some degree, although no better than that of anterior column. However, polyaxial pedicle screw distracted mainly middle and posterior column of vertebral body, with significantly weaker distraction effects in anterior column of vertebral body compared with the first two screws. There were significant differences (P<0.05). Conclusion:Novel percutaneous pedicle screw system has shown a good distraction effects. Modified and monoaxial pedicle screws distracted mainly vertebral anterior column, while polyaxial screw distracted mainly middle and posterior column of vertebral body, but distraction effects to anterior column is not better than that of modified and monoaxial pedicle screws.Part Ⅳ Biomechanical Studies of Modified Pedicle Screw Fixation System for Thoracolumbar Compression FracturesObjective:To evaluate the biomechanical properties of modified pedicle screw system for short-segment fixation of vertebral fracture in order to provide experimental evidence in late-stage clinical applications. Methods:15fresh calf (6-8w) lumbar specimens (L1-L5) were collected and stored at-20℃refrigerator. Before the test, the specimens thaw at room temperature. resected muscles and soft tissue around the vertebral body with ligament tissues and intervertebral disc retained. The L3vertebral body was pretreated referring to modified Gepstein method, and then specimen was fixated in material testing machine and loads were applied. Fracture specimens meeting the requirements were randomly divided into three groups (monoaxial screw fixation group, modified screw fixation group and the polyaxial screw fixation group) with5models in each group. three-dimensional motion testing of each specimen were performed in material testing machine (MTS858)in turn three kinds of states(intact, fractured, instrumented). At the same time, the digital motion capture and analysis system of gait analysis instrument recorded range of motion in6directions:anteflexion, postexion, left bending (LB), right bending (RB), left rotation (LR) and right rotation (RR), and calculated the mean value. Results:The stabilities of all specimens in intact or fractured states showed no significant differences (P>0.05), but there were significant differences in instrumented states of different types of screws. The stability of fixation segments ultilizing modified screw and monoaxial screw are similar (P>0.05) in anteflexion or postexion. but stronger than polyaxial screw (P<0.05). The stabilities of LB, RB, LR, RR in three kinds of screw to fixed segments are similar with no significant differences (P>0.05). Conclusion:The stability of modified screw and monoaxial screw to fixation segments in anteflexion or postexion are similar (P>0.05) but stronger than polyaxial screw (P<0.05). The stabilities of LB, RB, LR, RR three kinds of screw to fixed segments are similar with no significant differences (P>0.05). Digital motion capture and analysis system of gait analysis instrument can improve the accuracy of measurement results.Part V Biomechanical Safety Study of Modified Pedicle ScrewObjective:To assess the mechanical behavior of monoaxial pedicle screw in order to provide experimental evidence in late-stage clinical applications. Methods:The test blocks of corpectomy models were established with Ultra High Molecular Weight Polyethylene (UHMWPE) according to the American Society for Testing Materials (ASTM) published a testing standard for vertebral fixation based on a similar model (F1717-04) for present study. The UHMWPE blocks were instrumented with the spinal implants following the manufacturers'recommendations exactly.10Nm of tightening torque was applied to combine screws and rods with set screw. The posterior part of each block was obliquely cut to maintain consistent insertion for each pedicle screw perpendicular to the facet. The horizontal distance between the insertion points of pedicle screw was consistently40.0mm, while the longitudinal distance between the pedicle-screw axes was consistently76.0mm. The pedicle screws inserted formed a45-mm long lever arm, extending from the centerline of the posterior longitudinal rod to the actuator axis of the testing machine. Then the construct and a jig were mounted on the mechanical testing machine (Minibionix858; Eden Prairie, MN, U.S.A.). Transverse-connectors were not used in all implant constructs (Fig.2). All implants were unused before testing and not retested. All screws had similar fixation system by means of internal screw and nut, with the diameter of5.5mm,40.0mm in length and identical thread designs adopted. Spinal implant constructs were divided into three groups:Group1, constructs were instrumented with monoaxial screws, Group2with modified monoaxial screws, and Group3with polyaxial screws. Six subgroups (n=5) were constructed for each kind of screw. Three subgroups were used for static test (compression, tension and torsion) and three subgroups for Dynamic compression test (25%,50%, and75%load levels). All mechanical tests were performed on MTS. Static mechanical tests were performed in air under ambient conditions. Load the test apparatus at a rate of25mm/min for axial compression test and60°/min for torsion test. Record the load-displacement curves, compressive bending yield load (N), compressive bending stiffness (N/mm), compressive bending ultimate load (N) and torsional stiffness (Nm/degree). The mode of failure of each construct was noted after testing. Dynamic compression test was carried out with the assembly submerged in a saline bath at37℃. The fatigue test applies a sinusoidal displacement to the spinal construct (R≥10). Fatigue frequency is5Hz, and displacement is3mm (displacement were monitored through out the test). Fatigue test should be50%and75%of the compression bending ultimate strength as determined in the static compression bending test, respectively. Note the modes of failure and deformations of components and evaluate all surface changes. Results:there were no differences in compressive bending stiffness (N/mm), compressive bending yield load (N) and compressive bending ultimate load (N) between modified monoaxial screw and monoaxial screw(P>0.05), but differed significantly from polyaxial screw (P<0.05) in their static mechanical properties. Modified monoaxial and monoaxial screw assembly demonstrated plastic deformation of the longitudinal rods. In addition, two monoaxial screws occurred curved deformity at the insertion point when the compressive bending ultimate load was exceed. Angular deformity occurred at longitudinal axis of screw body and screw head because of rotational slip of the multiaxial link in polyaxial screw assembly. The measured torque stiffness was7.3/6.8/5.4N-m/degree for monoaxial, modified monoaxial and polyaxial screw, respectively. Analysis of variance demonstrated that the stiffness of modified monoaxial screw was similar to that of monoaxial screw (P>0.05), but which was stronger than that of polyaxial screw (P<0.05). Fatigue tests showed that failure of modified monoaxial and monoxial screw body occurred at the insertion point of UHMWPE blocks. While the results of polyaxial screw group in fatigue test showed that screw body swung up and down around the screw head because of ball-in-cup mechanism loosening. Conclosion:The modified monoaxial designs of this modified monoaxial pedicle screw, adopting the implant way of polyaxial screw, facilitates the assembly of connecting-rod through adjusting only the directions of screw head. Meantime, Modified monoaxial screw could provide sufficient mechanical stabilities similar to monoaxial screw for fixation segment. The biomechanical stabilities of modified monoaxial or monoaxial pedicle screw are stronger than that of polyaxial screw, however, the failure resistance of the latter might be better.