Design And Biomechanical Evaluation Of A Pedicle Screw With Capability Of Directional Injection

1.BackgroundSince the introduction of the transpedicular screw system by Boucher,because of the strong fixation through three columns,the applications of this system in the treatment of degenerative disorders,unstable fractures,deformities and tumors of the spine have become very popular in the last two decades.It is also the characterization of biomechanical theory in the practice of spinal internal fixation.The advantages of pedicle screw fixation are dependent on their ability to retain bony purchase until the fusion mass is stable.The bone-screw interface is a major determinant in the stability of spinal instruction systems.Loosening and failure of the screws are among the most common complications reported,especially for osteoporosis.Besides that,during the operations,because surgeons can not successfully insert screws into proper position at the first time,the turning back of the screws will be necessary.It also reduces the holding strength.Revision is often necessary.More and more loosening,prolapse and pseudoarthrosis are being reported along with the popularized employment of transpedicular fixations.It has been one of the focal points within spinal surgery how to improve the stability of pediele screws.Increasing the diameter and/or length of the pedicle screws appears to provide the best solution.However,increasing screw diameter may not always be possible because of anatomical constraints.There is an increased risk of pedicle fracture with possible neural injury if larger screws are used.The use of longer screws increases the risk of anterior body penetration with possible vascular or visceral injury.Besides that, to enhance fixation of salvage screws in case of severe bone loss,some surgeons have chosen to fill the void with a variety of materials,including corticocancellous bone grafts,polymethyl methacrylate(PMMA),and so on.But there are always somewhat defects in the revisions with filling materials.So it has been a highlight on how to increase the bone-screw interface strength.Filling and strengthening with PMMA is the most popular revision methods at present.It has been confirmed that pull-out force can be enhanced by 49%-183%according to different strengthening mode. However,PMMA is not frequently used in spine surgery because of the potential danger to adjacent nutrient vessel and nerve roots if leakage into the spinal were to occur.Immediate risks resulting from leakage into the spinal canal are the result of the exothermic reaction present in the curing process of PMMA,whereas long-term risks are secondary to a non-degradable foreign body in the spinal canal.In view of this,our goal in current study was to design a new type of pedicle screws with the capability of directional injection and evaluate the mechanical properties of it.With the characteristic of oriented injection,the incidence of bone cement leakage can be reduced,which lead to spinal compression and nerve root's denaturation.We hope this kind of screw can afford new options for revisions and augmentations.2.Objectives2.1 To design a model of the pedicle screw with capability of directional injection.2.2 To compare if there are some differences of the property and mechanics of materials between solid screws and hollow ones,then amend the model and get it into realization.2.3 To evaluate if there are some differences of biomechanics property and determine whether new options can be offered to revisions and augmentations of transpedicular operations or not..2.4 To simulate the directional injection with Fluent and then vertificate in experiments.3.Materials and methods3.1 Designing and modeling of the pedicle screws with capability of directional injection.A new type of hollow pedicle screw with lateral holes was designed with Pro/ENGINEER,a CAD(computer aided design) software.Then a model was established(Concrete steps included sketch,valuation,assignment,finishing semi-section,revolving to create billet,cutting screw threads,extruding and drilling holes,directional marks and etc).Then virtual assembling and moving were performed to find out if the function of directional injection can run and whether there was any mutual interference.3.2 Structural and materials analysis of directional-injection pedicle screws.3.2.1 Computer aided structural analysisFinished model was imported into the module of Pro/MECHANISM and simplified as a projecting beam.Then the model was defined with materials, constrains and loads before divided into meshes and performed with structural analysis.Where stress concentrated or motion contradicted was find out to amend.At the same time,stress and strain distribution of solid and hollow screws has been examined.3.2.2 Manufacturing of the screwsThe screws were made of Titanium Alloys TC4 by STB Surgical implants,LTD. In Foshan.The axial and lateral holes were drilled by DMU 60T high-speed DECKEL MAHO in Guangdong University of Technology. 3.2.3 Material mechanics tests3.2.3.1 Three points bending test10 hollow and 10 solid screws were investigated by material testing machine with three point bending test under the quick loading to 4000N.3.2.3.2 Shear test10 hollow and 10 solid screws were investigated by material testing machine with shear test under the quick loading to 4000N(30N/s).The information was captured and analyzed.Then two sample t-test was performed.3.3 Biomechanical analysis3.3.1 SpecimensSix fresh thoracolumbar spine specimens(5 males,1 femals) were divided into 36 vertebraes and 30were employed.3.3.2 Grouping and implantingAccording to random num table,30 vertebraes were divided into three groups: Control Group(Group C),Augmentation Group(Group A) and Restoration Groups (Group R).Entry point of screws was at the"人"shape crest.3.3.3 Vertebrae embedding and F-max testVertebraes implanted with screws were embedded into rackets with dental base acrylic resin powder before F-max test.Then the peak pull-out forces and Linear Displacements(LD) were recorded.And analysis of variance(ANOVA) test of a randomized block were employed.3.4 CFD simulation and experimental verification for directional injectionA model of directional injection was established with Gambit,and then solved in Fluent.Compression fracture model was employed with MTS.PMMA was injected into the fractured vertebrae via directional screws and then get X-ray photo captured.4.Results4.1 Designing and modeling of the pedicle screws with capability of directional injection. After virtual assembling and moving were performed,there was no mutual interference found out in the defined degree of freedom.The function of directional injection ran precisely.4.2 Structural and materials analysis of directional-injection pedicle screws.4.2.1 Computer aided structural analysisStress concentrated of the model was find out and amended at the end.The curves of stress,strain,strain energy and displacement of solid and hollow screws has been compared but little differences were find out.4.2.2 Material mechanics tests4.2.2.1 Three points bending testNo broken screw was found.The strains of two kinds of screws were just identical(0.17),and no decent curve was recorded in the course of quick loading to 4000N.4.2.2.2 Shear testAll screws broke when the quick loading added up to 4000N(30N/s). Coincidence stress-strain curves of solid and hollow screws were reported by the data. The max shear forces of the hollow and solid screws were(3983.17±10.28)N and (3992.48±9.68)N.There was no significant difference was found between them (t=2.085,P=0.052).4.3 Biomechanical analysis4.3.1 F-max testFor hollow screws,peak pullout forces were(798.24±139.86) N in Group C, (1476.21±223.09) N in Group R,(1741.33±317.79)N in Group A;For solid screws, peak pullout forces were(904.37±212.03)N in Group C,(1828.42±239.68)N in Group R and(1783.37±250.49)N in Group A.The peak pull-out force of Group C was significantly lower than the other two groups(P=0.000),but there was no significant differences between Group R and Group A(P=0.330).For hollow screws,LD were(1.68±0.24)mm in Group C,(3.16±0.70) mm in Group R,(3.13±0.62)mm in Group A;For solid screws,the LD were (1.85±0.37)mm in Group C,(3.43±0.98)mm in Group R and(3.36±0.98)mm in Group A.The LD of Group C was significantly lower than the other two groups (P=0.000),but there was no significant differences between Group R and Group A (P=0.971).4.3.2 About bone cement leakageNo PMMA was found in the vertebral canal or on the surface of pedicle of vertebral arch when PMMA was infused through the hollow screws,while PMMA was easily found in the vertebral canal or on the surface of the pedicle when using solid screws.4.4 CFD simulation and experimental study of directional injectionDirectional injection was simulated by CFD,static pressure and velocity distribution obtained,realized in experiment,vertifieated by X-ray photography.5.Conclusions1.Parametric modeling and real time dynamic simulations with Pro/ENGINEER afforded a precise and Intuitive way to realize the design thinking,directional injection.2.With computer aided engineering(CAE),convenient structural analysis and model revision had been performed.It has been confirmed by theoretical derivation, computer simulation,material mechanics testing,and statistical inference that the strength of hollow screw has no significant difference with the solid ones'.3.PMMA can significantly increase the stabilization of vertebral arch when it is infused in with directional injection screws,which offer a new and safe option for revision.