The Design And Biomechanical Study Of Lumbar Dynamic Fixation System

BackgroundFor the treating of the degenerative disc disease,open surgical intervention and fusion are main techniques to stabilize the spinal column.Advancements in technology have added to the surgeon's fusion rate and the clinical result,however, there are also some problem related to the fusion.It is widely accepted that,after the fusion,the discs in adjacent segments may also become affected a few years after surgical fusion of a degenerated disc.It is casually hypothesized that the load reduction of a slightly degenerated disc may postpone fusion surgery and adjacent disc disease.Several flexible posterior spinal fixation systems are clinically used.These so-called dynamic stabilization devices are designed to reduce the disc load.In contrast to solid fusion,the non-fusion systems are intended to maintain intersegmental motions or reduce them to magnitudes found in the intact spine.There have been a number of dynamic stabilization devices trialed in lumbar spinal disease, many with differing biomechanical principles.Some examples include the interspinous implants;the Graf Ligament,the Dynesis Spinal System(Zimmer Spine, Warsaw,IN),and the FASS system(AO International,Davos,Switzerland),all of which are pedicle screw based.Because of the shape memory and superelastic property,At present,the shape memory nickel-titanium alloy is clinically used in wires for orthodontic tooth alignment,vascular applications,osteosynthesis staples.The nickel-titanium alloy has a unique shape-memory phenomenon,which can memorize a particular shape at a particular temperature,when it was cooled below a transformation temperature,the alloy becomes malleable,and the shape can be changed.However when it is reheated to above the transformation temperature,the original shape is restored.Another unique characteristic of nichel-titamium alloy is its superelasticity,in which, regardless of whether the material is under loading or unlading,and within a range of deformation,the recovery force produced is constant.NiTi shape alloy also has excellent biocompatibility,corrosion resistance and fatigue endurence.It not only has the biomechanical superiority over the other material,the clinical usage is also very easy.Because of its special properties,the nearly equiatomic nickel-titanium alloy is widely used in the orthopaedics field.Recently,with the spinal surgery developing very fast,more and more spinal instruments came into use,and so much attention,was payed to the NiTi alloy.In recent years,our department have reported an original technique using a "C" shape nickel-titanium alloy intrasegmental fixation instrument for spondylolysis repair and have used the device in the spondylolysis patients successfully.Because the alloy can become malleable and the shape can be changed in the cold condition of 0±4℃,it can be fixed on the spondylolisis vertebrae, when the temperature was up to 37℃of the human body temperature,the shape-memory effect of the instrument was activated and the par defects can be fixed by the constantly stress of the shape-memory effect.The biomechanical study showed that after the "C" shape nickel-titanium alloy intrasegmental fixation instrument was fixed to the spondylolysis specimen,the biomechanical property is similar to the intact spine.Based on the above theory and the NiTi alloy study before,this study planed to invent an original technique using a "U" shape nickel-titanium alloy stick and titanium pedicle screw to form a dynamic fixation instrument for degenerative disc disease.We hepothesis that the "U" shape lumbar dynamic fixation instrument may have the following characteristics:1 the shape of the instrument abides by the mechanical characteristics of the lambar spine.Superimposing the shape-memory material,the instrument can produce stable and persistent elastic strength for the lumbar fixation segment.2 It is three-dimension fixation,which makes the fixation firm,avoiding the chance of gliding and shedding of the fixation instrument.3 The operative procedure is simple.It is a simple,safe,time-saving and labor-saving procedure.This study was divided into 4 part:PARTⅠ:Design of the "U" shape lumbar fixation device from NiTi alloyObjective:Traditionally,spinal fusion has been the mainstay of surgical approaches to the management of low back pain or lumbar instability.Advances in biomedical technology,including pedicle screw fixationl and bone morphogenic proteins,have enabled surgeons to achieve fusion rates between 90%and 100%. However,despite the improvement in radiographic fusion rates,there are some authors who think that there has not been a corresponding improvement in clinical outcomes.Furthermore,there is some evidence that fusion may increase the biomechanical stresses imposed on the adjacent segments leading to transitional disease,which may occur at an earlier rate in instrumented fusion cases.These issues have led some investigators to explore novel approaches to "stabilize" the lumbar spine.One such concept is that of "dynamic stabilization," or "soft stabilization." Dynamic stabilization has been defined as:"a system that would alter favorably the movement and load transmission of a spinal motion segment,without the intenvention of fusion of the segrnent."In other words,such a system would restrict motion in the direction or plane that produces pain,or painful motion,but would otherwise allow a full range of motion.Recently,the use of NiTi shape memory alloy in medical field is more and more extensive.In order to find a new way to treat the degenerative disc diseases,we attempted to utilize the characteristics of hyper-elasticity and shape memory effect of NiTi alloy and attempted to design a new dynamic fixation system.The design concept:The dynamic fixation system include the pedicle screws,which can fix to the pedicle of the lumbar vertebral and the sticks,which can connect the pedicle screws.the middle part of the stick is bend to a "U" shape.The dynamic fixation system abide by the anatomy and biomechanical character of the lumbar spine.the design adopt screw and stick binding concept,bending the middle part of the stick to a "U" shape can make the stick a good flexible effect.Conclusion:By using the super-elastic and shape memory feature of the NiTi alloy,and based on the pedicle screw fixation concept,we can design a "U" shape dynamic fixation system.We assume this system can avoid the disadvantages of the traditional fixation system.PartⅡ:Finite element analysis OF the "U" shape lumbar dynamic fixation systemObjective To evaluate the biomechanical properties of the "U" shape dynamic fixation system for lumbar with finite element method.Method The L4,5 motion segment data were obtained from CT scans(at 1 mm width increments) of the lumbar spine of an adult man.After these images were processed,a three-dimensional finite element model of the L4,5 was established by Mimics11.11.Meanwhile,the three-dimensional finite element model of the vertebral pedicle screw and "U" rod were established by Mimics11.11.According to the above mentioned,the L4,5 model was combined with the model of the vertebral pedicle screw and "U" rod,and established the three-dimensional finite element model of the L4,5 "U" shape dynamic fixation.Loads used in this study were axial compressive,flexion,extension,lateral bending,and rotation forces.The distribution of von mises stress of the fixation segment were analyzed and compared.Result In the same shearing load of 500 N,the stress of the"U" shape dynamic fixation system was lower at axial compressive than flexion,extension,lateral bending,and rotation forces(P＜0.001).The stress of the "U" shape dynamic fixation system for lumbar mainly concentrated on the "U" shape elastic rod,and the stress distribution of the vertebral pedicle screw was comparatively uniform. Conclusion The "U" shape dynamic fixation system has a unique design, which can reduce the stress block and stress concentration of the vertebral pedicle screw,may be a better fixation instrument for the lumbar degenerative disease.PartⅢ:Biomechanical comparison between the "U" shape lumbar dynamic fixation system and traditional fixation systemObjective:To compare the biomechanics features between a new "U" shape lumbar dynamic fixation system and traditional rigid fixation system.Method:Finite element analysis of the two fixation system were created on the basis of a three-dimensional model reconstructed from the images of CT scanning of an adult cadaveric lumbar at 1 mm interval.The stress and displacement of every nodule and element in the course of deformation of the internal fixation were tested.Result:In the same shearing load of 500 N,The force of the dynamic fixation system was concentrated on the "U" shape elastic zone,The force of the rigid fixation system was concentrated on the posterior 1/3 of the lower screw.Conclusion:We demonstrates the new "U" shape lumbar dynamic fixation system has a totally different biomechanical property compare to the traditional rigid fixation system.This dynamic fixation system may be a choice to replace the rigid fixation system for the lumbar degenerative disease.PartⅣ:Finite element analysis of a "U" shape lumbar dynamic fixation stickObjective:To evaluate the biomechanics features of a new "U" shape lumbar dynamic fixation stick.Method:Finite element analysis of the "U" shape stick were created on the basis of a three-dimensional model reconstructed from the images of CT scanning.The stress and displacement of every nodule and element in the course of deformation of the stick were tested. Result:In axial compression,extension and flexion load of 300 N,The force of the dynamic stick was concentrated on the "U" shape elastic zone.Conclusion:This article demonstrates the "U" shape stick has a good biomechanical property.This dynamic fixation system may be a choice to replace the rigid fixation system for the lumbar degenerative disease.