| BackgroundAlong with our country entering into aging society,more and more lumbar degenerative diseases(LDD) occurred,leading to the number of adults and old people needing operation increasing.At present,the main operation methods are neural canal decompression with or without interbody fusion accompany supplement with internal fixation.For the past decade years,the lumbar non-fusion techniques have make a great progress worldwide.However,the non-fusion techniques and equipment are in early stage and not enough to gain popularity because of it’s deficiency of prospective investigation and expensive instrument.At the same time,those techniques have some defects in instrument design,and are based on vague recognition about principal of between mobile lumbar vertebra and LDD.Since Chandler first report post-lateral fusion of lumbar vertebra at 1929,many kinds of fusion methods of lumbar spinal fusion had come out.Now,lumbar interbody fusion with stuffing bone graft cages and supplement with pedicle screws fixation had become main stream of operation method.The argument about lumbar spinal fusion and firm internal fixation are inceasing since emergence from these techniques.Lumbar interbody fusion supplement with pedicle screws fixation have major shortages as follow:1.stress-shielding from rigid internal fixation may lead to lumbar fusion failure and accelerating degeneration of vicinity intervertebral discs(IVD).2.Stress concentration of pedicle screws may cause breakage and displacement of screws or rod.Perforation of pedicle of vertebra result in nerve root lesions.3.The fixation system will increase operation time,soft tissue injury,bleeding volume,complication after operation and reoperation rate obviously.At the same time,it will make more expense by pedicle screws fixation.Although pedicle screw system can be implanted by mircoinvasive methods,it is hardly to widespread application in the near future still in our country,because this technique needs more expensive equipments,longer and steeper learning-curve, more radiation to doctors and patients,in addition to intrinsic risk of pedicle screws itself.In terms of tremendous lumbar disc herniation,unstable IDD and part of lumbar spondylolisthesis,the lumbar interbody fusion with internal fixation has distinct superiority and gain preferable result of operation certainly.For this reason,it has become an interesting problem that how to undertake lumbar fusion and internal fixation operation supplement with effective,appropriate,cheap and mirco-invasive methods.Facet joint(zygapophysial joint) is the only real active joint in lumbar vertebra, and share axial loading approximately 10 to 20 percent of total loading.The function of facet joint is against rotation as well as against shearing force.Immobilization of facet joint can be got more ability of anti-rotation,anti-shearing force and anti-extension.Therefore,Immobilization of facet joint with screw is one of reasonable methods offer by Heggeness MH.In 1959,Boucher attempted to lumbar spinal fusion with transfacetopedicle screw (Boucher screw,TFPS).Magerl introduced transliminarfacet screw(Magerl screw, TLFS) at first in lumbar posterlateral fusion of spine in 1983.These facet screws have below merits:installing convenience,cheaper cost,lower risk of nerve root lesions,shorten operation time,diminishing complication and rising fusion ratio.In 2003,Kandziora created a new facet screw:lumbar facet interference screw(TFIS), but no clinical report was found about this new facet screw in lumbar interbody fusion so far.For the past few years,it was discovered that segment stability of lumbar vertebra has no significant difference between interbody fusion supplement with facet screws fixation and with pedicle screws fixation.Moreover,campared to pedicle screws fixation,adjunct segment degeneration was later and lighter with facet screws fixation.It was conjectured that this phenomenon is because that facet screws fixation is a semi-rigid internal fixation.Implantation of these facet screws needs integrity of facet joint.Therefore,the past clinical application reports of facet screws were performed with anterior lumbar interbody fusion(ALIF) by percutaneous implantation mainly.The application was restricted because clinician worried about facet screw didn’t meet the biomechanics requirement,which will lead to screws breakage and failure of lumbar spinal fusion.Owing to the feature of transforaminal lumbar interbody fusion(TLIF) operation, which destroy facet joint of approach side,the lumbar vertebra of approach side segment need pedicle screws(PS) fixation.Theoretically,contralateral facet joint fixation supplements with facet screws can stabilize whole function segment. Biomechanical investigation had discovered that effectiveness of the posterior asymmetric fixation on the base of TLIF,one side using PS fixation and the other side using facet screw(TLFS and LFIS) was similar to double side pedicle screws fixation.The posterior asymmetric fixation on the base of TLIF can meet mechanical requirement of clinical application.What are stress attribution feature of facet screws fixation accompany with ALIF? Compared with pedicle screws fixation,whether facet screws fixation can reduce stress-shielding effect or not? What kind of facet screw is more secure than the other? How is the TFPS security and segment stabilization in posterior asymmetric fixation after TLIF? No answers are found about these questions so far,and it deserves to study further.Currently,the stress distribution of internal fixation and Cages can be analyzed by 3-D finite element(FE) model,which method has the characters of reproducibility and comparability.We plan to analyze the mechanical property of facet screws on basis of lumbar interbody fusion(ALIF\TLIF) by finite element method(FEM) and to evaluate stability of the poster pedicle screws plus TFPS fixation after TLIF with biomechanical experiments.These can offer theory proofs for the better clinical application.Objectives1.To develop a 3-D finite element model of the different facet screws fixation and pedicle screws fixation and to evaluate the stress feature of three kind screws and cages in ALIF with same loading and different moments.2.To develop three 3-D finite element models:posterior unilateral pedicle screws supplemented with contralateral TFPS fixation,posterior unilateral pedicle screws supplemented with contralateral TLFS fixation and posterior bilateral pedicle screws fixation after TLIF respectively.To evaluate the stress of three kinds of screws combined fixation and cages in TLIF.3.The biomechanical testing in vitro aimed to evaluate the stability of posterior unilateral pedicle screws plus TFPS fixation after TLIF,and then compare it with that of intact segment,unilateral pedicle screws and bilateral pedicle screws to indentify the biomechanical features of the novel construct after TLIF.Methods 1.The geometrical model was created by Mimics 11.1 based on CT data of L4-5 motion segment from a health adult man.Different facet screws,pedicle screws and cages geometrical model were dealt with as the same procedure.Subsequently,the above models were imported into the Simpleware 3.1 in order to establish the 3D finite element models of ALIF with double fusion cages and TLFS,TFPS and pedicle screws fixation respectively.The 3D finite element models were imported into Abaqus 6.8 and analyzed.At first,500N pressure was loaded on the upper surface of L4 of ALIF supplement with TLFS and TFPS to simulate lumbar axial compression,then,10Nm moments was loaded to simulate lumbar anteflexion,extension,left lateral bending and left axial rotation.The mean Von mises stress of the facet part of facet screws was recorded and inputted into Spss 13.0 to analyze the difference of stress of the two different fixations.Then,500N\6 Nm loading was loaded on the upper surface of L4 of ALIF supplement with three types screws to simulate lumbar flexion, extension,left lateral bending and left axial rotation.Both the peak Von raises stress and stress distribution imagerys of the three kinds of screws fixation and cages were recorded,and the feature of different stress distribution of the three fixations were analyzed.2.The geometrical model was created by Mimics 11.1 based on CT data of L4/5 motion segment from an adult man.Different fixation models after left TLIF including the following sequentially test configurations:1) ipsilateral PS plus contralateral TLFS;2) ipsilateral PS plus contralateral TFPS;3) Bilateral PS. Subsequently,the above models were imported into the Simpleware 3.1 in order to establish the 3D finite element models.The 3D finite element models were imported into Abaqus 6.8 and analyzed.500N\6Nm loading was loaded on the upper surface of L4 of TLIF supplement with three types of fixations to simulate lumbar flexion, extension,lateral bending and axial rotation.Both the peak Von mises stress and stress distribution imagerys of the three kinds of fixations and cages were recorded, and the features of different stress distribution of the three fixations were analyzed.3.Range of motion(ROM) testing of L3-4 motion segment was performed in 7 fresh-frozen health adult human cadaveric lumbar spine motion segments in flexion, extension,lateral bending and axial rotation using 10.0 Nm torques on the Spine 2000 3-D motion test machine,including the following sequentially test configurations:1) intact segment;2) left TLIF and ipsilateral PS;3) TLIF and ipsilateral PS plus contralateral TFPS;4) TLIF and bilateral PS.Analysis of ROM dates determined construct stability.4.Statistical analysis of the dates was performed using the SPSS 13.0 software package.For mean stress comparison of facet parts between TLFS and TFPS under different motions under 500N\10Nm loading,Statistical analyses of the data was performed using factorial analysis,followed by independent-samples t-Test at different fixations under same motion.Statistical analyses of the ROM dates in biomechanical experiments were performed using a repeated-measures analysis of variance.At same motion\fixation,statistical analyses of the mean ROM under different fixations\motions were performed using analysis of univariate,followed multiple comparsion at different fixations by Bonferroni method under same motion. Significance was assumed for probability values 0.05.Results1.It was obviously that the Von mises of the screws concentrated on the facet part. Among of mean Von mises stress of all loadings of TLFS and TFPS fixation,axial compression load was minimal and the lateral bending was maximal.The mean Von mises stress of TLFS fixation of different loading was larger than that of TFPS fixation(P<0.01). The peak stress of pedicle screws in extension\flexion were less than the stress of facet screws,but the stress of pedicle screws in bending\axial rotation were bigger than the stress of facet screws under 500N\6Nm loading.The stress features were similar to two types of facet screws.There was stress-shielding effect at cages because of pedicle screw robust fixation.The risk of screws fatigue breakage of two kinds of screws is quite lowly.2.Left facet joint was removed in TLIF procedure,which leads to asymmetry stress attribution of internal implants.For rigid PS fixation,they were more influenced by asymmetry stress attribution,especially under left axial rotation.The stress-shielding effect of cage was not obvious in bilateral PS fixation after TLIF.In asymmetry fixation,the stress of PS side was increasing accordingly,however,the breakage risk of facet screw remain lower under rotation controling properly.3.Both fixation and motion status have significant influence on the value of ROM (P=0.000).The reciprocational effect between fixation and motion status were also obvious(P=0.000).The motion of flexion and lateral bending was significant among such specimen as intact specimen,TLIF follow by unilateral PS,TLIF+PS+TFPS and TLIF+bilateral PS fixation.Except for ipsilateral PS in rotation and PS+TFPS in right axial rotation,the ROM of intact group had significant difference with other groups in different motions(P<0.05).After TLIF,the ipsilateral PS construct provided less segment stability than the novel asymmetric construct in left bending (P<0.05).In bending and rotation motion,the ipsilateral PS construct provided less segment stability than the bilateral PS fixation(P<0.05),and it allows for significant off-axial rotation motions,which could be detrimental to stability and the promotion for fusion.In flexion/extension,lateral bending and axial rotation motions,there were no measureable difference in ROM between the standard bilateral PS and the novel asymmetric construct fixation after TLIF. Conclusions1.The TFPS fixation after ALIF can be considered as a good alternative to TLFS fixation to avoid the breakage risk of the screws.It is necessary to limit patients’ activities by a waist-brace after operation.At the same torque,the safety of facet screws fixation after ALIF were similar to pedicle screws fixation.It may relieve stress-shielding effect of interbody cages by mean of facet screws fixation.2.For stepping down the risk of PS fatigue breakage,we suggest to restrict severely axial rotation after operation and to adopt bigger diameter PS on the side that facet joint was destroyed in posterior asymmetric fixation after TLIF.3.Placing ipsilateral PS plus contralateral TFPS provides the surgical advantages with stability compare to TLIF with bilateral PS fixation,which infers that the novel construct can be an alternative preferential in minimal invasive surgery. |
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