| 1 Objective: The purpose of this study is to design an intervertebral fusion device suitable for minimally invasive lumbar endoscopic surgery,which can be applied alone without additional posterior pedicle and screw fixation system to achieve immediate postoperative stabilization.Through the measurement of anatomical parameters of lumbar intervertebral space,especially the morphometry of endplate,we can provide accurate anatomical parameters for the design of minimally invasive lumbar interbody filling and fusion device.With preliminary concept of minimally invasive lumbar filling and fusion device,we establish the finite element models of minimally invasive lumbar interbody filling and fusion models with the ABAQUS/Standard module to analysis stress distribution simulation under the daily physiological activities.Comparing with the standard box-shaped PEEK(poly-ether-ether-ketone)interbody fusion cage fixation model to analysis biomechanical stability of minimally invasive lumbar interbody filling and fusion device,and optimization design of minimally invasive lumbar filling improved fusion device,but also provide the reference data for further research in vitro and in vivo.According to the results of anatomical measurement and finite element results,combined with the design concept of intervertebral fusion cage,a set of minimally invasive lumbar intervertebral filling and fusion device was designed.Selected calf lumbar spine as the biomechanical test specimens,to use the minimally invasive lumbar interbody filling and fusion device to simulate the interbody fusion surgery,and compared with clinical used box-shaped PEEK posterior lumbar interbody fusion to verify the feasibility and effectiveness of this device.2 Methods: 2.1 Retrospective analysis the image data of computerized tomography(CT)in outpatient picture archiving and communication system(PACS)from January 2005 to December 2015.A total of 67 patients were included in this study.Thin serial CT images were imported into Mimics 10.0 software to reconstruct into three-dimensional models.Lumbar spine were divided into upper vertebrae(from L1 superior endplate to L3 superior endplate)and lower vertebrae(from L3 inferior endplate to L5-S1),and the measurements parameters included endplate parameters(endplate sagittal diameter(SD),transverse diameter(TD)and endplate depth(EPD))and intervertebral disc parameters(intervertebral height(IH)and intervertebral wedge angle(IWA)).2.2 We selected the healthy male volunteer to perform lumbar computed tomography(CT)scan and the data were input into Mimics 10.01 to reconstruct into three dimensional bone structure of L3-4 segment.The 3D models was then put into forward engineering software 3-Matic for post-process to form L3-4 segment entity model.All part of L3-4 segment model were imported into Abaqus/CAE software for material properties assignation,part assembly,set step,defining contact and interaction,loadings and boundary constrains,meshing to establish the three dimensional finite element model of L3-4.800 N vertical load and 7.5Nm moment were applied to simulate lumbar physiology motion and calculate the L3-4 segment stiffness to verify the effectiveness of the model.Based on the intact model of L3-4 model,we established minimally invasive lumbar filling and fusion with 2.5mm wall-thickness PMMA(2.5mm PMMA model),minimally invasive lumbar filling and fusion with 5mm wall-thickness PMMA(5mm PMMA model)and box-shaped PMMA cage interbody fusion(PEEK model).400 N vertical load and 7.5Nm moment were applied to simulate stand,flexion,extension,lateral bending,axial rotation on all models.To calculate distribution of stress and strain at L3-4 segment to verify the effectiveness of the minimally invasive lumbar filling and fusion device.2.3 Based on the results from lumbar intervertebral anatomic measurement and finite element study,to design and manufacture the minimally invasive lumbar interbody filling and fusion device.2.4 Twenty fresh frozen calf vertebral thoracolumbar spines were dissected into functional spinal unite(FSU)and randomly divided into 4 groups(n=5 each):(1)intact control;(2)partial discectomy;(3)minimally invasive lumbar filling and fusion device(PMMA group)and(4)box-shaped PEEK cage(PEEK group).7.5 Nm torque was applied in flexion,extension,lateral bending,axial rotation for each test condition using the materials testing machine to calculate range of motion,stiffness,neutral zone stiffness.3 Results 3.1 From L1 to S1,lumbar endplates appeared to have constant concave morphology,superior EPD were significantly smaller than inferior EPD(P<0.01),except L1(P>0.05).For the superior EPD,the minimum value was locate at S1(0.57±0.42mm),maximum value was at L3(1.72±0.68mm),while for the inferior EPD,the minimum value was at L1(1.47±0.46mm),the maximum value was at L4(2.33±0.71mm).From L1-2 to L5-S1,AIH was significant greater than the PIH at each level(P<0.01),AIH increased from 7.55±1.34 mm at L1-2 to 12.06±2.34 mm at L5-S1;PIH increased from 5.43±1.13 mm at L1-2 to 7.70±1.19 mm at L4-5,decreasing slightly to 6.71±1.29 mm at L5-S1.There is a constant increase from 2.52±2.13° at L1-2 to 12.60±4.55° at L5-S1 for the IWA.The upper lumbar(L1-2 and L2-3)IWA was significantly larger than the lower lumbar spine(L3-4 to L5-S1)(P<0.01).3.2 The deformation of L3-4 segment with 2.5mm PMMA model,5mm PMMA model and PEEK model decreased as compared to the intact lumbar model,except for the extension moment.Compared with the 2.5mm PMMA model and 5mm PMMA model,PEEK model had obviously stress concentration with the peak stress of 27.99 Mpa at stand position,107.40 Mpa at flexion position.2.5mm PMMA model share more load than the 5mm PMMA model during flexion,extension,lateral bending and axial rotation.3.3 The novel minimally invasive lumbar interbody filling and fusion device were successfully designed and manufactured.Minimally invasive surgical instruments of lumbar intervertebral filling fusion device is mainly composed of three parts: the expandable balloon catheter,bone cement filling bag and matching of minimally invasive surgical instruments.3.4 The PMMA group significant reduce the ROM in flexion,extension,lateral bending and axial rotation compared with the intact spine(P<0.05),and the mean decreased ROM at 7.5Nm was 85.7% during flexion,58.3% during extension,75.0% during lateral bending,39.3% during axial torsion.By contrast,the PEEK group showed no statistically significant in extension and axial rotation compared with the intact spine(P>0.05).For all motions,neutral zone stiffness of PMMA group was greater than the PEEK group in all motions.4 Conclusion 4.1 Minimally invasive lumbar interbody filling and fusion device is a feasible and effective technique for immediate stability of the lumbar spine.4.2 Minimally invasive lumbar interbody filling and fusion device designed according to the anatomical structure of lumbar spine and principle of distraction-compression,can get through 8mm diameter approach to establish large footprint structural support and central bone graft area.4.3 Compared with the box-shaped PEEK cage,minimally invasive lumbar interbody filling and fusion device has superior lumbar FSU stabilization in all ROM of movement,especially in extension and axial rotation... |
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