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Journal of Orthopaedic Science xxx (2017) 1e8

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Journal of Orthopaedic Science

journal homepage: http: / /www.elsevier .com/locate / jos

Original Article

Effect of two-level pedicle-screw fixation with different rod materialson lumbar spine: A finite element study

Jayanta Kumar Biswas a, Masud Rana b, Santanu Majumder a, Santanu Kumar Karmakar b,Amit Roychowdhury a, *

a Dept. of Aerospace Engineering & Applied Mechanics, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, 711103, Indiab Dept. of Mechanical Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, 711103, India

a r t i c l e i n f o

Article history:Received 12 May 2017Received in revised form23 September 2017Accepted 19 October 2017Available online xxx

* Corresponding author. Department of AerospaMechanics, Indian Institute of Engineering SciencHowrah, 711103, West Bengal, India. Fax: þ91 33 266

E-mail address: [email protected] (A. Roychow

https://doi.org/10.1016/j.jos.2017.10.0090949-2658/© 2017 The Japanese Orthopaedic Associa

Please cite this article in press as: Biswas JKfinite element study, Journal of Orthopaedic

a b s t r a c t

Background: Pedicle-screw-rod fixation system is very popular surgical remedy for degenerative discdisease. It is important to observe load vs. spinal motion characteristic for better understanding ofclinical problems and treatment of spinal instability associated with low-back pain.Objective: The objective of this study is to understand the effect [range of motion (ROM) and interver-tebral foramen height] of pedicle-screw fixation with three rod materials on lumbar spine under threephysiological loading conditions.Method: A three-dimensional finite element (FE) model of lumbar to sacrum (L1-S) vertebrae withpedicle-screw-rod fixation at L3-L5 level is developed. Three rod materials [titanium alloy (Tie6Ale4V),ultra-high molecular weight poly ethylene (UHMWPE) and poly-ether-ether-ketone (PEEK)] are used fortwo-level fixation and the FE models are simulated for axial rotation, lateral bending and flexion-extension under ±10 Nm moment and 500 N compressive load and compared with the intact (natu-ral) model.Result & discussion: For axial rotation, lateral bending and flexion, ROM increased 2.8, 4.5 and 1.83 timesrespectively for UHMWPE, and 3.7, 7.2 and 2.15 times respectively for PEEK in comparison to Tie6Ale4V.As ROM is 49, 29 and 31% of the intact model during axial rotation, lateral bending and flexionrespectively, PEEK rod produced better motion flexibility than Tie6Ale4V and UHMWPE rod. Foramenheight increased insignificantly by 2.21% for the PEEK rod with respect to the intact spine during flexion.For the PEEK rod, maximum stress of 40 MPa during axial rotation is much below the yield stress of98 MPa.Conclusion: Tie6Ale4V pedicle-screw-rod fixation system highly restricted the ROM of the spine, whichis improved by using UHMWPE and PEEK, having lower stiffness. The foramen height did not varysignificantly for any implant materials. In terms of ROM and maximum stress, PEEK rod may beconsidered for a better implant design to get better ROM and thus mobility.

© 2017 The Japanese Orthopaedic Association. Published by Elsevier B.V. All rights reserved.

1. Introduction

Lowback painmay be caused by themechanical degeneration ofdiscs, ligaments, facet joints or due to severe external loads. It isnecessary to study the load vs. displacement behaviour of the spineto understand the motion characteristics of the same. The knowl-edge of the physical characteristics of the spine becomes important

ce Engineering & Appliede and Technology, Shibpur,84564.dhury).

tion. Published by Elsevier B.V. All

, et al., Effect of two-level peScience (2017), https://doi.o

for better understanding of the spinal problems and treatments ofspinal instability in association with low-back pain [1]. Moramarcoet al., concluded that disc failure generally starts from interfacebetween disc and vertebra [2]. This type of disc failure may occurdue to material discontinuity between the disc-vertebrae andstressed interfaces. Costi et al., have shown that maximum shearstress may cause failure in disc tissue [3]. One of the major medicalremedy for this kind of problem is arthrodesis (fusion), which is aneffective tool for treatment of spinal instabilities and painful con-ditions [4]. Pedicle-screw fixation (fusion) is widely used surgicalremedy for treating these conditions [5]. The most common

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Fig. 1. Stressestrain curve used for inter-vertebral disc [24].

J.K. Biswas et al. / Journal of Orthopaedic Science xxx (2017) 1e82

problems in pedicle-screw fixation systems are screw loosening,bending, breakage and reducing mobility of the spine [6].

Due to the complexity of the spine structure, sometimes it maybe very difficult to understand the mechanical causes of lower backpain. Most commonways to investigate these kinds of problems areexperimental and finite element (FE) method. Experimentalmethod is the direct way for analysing different problems related tospinal biomechanics. But most of the cases it is very costly, timeconsuming and more over, it is difficult to apply load realistically[7]. It is also very difficult to get the information concerning thedistribution of forces within the spine. FE analysis is a very usefultool to explore variety of cases which are difficult to performexperimentally. In this method, a virtual environment is createdand the desired information can be obtained. FE technique has thepotential to reduce costs and to save time during the developmentof new spinal treatment methods or implants [8,9].

Spine kinematics may be expressed as the rotation of spinearound a specific segment and it is referred as 'range of motion'(ROM) which is also a measurement of joint flexibility. There aresome FE analysis which measured the ROM on lumbar spine to findout spinal stability [9e14]. Similar type of studies have been per-formed experimentally on human cadaver [1,15]. Previously, onlymetals (stainless steel, titanium) and their alloys were used asimplant material, which highly restricted themotion of spine in theregion of pedicle screw fixation. Now-a-days, some semi rigidmaterials are introduced to overcome this problem to some extent.Some biocompatible polymeric materials and composites of lowerstiffness are being used for pedicle screw and rod implants.

In case of orthopaedic implants, poly-ether-ether-ketone (PEEK)and carbon fibre reinforced PEEK (CFRP) composites has been clin-ically used as cage to support the lumbar and cervical vertebraewitha good biomechanical and clinical result [30e33]. Chou et al. clini-cally observed the effect of the PEEK cages with bone grafts forfusion of cervical vertebrae (one year follow up) in 55 patients [31].Some FE studies compared PEEK rod with Titanium rod, for pediclescrew fixation under different physiological boundary condition[33e35,9]. As the PEEK's modulus of elasticity is between that ofcortical and cancellous bone, this polymer would offer adequaterigidity for pedicle screw fixation but would develop stresses lessthan metallic rods [35]. The chemical structure of PEEKconfers sta-bility, resistance to chemical environment and compatibility withmany reinforcing agents (such carbonfibres) thanmanymetals [36].

Rohlmann et al. simulated 500 N compressive loads for standingwith intact bone [13]. This load was increased up to 1000 N undernormal activity (sitting, standing and walking) and more (1000 Nand 1600 N) under many daily dynamic activities such as carryingweight, jogging etc [16]. Galbusera et al., studied the ROM of L2-L5spinal segment with single level pedicle screw at L4-L5 level withstainless steel, titanium, PEEK and the composite ostaPek as rodmaterials under bending moment of 7.5 Nm in flexion, extension,right lateral bending and right axial rotation and no compressiveload was applied [9]. But there is no comparison of rod materialflexibility with two level pedicle screw fixation under wide range ofbending moment combined with the physiological compressiveload due to upper body weight.

Intervertebral foramen is a roughly elliptical shaped openingbetween every pair of vertebrae through which some structurespass including nerve roots. It's top and base are constructed byinferior vertebral notch of upper vertebrae and superior vertebralnotch of lower vertebrae [17]. Cosar et al., measured the right andleft side foraminal areas and also studied how the foraminal areaswere affected from the postoperative angulation on the cervicalvertebra [18]. Based on MRI measurements, Richards et al., quanti-fied the effect of a different type of implant (X-Stop place betweenL3-L4) on the dimensions of the spinal canal and intervertebral

Please cite this article in press as: Biswas JK, et al., Effect of two-level pefinite element study, Journal of Orthopaedic Science (2017), https://doi.o

foramen area for flexion and extension [19]. They also showed thattheir implant prevents narrowing of the spinal canal and foraminaduring extension, but there is no significant effect inflexion. It is alsoimportant to study the effect of semi rigid rodmaterials for changingthe foramen height in case of two level pedicle screw fixation.

The objective of the present FE study is to use three different rodmaterials [titanium alloy (Tie6Ale4V), ultra high molecular weightpoly ethylene (UHMWPE) and PEEK] for two-level fixation and findits effect on ROM of the lumbar spine under three physiologicalloading conditions. It is also aimed to study the effect of semi rigidrod materials on intervertebral foramen height.

2. Material and method

2.1. Finite element modelling

The geometry of the lumbar spine with sacrum (L1-S) is ob-tained from CT images in DICOM format (Phillips/Brilliance 64model; 120 kV; 230 slices, 0.684 mm � 0.684 mm, 0.5 mm gap).This data is used in MIMICS® (Materialise Inc, Belgium), an imageprocessing software to build the spine geometry, including thevertebral bodies and inter-vertebral discs [20]. With the help ofANSYS® software, a three-dimensional FE model of implant(pediclescrew & rod)is developed and fixed at L3-L5 level. The cylindricalpedicle screw having V-shaped thread, with diameter 6mm, threadpitch 2.95 mm and thread depth 1.75 mm [21]. The vertebralbodies, inter-vertebral discs and implants are meshed using ten-noded tetrahedral elements. The convergence analysis is done byresetting the element size controls until the error is less than3%.Ligaments are also included in the present study. Seven types ofligaments are modelled considering tension only node to node linkelements. Those are anterior longitudinal (ALL), posterior longitu-dinal (PLL), ligamentum flavum (LF), interspinous (ISL), supra-spinous (SSL), intertransverse (ITL), and facet capsulary (FCL).

Some researchers assigned material properties of cancellousand cortical bone for each element by calculating bone density fromCT scan data [20, 22, 23]. In this study, material property of thecancellous and cortical bone is considered as linearly elastic.Young's modulus and density are assigned from MIMICS based onCT scan data [20, 22, 23]. Eqs. (1) and (2) used for attributing theYoung's modulus of each and every element which is basicallybased on intensity value (Hounsfield unit) [20].

Density (r) ¼ 1.122*(Hounsfield Unit) þ 47 (1)

and

Elastic Modulus (E) ¼ 0.02*(Density)1.69 (2)

Non-linear stressestrain curve is used as shown in Fig. 1 for thematerial property of inter-vertebral disc [24]. The cross-sectional


J.K. Biswas et al. / Journal of Orthopaedic Science xxx (2017) 1e8 3

area, Young's modulus and Poisson's ratio for the ligaments aretaken from literature as given in Table 1 [14,25]. For the implant,Tie6Ale4V is used for the pedicle-screw part and Tie6Ale4V,UHMWPE and PEEK are used for the rod part. Material propertiesand dimensions of the rod and screw are given in Table 2. The FEmodel consisted of 301 314 elements connected through 450 757nodes as shown in Fig. 2. All interfaces in the FE model are assumedto be bonded except the interfaces between the pedicle-screw andthe lumbar vertebrae. The friction contact elements are used in thepedicle screwevertebra interface and frictional coefficient is set to0.3 for non-linear analysis [26].

2.2. Boundary and loading conditions

In this study, six loading conditions such as flexion, extension,left and right lateral bending and clockwise and anticlockwise axialrotation are analysed and shown in Fig. 3. For the FE model, sideportion of the sacrum has been fixed in all directions to prevent allmovements as shown in Fig. 2. To simulate physiological flexion,extension, lateral bending and axial rotational motions, 2, 4, 6, 7.5and 10 Nm bending moments are applied to implanted and intactmodels. For combined load case, fixed compressive load of 500 N isapplied on the upper surface of the L1 vertebra along with theabove mentioned bending moments. Such values for bendingmoment are recommended by Panjabi et al. and Goel et al. forspinal implant simulation [1,12] and compressive load by Rohl-mann et al. for intact bone [13]. Frictional contact pair is generatedto perform non-linear analysis of the boneeimplant interface. Fornodal contact, upper surface of the screw is selected as the contactsurface and the screw hole in the vertebrae is selected as the targetsurface. Friction contact pair is defined between these two surfacesfor non-linear analysis. A total of 240 FE simulations are performed.ROM at various levels are computed for all the cases and comparedwith the intact model.

2.3. ROM e an analytical method

To find out the ROM between the vertebrae, three nodes in X-Zplane are selected nearest to the centre of gravity and these threenodes create a plane. Another new plane is considered after

Table 1Mechanical properties of seven ligaments, taken from Shin et al. [14] and Tsuanget al. [25].

Ligaments Young'sModulus (MPa)

Poisson'sratio

Area(mm2)

Anterior longitudinal ligament (ALL) 20 0.3 63.7Posterior longitudinal ligament (PLL) 20 0.3 20Ligamentum flavum (LF) 19.5 0.3 40Interspinous ligament (ISL) 12 0.3 40Supraspinous ligament (SSL) 15 0.3 30Intertransverse ligament (ITL) 59 0.3 1.8Facet capsulary ligament (FCL) 33 0.3 30

Table 2Material properties and dimensions of the pedicle screw and rods are taken fromBiswas et al. [20] and Lo et al. [26].

Material Young'smodulus (MPa)

Poisson'sratio

Dia(mm)

Length(mm)

Screw Tie6Ale4V 113 000 0.3 6 35Rod Tie6Ale4V 113 000 0.3 6 58

UHMWPE 11 000 0.46 6 58PEEK 3500 0.3 6 58


deformation taking same nodes. The angle between the two planesis equal to the angle between their normal vectors.

A1x þ B1y þ C1z þ D1 ¼ 0 is the plane before deformation.and A2x þ B2y þ C2z þ D2 ¼ 0 is the plane after deformation.Then the angle ‘a’ between the two planes found by using the

following formula.

cosa ¼ jA1$A2 þ B1$B2 þ C1$C2jffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�A21 þ B21 þ C21

�r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�A22 þ B22 þ C22

�r

Fig. 2. Implanted FE model with pedicle-screw and rod.


Fig. 3. Schematic representation of (a) Axial Rotation, (b) Lateral Bending and (c) Flexion-Extension.


The angle between these two planes is calculated using a Cprogram with the software Turbo Cþþ.

3. Results

3.1. Axial rotation

ROM is measured at L3 vertebra with respect to L5 vertebra.ROM under axial rotation is shown in Fig. 4. ROM for the intact(natural) model is compared with the implanted models havingdifferent rod materials. For the intact model, ROM varied from �2�

to �5.6� for clockwise rotation and 1�e5.8� for anti-clockwiserotation with the increase of only moment from 2 Nm to 10 Nm.

Fig. 4. During axial rotation, variations of ROM for Tie6Ale4V, UHMWPE, PEEK rods aloncompressive load.


But under combined loading (moment and axial compressive loadof 500 N), ROM varied from �1.1� to �5.4� for clockwise rotationand 1�e4.9� for anti-clockwise rotation. In case of implantedmodels under combined loading, ROM varied from 0.1� to 0.7� forTie6Ale4V, 0.4�e1.9� for UHMWPE and 0.5�e2.5� for PEEK rodduring anti-clock clockwise rotation.

3.2. Lateral bending

Under lateral bending, the intact model exhibited ROM from 1.8�

to 8.7� during left side bending and�1.7� to�8.4� during right sidebending with increase of moment from 2 Nm to 10 Nm as shown inFig. 5. But in case of combined loading, ROM varied from 1.2� to 7.6�

g with the intact spine under �10 Nm to þ10 Nm moments with and without 500 N


Fig. 5. During left-right lateral bending variations of ROMs for Tie6Ale4V, UHMWPE, PEEK rods along with the intact spine under �10 Nm to þ10 Nm moments with and without500 N compressive load.


for left bending and �1.2� to �7.6� for right bending. Withimplanted models under combined loading, ROM varied from 0.03�

to 0.3� for Tie6Ale4V, 0.3�e1.4� for UHMWPE and 0.4� to 2.2� forPEEK rod in left side bending.

3.3. Flexion-extension

ROM for the L3-L5 segment under flexion-extension is shown inFig. 6. With increase of moment from 2 Nm to 10 Nm for the intactmodel, ROM varied from 2.8� to 13.5� during flexion and �3�

to�14.5� during extension. But ROM varied from 9.9� to 20� duringflexion and 4.4�e7.7� during extension under combined loading.Implanted models under combined loading exhibited ROM from2.3� to 2.9� for Tie6Ale4V, 3.8�e5.3� for UHMWPE and 4.2�e6.2�

for PEEK rod during flexion and 2�e1.3� for Tie6Ale4V, 3�e1.5� forUHMWPE and 4.2�e1.1� for PEEK rod during extension.

Fig. 6. During flexion-extension variations of ROMs for Tie6Ale4V, UHMWPE, PEEK rods alocompressive load.


3.4. Intervertebral foramen height

Foramen heights (as shown in Fig. 7) i.e., distances between thetop and bottom notch for L3-L4 segments are measured fordifferent rod materials under extreme load cases (10 Nm momentand 500 N compressive load). For the Tie6Ale4V, UHMWPE, PEEKand intact models, the foramen heights are found to be 15.88, 15.72,15.70 and 15.36 mm respectively and at L4-L5 17.9, 17.7, 17.7 and17.36 during flexion.

4. Discussion

There are few limitations to this study. The muscles are notincluded in the FE model which would help in defining properphysiological boundary conditions and the nucleus polpusus arenot modelled within the disc which would reflect realistic

ng with the intact spine under �10 Nm to þ10 Nm moments with and without 500 N


Fig. 7. Inter-vertebral foramen height.


deformation of the disc. To validate this intact FE model, a fulllumbar spine including sacrum is taken and compared with anexperimental analysis Yamamoto et al. [27], FE analysis Shin et al.[14] and stereoradiographic results of Pearcy and Tibrewal [28],under 10 N-m loading conditions of flexion, extension, lateralbending and axial rotation (Fig. 8 and Table 3).

In this study, two FE models are considered for an intact lumbarspine and an implanted lumbar spine. Tie6Ale4V, UHMWPE andPEEK are used to check for their suitability as implant materials interms of flexibility. For realistic analysis, 500 N compressive load isapplied in addition to moment of �10 Nm to 10 Nm in case ofcombined loading. It is also observed that difference of ROM be-tween the intact and implanted models increases as the load isincreased. But, ROMs under combined load are insignificantly lowerthan those under pure moment except the case of flexion andextension. Due to the additional compressive load during flexion

Fig. 8. a) Full lumbar spine including Sacrum with Flexio


and extension, significant increase and decrease are found in ROMsrespectively for the intact model, implant with Tie6Ale4V,UHMWPE and PEEK rod materials. This may be due to the existingnatural curvature of lumbar spine and some forward displacementof the spine due the compressive load.

In this study, ROM for all the implanted models is much lesserthan the intact model in axial rotation. Under combined loading,when UHMWPE and PEEK are used as rodmaterials, there is a hugechange in ROM as compared to Tie6Ale4V. ROM increased 2.8times for UHMWPE and 3.7 times for PEEK. In comparison toTie6Ale4V and UHMWPE, better motion perseverance is obtainedwith PEEK material during axial rotation as the ROM has 49% of theintact model (Fig. 9). Galbusera et al. studied the ROM of L2-L5spinal segment with single level pedicle screw at L4-L5 level.Stainless steel, titanium, PEEK and the composite ostaPek wereconsidered as rod materials. The models were subjected to puremoments of 7.5 Nm in flexion, extension, right lateral bending andright axial rotation. Stainless steel, titanium and ostaPek rodsreduced the ROM strongly (>70%), while the PEEK rod induced asmaller ROM reduction (32%) [11]. Dmitriev et al. studied experi-mentally cadaveric human lumbar spine (L2-S1) with multi levelpedicle screw (CD Horizon™) fixation at L3-L4-L5. They loaded themodel with ±8 Nm in flexion-extension, lateral bending and axialrotation. ROM at L3-L4 and L4-L5 level were 1.5� and 1.9� respec-tively in axial rotation [15]. Whereas in this study, ROM at L3-L5level is observed as 1.2� for Tie6Ale4V, 3� for UHMWPE and 4� forPEEK rod under ±7.5 Nm load in axial rotation.

In case of lateral bending, intact model and implanted modelwith Tie6Ale4V, maximum ROM of 7.6� and 0.3� respectively areobserved under combined loading. While UHMWPE and PEEK areused as rodmaterials, ROM is increased 4.5 times for UHMWPE and7.2 times for PEEK. As the ROM is 29% of the intact model, PEEKimplant produces better motion flexibility compared to Tie6Ale4Vand UHMWPE as shown in Fig. 9. Alapan et al. studied the ROM ofL4eL5 intact spine segment. The FE model was simulated under a

n. b) Deformed shape of the full intact lumbar spine.


Table 3Comparison, of ROM for the present study with previous research works, for intact lumbar spine under 10 Nm moment.

Intact model Present Study Yamamoto et al. [27] Pearcy and Tibrewal [28] Shin et al. [14]

Flex þ Ext L.B A.R Flex þ Ext L.B A.R Flex þ Ext L.B A.R Flex þ Ext L.B A.R

L1-L2 11.6 5 3.19 10.1 4.9 2.1 13 5.0 1.0 e e e

L2-L3 11.9 6.2 3.3 10.8 7.0 2.6 13 5.5 1.0 10.4 6.4 e

L3-L4 12.1 5.3 2.4 11.2 5.7 2.6 13 5.0 1.5 11.2 6.3 e

L4-L5 15.2 4.3 2.2 14.5 5.7 2.2 16 3.0 1.5 14.5 6.4 e

L5-S 15.9 3.5 1.9 17.8 5.5 1.3 14 1.5 1.0 e e e

Fig. 9. Comparison of maximum ROMs for Tie6Ale4V, UHMWPE, PEEK rods during flexion-extension, lateral bending and axial rotation.

Table 4Maximum von-Mises stress for the PEEK rod during axial rotation, lateral bending,flexion and extension.

500 N Compressiveand ±10 Nm loading

Upper(L3-L4) Lower(L4-L5)

Stress (MPa) Stress(MPa)

Left rod Right rod Left rod Right rod

Axial rotation 32 30 40 35Bending moment 11 10 23 17Flexion 20 25 20 15Extension 22 24 25 18


7.5 Nm moment in flexion, extension, lateral bending and axialrotation. They obtained a ROM of 9.4� during lateral bending [10].

Panjabi et al. studied experimentally with cadaveric humanlumbar spine (L1-S1) under ±10 Nm moment and axial compres-sive pre-loading of 100 N [1]. They observed ROM of 16.7� inflexion and �6� in extension. But, in this study, it is found to be 20�

in flexion and �7.7� in extension for the intact model. This dif-ference may be due to different compressive load of 500 N. WhileUHMWPE and PEEK are used as rod material, ROM is increased1.83 times for UHMWPE and 2.15 times for PEEK. Compared toTie6Ale4V and UHMWPE, better mobility has shown by PEEK rodduring flexion as the ROM is 31% of the intact model. Similarly,Goel et al. performed FE analysis on L3-S1 spine segment withposterior dynamic stabilizer (PDS) made of cobaltechromium alloyat L4-L5 level [12]. Under 10 Nm moment with 400 N compressiveload in flexion, they found the ROM at L4-L5 to be 4.5� in case ofonly PDS fixation. But with combined pedicle-screw (Titanium rod,110 GPa) and cage (PEEK, 3.6 GPa) fixation at L4-L5 level for a L3-L5spine segment, obtained a ROM of 0.1� in flexion under 10 Nmmoment [11].

With one-level (L3-L4) and two-level (L3-L5) pedicle screwfixation, having PEEK, CFRP and Titanium rods under 7.5 Nm puremoment, Kang et al. also conclude that the PEEK and CFR-PEEK rodsystems reduce the possibility of breakage of the pedicle screw andprovide more flexibility to the lumbar spine, compared to Titaniumrod [34]. In a similar type of study, Chang et al. compared the PEEKrod system to Titanium rod at (L3-L4) level under 10 Nm puremoment and also conclude the same [35].


From the above discussion, it is clear that the flexibility of thePEEK implant is better than the other twomaterials. But apart fromthe ROM criteria, the implant must be checked for the yield stresscriteria. It is observed that the maximum von-Mises stresses in thelower level (L4-L5) PEEK rod under 500 N compressive load and10 Nm moment are 40, 23, 20 and 25 MPa during axial rotation,lateral bending, flexion and extension respectively as given inTable 4. These values arewell below the yield stress value of 95MPafor the PEEK [29]. Among three rod materials, it is found that fo-ramen height is maximum for the Tie6Ale4V rod andminimum forthe PEEK rod during flexion. For axial rotation and lateral bending,the changes in foramen height for the implanted models arenegligible compared to the intact model. Foramen height decreasesinsignificantly as the Young's modulus decreases such as 0.97% forUHMWPE and 1.14% for PEEK, as compared to Tie6Ale4V. Itincreased by only 2.21% for the PEEK rod with respect to the intactspine.



5. Conclusion

In this study of the effect of pedicle-screw fixationwith differentmaterials during axial rotation, lateral bending and flexion-extension, it is observed that the ROM is very less in the implan-ted model with Tie6Ale4V rod in comparison with other materialsand intact spine. This indicates that the Tie6Ale4V rod is stiffcompared to UHMWPE and PEEK rods. So it is suggested that PEEKrod may be used to get better ROM and thus mobility. It is impor-tant to state that for pedicle-screw fixation, the foramen heightdoes not vary significantly for any implant material. So it may notdisturb the nerve roots passing through the foramen for theimplanted condition. This study may be useful for prediction of theROM and foramen height under other loading conditions too. Interms of ROM and maximum stress, PEEK rod may be consideredfor a better implant design after subsequent fatigue testing andexperimental validation.

Conflict of interest

The investigators of the project and the authors have no conflictof interest with the funding agency and also among themselves.

Acknowledgement

This study was a part of a research project (DRC/DBT/AE&AM/ARC/020/11-12) funded by Department of Biotechnology (DBT),Govt. of India. The investigators of the project and the authors haveno conflict of interest with the funding agency and also amongthemselves.

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