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Yann-Philippe CHARLES, Sylvain PERSOHN, Philippe ROUCH, Jean-Paul STEIB, E.A.
SAULEAU, Wafa SKALLI - The effect of posterior non-fusion instrumentation on segmental shear
loading of the lumbar spine - Orthopaedics and Traumatology - Surgery and Research - Vol. 100,
n°5, p.461-467 - 2014
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Original
article
The
effect
of
posterior
non-fusion
instrumentation
on
segmental
shear
loading
of
the
lumbar
spine
Y.P.
Charles
a,b,∗,
S.
Persohn
a,
P.
Rouch
a,
J.-P.
Steib
b,
E.A.
Sauleau
c,
W.
Skalli
aaLaboratoiredeBiomécanique,ArtsetMétiersParisTech,151,boulevarddel’Hôpital,75013Paris,France
bServicedeChirurgieduRachis,HôpitauxUniversitairesdeStrasbourg,FédérationdeMédecineTranslationnelle(FMTS),1,placedel’Hôpital,BP426,
67091StrasbourgCedex,France
cDépartementdeSantéPublique,HôpitauxUniversitairesdeStrasbourg,FédérationdeMédecineTranslationnelle(FMTS),1,placedel’Hôpital,BP426,
67091StrasbourgCedex,France
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Accepted27May2014 Keywords: Degenerativespondylolisthesis Facetectomy Undercuttinglaminectomy Non-fusioninstrumentation Anteriorsheara
b
s
t
r
a
c
t
Background:Lumbarstenosisandfacetosteoarthritisrepresentindicationsfordecompressionand instru-mentation.ItisunclearifdegenerativespondylolisthesisgradeIwitharemainingdischeightcouldbean indicationfornon-fusioninstrumentation.Thepurposeofthisstudywastodeterminetheinfluenceofa mobilepediclescrewbaseddeviceonlumbarsegmentalshearloading,thussimulatingtheconditionof spondylolisthesis.
Materialsandmethods:Sixhumancadaverspecimensweretestedin3configurations:intactL4–L5 seg-ment,thenfacetectomyplusundercuttinglaminectomy,theninstrumentationwithlesion.Astaticaxial compressionof400NwasappliedtothelumbarsegmentandanteriordisplacementsofL4onL5were measuredforposterior-anteriorshearforcesfrom0to200N.Theslopeoftheloadingcurvewasassessed todetermineshearstiffness.
Results:Homogenousload-displacementcurveswereobtainedforallspecimens.Theaverageintact ante-riordisplacementwas1.2mm.Afterlesion,thedisplacementincreasedby0.6mmcomparedtointact (P=0.032).Theinstrumentationdecreasedthedisplacementby0.5mmcomparedtolesion(P=0.046). Thestiffness’swere:162N/mmforintact,106N/mmforlesion,148N/mmforinstrumentation.The differencewasnotsignificantbetweeninstrumentedandintactsegments(P=0.591).
Conclusions: Facetectomy plusundercutting laminectomy decreases segmental shear stiffnessand increasesanteriortranslationalL4–L5displacement.Shearstiffnessoftheinstrumentedsegmentishigher withthedeviceandanteriordisplacementsundershearloadingaresimilartotheintactspine.This con-ditioncouldtheoreticallybeinterestingforthesimulationofnon-fusioninstrumentationindegenerative spondylolisthesis.
©2014ElsevierMassonSAS.Allrightsreserved.
1. Introduction
Lumbar non-fusion instrumentation systems are aimed to reduce theriskof adjacentsegment degenerationsecondary to fusion [1]. Total disc replacement can be efficiently indicated in low-backpain caused bydiscopathy. Nevertheless, the load-sharingcomplexbetweenthediscand facetjointsmaylead to recidivatingpainifadditionalmoderatefacetdegenerationisnot diagnosed preoperatively [2]. This has spawned an interest in
∗ Correspondingauthorat:LaboratoiredeBiomécanique,ArtsetMétiers Paris-Tech,151,boulevarddel’Hôpital,75013Paris,France.Tel.:+33388116826; fax:+33388115233.
E-mailaddress:yann.philippe.charles@chru-strasbourg.fr(Y.P.Charles).
the development of posterior facet preserving non-fusion sys-tems,whichmaydecreasesegmentalmotionwithoutsuppressing it [3–5]. Facet resurfacing and replacement devices have been designed toaddress severe facetosteoarthritis and subsequent stenosis[6].
Instrumentationisrequiredafterfacetectomyorarthrectomy becauseofsegmentalincreaseofmotioninaxialrotationandunder shearloading[7,8].Invitrostudiesandfiniteelementmodels indi-cate that posterior non-fusiondevices could stabilizea lumbar segmentandmaintainmobilityafterpartialortotalfacetresection andlaminectomy[9–12].Firstclinicaltrialsshowedthat decom-pressionandnon-fusioninstrumentationmightimproveback-and leg-pain, and thequalityoflife indegenerative spondylolisthe-sis [13–15]. However,these devices are restricted to segments witha sufficient discheight,and it isnot cleartowhat extent http://dx.doi.org/10.1016/j.otsr.2014.05.005
462 Y.P.Charlesetal./Orthopaedics&Traumatology:Surgery&Research100(2014)461–467
Fig.1.Non-fusioninstrumentationwithpolyaxialconnectorlinkingtherodsto caudalscrews,thusallowingathree-dimensionalmovementandstabilizationafter medialfacetresection.
decompressionshouldbeperformed,sinceshearforcesare trans-mitted through the implant, which may lead to device-related complications[16].
The NeoFacetTM (Clariance, Dainville, France) represents an
implant,whichisdesignedforposteriorelementsupplementation ifafacetresectionisrequiredinadditiontoundercutting laminec-tomy.Itmightbeindicatedforlow-backpain,mainlyduetofacet osteoarthritis,andsciaticaduetolateralrecessand/orforaminal stenosis.Thissystemutilizesfourpediclescrewswithtwo angu-latedrodsfixedcranially.Thisimplantismadeofimplantablegrade metalcomponents,whichaddresstheanatomicalrequirementsof thesegmentsL3–L4andL4–L5.Traditionalpediclescrewfixation isused.Tworods(30◦ or45◦)areinsertedandfixedatthe
cra-nialvertebrausingpolyaxialpediclescrews.Theserodsarelinked tocaudalpediclescrewsusingapolyaxialconnectoroneachside, whichallowsmovementsinflexion-extension,lateralbendingand axialrotation.Across-linkconnectsbothrodstoeachother,thus avoidingexcessiveaxialrotation(Fig.1).Pediclescrewsare man-ufacturedof titaniumalloy.A titaniumplasma spraycoating is
appliedtotheboneinterfacesurfacesofthescrews.Theother com-ponentsoftheimplantaremanufacturedfroma wear-resistant cobalt-chromium-molybdenumalloy.
Apreviousinvitrostudydemonstratedthatthisdevicecould preserveflexibilitybetweenlumbar vertebraewhile restraining motioninaxialrotationafterfacetectomy[17].Itisalsoimportant toinvestigatetheshearbehaviorofthisimplant,whichmaybe indicatedindegenerativespondylolisthesisgradeIwitha remain-ingdischeight.Thepurposeofthisstudywastodeterminethe influence of non-fusion instrumentation on a lumbar segment undershearloading,thussimulatingtheconditionsof degener-ativespondylolisthesistreatedbyfacetectomyplusundercutting laminectomy.
2. Materialsandmethods
Sixfresh-frozenhumancadavericL4–L5spinesegmentswere tested.Theaverageageofthedonorswas73.8yearsandranged from63to84years.Therewere5malesand1female.The speci-menswerefreshlydissected,sealedindoubleplasticbags,frozen, andstoredat−20◦Cuntiltesting.Thespecimenswerethawedto
6◦C12to14hoursbeforestartingthepreparationprocess.Soft
tis-sueswereremoved,leavingallligaments,jointcapsules,discsand bonystructuresintact.Spinaldeformities,damageorsevere degen-erationofthediscsandfacetjointswereexcludedmacroscopically andradiographically.Mediandischeightswere≥7mmonlateral radiographs.Theexperimentwasperformedatroomtemperature, whileusingasalinesolution(NaCl0.9%)tomoisturethedisc.
ThecranialhalfoftheL4vertebralbodyandthecaudalhalfof theL5vertebralbodywereembeddedin2metalcontainersusing polymethylmetacrylatecement(Technovit3040;Haerus,Hanau, Germany).ThemedianplaneoftheL4–L5discwasalignedwith ananteriorinclinationof10◦withregardtothehorizontalplane,
thusreproducingitssagittalalignmentinvivo.Biplaneradiographs wereusedtochecktheorientationofthespecimen.Shear load-ingtestswereconductedinaspecificspine-testingdevicethatwas designedforthispurpose.Thecaudalcontainer,fixedonL5,was rigidlyscrewedtoatable,whilethecranialcontainer,fixedonL4, wasmountedtoarail,allowingtranslationinthesagittalplane. Acompressivepreloadof400Nwasappliedtothemotion seg-ment[10,11,18,19].LoadswereappliedtoL4usingdeadweights placedattheendofloadingbars,cablesandpulleys,thus induc-ingananteriortranslationofL4onL5(Figs.2and3).Thissystem
Fig.2.ExperimentalsetupforinvitrosheartestingoftheL4–L5segment,withamobilerailfixedtoL4,alineartransducerformeasurementsoftranslationofL4onL5 obtainedbyanteriortractionviaacableandpulleysystemattachedtotherail.
Fig.3. InstrumentedL4–L5specimeninthespine-testingdevice.
Fig.4.LesionappliedtoL4–L5bymedialfacetectomyplusundercutting laminec-tomy.
allowedapplyingquasi-staticforcesinstepsof10N,withan inter-valof15secondsbetweeneachstep,untilamaximumof200N wasreached.Themaximalanteriordisplacementandbackto neu-tralpositionwerecompletedduringthesameloading-unloading cycle.Threepreconditioningcycleswereappliedtothespecimen usingthesameloadingprotocol,beforethemeasurementcycle wasstarted.Displacementsweremeasuredattheleveloftherail usingalineartransducer(VishaySfernice50L4D202W00235D 2k;VishayElectronicGmbH,Selb,Germany).Themeasurement accuracyofthis systemwasestimatedat0.1mmfor linear dis-placements.Load-displacementcurveswereobtainedforloading andunloadingcycles.Thestiffnesswascalculatedconsideringthe slopeofthelinearpartoftheloadingcurve.
Thespecimens weretestedin 3configurations:intact speci-men,specimenwithlesion,instrumentedspecimenwithlesion. ThelesionconsistedofanL4–L5medialfacetectomybyremoving theinferiorL4articularprocesses.Aninterlaminarfenestrationand yellowligamentresectionattherecessuswereperformedusinga Kerissonrongeur,thussimulatinganundercuttinglaminectomy (Fig.4).Theimplantwaspositionedsymmetricallybetweenright andleftsides.Pediclescrews,witha6.5mmdiameteranda45mm length,wereplacedparalleltothesuperiorendplateandalonga convergent trajectoryinthehorizontalvertebralplane.Therod systemandthepolyaxialconnectorsweremountedtoL4andL5 screws,respectively.Bothrodswereconnectedbyarigid cross-link.Thepositionoftheimplantwasdocumentedusingbiplane radiographs(Fig.5).
Fig.5.Antero-posteriorandlateralradiographsofinstrumentedL4–L5segment withlesionandvertebralbodiesembeddedinPolyMethylMethAcrylate(PMMA).
Statistical evaluation was performed with R Software Ver-sion 2011 (R Foundation for Statistical Computing, Vienna, Austria).Afterchecking homogeneityof variancesusing a non-parametricFligner-Killeentest,theWilcoxonranktestwasusedfor pairedsamplestocomparemaximaldisplacementsandstiffness’s betweendifferentconfigurations.Unilateraltestsfor superiority wereused.Thesignificancelevelwassetat0.05foralltests.
3. Results 3.1. Displacement
Load-displacement curves were obtained for anterior trans-lation and back to neutral position. A hysteresis phenomenon was observedfor each configurationsince the unloadingcurve wasnotsuperimposedontheloadingcurve.Loadingcurveswere comparable for the six intactspecimens (Fig.6).The variances werehomogenousformaximaldisplacements(P=0.194).Table1 demonstrates average,median andextremevalues obtainedfor each configuration.Fig. 7representsaverageload-displacement curvesandFig.8showsindividualvaluesobtainedforeach spec-imen.Thelesionledtoanaverageincreaseof0.6mmcompared totheintactL4–L5segment(P=0.032).Theinstrumentedsegment withlesionincreasedtheaveragedisplacementby0.1mm com-paredtotheintactspine(P=0.468).Theinstrumentationdecreased theaveragedisplacementby0.5mmcomparedtothelesionalone (P=0.046).
464 Y.P.Charlesetal./Orthopaedics&Traumatology:Surgery&Research100(2014)461–467
Fig.6.Load-displacementcurvesduringloadingfrom0Nto200Nforeachspecimen.
Table1
Maximalanteriordisplacement(mm)at200N.
Configuration Average Median Minimum Maximum Differencewithintact P-valuea
Intact 1.2 1.2 0.8 1.7 – –
Lesion 1.8 1.7 1.1 2.5 +0.6[+49%] 0.032
Instrumented 1.3 1.2 0.8 2.4 +0.1[+6%] 0.468
aWilcoxontestsignificantifP<0.05.
3.2. Stiffness
Thevarianceswerehomogenousforstiffness’s(P=0.717). Val-uesforeachconfigurationaredemonstratedinTable2.Thelesion decreased the stiffness by 56N/mm on average compared to theintactL4–L5segment(P=0.032).Thedifferencebetweenthe instrumented and theintact segment was14N/mm(P=0.591). The average stiffness of the instrumented segment increased by 42N/mm compared to the lesion without instrumentation (P=0.046).
4. Discussion
Theindicationforposteriornon-fusiondeviceshasbeen empha-sizedin lateralrecessand/orforaminalstenosisassociatedwith osteoarthritis of thefacet joints. Thesurgical treatmentof this lumbardegenerativepathologyusuallyrequiresadecompression of thecanal,and anadditional partialorcomplete resectionof thefacetjointsifthesearemainlyresponsibleforbackpain.This impliesastabilizationusingaposteriorinstrumentationandfusion to avoid segmental hypermobility of the treated segment. The
Fig.8. Maximaldisplacements(Dmax)at200Nforeachspecimenandtheiraverageforthedifferentconfigurations.
rationale behindpediclescrewbased mobilesystemswould be thestabilizationof thelumbar segmentwhilepreservinga cer-tainamountofsegmentalmobility,andthuspreventingadjacent segmentdegeneration[18].Nevertheless,theprotectiveeffectof thesenon-fusiondevicesonadjacentlevelshasnotbeenclearly demonstratedtodate.Theinfluenceofthesedevicesonsegmental kinematicsofthelumbarspinemainlydependsontheimplant’s design:flexion-extensionandlateralbendingareusuallyslightly decreased,whereasaxialrotationisonlylimitedbysystemswith across-linkcomponent[9–12,20].However,thispropertyseems essentialforthestabilizationofthelumbarsegmentwhen perform-ingafacetectomy[7].Ontheotherhand,shearloadingincreases mobilityofthelumbarsegmentafterfacetresection[8,19],and posterior-anterior displacementsmay havean influence onthe implant’s function.Thiswouldbeclinically relevantif degener-ative spondylolisthesis grade Iwithan associated stenosis was consideredasanindicationforposteriornon-fusion instrumenta-tion.
Hasegawaetal.[20]investigatedlumbarsegmental hypermo-bilityinvivobyusingintra-operativebiomechanicaldatacompared topreoperativeradiologicalparameters.Openingofdegenerated facetjointsonaxialcomputedtomographyimagesand degenera-tivespondylolisthesiswerefoundtobethestrongestpredictors for an unstable segment. Furthermore, thePfirrmann grade of degenerated discs onmagnetic resonance imaging was investi-gated.Segmentswithgrades3and4,whichcorrespondtomild andmoderatediscdegeneration,weremorepronetobeing hyper-mobilethanthosewithagrade5.Theconceptofposteriormobile instrumentation couldbeinteresting for patientswithstenosis and moderatediscopathy, presenting theserisk factorsfor seg-mentalhypermobility.Wethereforefocusedonaninvitromodel
analyzinganteriorshearstress,whichmightreproducethe clini-calindicationofdegenerativespondylolisthesiswithmoderatedisc degeneration,treatedbyposteriordecompressionandnon-fusion instrumentation.
Bothinvitroand finiteelementstudieshave shownthatthe shear stiffness of a lumbar segment is reduced after posterior decompressiontechniques[8,19,21,22].vanSolingeetal.[21]used aporcinemodelofthelumbarspinetodemonstratethat laminec-tomy and partial facetectomy resulted in a decrease of shear stiffnessof9%atapreloadof1600N.Bisshopetal.[22]useda simi-lartestingprotocolforhumancadavericL2–L3andL4–L5segments, andshowedthatshearstiffnesswasdecreasedafterlaminectomy comparedtotheintactspineinmilddiscdegenerationbasedonthe Pfirrmanngrade.Incontrasttothat,severedegenerationappeared toenhanceshearstiffnessratherthanreducingit.Moreover,the amountofpreloadneedstobeconsideredsinceaxial compres-sioninfluencesthestiffness,thehysteresisareaandthelinearityof theload-displacementrelationshipofthelumbarmotionsegments [23].Luetal.[19]analyzedtheinfluenceofposteriorversus ante-riorelementresectiononshearstiffnessofhumanlumbarmotion segments.Thecompleteremovaloffacetjointsandposterior lig-aments ledtoanaveragedecrease ofstiffnessinanteriorshear of 77.7%comparedtoanintactspine.Aftercompletesectionof thediscincludinganteriorandposteriorlongitudinalligaments, anteriorshearstiffness decreasedby22.8%onaverage. Further-more,theresectionofposteriorelementsapproximatelydoubled anteriordisplacements(+117%)whenthespecimenswereloaded to250N.Theseresultsstresstheimportanceofthefacetjoints, thesupraspinous,interspinousandyellowligamentsforshear sta-bility. Nevertheless,anterior and posterior elementsdo not act independentlyofoneanotherinresistinganteriorshear,butthe
Table2
Stiffnessduringposterior-anteriorloading(N/mm).
Configuration Average Median Minimum Maximum Differencewithintact P-valuea
Intact 162 164 112 242 – –
Lesion 106 113 81 172 −56[−34%] 0.032
Instrumented 148 165 81 240 −14[−8%] 0.591
466 Y.P.Charlesetal./Orthopaedics&Traumatology:Surgery&Research100(2014)461–467 intervertebralsegment is rather a composite structure withits
differentcomponentsfunctioningincooperation, whichare fur-therguidedbysurroundingmusclesinvivo.Thefindingsin our studyareinlinewiththepreviouslymentionedresults,showing reducedshearstrengthandstiffnessofthespineafterrealizinga medialfacetectomyincombinationwithanundercutting laminec-tomy.Theconfigurationofamoderatediscopathyleadingtolower shearstiffnessinvitrohasconsequencesfortheinstrumentation ofahypermobilesegmentinvivo.Thiswouldreflecttheclinical indicationofadegenerativespondylolisthesiswithadisc degen-erationgradePfirrmann3or4,facetosteoarthritis,lateralrecess and/orforaminalstenosis,whichwouldbecarriedoutforposterior non-fusioninstrumentation.
Theinfluenceofnon-fusioninstrumentationsonshearstress isnotwellunderstoodtodateandithasneverbeenanalyzedfor posteriormobiledevicestoourknowledge,althoughload trans-missioninposterior-anteriordirectionandaxialrotationappears crucialfor theirfunction.Schillinget al.[24]haveanalyzedthe effectofdesignparametersofposteriordynamicstabilization sys-temsanddemonstratedacorrelationbetweenaxialstiffnessand inter-segmentalmotionrestrictioninthesagittalplane,butnotin thetransversal plane.Thismaybeduetothefactthatdynamic stabilizationsystemsarenotprovided witha cross-linkin con-trasttothemobilesysteminourstudy.Furthermore,theseauthors showedthatthespecificdesigndictatedtheimplantsshear prop-erties.Implantsusingaspacerlockedintoplacebyacordbetween thescrewshadalowershearresistancethanthoseusingaspring mechanismrestricting translationalmovements.The implantin thepresent studyrestrictedanteriorsheardisplacementsofthe lumbarsegmenttreatedbymedialfacetectomyandundercutting laminectomy.Thisindicated thatthepolyaxial connectormight allowmovementsinflexion-extension,lateralbendingandaxial rotation,butthatitalsolimitstheeffectofanteriorshearstress. Furthermore,thepropertiesoftheroditselfareimportantforthe loadtransfercharacteristicsbetweentheimplantandthelumbar spine.Melnyketal.[25]investigatedtheinfluenceofrod mate-rialandgeometryonshearstiffnessupto250N(under300Naxial compression)inhumanlumbarsegmentsthathadbeentreated invitrobypartialfacetresection,undercuttinglaminectomyand nucleotomy.Theimplantssupportedgreatershearforcesasthe specimenwasdestabilized.Lowershearloadsweretransferredto thespinewith5.5mmtitaniumrods(stiffestconfiguration) com-paredto6.35×7.2mmoblongPEEKrods(intermediatestiffness) and5.5mmroundPEEKrods(lowstiffness).Themeasuredanterior displacementswereinferiorto2mmandcomparabletotheresults oftheinstrumentspineinourstudyatsimilarloadingconditions. Therodsofthepresentnon-fusiondevicehavea5mmdiameter andaremadeofacobalt-chromium-molybdenumalloy,whichhas ahigherstiffnessthantitanium.
Althoughthetechnicalcharacteristicsoftheimplantare impor-tantforshearresistance,thesurgicalprocedureitselfalsoneedsto beadapted,thusprovidingacompleteneurologicaldecompression butthelowestpossibledestabilizationofthelumbarsegment.The undercuttingorcompletelaminectomyitselfdoesnotcreateand unstablesituationifthereisnoolisthesisofthetreatedsegment. Onlythecombinationwithafacetresection,requiredforfarlateral andforaminaldecompression,maynecessitatestabilizationbyan implant.
5. Conclusion
Thecombination of partialfacetresectionplusundercutting laminectomydecreasessegmentalshearstiffnessandincreasesthe anteriortranslationaldisplacementofL4onL5undershear load-ing.Theshearstiffnessoftheinstrumentedlumbarsegmenttends
toincreasewiththeposteriornon-fusiondeviceandanterior dis-placementtendstodecreaseundershearloading.Thiscondition couldtheoretically beinterestingforthesimulationofposterior non-fusioninstrumentationindegenerativespondylolisthesis. Disclosureofinterest
TheLBMArtsetMétiersParisTechreceivedinstitutionalsupport fromClariance.
Y.P.CharlesisconsultantforClariance. J.-P.SteibisaboardmemberofClariance. Acknowledgements
Clariancefortechnicalsupport. References
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