Biomechanical evaluation of predictive parameters of progression in adolescent isthmic spondylolisthesis: a computer modeling and simulation study

Xiaoyu Wang, Sevrain Amandine, Aubin Carl-Eric, Gharbi Hicham, Labelle, Labelle Hubert

Description

Pelvic incidence, sacral slope and slip percentage have been shown to be important predicting factors for assessing the risk of progression of low- and high-grade spondylolisthesis. Biomechanical factors, which affect the stress distribution and the mechanisms involved in the vertebral slippage, may also influence the risk of progression, but they are still not well known. The objective was to biomechanically evaluate how geometric sacral parameters influence shear and normal stress at the lumbosacral junction in spondylolisthesis. Methods A finite element model of a low-grade L5-S1 spondylolisthesis was constructed, including the morphology of the spine, pelvis and rib cage based on measurements from biplanar radiographs of a patient. Variations provided on this model aimed to study the effects on low grade spondylolisthesis as well as reproduce high grade spondylolisthesis. Normal and shear stresses at the lumbosacral junction were analyzed under various pelvic incidences, sacral slopes and slip percentages. Their influence on progression risk was statistically analyzed using a one-way analysis of variance. Results Stresses were mainly concentrated on the growth plate of S1, on the intervertebral disc of L5-S1, and ahead the sacral dome for low grade spondylolisthesis. For high grade spondylolisthesis, more important compression and shear stresses were seen in the anterior part of the growth plate and disc as compared to the lateral and posterior areas. Stress magnitudes over this area increased with slip percentage, sacral slope and pelvic incidence. Strong correlations were found between pelvic incidence and the resulting compression and shear stresses in the growth plate and intervertebral disc at the L5-S1 junction. Conclusions Progression of the slippage is mostly affected by a movement and an increase of stresses at the lumbosacral junction in accordance with spino-pelvic parameters. The statistical results provide evidence that pelvic incidence is a predictive parameter to determine progression in isthmic spondylolisthesis.

Informations

• Publié par: biomed
• Publié le: 01 janvier 2012
• Nombre de lectures: 0
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

Sevrain
etal
.
Scoliosis
2012,
7
:2
http://www.scoliosisjournal.com/content/7/1/2

RESEARCH

OpenAccess

Biomechanicalevaluationofpredictive
parametersofprogressioninadolescentisthmic
spondylolisthesis:acomputermodelingand
simulationstudy
AmandineSevrain
1,2
,Carl-EricAubin
1,2*
,HichamGharbi
1,2
,XiaoyuWang
1
andHubertLabelle
2

Abstract
Background:
Pelvicincidence,sacralslopeandslippercentagehavebeenshowntobeimportantpredicting
factorsforassessingtheriskofprogressionoflow-andhigh-gradespondylolisthesis.Biomechanicalfactors,which
affectthestressdistributionandthemechanismsinvolvedinthevertebralslippage,mayalsoinfluencetheriskof
progression,buttheyarestillnotwellknown.Theobjectivewastobiomechanicallyevaluatehowgeometricsacral
parametersinfluenceshearandnormalstressatthelumbosacraljunctioninspondylolisthesis.
Methods:
Afiniteelementmodelofalow-gradeL5-S1spondylolisthesiswasconstructed,includingthe
morphologyofthespine,pelvisandribcagebasedonmeasurementsfrombiplanarradiographsofapatient.
Variationsprovidedonthismodelaimedtostudytheeffectsonlowgradespondylolisthesisaswellasreproduce
highgradespondylolisthesis.Normalandshearstressesatthelumbosacraljunctionwereanalyzedundervarious
pelvicincidences,sacralslopesandslippercentages.Theirinfluenceonprogressionriskwasstatisticallyanalyzed
usingaone-wayanalysisofvariance.
Results:
StressesweremainlyconcentratedonthegrowthplateofS1,ontheintervertebraldiscofL5-S1,and
aheadthesacraldomeforlowgradespondylolisthesis.Forhighgradespondylolisthesis,moreimportant
compressionandshearstresseswereseenintheanteriorpartofthegrowthplateanddiscascomparedtothe
lateralandposteriorareas.Stressmagnitudesoverthisareaincreasedwithslippercentage,sacralslopeandpelvic
incidence.Strongcorrelationswerefoundbetweenpelvicincidenceandtheresultingcompressionandshear
stressesinthegrowthplateandintervertebraldiscattheL5-S1junction.
Conclusions:
Progressionoftheslippageismostlyaffectedbyamovementandanincreaseofstressesatthe
lumbosacraljunctioninaccordancewithspino-pelvicparameters.Thestatisticalresultsprovideevidencethatpelvic
incidenceisapredictiveparametertodetermineprogressioninisthmicspondylolisthesis.
Keywords:
Isthmicspondylolisthesis,finiteelementmodel,pelvicincidence,biomechanicalmodel

Background
clinicalpredictionandtheimprovementofitstreatment.
SpondylolisthesisisaspinalconditioncharacterizedbyaSpondylolisthesiswasthoughttobecloselyrelatedto
posteroanteriorslippageofonevertebraovertheverte-spondylolysiswhichisaunilateralorbilateralpars
braimmediatelybelow[1-4].Thebiomechanicsofitsdefectofavertebrathataffects5-6%ofthepopulation
occurrenceandprogressionisyettobefullystudied.[5].Otherfactors,suchasdischerniationorchangesin
Theknowledgeonitsbiomechanicsisessentialforitsspinopelvicmorphology,alsohaveanimportantrolein
theoccurrenceanddevelopmentofspondylolisthesis
*Correspondence:[e-mail]
[1-4].Approximately80%ofpatientswithspondylolysis
1
ÉcolePolytechnique,UniversitedeMontreal,P.O.Box6079,StationCentre-
atL5havetheisthmictypeofspondylolisthesis,and
Ville,Montréal(Québec),H3C3A7CANADA
20%ofthesesamepatientsshowaslippagethatexceeds
Fulllistofauthorinformationisavailableattheendofthearticle
©2012Sevrainetal;licenseeBioMedCentralLtd.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons
AttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductionin
anymedium,providedtheoriginalworkisproperlycited.

Sevrain
etal
.
Scoliosis
2012,
7
:2
http://www.scoliosisjournal.com/content/7/1/2

25%[6].Inpediatricpatients,elevatedstressinthe
structuressurroundingthegrowthplatemaycauseepi-
physealseparation,apophysealbonyringfracture,slip-
pageatthegrowthplatewithoutprovokingdisc
degenerationandformationofasacraldome,butthe
etiologyandpathomechanismsofspondylolisthesis
remainunclear[7-12].
Studieshavebeenreportedonhowtheoccurrence
anddevelopmentofspondylolisthesismightbeinflu-
encedbyvariousspinalparameters.Retrospectiveand
prospectiveinvestigationshavebeenperformedto
determinespinalparametersthatmayincreasetherisk
ofspondylolisthesisprogressionusingradiographic
measurements.Pelvicincidence(PI)hasbeenshownto
beanimportantpredictingfactorforassessingtherisk
ofprogressionoflow-gradespondylolisthesis[1,2].In
high-gradespondylolisthesis,sagittalplanepelvic
orientationparameters,suchassacralslope(SS)and
pelvictilt(PT),aremorerelevantsincethePIisalways
high[13].Spinalandlumbosacralparameters,essential
tomaintainglobalsagittalbalance(suchasslipangle,
thelumbarindexorthesacralcontour),arethoughtto
besecondarychangesastheirroleisrelatedtothe
degreeofdysplasiaratherthanthecauseofslippage
[1,14,15].Mac-Thiongetal.proposedaclinically
orientedclassificationmethod,whichdetailedthatin
low-gradespondylolisthesis,patientswithhighPI/high
SSwereclassifiedas
“
shear-type
”
,andthosewithlow
PI/lowSSwereclassifiedas
“
nutcracker-type
”
[7].In
highgradespondylolisthesis,patientswithhighSS/low
PTwereclassifiedas
“
balancedpelvis
”
,whilethose
withlowSS/highPTwereclassifiedas
“
retroverted
pelvis
”
(Figure1).Sincespondylolisthesisismainly
assessedusingradiographicmeasurements,forces
responsiblefortheprogressionofthedeformityremain
unclear.

Page2of9

Discussionswerealsoreportedonthebiomechanical
aspectsoftheoccurrenceandprogressionofthespon-
dylolisthesis.Mac-Thiongsuggestedthatdysplastic
changesaffectthedirectionandmagnitudeofstressand
soincreasetheriskofprogression.Therefore,thepre-
senceofdifferentpatternsofsagittalspinopelvicbalance
suggeststhatbiomechanicalfactorsmayinfluencethe
riskofprogressioninspondylolisthesis.Furthermore,a
mechanismofslippagethroughthegrowthplatehas
beendocumentedafteraphysisstressfractureofthe
vertebralbody,whileothersfoundthattheslippage
occursatthedisclevel[8,9,11,16-18].
Severalfiniteelementmodelsofspondylolysisand
spondylolisthesishavebeenreportedintheliterature
whichwereaimedatunderstandingofthebiomechanics
ofspondylolisthesis[3,8,9,16,19-22].Howevermostof
thesestudiesdidnottakeintoaccounttheoverallsagit-
talbalance,northeinfluenceofthesurroundingmuscu-
lature,whichaffectsthestressdistributionofthe
anatomicalstructuresunderconsideration,andthe
mechanismsinvolvedintheslippageforthelow-grade
spondylolisthesis.Therefore,severalbiomechanicalfac-
torsandimportantinformationarestillmissinginthe
currentfiniteelementmodels;themechanismofpro-
gressionfromlowgradetohighgradespondylolisthesis
inadolescentsremainspoorlyunderstoodsincenolong-
itudinalbiomechanicalstudieshavebeenperformed.
Theobjectiveofthisstudywastoassessthemechan-
ismsinvolvedatthelumbosacraljunctioninthe
progressionofslippagefromlowtohighgradespondy-
lolisthesisandidentifythespino-pelvicparametersthat
arepredictiveoftheprogression.
Methods
Thegeometryofthefiniteelementmodel(FEM)was
constructedonthebasisofpatientspecificcharacteristics

Figure1
Classificationoflow-gradeandhigh-gradespondylolisthesis
.

Sevrain
etal
.
Scoliosis
2012,
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obtainedfromamulti-viewradiographicreconstruction
technique.Thismethodprovided3Dcoordinatesof17
pointspervertebra,11perriband23onthepelvis
whichwerethencomputedusingaself-calibrationand
optimizationalgorithm[23,24].Adetailedgeometric
modelofnormalvertebraewasthentransformedand
adjustedtomatchthoselandmarksusingadualkriging
freeformdeformationtechnique.Theaccuracyofthis
reconstructiontechniqueis3.3mmonaverage[24].The
progressionmechanismwasnotstudiedfromthenormal
state,butfromalow-gradetohigh-gradespondylolisth-
esis.Thepatientunderconsideration(age=14yearsold,
height=157.2cm,weight=45.5kg)possessesalow-
gradespondylolisthesis(GradeII)withaPI=61°anda
SS=52°.TheresultingFEMconsistedofapproximately
93,000elementsgovernedbylinearelasticbehaviour(Fig-
ure2).ThemodelwascomposedofasimplifiedFEM
withbeam-typeelementsforthesegmentT1-L3andthe
ribcageandadetailedvolumetricmodelfortheL4-pel-
vissegment[22,25-27].ThesimplifiedFEMincludes
1050beamelementstorepresentthevertebrae,theinter-
vertebraldiscs,theribs,thecostalcartilagesandthester-
num.Shells,springsandnonlinearcontactelements
wereusedtorepresenttheligaments.Thedetailedmodel
includesthefollowing:corticalshell,trabecularbone,
bilaterallysisintheparsofL5,sacraldomeconformed
tothepatient
’
sphysiology,annulusfibrosus,nucleuspul-
posus,allspinalligaments,andvertebralgrowthplates.

Figure2
FiniteElementModel
.

Page3of9

Morespecifically,thevertebralgrowthplateswerecon-
structedinthreesectionsinaccordancewithphysiologi-
calfindings(asensitivezone,anewlyformedbonelayer
andatransitionzone)[28,29].Thematerialpropertiesof
theseanatomicalpartitionsreflectfindingsfrompub-
lishedstudies(Additionalfile1).
Theappliedspinalforcesarebasedonloaddistribu-
tions,asreportedbySchultzetal.,anddefinedbya
bodyweight(BW)distributiononeachvertebralbodies
(Additionalfile2)[30].A
“
foll