The biomechanical effect of bone quality and fracture topography on locking plate fixation in periprosthetic femoral fractures
Andreas Leonidou
a,*, Mehran Moazen
b, Panagiotis Lepetsos
a,c, Simon M. Graham
a, George A. Macheras
c, Eleftherios Tsiridis
a,daAcademicDepartmentofOrthopaedicsandTrauma,DivisionofSurgery,AristotleUniversityMedicalSchool,UniversityCampus,54124Thessaloniki,Greece
bMedicalandBiologicalEngineering,SchoolofEngineering,UniversityofHull,HullHU67RX,UK
c4thDepartmentofTrauma&Orthopaedics,KATHospital,Nikis2,Kifissia,14561Athens,Greece
dDepartmentofSurgeryandCancer,DivisionofSurgery,ImperialCollegeLondon,B-blockHammersmithHospital,Du-CaneRoad,LondonW120HS,UK
Introduction
Totalhiparthroplasty(THA)iscomplicatedbyperiprosthetic femoralfractures(PFFs),withincidencevaryingfromabout1%for cementedand5.4%foruncementedprimaryprostheses[1,2].The majorityofthePFFsarelocatedaroundthetipofthestemandare subdividedasB1withthestemstable,B2withthestemunstable and B3,withsignificant bone loss,according totheVancouver classification[3].OptimalmanagementofVancouverB1peripros- theticfemoral fracturesarounda wellfixed prosthesis remains controversialasadequatefixationneedstobeachievedwithout compromisingthestabilityoftheprosthesis[2].
Lockingplateshavebeenfrequentlyusedinthemanagementof B1PFFswithvariablepublishedresults[2,4,5].Overrecentyears therehasbeenanincreaseinnumberofPFFfixationfailurereports including locking plates [2,4,5]. While various studies are investigating development of new fixation methods for these fractures,patientbonequality,stemstabilityandfracturelevelare alsoconsideredascontributingfactorstothesuccessorfailureof PFFfixations[1,2].
A recent clinical study by Leonidou et al. highlighted that fractureanglemayalsoneedtobeconsideredasanindependent parameterwhencontemplatingatreatmentmethodforVancouver typeB1fractures[1].Regardless,thedefinitionofbonequalityand fracturelevelinB1fracturesisalsonotclear[1].
Therefore the underlyinghypothesis of this study was that, bone quality,fractureangleandlevel(fracturetopography)can change thebiomechanicsof PFFfixation in B1fractures where ARTICLE INFO
Articlehistory:
Accepted22October2014
Keywords:
Periprostheticfemoralfracture Bonequality
Fractureangle Fracturelevel Fracturetopography Finiteelementanalysis
ABSTRACT
Optimal management of periprosthetic femoral fractures (PFF) around a well fixed prosthesis (VancouverB1)remainscontroversialasadequatefixationneedstobeachievedwithoutcompromising thestabilityoftheprosthesis.Theaimofthisstudywastohighlighttheeffectofbonequalityi.e.canal thicknessratio(CTR),andfracturetopographyi.e.fractureangleanditspositioninrelationtothestem, onthebiomechanicsofalockingplateforaVancouverB1fracture.Apreviouslycorroboratedsimplified finiteelementmodelofafemurwithacementedtotalhipreplacementstemwasusedinthisstudy.
Canalthicknessratio(CTR)andfracturetopographywerealteredinseveralmodelsandtheeffectof thesevariationsonthevonMisesstressonthelockingplateaswellasthefracturedisplacementwas studied.IncreasingtheCTRledtoreductionofthevonMisesstressonthelockingplateaswellasthe fracturemovement.Inrespecttothefractureanglewiththemedialcortex,itwasshownthatacute anglesresultedinlowervonMisesstressontheplateasopposedtoobtuseangles.Furthermore,acute fractureanglesresultedinlowerfracturedisplacementcomparedtotheotherfracturesconsideredhere.
Fracturesaroundthetipofthestemhadthesamebiomechanicaleffectonthelockingplate.However, fracturesmoredistaltothestemledtosubsequentincreaseofstress,strain,andfracturedisplacement.
Resultshighlightthatingoodbonequalityandacutefractureangles,singlelockingplatefixationis perhapsanappropriatemanagementmethod.Onthecontrary,forpoorbonequalityandobtusefracture anglesalternativemanagementmethodsmightberequiredasthefixationmightbeunderhigherriskof failure.ClinicalstudiesforthemanagementofPFFarerequiredtofurthersupportourfindings.
ß2014ElsevierLtd.Allrightsreserved.
* Correspondingauthor.Tel.:+447707695139.
E-mailaddress:[email protected](A.Leonidou).
ContentslistsavailableatScienceDirect
Injury
j ou rna l h ome p a ge : w ww . e l se v i e r. co m/ l oc a te / i n j ury
http://dx.doi.org/10.1016/j.injury.2014.10.060 0020–1383/ß2014ElsevierLtd.Allrightsreserved.
somecasesmightbeathigherriskoffailure.Thespecificaimof thisstudywastoillustratetheeffectofbonequality,fractureangle andlevelina simplifiedfiniteelement(FE)modelreplicating a VancouverB1fracture.Itmustbementionedthatthesimplified model has been widely used in literature to understand the biomechanics of fracture fixation and was validated against a clinicalcasestudyinoneofourrecentstudies[6].Nevertheless, duetoitssimplified natureourresultshere haveintendedasa preliminaryinvestigationwheretheemphasishasbeenputonthe patternofFEresultsratherthantheirexactpredictions.
Materialsandmethods Modeldescription
A simplified parametricfiniteelement modelof a cemented totalhipreplacement(Fig.1)wasdevelopedpreviouslybyMoazen etal.,andwasadoptedinthisstudy[7].Themodelwasvalidated againstaclinicalcasestudy[6].Thebone,stemandthecement weremodelledasconcentriccylinders.Atransversefracturebelow thetipofthestemwascreatedandfixedlaterallywithatenholes plate.Inordertoachieveanappropriatebridginglength,twoholes inthecentreoftheplate(i.e.acrossthefractureline)wereleft empty[8].The fracturewasheld inplace withfourunicortical screwsproximallyand fourbicorticalscrewsdistally[9,10].The screwswere modelledas cylinders witha diameter of 4.5mm correspondingtocommondiameteroflockingcorticalscrewsused inthefemur[11].Theplatewasplaceddirectlyonbone,asithas beenshownthatdirectcontactofthelockingplatewiththebonein combination with two holes of working length resulted in
decreased stresses on theconstruct [8]. Allparts of the model were assigned isotropic materials properties with a Young modulus of2GPa forcement, 20GPa forbone and200GPafor themetalplateandscrews[12].APoisson’sratioof0.3wasused forallmaterials.
Boundaryconditionsandloads
Thestem–cement,cement–bone,andscrewhead–plateinter- facesweretiedtogether.Interactionattheplate–boneinterfaced wasmodelledwithcontactelementswithcoefficientoffrictionof 0.3[13].Interfacesatthefracturesitewerealsomodelledwith contactelementswithcoefficientoffrictionof0.04,corresponding toearlystages of fracture healing,where no callushasformed betweenthetwofragments[7].Thedistalpartofthemodelwas rigidlyfixedandtheproximalpartofthestemwasloadedwitha transverseforceequaltoP=5Wsin
u
,whereWwasthebody weightandu
wastheloadinganglebetweenthelineofactionof gravity and the long axis of femur.Body weight of 600N and loadingangleof118wereusedinthisstudy[12].Meshsensitivity
The model was meshed with Tetrahedral (C3D4) elements.
Convergence wastested by increasingthe numberof elements from42,000to1,600,000infivesteps.Thesolutionconvergedon theparametersoftheinterest(5%-forvonMisesstressacrossthe twoemptyscrewholeswheretheplateisathighriskoffailure [7,12,13])withapproximately300,000elements.Modelswiththis number of elements or more were used for each of the cases presented.
Bonequalityandfracturetopography
Threemodelswithdifferentcanalthicknessratio(CTR)were developedrepresentingpoor,averageandbestbonequalitywith respectiveCTRsof0.44,0.88and1.46(Fig.2).Thiswasdonebased onourpreviousstudy,whichindicatedthatbonequalitycanvary within Vancouver B1 fractures [1]. Further three models were developed with angle fractures varying from the unstable transverse(08)andshortoblique(1468)tothestablelongoblique configuration(768)(Fig.3).Finally,threemodelsweredeveloped withthefractureatthetipofthestem,4mmand14mmbelowthe tipofthestem(Fig.4).
Simulationsandmeasurements
Themodelsweresolvedandanalysedusinga finiteelement simulationpackage (ABAQUSv.6.9,SimuliaInc.,Providence,RI, USA).Thepatternof vonMisesstressontheplateandfracture movementwas comparedacrossthecases. Fracture movement wasquantifiedastherelativedisplacementofthemostdistalpoint oftheproximalfragmentandthemostproximalpointofthedistal fragmentonthemedialsideofthebone.
Fig.1.SimplifiedmodelascreatedinAbaqusCAESoftware.
Fig.2.Sagittalviewofthemodelshowingdifferentbonequalityconfigurations.(A) Worstbonequality(CTR=0.44),(B)averagebonequality(CTR=0.88),(C)bestbone quality(CTR=1.46).
Results
Increasingthebonequality(CTR)ledtoreductionofvonMises stressesonthelockingplate.Thefracturedisplacementwasalso decreased.Inrespecttothefractureanglewiththemedialcortex,it wasshownthatacuteanglesresultedinlowerlevelofvonMises stressontheplateasopposedtoobtuseangles.Furthermore,acute fracture angles resultedin lower fracture movement. Fractures around the tip of the stem (0 and 4mm) had the same biomechanicaleffectonthelockingplate.However,asthedistance ofthefracturefromthetipwasincreasedat13mmthevonMises stressesandfracture movementwerealsoincreased.Numerical resultsoftheFEaresummarizedinTable1.PatternofvonMises stressaroundtheemptyscrewholesoftheplateforallthemodels isshowninFigs.5–7.
Discussion
Theresultsofthisstudyconfirmourhypothesisthatpoorbone quality,unstablepatternsoffractures,andfracturesbelowthetip of thestem in VancouverB1fractures increasethe mechanical stresson thelockingplateaswellasfracturemovementunder loading.
Finiteelementanalysisresultsfromthisstudyhighlightedthat theworsebonequalitymodelexhibitedincreasedlevelsofstress aroundtheemptyscrewholes(i.e.potentiallyleadingtofixation failure [7,8,14]), aswellasincreasedfracture displacement(i.e.
potentially leading to non-union). Considering that Vancouver classificationdoesnottakeintoaccountthevariationthatexistsin thebone qualityin B1fractures,thepreliminaryresultsofthis studysuggestthatperhapsB1fractureswithlowCTRneedtobe treatedwithcautions.Nevertheless,whatclassifiesasalowCTR basedonaplanX-rayrequiresfurtherinvestigation[1].
Thefractureanglewasshowntohaveaconsiderableeffectover thedistributionofthevonMisesstressonthelockingplate.The worseresultswererecordedwiththeshortoblique(1468)model and the best oneswiththelong oblique (768)model withthe transversefractureresultsbeingwithintherangeoflongandshort obliquefractures.Theseresultsconfirmtheinherentinstabilityof thetransverseandshortobliquePFFandquestionthesuitabilityof singlelockingplatealoneforthemanagementoftheseinjuries.
The orthopaedic surgeon might therefore consider that short obliqueandtransversefracturesareunstableandrequirestronger andmoresecurefixationinallplanes[15,16].
ThelevelofthePFFisanimportantparameterofitspersonality.
In thisstudyitwasshownthat vonMisesstressesand fracture movementwereincreasedasthefractureoccurredmoredistalto thetipofthestem(Table1).Morespecifically,forafracture14mm Fig.3.FiniteelementmodelsandtherelevantanglesofthePFF.Aisatransverse
fracture,Bequalsto768andCisafracturewitha1468angle.
Fig.4.FiniteelementmodelsandtherelevantlevelofthePFF.(A)Fractureatthetip ofthestem,(B)4mmbelowthetipand(C)14mmbelowthetip.
Table1
ResultsofFEforthedifferentmodels.
vonMises(MPa) Fracturedisplacement(mm) Bonequality
Model1(worst) 168 0.53
Model2(average) 111 0.37
Model3(best) 104 0.27
Fractureangle
Model1(08) 111 0.37
Model2(768) 60 0.10
Model3(1468) 342 0.72
Fracturelevel
Model1(tip) 114 0.30
Model2(4mmbelow) 111 0.37
Model3(14mmbelow) 139 0.75
awayfromthetipofthestemthefracturedisplacementwastwice higherasopposedtoafractureatthetipofthestem.Thishighlight theimportanceofidentifyingaclearborderbetweenVancouverB andCtypefractures.Regardless,ourresultssuggestthatsingle lockingplateindistalfemoralfracturescanbeunderhighlevelof stress[5].TheimportanceoffracturelevelwasidentifiedbyBryant etal.,astheyclaimedthattheB1fracturesaroundthetipofthe stemarethemostchallengingtotreat[17].Theyfurtheradvocated theuse of longlocking plates spanning the whole femur with multiple screws for the management of this type of fracture [17].Choietal.furthersuggestedthatsinglelockingplatefixation maynotbeadequateforcomminutedB1PFFoccurringatornear thetipofthestem[18].Theysuggestedsupplementationofthe lockingplatefixationeitherwithanallograftorwithananterior plate[18].Resultsobtainedinthisstudyfurthersuggestthatdistal fracturescanincreasethestresslevelonthesinglelockingplate fixation.Thiscouldpotentiallyexplainreportedcasesoflocking platefailureandpulloutwhenusedforthemanagementofdistal PFF[5,19].
FracturedisplacementwasstudiedintheFEmodelsandisof paramountimportanceasitdisturbsthecontinuityofthemedial cortexofthefemurandcanpotentiallyaffectthehealingprocess.
TheimportanceofthemedialcortexwasoutlinedbyCortenetal., astheysuggestedthatalateralplatealonewassuitable forthe fixationofa B1PFFonly ifthemedialcortexwasanatomically
reducedandnotcomminuted[20].Theyfurtheradvisedbiplanar fixationifthemedialcortexwasdisrupted,astheythoughtthat thisfractureconfigurationwouldbemoreunstable[20].Further- more,fracturedisplacementandmovementareknownriskfactors for delayed healing, which can subsequently interpret the documented delayed and non-unions of B1 PFF treated with lockingplates[4,11,21].
Thisstudyistothebestofauthors’knowledgethefirstoneto investigatetheeffectoffracturetopographyandbonequalityon locking plate fixation for a Vancouver B1 PFF. One of thekey advantagesofthesimplifiedmodelusedinthisstudywasthatthe effectofdifferentparameterson thestresslevelof thefixation couldbeisolated.Atthesametime,oneofthemainlimitationsof this FE model is that it has not been corroborated with a biomechanical experimental model yetwas validatedagainst a clinical case study [6]. Nevertheless,similar simplified femoral modelshavebeenextensivelyusedintheliteratureandhavebeen validated both on theoretical and on biomechanical grounds [8,22,23]. More specifically,both Iesakaet al. and Stoffel etal.
corroboratedtheirsimplifiedfemoralmodelagainstexperimental model [8,23]. Furthermore, published data frombiomechanical andclinicalstudies–withdifferentexperimentalsetup–revealed increasedstressesandsubsequentfailurethroughtheemptyholes ofthelockingplate,whichagreewiththefindingsofthecurrent study [4,8,24,25]. Therefore, despite the limitations of the Fig.5.VisualDistributionofvonMisesstressaroundtheemptyscrewholesofthe
plate.Ademonstratesworstbonequality,BequalstoaverageandCtobestbone quality.
Fig.6.VisualDistributionofvonMisesstressaroundtheemptyscrewholesofthe plate.Ademonstratesthetransversefracture,Bequalsto768(longobliquefracture) andCisthefracturewitha1468angle(shortobliquefracture).
validationprocess,relative comparisonsthatweremadein this studybetweenthemodelsremainvalid.
This study suggests that the orthopaedic surgeon should contemplate on the PFF topography and bone quality and not relymerelyontheVancouverClassificationinordertoformulatea treatment plan. More specifically, in Vancouver B1 fractures with unstable patterns, away from the tip of the stem with poorunderlyingbonestock,thesurgeonmightconsideralterna- tive methods to enhance the single locking plate fixation.
This can include increasing the plate length, revision to long stemor applyingbone allograft.Furtherclinical,biomechanical and computational studies need to be conducted focusing on the fracture personality and not only on the locking plate configuration.
Conflictofinterest Nonedeclared.
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Fig.7.VisualdistributionofvonMisesstressaroundtheemptyscrewholesofthe plate.Aistheplatewiththefractureatthetipofthestem,Bat4mmbelowthetip andCat14mmbelowthetip.