the use of computed tomography to determine femoral...

49 BulletinoftheHospitalforJointDiseases Volume63,Numbers1&2 2005

Abstract

Computed tomography is used to assess whether ideal femo-ral component size in knee arthroplasty can be made more accurately. Ideal femoral component size was measured on radiography, computed tomography, and by direct measurement of 20 cadaver femora and analyzed statistically. There was no significance between the tomographically anticipated ideal femoral component size and ideal femoral size values (Wilcoxon W 388.5; p 0.565). There was difference between radiographi-cally anticipated ideal femoral size and ideal femoral size values (Wilcoxon W 324.5; p 0.020). Anticipating the ideal femoral size can be made more precisely with computed tomography than radiographs in cadaver femora. Anticipating the ideal femoral component size by computed tomography may solve the problems in “in-between sizes.” Future total knee designs may be manufactured in more anatomic sizes.

Componentsizingisoneofthefactorsaffectingtheoutcomeinkneearthroplasty.1,2Inthetibiaandpa-tellaitisrelativelysimple.However,femoralsizing

hasaspecificfunction,which is itseffectonflexionand

extensiongaps.3Duringpreoperativeplanning, templatesareusedforanticipatingthefemoralcomponentsizefromradiographs.Howevermagnificationofbonystructuresinradiographs may mislead the surgeon.4 Definite femoralsizing ismadeby instruments during theoperation. It ismadebyreferencingeithertheposteriorfemoralcondyle(posteriorreferencing)oranteriorfemoralcondyle(anteriorreferencing).Theposteriorreferencesystemistherecom-mendedmethodforpreventingcomplications.3 Scans obtained in computed tomography (CT) aredescribed as scout-views and used for measurement ofdistancesandangleswithsoftwareprograms.5Precisemea-surementsinbonystructuresandimplantsizemeasurementscanbedonereliablyindentalsurgeryusingthequantitativepropertiesofCT.6Therefore,insearchofamoresensitiveandreliabletoolforpreoperativefemoralsizing,ananalysisoftheefficacyofusingCTmaybeuseful.

Materials and MethodsTwentycadaverfemurs(10rightand10left)wereincludedinthestudy.

Step 1Directtruelateralradiographsofthefemurweretakenpayingspecialattentiontotheconjunctionofthecon-tours of the posterior femoral condyles (PFC) on oneanother.7Theradiographicsettingswereasfollows:40kW,2.5mAs,44ms.Thedistancebetweenthebeamandthefemurwas1meter.Thesameoperator(MU)identi-fied the landmarks and made the measurements. ThemostposteriorpointofthePFCandtheanteriorfemoralcortexsuperiortoanteriorfemoralcondyle(AFC)weredetermined.Atangentlinetoanteriorfemoralcortexwasdrawn passing through theAFC.A perpendicular linefromthePFCtothetangentlinewasdrawnanditslengthwasdetermined inmillimetersbya rulerdesigned for

The Use of Computed Tomography to Determine Femoral Component SizeA Study of Cadaver Femora

Murad Uslu, M.D., Baris Ozsar, M.D., Tuba Kendi, M.D., Simay Kara, M.D., Ibrahim Tekdemir, M.D., and O. Sahap Atik, M.D.

Murad Uslu, M.D., is anAssociate Professor at the KirikkaleUniversity Medical Center, Department of Orthopaedics andTraumatology,Ankara,Turkey.BarisOzsar,M.D.,isaresidentintheDepartmentofOrthopaedicsandTraumatologyatKirikkaleUniversityMedicalCenter,Ankara,Turkey.TubaKendi,M.D.,isanAssistantProfessorintheDepartmentofRadiologyandRadio-diagnosticsatKirikkaleUniversityMedicalCenter,Ankara,Turkey.SimayKara,M.D.,isanAssociateProfessorintheDepartmentofRadiologyandRadiodiagnosticsatKirikkaleUniversityMedi-calCenter,Ankara,Turkey. IbrahimTekdemir,M.D.,ProfessorintheDepartmentofAnatomyattheAnkaraUniversityMedicalCenter,Ankara,Turkey.O.SahapAtik,M.D.,isaProfessorintheDepartmentofOrthopaedicsandTraumatologyatGaziUniversityMedicalCenter,Ankara,Turkey.Correspondence: Murad Uslu, M.D., Ozdoya Sitesi No 25,AlacaatliKoyu,Cayyolu,Ankara,06530Turkey.

50 Bulletinofthe HospitalforJointDiseases Volume63,Numbers1&2 2005

accuratemeasurementwithregardtothemagnificationpropertyofradiography(TemplateforOsteonics®Series7000 Total Knee, Printed in U.S.A, Osteonics Corp.,1994).Themeasurementsforeachfemurwererecordedasthe“radiographicallyanticipatedidealfemoralsize”(RAIFS)(Fig.1).

Step 2FemurswereplacedinthecenterofthegantryoftheCTmachineandlateralscansofthefemurweretakenpayingspecial attention toconjunctionof thecontoursof thePFCononeanotherandplacing the femoralcondylesin the center of the CT gantry. The scans were taken

withthefollowingsettings:120kvp,50mA,matrix512andlength500mm.Thesameoperator(TK)identifiedthe landmarks and made the measurements.The mostposterior point of the PFC and the anterior femoralcortexsuperiortoanteriorfemoralcondyle(AFC)weredetermined.A tangent line to anterior femoral cortexwas drawn passing through theAFC.A perpendicularline from the PFC to tangent line was drawn and itslength was determined in millimeters by CT process-ing.The measurements for each femur were recordedasthe“tomographicallyanticipatedidealfemoralsize”(TAIFS)(Fig.2).

Step 3Thefemoralsizingandanteriorfemoralresectionwasmade by using the total knee instrumentation system

Figure 1Directlateralradiographyofthedistalfemur.•Ante-riorfemoralcondyle(AFC).*Posteriorfemoralcondyle(PFC).Intermittantline:tangentlinetoanteriorfemoralcortexpassingthroughtheAFC.StraightlineperpendicularlinefromthePFCtothetangentline.Measurementofradiographicallyanticipatedidealfemoralsize(RAIFS)withregardtothemagnificationpropertyofradiographybyaruler.

Figure 2CTofdistalfemur.•Anteriorfemoralcondyle(AFC).*Posteriorfemoralcondyle(PFC).Intermittantline:tangentlinetoanteriorfemoralcortexpassingthroughtheAFC.StraightlineperpendicularlinefromthePFCtothetangentline.Measurementoftomographicallyanticipatedidealfemoralsize(RAIFS)byCTprocessing.

Figure 3Anteriorfemoralresectionwithpos-terior referencing.The stylus was lowered tothelateralanteriorcortexandatthispointtheindicatedsizewasnottakenintoconsiderationandtheresectionwasmadefromwhereverthestyluspointed.


(Genesis2TotalKneeSystem,SmithandNephewInc.)by the authors (MU and BO). The femoral canal wasopenedwitha9.5mmdrillandan intramedullary rodwasplacedwithavalgusalignmentguideandthebushingpositionedtoeitherleftorrightside.Posteriorpaddlesofthevalgusalignmentguidewereincontactwiththeposteriorcondyles.Thefemoralsizingguideforposteriorreferencingwasattachedtothevalgusalignmentassem-bly.Thestyluswasloweredtothelateralanteriorcortexandanteriorfemoralresectionwasmade.Atthispointtheindicatedsizewasnottakenintoconsiderationandtheresectionwasmadefromwhereverthestyluspointed(Fig.3).ThedistancebetweentheanteriorcutsurfaceandthePFCwasmeasuredbydigitalcalipersandrecordedasthe“idealfemoralsize”(IFS)(Fig.4).

Statistical AnalysisStep 1:Pearsonbivariatecorrelationanalysiswasper-formed between IFS, RAIFS andTAIFS to search forcorrelation.Step 2: IFS,RAIFS,andTAIFSvalueswereanalyzedwithnon-parametrictests(Kruskal-WallisTest,Mann-WhitneyTest)tosearchfordifferences.Step 3:Foreachfemurthedifferencebetweentheantici-patedvalues(TAIFSandRAIFS)andIFSwascalculatedandshowninFigure5.

ResultsMinimum, maximum, mean, and standard deviationvaluesforIFS,RAIFS,andTAIFSareshowninTable1.TAIFSvalueswerecorrelatedwithIFSvalues(r:0.940)andRAIFSvalueswerecorrelatedwith IFSvalues (r:0.874). The difference between three groups (RAIFS,TAIFSandIFS)wasanalyzedwiththenon-parametric

Kruskal-WallisTest.Therewasasignificantdifferencebetweenthethreegroups(chi-square:9,299;p0.01).Inordertodemonstratethedifference,theMannWhitneyTestwasused.Therewasnosignificantdifferencebe-tweentheTAIFSandIFSvalues(WilcoxonW388.5;p0.565).TherewasadifferencebetweenRAIFSandIFSvalues(WilcoxonW324.5;p0.020).

DiscussionPreoperativeplanningisoneofthemostimportantstepsof arthroplasty.8Although, contemporary arthroplastydesignsprovidepreoperativetemplatesforanticipatingcomponent size from radiographs, in hip arthroplasty,anticipatedmagnificationmaydifferfromactualmag-nificationandin17%ofcasesitwasfoundtoaffectthechoiceofimplantsize.4Inkneearthroplasty,anticipatingthefemoralcomponentsizeismorechallenging.Usingposteriorcondylesasareferencethesurgeonhastomakeadecisionregardingtheanteriorfemoralresectionlevelin order to achieve the “downsize without notching”goal.3Thedetailedmethodforfemoralsizingwithcur-rentfemoralcomponentsizesresultsincontradictions,especiallyforcomponentsthatare“in-between”sizes.Asopposedtothepatellaandtibia,femoralcomponents

Table 1 Minimum,Maximum,MeanandStandardDeviationValuesforIFS,RAIFSandTAIFS

n Minimum Maximum Mean Stddev

IFS 20 48.73 62.00 55.10 3.86 RAIFS 20 47.00 59.00 51.98 3.45 TAIFS 20 49.10 62.90 55.73 3.81

Figure 4ThedistancebetweentheanteriorcutsurfaceandthePFC was measured by digital caliphers and recorded as idealfemoralsize.

Figure 5ThedifferencebetweenIFSandanticipatedvaluesforeachfemur.

Case Number

20 19

18 17

16 15

14 13

12 11

10 9

8 7

6 5

4 3

2 1

Diff

eren

ce/m

m

10

8

6

4

2

0

-2

-4

IFS-TAIFS

IFS-RAIFS

52 Bulletinofthe HospitalforJointDiseases Volume63,Numbers1&2 2005

havebeenfoundtodifferinsizein6.7%ofpatientsbe-tweenrightandleftkneesofthesamepatientundergoingbilateral knee replacement.9 For those reasons severalinvestigatorshavelookedforabettermethodtoestimatemagnificationandbonemorphologyofthedistalfemur.Macherandcolleaguesdemonstratedthatbonemodelscreatedbyanalysisofthree-dimensionalultrasoundim-agesareusefulforthesimulationofkneereplacements.10Menschandassociatesdevelopednomogramsforprecisepreoperative morphologic evaluation.7 However, theclinical validity of these methods is controversial. Inthecurrentstudy,bothTAIFSandRAIFSvalueswerecorrelatedwithIFS.HoweverthecorrelationofIFSwashigherwithTAIFSthanRAIFS.Inaddition,therewasnostatisticaldifferencebetweentheTAIFSandIFSvalues(p0.565).Thecurrentstudydemonstratesthatanticipat-ingtheidealfemoralsizecanbemademorepreciselywithCTscansthanradiographsincadaverbonesandCTscanscanbeusedasanalternativemethodforpreopera-tivesizeanticipation. Currentkneearthroplastydesignsuseanteroposterior(AP) height in the production of knee prosthesis andmediolateral (ML)width iscalculatedwithanAP/MLratioof0.8:1.0.11Anteroposteriorheighthasakeyrolein femoral sizing.The femoral components come in aspectrumofsizesandanteroposteriorsizesusuallyvarybetween3mmand5mm.Howevercontemporarydesignsdonotprovidesurgeonswiththeidealfemoralsizeinallcases.Thesurgeonisusuallyexpectedtotailorthefemuraccordingtothesizeofthepre-manufacturedprosthesis.Thisprocessmayrevealcomplicationssuchasanteriorfemoralnotchingorpatellafemoraldisordersat“inbe-tweensizes.”3Sizeincrementsincontemporaryfemoraldesignsdonotseemtoprovideanidealfitofprosthesistofemurinallcases.Intraoperativemeasurementofdistalfemursrevealsthatthereisaneedtogeneratefemoralcomponentsizesthataremorelikelytofitacrossthewidevariationofcases.11PrecisepreoperativedeterminationofAPheightmaycontributetotheefficacyofpreoperativeplanningandservethispurpose. Radiographshaveaseriousmagnificationproblemthatcannotbeeasilystandardized.Thissituationarisesfromthefactthattheradiographicimageisnotanexactpictureofthebone,butratheraprojectionofthebone.Andthisprojectionismagnifiedbyseveralvariablesintheprocess,likeobjectfilmdistance,focalfilmdistance,and object size. The magnification ratio in standardlateral knee radiographswas found tobe3.8% inonestudy.7 However, the standard measurement templatesare manufactured with a magnification ratio of 10%.ThismaybeoneofthefactorscausingtherelativelypoorcorrelationbetweenRAIFSandIFSvalues.Additionally,sinceX-rayshaveadivergingprojectiongeometry,mag-nificationratiosofmedialandlateralcondylesinlateralradiographsdifferandmaketheradiographicassessment

ofimplantsizemoredifficult.7 ACTscan isascoutviewanda two-dimensionalprojectionofathree-dimensionalstructurelikearadio-graph.However,inCTtheradiationsourcemovesatafixeddistancetothecenteroftheobject,whichisattheintersectionofthesagittalandfrontalplanes.Magnifica-tioninthecentraltomographicplanehasbeenreportedas1:1.5ThehighlycorrelatedTAIFSvalueswithidealfemoralcomponentsizesfoundinthecurrentstudysup-portthatthemagnificationinCTscansisminimalundercontrolledconditions.ClinicalstudiesmustbecarriedoutinordertofurtherresearchtherelevanceandutilityofCTinfemoralsizing. Anterior referencingorcutting the femur in3de-grees flexion are alternatives to femoral sizing usingposterior referencing systems in “in-between sizes.”Howeverthesesolutionsbringtheirowncomplicationswith them.Anterior referencing may result in flexioninstability.3,9 Cutting the femur in 3 degrees flexionmayresult inpost-impingementintheposteriorcruci-atesubstitutingsystems.3Manufacturing the“betweensize”femoralcomponentsmayprovidemorecongruousfemoral component implantation. In such a situation,precisepreoperativeanticipationof femoralsizesmayprovideadvantagetothesurgeonandthepatient.

ConclusionPreoperativeanalysisofCTscansandanticipatingtheidealfemoralcomponentsizemorepreciselymayhelptosolvetheproblems inherent inpatients that require in-betweensizedimplants.Ifprecisepreoperativemethodsforanticipat-ingcomponentsizeandtheirclinicaluseimprove,futureimplantdesignsmaybemanufacturedinawiderrangeofanatomicsizes.

AcknowledgmentWethankCumhurBoratav,M.D.,forhiskindassistanceinthestatisticalstudyofthisarticle.

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