osteosynthesis for distal femoral fractures — A biomechanical analysis

Impact of cement-augmented condylar screws in locking plate

osteosynthesis for distal femoral fractures — A biomechanical analysis

Christopher Bliemel, MD*, Ludwig Oberkircher, MD, Benjamin Bockmann, MD, Eric Petzold, Rene Aigner, MD, Thomas Jan Heyse, MD, Steffen Ruchholtz, MD, Benjamin Buecking, MD

CenterforOrthopaedicsandTraumaSurgery,UniversityHospitalGiessenandMarburg,LocationMarburg,Germany

ARTICLE INFO

Keywords:

Distalfemoralfracture Polyaxialangularstableplate osteosynthesis

Cementaugmentation Osteoporosis Biomechanicalanalysis

ABSTRACT

Introduction:Compromisedbonequalityandtheneedforearlymobilizationcontinuetoleadtoimplant failureinelderlypatientswithdistalfemoralfractures.Thecement augmentationofscrewsmight facilitateimprovingimplantanchorage.Theaimofthisstudywastoanalysetheimpactofcement augmentationofthecondylarscrewsonimplantfixationinahumancadavericbonemodel.

Materialandmethods:Tenpairsofosteoporoticfemora(meanage:90years,range:84–99years)were used.A2-cmgaposteotomywascreatedinthemetaphysealregiontosimulateanunstableAO/OTA33- A3fracture.Allspecimensweretreatedwithapolyaxiallockingplate.Specimensrandomlyassignedto theaugmentedgroupreceivedanadditionalcementaugmentationofthecondylarscrewsusingbone cement.Aservohydraulictestingmachinewasusedtoperformincrementalcyclicaxialloadingusinga load-to-failuremode.

Results:Allspecimenssurvivedatleast800Nofaxialcompressiveforce.Themeancompressiveforces leadingtofailurewere1620N(95%CI:1382–1858N)inthenon-augmentedgroupand2420N(95%CI:

2054–2786N)inthegroupwithcement-augmentedcondylarscrews(p=0.005).

Deformationwithcuttingoutofthecondylarscrewsandcondylarfracturewerethemostcommon reasonsforfailureinbothgroups.Whereasaxialstiffnesswascomparablebetweenbothosteosyntheses (p=0.508),significantdifferenceswereobservedfortheplasticdeformationoftheconstructs(p=0.014).

Conclusion:Theresultsofthepresentstudyshowedthatthecementaugmentationofthecondylarscrews mightbeapromisingtechniqueforthefixationofdistalfemoralfracturesinelderlypatientswith osteoporoticbones.

ã2016ElsevierLtd.Allrightsreserved.

Introduction

Osteoporosis-associatedfracturesrepresentoneofthelargest socio-economic burdens in western industrial countries [1].

Because of ongoing demographic changes, the incidence of osteoporoticfractures will most likely continue toincrease. In this context, distal femoral fractures occurring among elderly patientsare typically considered to berelated toosteoporosis.

Because these fractures are less frequent than fractures of the proximalhumerus, distal forearm,spine,or hip,theymight be underestimated[2].Nevertheless,fracturesofthedistalfemurare particularlydifficulttotreat,especiallyingeriatricpatientswith reducedbonequality.

Such challenges in the treatmentof geriatric distal femoral fractures,whicharemostlyrelatedtoconventionalplateosteosyn- thesis, have led to innovations in implant technology [3–5].

Currently,lockingplatesarea standard tooltoachieve fracture fixation in osteoporotic bone. Clinical and biomechanical trials havedemonstratedthatangularstableplatesaresuperiorinterms of axial stability and load tofailure compared toconventional plates[5–9].Whereasfirst-generationlockingplatesprovidedonly monoaxial screw fixation, further developments led to the introductionofpolyaxial angularstable plates.Theseplates are distinguishedbytheirincreasedflexibilityinthepositioningofthe screws, allowing the regions with the best bone stock to be

*Corresponding author at: Center for Orthopaedics and Trauma Surgery, UniversityHospitalGiessen Marburg,Marburg,Baldingerstrasse,35043Marburg, Germany.

E-mailaddresses:[email protected](C.Bliemel),

[email protected](L.Oberkircher),[email protected] (B.Bockmann),[email protected](E.Petzold),[email protected] (R.Aigner),[email protected](T.J.Heyse),

[email protected](S.Ruchholtz),[email protected] (B.Buecking).

http://dx.doi.org/10.1016/j.injury.2016.10.013 0020-1383/ã2016ElsevierLtd.Allrightsreserved.

ContentslistsavailableatScienceDirect

Injury

j o u r n al h o m ep a g e: w w w . el s e v i e r . c o m / l o c at e / i n j u r y

accessed,thusenabling firmer implantanchoragein bothweak cancellousboneandcomplexanatomicalconditions,suchasthe condyleregionofthedistalfemur[10].

Despite these innovations in implant technology, problems continue to affect the fixation of far-distal femoral factures, especiallyinseverelyosteoporoticbone.Arelativelynewapproach to improving implant anchorage is to increase the interface betweentheimplantandtherarefiedbonebycementaugmenta- tion of screws, as performed in osteoporotic fractures of the proximalhumerus,spineandhipintheclinic[11–13].Theresults are promising, suggesting the potential development of this strategyfor thefixation of distal femoralfractures [14].In this context,Fig.1apresentstheclinicalcaseofan82-year-oldwoman withhighlyosteoporoticbonestructureandaperi-implantdistal femoral fracture. Fig.1b shows the fracture fixation via plate osteosynthesisandcement-augmentedcondylarscrews.

The purpose of the present biomechanical study was to compare a polyaxial locking plate with augmented condylar screwsand onewithnon-augmentedscrewsfor thefixationof distalfemoralfracturesinahumancadavericbonemodel.

Thecement augmentation of condylarscrews was hypothe- sized to improve implant fixation in osteoporotic bone and, therefore,increasetheload-to-failurevaluesoftheosteosynthe- ses.

Materialsandmethods Specimens

Thisstudywasconductedon10pairsofadultfemora.These femora were obtained from human cadavers that had been embalmedwithasolutionof96%ethanoland<2%formaldehyde, aspreviouslydescribed[15].Thespecimenswerestoredforatleast oneyearbeforeuse.ThefemorawereprovidedbytheInstitutesof

Anatomy and Cell Biology of Philipps University, Marburg, Germany.Alldonorsgavewrittenconsentbytheirownfreewill fortheuseoftheirbody forresearchpurposes.Allexperiments wereconductedinaccordancewiththelocalethicscommittee(AZ 157/14).

Thesamplesoriginatedfrom2maleand8femaleadultswithan averageageof90years(range:84–99years).Afterthesurrounding softtissuewasstrippedoff,thespecimenswerewrappedintowels moistened with the aforementioned embalming solution and storedinacoolingchamberat4Ctoavoidartefactsfromdrying.

Assessmentofbonequality

To exclude damage related to pre-existing fractures or osteolyses, allfemora weresubjected toa clinical examination andanX-rayexaminationusingac-armunit.Subsequently,the bonemineraldensityofeachfemurwasmeasuredbyDual-energy X-rayAbsorptiometry(DXA).

Implantsandsurgicaltreatment

TheimplantusedinthisstudywastheNon-ContactBridging plate for Distal Femur (NCB-DF¹, Zimmer Inc., Winterthur, Switzerland) (CE marketing 0086).The NCB-DF¹ is a polyaxial lockingdevicemadeoftitaniumalloy.Thisplateisanatomically preformedtoensureanoptimumfittothelateral cortexofthe distalfemur.Inthepresentsetup,onlyNCB-DF¹nine-holeplates were used.Shaft fixation was bicortically performedusing five screws(diameter:5mm)locatedintheproximalfiveholesofthe plate.Fivescrews(diameter:5mm)werealsopositionedinthe cancellousboneofthecondylarregion.Allosteosyntheseswere performed by the same study surgeon (BC). Subsequently, a standardized osteotomywas createdusinga surgicaloscillating saw.Anunstable supracondylarfemoralfracturewas simulated (33-A3accordingtotheOrthopaedicTraumaAssociationOTA/AO classification)asthefracturemodel.A2-cmgaposteotomywas createdperpendiculartotheanatomicaxisofthefemur.Thedistal cutwas madeatthelevelofthree-quartersofthewidthofthe distalfemurabovetheintercondylarnotch.Oneosteosynthesisof each pairof femorawas randomly assigned tothe augmented group,andtheotherwasassignedtothenon-augmentedgroup.

The specimens randomizedintotheaugmented groupreceived additionalcementaugmentationofthefivecancellousscrewsin the distal part of the construct. Cement augmentation was performed by removing one cancellous screw at a time and injecting 2mlof polymethylmethacrylate(PMMA)bonecement (Kyphon¹ HV-R¹ bone cement; Medtronic Inc., Sunnyvale,CA, USA)(CEmarketing0473)intoeachscrewhole.Allofthesurgical stepswereperformedunderfluoroscopicimagingintwoplanesto

Fig.1.Clinical caseof an 82-year-old womenwith highlyosteoporotic bone structureandthepresenceofaperi-implantdistalfemoralfracture(a).Fracture fixationwasconductedwithaplateosteosynthesisandcementaugmentationof thecondylarscrews(b).

Fig.2.Typicalpostoperativex-raypicturesshowingapairofsampleswithcement augmentedcondylarscrewsintherightfemur(a)andnon-augmentedscrewsin theleftfemur(b)-.

assureaccuratehardwarepositioningandproperfitoftheinjected cementwithoutanyleakage(Fig.2aandb).

Testsetupandloading

After osteosynthesis, the proximal part of each femur was resectedwithanoscillatingsaw.Theosteotomywasmade6cm proximaltothe plate.Subsequently, the proximal part of each specimenwas embedded in special containers using Technovit 3040(Heraeus,Wehrheim,Germany),which is athermosetting resin. Using a custom-made holding device, the femora were positionedupside down in an Instron 5566 universalservohy- draulic testing machine (Instron Cor., Darmstadt, Germany). A metalplatethatwasmovableintwodirections(anterior-posterior and medial-lateral) was used to exert the pressure. After the femorawerealignedinananatomical,slightlyvalguspositionof5– 7,loadingwasappliedalong themechanicalaxisofthefemur (Fig. 3). Before testing, the samples were thawed at room temperature.

Subsequently, each femur was subjected to cyclic loading followinga standardizedprotocol.Each femurwassubject toa preloadof100Ntocompressthespecimenandavoiddeformation artefacts in the bone construct. Cyclic loadingtests were then conducted at a constant speed of 125mm/min. Loading was applied at this speed until the desired maximum force was achieved;amaximumcyclingfrequencyof1Hzwasreached.As thecompressionforceincreased,thefrequencydeclined.Atthe endofthetest,theloadingmachinestoppedautomatically.The testsequencestartedwith500cyclesat600N,andtheloadwas increasedinstepsof200Nevery500cyclesuntiltheosteosyn- thesisfailed.Construct failurewas defined as a suddenloss of measured force (>30%) and major deformation of the bone construct (>20mm). Testing was conducted in displacement controlmode.

Datacollectionandstatisticalanalysis

The data were collected at 100ms intervals using the instrument-specificBluehillSoftware.Inalltests,plasticdeforma- tion,asameasureofirreversibledeformationundertheinfluence offorce,wasrecordedautomaticallyasthemaximumvalueinthe previouscycle.

Loading(N),compressionset(mm),andthenumberofcycles werealsorecorded.Inaddition,95%confidenceintervals(CIs)were determined. Thestiffness of each osteosynthesis constructwas calculatedfromthecompressionsetundertheappliedloading.

An a priori poweranalysis was performed: For the load to failure(i.e.,theprimaryoutcome measure),a samplesizeof10 pairsoffemorawascalculated.Aclinicallymeaningfuldifference was defined as 100% failure of the osteosynthesis in non- augmentedvs.40%failureinaugmentedspecimensataloadof 2000N.Thepowerwassetat0.80withanalphaerrorof0.05.The data were statistically analysed using IBM SPSS statistics 22 (Statistical Package for the Social Sciences, IBM Cooperation, Armonk,NY,USA).Thedatawereexaminedfornormalityusingthe Kolmogorow–Smirnow test. Depending on the result, a paired Student’st-testortheWilcoxonranksumtestforpairedsamples wasappliedforfurtherdataanalysis.Statisticalsignificancewas setatp<0.05.

Results Bonequality

DXArevealedcompromised bonequality in alltestedspeci- mens witha meanT-scoreof 3.31(CI: 3.65to 2.96).Bone mineraldensity was comparable inall of thematched pairsof cadaverfemoraexamined(p=0.543).

Cyclictestingandfailuremode

Allspecimenssurvivedatleast800Nofaxialcompressiveforce.

Whereas90%ofthespecimenswithaugmentedcondylarscrews survivedaloadof2000Normore,noneofthespecimensinthe non-augmented group survived a load of 2000N. The mean compressiveforcesleadingtofailurewere1620N(95%CI:1382– 1858N)inthenon-augmentedgroupand2420N(95%CI:2054– 2786N)in the groupwithcement augmentedcondylar screws (Fig.4).Thisdifferencewasstatisticallysignificant(p=0.005).

Fig.3. Testsetupdemonstratingforceapplicationoverthecondylarregionwitha metalplate.Thefemoralshaftwasstaticallyfixedincontainersembeddedwith Technovit.

Fig. 4. Failure loads based on the cyclic loading tests, comparing plate osteosynthesiswithandwithoutaugmentationofthecondylarscrews.

Failureinthecement-augmentedgroupoccurredbycondylar fracturesinsixcases,cuttingoutofthecondylarscrewsinthree cases,andshaftfractureinonecase.Inthenon-augmentedgroup, deformationwithcuttingoutofthecondylarscrewsandcondylar fractureseachoccurredfivetimes(Fig.5a–c).

Axialstiffnessandplasticdeformationataloadof800N

Based on a load of 800N (the highest value at which all specimensremainedintact),aseparateanalysisofthestiffnessand plasticdeformationoftheconstructswasperformed.Atthisload, stiffnesswascomparableforosteosyntheseswithaugmentationof thecondylarscrews (mean:2.71kN/mm;95% CI:1.69–3.72kN/

mm) and without augmentation (mean: 2.07kN/mm; 95% CI:

1.55–2.59kN/mm)(p=0.508)(Fig.6).Asshownin thebox-plot (Fig. 7), analysing the data revealeda higher variability in the plasticdeformationoftheconstructinnon-augmentedosteosyn- theses (mean: 1.69mm; 95% CI: 0.43–2.96mm). In contrast, augmentation of the condylar screws led to smaller plastic deformation (mean: 0.67mm; 95% CI: 0.45–0.89mm). Under a loadof800N,thisdifferenceintheplasticdeformationbetween thetwotypesofosteosynthesiswasnotsignificant(p=0.098).

The plastic deformation of the constructs was also tested between600Nand 1800N.Loadsgreaterthan1800Nwerenot tested because all non-augmented specimens failed before reachingthisload.Analysingtheplasticdeformationinthisrange revealed significant differences between the two types of osteosynthesis(p=0.014)(Fig.8).Osteosynthesiswithaugmented condylarscrewsexhibitedameanplasticdeformationof0.98mm (95% CI: 0.68–1.28mm), whereas osteosynthesis with non- augmentedcondylar screwshad a meanplastic deformation of 1.45mm(95%CI:1.17–1.73mm).

Discussion

This biomechanical study analysed the impact of implant augmentation using a polyaxial lockingplate for theosteosyn- thesis of osteoporotic distal femoral fractures. The principle findingsofthisstudyrevealedthatthecementaugmentationof condylar screws significantly decreased the construct’s plastic deformationandsignificantlyincreaseditsloadtofailurerelative tothoseofnon-augmentedspecimens.

Fig.5. Photographsshowingthedifferentfailuremodes.Intheaugmentedgroupdeformationwithcuttingoutofthecondylarscrews(a),condylarfractures(b)andshaft fracture(c)occurred.Inthenon-augmentedgroupdeformationwithcuttingoutofthecondylarscrews(a)andcondylarfractures(b)wereseen.

1 1,5 2 2,5 3 3,5 4 4,5 5 5,5

0 1 2 3 4 5 6 7 8 9 10

Pair of samples

Stiffness (kN / mm)

non-augmented screws augmented screws

Fig.6.Stiffnesscalculationofplateosteosynthesiswithaugmentedscrews(black dots)andplateosteosynthesiswithnon-augmentedscrews(greydots)ataloadof 800N.

Fig.7.Plasticdeformationofplateosteosynthesiswithaugmentedscrewsand plateosteosynthesiswithnon-augmentedscrewsataloadof800N.

Previous studies on implant augmentation in distal femoral fracturesfroma biomechanical perspectivewere conductedby Wähnertet al. using monoaxial locking plates in artificial and humancadavericbones[14,16,17].However,theexistingstudieson implantaugmentationindistal femoralfracturesarelimitedby somemethodicaldrawbacks.Inthis context,it shouldbenoted that synthetic bone models do not capture interspecimen variabilityandmaterialinhomogeneity,bothofwhichmayaffect implant performance [18]. Additionally, particularly in severe osteoporoticbone,wherethecementaugmentationof implants shouldbeconsideredforfracturefixation,polyaxialimplantsare preferred.Inthiscontext,themaindisadvantageofunidirectional angularstableplatesisrelatedtotheperpendicularconfiguration ofthethreadsintheplate,whichresultinpredeterminedscrew positioning.Thus, screwpositioning oftenoccurs in areas with inferior bone quality, possibly leading to secondary loss of realignmentandcuttingoutofthescrews[10].Inaprospective multicentrestudy,monoaxialimplantswereshowntobeinferior tomultidirectionalangularstableplatesforthefixationofdistal femoralfractures[19].

Having overcome the method-related drawbacks affecting previousexaminations,ourstudyreportsthefirstbiomechanical approachwithdatathatareascloseaspossibletotherealsituation ingeriatricpatientswithosteoporoticbonestructures.Augmen- tationof thecondylarscrewssignificantlyincreasedtheloadto failureindistalfemoralfractures.Theseresultscanbeexplainedby the enlargement of the bone-implant interface by the cement augmentation,leadingtoincreasedstabilityinosteoporoticbone [13].Nevertheless,ourresultsarecontrarytothoseof Wähnert etal.,likelybecauseoftheuseofhybridspecimensconsistingofa combinationoffresh-frozendistal femoraandaPMMA femoral shaft [17].Unfortunately, noDXA scan was performedin their study,andthus,itcannotberuledoutthatthedifferencesinthe load-to-failure results are related to non-osteoporotic bone structures in some of their specimens. The use of the hybrid modelitselfmightalsoexplainthesedivergentfindings.

Theabovementionedenlargementofthebone-implantinter- facebythecementaugmentationofthecondylarscrewsmayalso havecontributedtothefailuremodeoftheosteosynthesisandthe

significantdifferencesintheplasticdeformationofthetwotypes of osteosynthesis studied here. Whereas cutting out of the condylarscrewsoccurredin50%ofthenon-augmentedosteosyn- theses, it was less common in the cement-augmented group.

Additionally, because of the increased pull-out strength, less plastic deformationoftheconstructoccurredin osteosyntheses withcement-augmentedscrews.Reducedratesofcuttingoutof augmentedcondylarscrewsandminorplasticdeformationhave beenreportedinpreviousbiomechanicalinvestigationsaddressing distalfemoralfractures[14,16].

Thepresentbiomechanicalanalysisrevealedthatthecement augmentationofcondylarscrewsdidnotincreasethestiffnessof theconstruct.Therefore,mostofthemotionoftheosteosynthesis likely occurs throughthe bendingof the plateitself under the influenceofaxialforceratherthanthroughmicromotionofthe screwsinthebone. Thefactthataugmentation ofthecondylar screwsdidnotsignificantlychangethestiffnessesoftheosteosyn- thesis constructs must be considered to be a positive result because secondary bone healing strongly dispends on the existenceofaxialmicro-movements[20].

Despitethesepromisingbiomechanicalresults,thediscussion aboutcement-relateddisturbancesofbone metabolismremains ongoing.BecausePMMAcementishardenedthroughanexother- micreaction,theheatreleasedduringthecementaugmentationof thecondylarscrewsmightincreasetheriskofboneandcartilage necrosis.Forexample,temperaturesupto45Cwereobservedin anin vitroinvestigation ofcement-augmentedhipscrews[21].

Furtherinvitroexperimentsaddressingthecementaugmentation of proximalhumerus plate screwsconducted byBlazejak et al.

revealedthatPMMAleadstolocallylimitedheatdevelopmentin thecementcloudandthesurroundingtissue[22].Mostoftheheat releasedduringpolymerizationof PMMAwasdissipated bythe implant itself, and the restaccumulated mainlyin the cement cloud. Therefore, thecritical threshold values for cartilage and subchondral bone necrosis were not reached, and cement augmentationwas determinedtobesafe.Althoughsuchinves- tigations havenot yet beenperformed in thedistal femur, the resultsobtainedinotherpartsofthebodycanbeassumedtobe generalizabletothistypeoffracture.

-0,5 0 0,5 1 1,5 2 2,5 3 3,5 4

600 800 1000 1200 1400 1600 1800

Load in Newton (N)

Deformation in Milimeter (mm)

Mean plastic deformation (non-augmented screws) Mean plastic deformation (augmented screws)

Fig.8.Meanplasticdeformationofplateosteosynthesiswithaugmentedandnon-augmentedcondylarscrewsplottedagainsttheappliedload.

Foreachmeanvalue,halfbarsillustratingthestandarddeviationareshown.

Forthemeanvaluesofplateosteosynthesiswithnon-augmentedcondylarscrews,thehalfbarsgoupwards.Forthemeanvaluesofplateosteosynthesiswithaugmented condylarscrews,thehalfbarsgodownwards.

Limitations

Despite its careful design, the present study has some limitations in terms of theoretical or computational analyses.

Althoughalargescopeexistsforthevalidationofourstudyresults, finiteelementanalyseswerenotperformedinthisbiomechanical investigationandshouldbeaddressedinthefuture.Furthermore, theinvitrosetupinwhichonlyaxialloadingofthespecimenwas testedalsorepresentsapotentiallimitation.Theserestrictionsof theloadingconditionswiththeabsenceoftorsionalandbending loads might have influenced theresults concerning thefailure modeandload.Additionally,thenumberoftestedsamplesislow.

Nevertheless,amongtheexistingstudiesonthistopic,thepresent oneisthelargesttodate.Moreover,accordingtoapoweranalysis performedpriortotesting,thisstudyachievedsignificantresults.

Conclusion

Theresultsofthisstudyshowedthatthecementaugmentation of the condylar screws might be a promising strategy for the implantfixationofdistalfemoralfracturesinelderlypatientswith osteoporotic bones.Thus, theaugmentation of implants, which increasestheirloadingcapacities,mightcontributetowinningthe racebetweenfracturehealingandosteosynthesisfailure.

Conflictofinterest

Thecorrespondingauthordeclaresonbehalfofallauthorsthat therearenoconflictsofinterest.

Acknowledgments

Zimmer Inc. Winterthur, Switzerland is acknowledged for providingtheimplants.Nofurtherfundingwasreceivedforthis work.Noadditionalfinancialsupportfortheexecutionofthestudy wasreceived.

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