Research
Paper
Trauma
In
vitro
mechanical
assessment
of
2.0-mm
system
three-dimensional
miniplates
in
anterior
mandibular
fractures
K. P. de Oliveira,P.H. de Moraes,J.S.P. daSilva,W.F. deQueiroz,A.R. Germano:Invitromechanicalassessmentof2.0-mmsystemthree-dimensional miniplatesinanteriormandibularfractures. Int.J.OralMaxillofac.Surg.2014;43: 564–571. CrownCopyright#2013PublishedbyElsevierLtdonbehalfof InternationalAssociationofOralandMaxillofacialSurgeons.Allrightsreserved.
K.P.deOliveira,P.H.deMoraes, J.S.P.daSilva,W.F.deQueiroz, A.R.Germano
DepartmentofOralandMaxillofacialSurgery,
HospitalUniversita´rioOnofreLopes,Federal
UniversityofRioGrandedoNorte,Natal,Rio
GrandedoNorte,Brazil
Abstract. Thisstudyconstitutedacomparativeassessmentofthemechanical resistanceofsquareandrectangular2.0-mmsystemthree-dimensionalminiplates ascomparedtothestandardconfigurationusingtwostraightminiplates.90 polyurethanereplicamandibleswereusedforthemechanicaltrials.Groups1,2, and3simulatedcompletesymphysealfracturescharacterizedbylinearseparation ofthecentralincisors;groups4,5,and6simulatedparasymphysealfractureswith anobliqueconfiguration.Groups1and4representedthestandardmethodwithtwo straightminiplatessetparalleltooneanother.Squareminiplates wereusedin groups2and5,andrectangularminiplatesingroups3and6.Auniversaltesting machinesettoavelocityof10mm/minanddeliveringaverticallinearloadtothe firstleftmolarwasusedtotesteachgroup.Maximumloadvaluesandloadvalues withpre-establisheddislocationof5mmwereobtainedandsubmittedtostatistical analysisusingacalculatedreliabilityintervalof95%.Themechanical
performancesofthedevicesweresimilar,exceptinthecaseofrectangularplates usedintheparasymphysealfractures.Theinnovativefixationmethodsusedshowed significantly betterresultsinthe caseofsymphysealfractures.
Keywords:internalfracturefixation;
mandibu-lar fractures; biomaterial; biomechanics;
trauma.
Acceptedforpublication8October2013
Availableonline13November2013
In recent years, different methods have been proposedforthe surgicaltreatment of mandibular fractures. Rigid internal fixation (RIF)isusedtoachieveastable anatomical reduction, thereby reducing theriskofpostoperativedisplacementof fractured bone fragments, avoiding the need for maxillomandibular fixation1,2 and favouring an early return tonormal functioning.3,4
Themandibleissubjecttoforces gen-eratedbythechewingmusclestransmitted via theteethandthetemporomandibular joints. During treatment, tensions and deformations occuraccordingtothe dis-tribution of the external forces and the properties and geometry of the material being used.5 It is well known that the mandibleisnormallysubjectedto tension-ingforcesonitssuperiorborderand
com-pression forceson its inferior border.5–8 However,thatisonlytrueforfracturesin thebodyandangleofthemandible;inthe caseof fractures in the symphyseal and parasymphyseal regions, the opposite situation prevails and a single form of biomechanicalbehaviourcanbeexpected forthislatterregionasawhole.6
Irrespectiveofthefixationmethodused, stabilityisakeyfactorforthesuccessful
http://dx.doi.org/10.1016/j.ijom.2013.10.008,availableonlineathttp://www.sciencedirect.com
treatmentofthe symphyseal fracture.9,10 Thiscanbeevaluatedbymechanicaltests thatsimulatetheforcesthatthe mandib-ularfractureswillbesubjectedto,making itpossible todeterminetheresistanceof thefixationmaterialandthebehaviourof thefracturedregion.4
Thesymphysisisoneofthemost com-mon mandibular fracture sites, with reports of prevalence varying from 9% to57%;itis onlysurpassedbyfractures of the condyle or of the angle.5–9 Each mandibular regionhas itsown peculiari-ties, including variations in the forces exerted by the chewing muscles, zones offragility,and thepossiblepresence of mechanical forces acting in different directions. These factors determine the extent of a region’s susceptibility to traumaanditspropensityforafavourable responsetotreatment.5,11
RecentworkdonebyMadsenetal.6and byOliveiraandPasseri4hasinvolvedthe comparativebiomechanicalassessmentof different fixation techniques applied to symphysealandparasymphyseal mandib-ularfractures,buttheydidnotmakeuseof three-dimensional(3D)fixations.
Farmand11,whowasthefirstto under-takeabiomechanicalinvestigationof3D platesin1996,studiedtheperformanceof aplateintheshapeofthefoursidesofa squareopeninthemiddle.Inhisview,the device,whichwasfixedbyscrews,would fosterstabilityinthreedimensions,andits biomechanical characteristics were com-parable to those of conventional mini-plates. The open-centred square configurationwouldbethesmallest pos-sibleonefora3Dplatecomponent.Inthat study, the clinical results and investiga-tionsshowedthat3Dplatesprovidedgood stabilityduringosteosynthesisassociated withmorecomplicatedcases of mandib-ularfracture.Theauthor also stated that the1-mmprofileoftheconnectingarmsof thedevicemadeitsadaptationtothebone without distortions easier, and that the untrammelledareasbetweenthe connect-ingarmsensuredagoodbloodsupplyto thebone.
Inspiteofthesmallnumberofinvitro studiesusing3Dplatesinfracturesinthe anteriorregionofthemandible,thereare someauthorswhohavereportedthe effi-cacyofthemethod.Intwoclinicalstudies, Jainetal.demonstratedtheeffectiveness of3Dminiplatesforfixationinthe treat-ment of mandibular fractures in that region and analysed their advantages and disadvantages over a 2-month fol-low-up period.Inthe configurationused intheirwork,thematerialusedprovedto be less costly and readily adaptable, as
well asreducing the operation timeand providinggreaterstability.12,13
Whiletherehavebeensome investiga-tions using 3D plates as the fixation method, there is no scientific evidence concerning their use in the symphysis/ parasymphysis region.4,6,14–17 Thus the present workwas undertakentoperform a laboratory evaluation ofthe resistance andperformanceofsquareandrectangular 2.0-mm system 3D miniplates used to stabilize fractures in the anterior region ofthemandible,ascomparedtothe per-formance of standard pattern plates, namely two straight plates also of the 2.0-mm(screwdiameter)system. Materialsandmethods
For thepurposesofthisstudy,tworigid polyurethanemandiblemodelswith com-pletesetsofteethwereprepared(Nacional Ossos, Jau´, Sa˜o Paulo, Brazil) and two different ‘fractures’ in the form of cuts weremadeinthemusingametaldiscat low speed rotation. The ‘osteotomies’ simulated symphyseal and parasymphy-sealfractures.Thesimulatedsymphyseal fracture consisted of a linear cut in the centreofthemandible,frombetweenthe medianincisorsdowntothebasalpartof the mandible. The parasymphyseal frac-ture was represented by an oblique cut originating between the median incisors and going down in a slightly posterior direction tothe baseofthe mandibleon therightside.Thetwomodelsweresentto themodelcompany,whichthenproduced 90standardizedreplicas,45foreachtype of‘fracture’.
Fourhundredandeighty titanium–alu-minium–vanadium alloy (Ti–6Al–V) screws(PROMM, Indu´stria deMateriais Ciru´rgicos,PortoAlegre, RioGrandedo Sul,Brazil)wereused,ofwhich240were 6mmlongand240were12mmlong,all compatiblewiththe2.0-mmsystem.120 miniplates were used as follows: 60 straightfour-holeminiplates thatarethe standardpatternforthe2.0-mmsystem,30 square four-holeminiplates, and30 rec-tangularfour-holeminiplates.Thesquare andrectangularminiplatesweredesigned bytheauthorsandmadetoorder bythe suppliers(PROMM, Indu´striade Materi-aisCiru´rgicos).
Samplepreparation
The rigid polyurethane mandibles were divided intosix groupsof15mandibles each for mechanical trialsinaccordance withastatistical designobtainedusinga programmeforsamplesizedetermination
(Diman 1.0); the confidence interval establishedwas95%.
Themandibularfracturesinthereplicas of groups1 and4 were stabilized using twostraightfour-holeminiplatesofthe 2-mmsystemoneach.Thesewerefixedin parallel, oneinthesuperior positionand theotherinaninferiorposition,withcare taken so that the superior plates were alwayslowerthanthelevelofthedental root apices.Thesetwogroupswere con-sideredtobethestandardpatternor con-trol groups. Fixation in groups 2 and 5 used square miniplates, and ingroups 3 and 6,rectangular miniplateswere used. Eachsquareandrectangularminiplatewas stabilizedwithfourscrews(Figs.1–3).In allgroups,thescrewsthatwerenearestto thedentalapiceswere6mmlong,while those insertedneartheinferiorborderof themandiblewere12mmlong.
The three different fixation methods wereused onbothtypesoffracture,i.e., onthelinearfracture(symphyseal)andthe oblique fracture (parasymphyseal). The symphysealfracturewaspresentingroups 1,2,and3,andtheparasymphyseal frac-tureingroups4,5,and6.
To ensure uniformity inthe miniplate positioning,eachonewaspre-mouldedto thesurfaceofthefracturedmandibleusing specificbendingtoolsandastandardized template, therebydiminishing the possi-bilityofalterationsamongthemthatmight result from manual preparation of the samples.
Atthetimeofminiplatefastening,the hemi-mandiblewasstabilizedusingdrops of prefabricated self-polymerizing resin and the miniplates positioned to make perforation possible. The same operator carriedoutallperforations.
Mechanicaltesting
Theequipmentused forloadtestingwas theAutographAG-X300kNmodelofthe Shimadzu UniversalMechanical Testing Machine (Shimadzu Corporation, Chiyoda,Tokyo,Japan).
For testing purposes, a specifically designedclampwasconstructedto stabi-lizethepolyurethanemandibleduringthe applicationofthetestloads(Fig.4).This clamp was perfectly adapted to tightly hold the posterior region of the right mandible, accommodating the border of theramusfromtheangletothecondyle. Tomaketestingfeasible,ametalrodwas prepared and adapted to the universal testing machine in such a way that the pointofapplicationofthetestloadswas alignedwiththecentralfossa ofthefirst leftmolar(Fig.5).
Thevelocityoftheloadtestwas cali-brated at10mm/minonthebasisofthe results of pilot studies experimenting with various velocities, and the test was orientated by the protocol used by OliveiraandPasseri.4Tworeadingswere obtained, one at the moment that the metal rod attained a displacement of 5mm under the constant load applied to the system at the pre-established
velocity, and the otherwas takenwhen themaximumloadthatthesystemcould resist was attained. Values were regis-tered inNewtons(N).
Statisticalanalysis
Aftertheaverageoftheloadreadingsat 5mm displacement of the rod and at maximum load had been obtained for
all the study groups, the data were subjectedtostatisticalanalysisfor quan-titative and comparative purposes. The data were also analysed after grouping bytype ofsimulatedfracture cut.SPSS software was used for the analysis (SPSS version 19 for Windows; IBM Corp.,Armonk,NY,USA),andthe cal-culated confidence interval was 95% (95% CI).
deOliveiraetal.
Fig.1. (a)Group1:symphysealfracture,straightminiplates.(b)Group4:parasymphysealfracture,straightminiplates.
Fig.2. (a)Group2:symphysealfracture,squareminiplate.(b)Group5:parasymphysealfracture,squareminiplate.
Results
LoadreadingsinNewtonswereobtained for each sample in the symphyseal and parasymphyseal fracture groups at the
moment that rod displacement reached the5mmmark.
For the symphyseal fractures, no statistically significant differences were observed in the average values and the
standard deviations among the different types offixation used. Atthat predeter-minedmomentofdisplacement,the high-estaveragevaluewasobtainedbygroup1 (straightminiplates),followedbygroups2 (square miniplate) and 3 (rectangular miniplate),respectively(Table1).
Inthecaseoftheparasymphyseal frac-tures,therewasnostatisticallysignificant difference between groups 4 (straight miniplates) and 5 (square miniplate). Howeverthevalueingroup6(rectangular miniplate)wassignificantlylowerthanin groups4and5(Table2).
Next, the maximum load values (N) were obtainedfor themoment when the systemstillsupportedtheload,butbeyond which its resistance started to decrease showingthatfixationfailurehadoccurred. Inthecaseofthesymphysealfractures, theaveragevaluesandthestandard devia-tionvaluesformaximumloadshowedthat themaximumforceattainedandappliedto the system was higher in group 1 (the standardorcontrolgroup)thaningroups 2and3.However,therewereno statisti-cally significant differences between groups 2 and 3 and the control group (Table3).
Inthecaseoftheparasymphyseal frac-tures, the highest load resistance figure was obtained for group 4 (the control group), followed by group 5 and group 6. The difference between group 4 and group 5was not statistically significant; however, the maximum load in group 6 wassignificantlylowerthanthatingroup 4. There was no statistically significant difference between group5 andgroup 6 figuresinthisrespect(Table4).
When the two fracture groups (sym-physeal vs.parasymphyseal) were com-pared, the maximum registered force supported by the system was found forthesymphysealfractures.The differ-ence in maximum force measurements betweenthesymphysisand parasymphy-sis groups was statistically significant (Table5).
Discussion
In vitro tests have been carried out to describe the biomechanical behaviour ofsupportingdevicesinabidtovalidate the various modes of fixation.4,6,18–25 Althoughscientificresearchhasbeen car-riedouttoassessthebiomechanical per-formanceofthedifferentfixationmethods used in oral and maxillofacial trauma, there has been far lessattention paid to fixationofthemandibularsymphysis,and thisistrueevenfortheuseof3Dfixation devicesinthatregion.
Mechanicalassessmentof3Dminiplates
Fig.4. (a)Devicedevelopedforthestudy,beforesamplepositioning;and(b)itsadaptationin theuniversaltestingmachine.
Anatomicallyaccuratemodels usedas test specimens can effectively simulate physiological conditions.23 In the case
ofpolyurethane,itsmodulusofelasticity, animportantfactorinchoosingthistype ofmaterial,isverysimilartothatofbony structures, thereby justifying its use in biomechanical testing. It hasa valuefor this modulusofaround 0.07GPa, which places it within the range of values obtained for human bone: Cordey20 reported values ranging from less than 0.1GPainbonemarrowto2GPain cor-ticalbonetissue.
The force mechanisms involved in chewing and the forces experienced by thebonesinvolvedarehighlycomplex.19
Axial loads arethose thatproduce com-pression forces or tensions in a struc-ture.20,26 They act at points of contact thatmayserveasascreenorfrictionzone andconsequentlytheymaybeinfluential ininducingrotationordisplacement.19In a clinical situation, itis important to be awarethateffectsofbendingforcesarefar moreimportantthanthoseofaxialforces. The third type of force involved is torque, which produces or tends to
pro-duce rotation or twisting.20 It is known thatbone is normally subject to tension forcesonits upperborder and compres-sion forces on its lower border.6,8,17,24 However, that is only true for fractures inthebodyandangleofthemandible,and inthecase ofsymphyseal and parasym-physeal fractures, the opposite situation prevails.Thusacomplexformof biome-chanicalbehaviourcanbeexpectedforthe regionasawhole.3,6
According to research conducted by Tamsetal.25, thesymphysealregionhas one ofthe highest negative moments of force (defined as being the reduction in the distance between the fragments on thealveolarborder)ascomparedtoother mandibular regions, and this generates great tension at the inferior border and compressionatthesuperiorborder.These researchersfoundlittleevidenceofshear forces in this region, but found that momentsofforcearemoreintensely pre-sentthaninotherregions.Thesamestudy reportedthatmomentsofflexure(bending) are1.5timesgreaterthantorqueforcesin theanteriorregionofthemandible.
Researchusing3Dmodelshasproved that when loads are applied to chewing pointsintheposteriorregionofthe mand-iblewherethemolarteetharelocated,the greatestintensityoftorqueforcesisfound inthe region ofthe symphysis. Further-more, studies using 3D models have shownthattheareasoftensionand com-pression are not fixed but are liable to alternatewithoneanother.6,18,26–28Other studies evaluating the use of RIF have applied the forces to the region of the centralincisors,29,30 whichoffers onlya very small area of contact, making it difficult to accommodate the testing device and increasing the likelihood of its slipping from the point of contact andalteringtheresults.19
Our study took heed of the various recommendations made by Tams etal.25,whodeterminedthemosteffective positioningoftheplates,and took parti-cularcareinthecaseofthesuperiorborder to respect the anatomical limits of the tooth root apices. However, due to the height ofthe anterior region of the test specimensandthefixedstandardlengthof theverticalbarsoftheminiplates,itwas notpossibletoinsertthe3Ddevicesonto thebone surfaceashighup aspossible, whichwouldhavebeenmorefavourable for neutralizing torque. This means that ideally, to maximize efficiency, plates with a variety of dimensions should be availabletothesurgeoninordertoaddress the anatomical variationsthat are found amongindividualsbeingtreated. deOliveiraetal.
Table4. Maximumforcemeasurementsforthegroupswithparasymphysealfractures.
Group No. Average SD Median Min Max 95%CI
G4 15 19.53a 3.50 20.50 12.35 27.23 17.59–21.47
G5 15 17.34a,b 4.80 16.59 9.35 25.27 14.68–20.00
G6 15 15.30b 3.12 15.69 8.10 19.17 13.57–17.03
G4,straightminiplates;G5,squareminiplate;G6,rectangularminiplate;SD,standard devia-tion;CI,confidenceinterval.
a,b
Differencesbetweentheresultsmarkedwiththeletters‘a’and‘b’arestatisticallysignificant. Statisticaltestingwasbasedonthecalculated95%CI.
Table1. Forcemeasurementsat5mmdisplacement:symphysealfracture.
Group No. Average SD Median Min Max 95%CI
G1 15 16.22 3.54 17.17 6.05 20.60 14.26–18.18
G2 15 15.32 3.20 16.50 7.68 19.79 13.54–17.09
G3 15 13.85 1.18 13.45 11.92 16.16 13.20–14.51
G1,straightminiplates;G2,squareminiplate;G3,rectangularminiplate;SD,standard devia-tion;CI,confidenceinterval.
Table2. Forcemeasurementsat5mmdisplacement:parasymphysealfracture.
Group No. Average SD Median Min Max 95%CI
G4 15 12.48a 2.19 12.49 7.20 16.83 11.27–13.70
G5 15 12.59a 3.19 11.54 7.68 18.50 10.82–14.36
G6 15 9.10b 2.09 9.30 3.62 11.54 7.94–10.25
G4,straightminiplates;G5,squareminiplate;G6,rectangularminiplate;SD,standard devia-tion;CI,confidenceinterval.
a,b
Differencesbetweentheresultsmarkedwiththeletters‘a’and‘b’arestatisticallysignificant. Statisticaltestingwasbasedonthecalculated95%CI.
Table3. Maximumforcemeasurementsforthegroupswithsymphysealfractures.
Group No. Average SD Median Min Max 95%CI
G1 15 26.88 5.42 27.13 12.02 35.38 23.88–29.89
G2 15 22.24 4.09 22.89 15.35 29.04 19.97–24.51
G3 15 22.86 2.26 22.79 19.17 28.32 21.61–24.11
G1,straightminiplates;G2,squareminiplate;G3,rectangularminiplate;SD,standard devia-tion;CI,confidenceinterval.
Table5. Maximumforcemeasurementsgroupedbythetwotypesoffracture.
Group No. Average SD Median Min Max 95%CI
Symphysis 45 23.99a 4.54 23.70 12.02 35.38 22.63–25.36
Parasymphysis 45 17.39b 4.17 17.69 8.10 27.23 16.17–18.64
SD,standarddeviation;CI,confidenceinterval.
Upuntilnowtherehavebeenveryfew reports publishedintheliteratureof stu-dies comparing fixations using standard miniplatesoftheChampyetal.31,32type andthoseusing3Dminiplatesinthe treat-mentofmandibular fractures,toidentify theirrespectiveadvantagesand disadvan-tages. Square and rectangular miniplates dohaveadisadvantageatthemomentof mouldingtothebonesurface,astheyhave tobebentinthreedimensions,whereasthe linearplatesonlyhavetobebentintwo.It ismoredifficulttogetaperfectadaptation with the 3D plates than with the linear onesbecausetheyareobjectsintheform of a plane that need to be adapted to a curvedsurfaceandnotjustanobjectinthe formofastraightline.13Thiswas corro-boratedinthepresentstudy.
Theadvantageofthe3Dplates isthat theyprovideimprovedmechanical stabi-lityascomparedtotheconventional mini-plates, although that additional stability may be lostin thecase ofoblique frac-tures.Thereasonforthisisthatinoblique fracturesitisdifficulttoreconcileallthe recommendations for an ideal fixation usingadevicethatconsistsofhorizontal barsandverticalbarsparalleltothe frac-ture line13. This mayexplain the
differ-encesinresistanceencounteredamongthe fixationdevicesinthisstudyaccordingto thetypeoffracture(Table5).
The principles and utilization of 3D miniplates in mandibular fractures have notyetbeenfullyestablished.Inarecent researchsurveyamong104Americanand Europeandentalsurgeonsbelongingtothe Association for Osteosynthesis/Associa-tion for the Study of Internal Fixation (AO/ASIF), only 6% declared that they madeuseofsuchmaterialsforfixation.33 Furthermore, there are no case series reportsavailableinthespecialized litera-tureandonlyasmallnumberofpublished studies investigating their advantages as compared totraditional plates and mini-platesforbonereconstruction.12,13
Withregardtothedisplacementvalue tobeadopted,thereareauthorswhohave standardized the use of a displacement value of 1mm,34,35 3mm,36 and 10mm,19orthemomentwhenthesystem collapses,ifthelattershouldoccurbefore the predetermineddisplacement is regis-tered.37Madsenetal.6usedacontinuous load right through to the moment of mechanical failure of the system, or to thelimitofdisplacement,whenevaluating symphysisfractures.VieiraeOliveiraand Passeri4 registered resistance values at intervals,oratthelimitofpredetermined displacement. As a simplified form for presentingtheresultsandtogiveenough
informationfortheevaluationofmultiple data,weadoptedapredetermined displa-cementvalueof5mmandusedthe max-imumloadtodemonstratethelimitsofthe resistanceeachsystemiscapableof.
Withregard tothedisplacement velo-city, different protocolscan be found in the literature, with great variations betweenthem.Inresearchwork compar-ing different RIF techniques, Asprino38 and Brasileiro29 proposed the use of a displacement velocityof1mm/min. Tri-vellato37 preferred a velocity of 2mm/ min.Inbiomechanicalstudiestoevaluate fixationsintheanteriorregionof polyur-ethane mandibles, Madsenetal.6 useda velocityof5mm/mintodeliverloadtothe edgeoftheincisorsand1mm/minwhen applying load to the left molar region. Oliveira andPasseri4adopteda predeter-mined displacement velocity of 10mm/ minforthefirstleftmolar.Thepilottests thatwerecarriedoutonspecimenspriorto thisstudyshowedgreateraccuracywhen the predetermineddisplacement valueof 10mm/minwasadopted,hencethisvalue was used as the standard in the present research.
Theresultsobtainedforthesymphyseal fracture groupsintermsofboththe pre-determined 5mm displacement values and themaximum force resisted (Tables 1and3),underscorethegoodperformance ofthesquareandrectangulardevicesused incomparisontothecontrolgroup.These are therefore viable options for use in fractures in thatparticular region ofthe mandible.Withregardtothe parasymphy-sealfracturegroups,intermsofthe5mm displacement,theresultsforthe rectangu-larminiplatesweresignificantlydifferent tothoseofthecontrolgroup,which actu-ally registered the highest values of all groups. The reduced level of resistance demonstrated by the rectangular plates when used in parasymphyseal fractures may berelatedtothe oblique configura-tion ofthe cut (which tends tointerfere with the equidistant positioning of the connectingarmsoftheminiplatesin rela-tiontotheslantingtrajectoryofthecut),to thelessernumberofscrewsinvolved,and tothelackofaperfectadaptationofthe miniplatetothepolyurethane model.
Whiletheloadwas beingapplied,the parasymphysealfractureswerethosethat sufferedthegreatestrotationeffectsdueto the configuration of the cuts, which favoured the axis of movement. Shear andcompressionforcesinthebasalregion are less active in oblique cuts than in straightlinecuts.Thismeansthattheleft mandiblesidetowhichtheforceisapplied undergoes greaterand more intense
dis-placement towards the opposite hemi-mandiblewhenthecutsareoblique. Gen-erallyspeaking,duetosuchforce compo-nentsactingintheobliquefracturegroups, the tendency to displacement is greater andtheuse offixation systemsendowed withgreaterrigidityshouldbeconsidered. What mustbe remembered, however, is that thiswas aninvitrotrial and conse-quently subject to certain limitations– thereisaneedformoreintense investiga-tionintotheuseof3Dplates,especiallyin the case of rectangular miniplates. Furthermore, it has already been made clear that the forces used in biting are considerably reduced after mandibular fracture treatment, so it may possibly notbenecessarytoemploysuchveryrigid systems to stabilize fractures during the healingperiod.39,40
With 3D plates, the spacing of the screws means that the loads or forces actingontheplatearesharedbytheupper and lower sectionsof the plate and this enhancestheresistancetorotation gener-ated bytorsion forces.Inthecaseofthe square plates, because the configuration situatesthescrewsatsmalldistancesfrom one another and from the geometrical centreoftheplate,thetendencyforscrews tolosetheirholdduringtheapplicationof thetestforcesisincreased.Inmechanical terms, the square format and its screw positionsislessstablethantherectangular formatbecauseitsgeometryprovidesless stability. However,therewereno detect-able statistically significant differences amongthesegroupsinthemeasurements taken atmaximum force ineitherofthe fractureconfigurations.Asignificant dif-ferencewas onlyobservedinone prede-termineddisplacementof5mm,inwhich situation the square plates were more effectiveintheobliquefractures.
It must be stressed that the greatest displacement registered in testing the groupswithfixationbasedonrectangular miniplates,forbothkindsofcut,iscaused bythegreaterdeformationtheysuffer,and thisisespeciallydemonstratedbythefact that in some of the tested samples the screws did not come out but remained inplaceuntilthelimitsofminiplate defor-mationhadbeenattained, atwhichpoint thetipoftheverticalrodslippedoutofthe central fossa of the first leftmolar. The explanation for this would be that the energyprovoking deformationstemming fromtheforcebeingappliedandtheway thatthesystemusesthatenergyin deform-ingtheplate,limitstheforcesthatmight eventually displace the screw so that it becomes an event that takes longer to occur. From the biomechanical point of Mechanicalassessmentof3Dminiplates
view,suchdeformationsintheplatesare actuallybeneficial,butonlyuptothepoint wheretheyreachthelimitsoftheir elas-ticity. Once that point has been passed, they no longer favour fixation stability because they are unable to go back to normal once the force has ceased and accordingly make the fracture reduction and fixation obtained by the treatment unfeasible.
Thefixationsshowedtheirbestresults in the symphyseal fractures with differ-ences that were statistically significant. Thus, extrapolating to the clinical situa-tion,wewouldobservethat3Dfixations withfourscrews,astestedinthestudyin symphyseal fractures, would probably function adequately in stabilizing such fractures,consideringthattheypresented a good mechanical performance.On the otherhand,theirlossofperformanceinthe parasymphysealfracturescouldminimize thesuccessof3Dfixationstestedin clin-icalsituations,especiallywhenthe rectan-gularplatesareused.Theincorporationof twomore screwsineach segmentofthe 3D plates wouldprobably lead to better resultsinfracturestabilization,butinthat casetheywouldnolongerhaveany clin-ical advantagesconsidering that straight platesmountedinparallelhavedelivered goodresultsforyearsandtheyareeasier tomould.Anotheraspecttoconsideristhe modification ofthe bitingforce after the trauma, becauseeven though the rectan-gularminiplatesmayhaveobtained infer-ior results,the stabilizationtheyachieve maybesufficienttoallowfortheskeletal unittofunctionduringtherepairprocess because they reduce the functional demand made on it during the first 4 weeks.Clinicalinvestigationscould con-firmthesefindingsandtheyareimportant and necessarytovalidatethisalternative form offunctionallystable internal fixa-tionintheanteriorregionofthemandible. Funding None. Competinginterests Nonedeclared. Ethicalapproval Notrequired.
Acknowledgement.Theauthorswouldlike toexpresstheirgratitudetoPROMM1 -Indu´stria de Materiais Ciru´rgicos, Porto Alegre, Rio Grande do Sul, Brazil, for
their kind support through donations of thetitaniumplatesandscrewsusedinthis research.
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Address: A.R.Germano
DepartmentofOralandMaxillofacial Surgery
HospitalUniversita´rioOnofreLopes FederalUniversityofRioGrandedo
Norte–UFRN Av.NiloPec¸anha 620–Petro´polis CEP59012-300 Brazil Tel.:+558432154130; Fax:+558432154130 E-mail:adrianogermanoufrn@yahoo.com.br
