Dégradation des propriétés mécaniques des alliages orthodontiques NiTi en milieu buccal :...
Transcript of Dégradation des propriétés mécaniques des alliages orthodontiques NiTi en milieu buccal :...
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� 2014 CEOPublished by / Edite par Elsevier Masson SAS
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Degradation of the mechanical properties oforthodontic NiTi alloys in the oralenvironment: An in vitro study
D�egradation des propri�et�es m�ecaniques desalliages orthodontiques NiTi en milieu buccal :�etude in vitro
Wiam RERHRHAYEa,*, Loubna BAHIJEa, Khalil EL MABROUKb, Fatima ZAOUIa,Nadia MARZOUKa
a �Equipe de biotechnologie et mat�eriaux du milieu buccal, facult�e de m�edecine dentaire de Rabat,universit�e Mohammed V, Souissi, MoroccobInstitute of nanomaterials and nanotechnology, MAScIR, Rabat, Morocco
/ D
SummaryAppropriate characterization studies are needed to demonstratethe mechanical and biological effects of interaction betweenarchwires and the oral environment. The aim of this studywas to investigate, in vitro, the impact of this acidic and fluo-ridated environment on the electrochemical behavior and themechanical properties of orthodontic alloys in nickel titaniumand in stainless steel (controls) for the following parameters:Young’s modulus (E), elastic limit (se) and the maximumtensile load (sm). Six samples of each archwire alloy were usedto assess these parameters. An Instron universal test apparatus(model – 88512) was used for the traction tests on the wiresafter immersion in solutions at different concentrations of fluo-ride and at various pH levels. Observations were made using anelectron scanning microscope (ESM) to evaluate the surface andan ICP (inductively coupled plasma) mass spectroscopy anal-ysis was made to quantify the substances released into theimmersion solution. For the NiTi archwires, immersion in thefluoridated and acidic medium showed a statistically signifi-cant reduction of the Young’s modulus (E), the elastic limit (se)and the maximum tensile load (sm). Similarly, a higher level ofreleased nickel proportionate to the increase in the fluorideconcentration and acidity was observed in the immersion solu-tions. ESM observations revealed the status of the surface of thedifferent alloys and the presence of corrosive pitting.
Available online: 13 August 2014
International Orthodontics 2014 ; 12 : 271-280http://dx.doi.org/10.1016/j.ortho.2014.06.006
R�esum�e
L’interaction des arcs avec le milieu buccal implique des�etudes de caract�erisation appropri�ees afin d’illustrer lescons�equences m�ecaniques et biologiques. L’objectif de cetravail a �et�e d’�etudier, in vitro, l’influence du milieu acide etfluor�e sur le comportement �electrochimique et les propri�et�esm�ecaniques des alliages orthodontiques en nickel titane et enacier inoxydable (t�emoins) pour les param�etres suivants : lemodule de Young (E), la limite �elastique (se) et la chargemaximale a la rupture (sm). Six �echantillons de chaque alliaged’arc ont �et�e utilis�es pour l’�evaluation de ces param�etres. Unemachine d’essai Instron universel (mod�ele – 88512) a �et�eutilis�ee pour les essais de traction des fils apr�es leur immer-sion dans des solutions a diff�erentes concentrations de fluora des degr�es de pH variables. Une observation aumicroscope�electronique a balayage a �et�e utilis�ee pour l’�evaluation de lasurface et une analyse par spectroscopie de masse ICP(inductively coupled plasma) a permis le dosage des �el�ementslib�er�es dans les solutions d’immersion. L’immersion dans lemilieu fluor�e et acide a montr�e, pour les arcs en Niti, unediminution statistiquement significative du module de Young(E), de la limite �elastique (se) et de la charge maximale a larupture (sm). De meme qu’une lib�eration plus importante dunickel dans les solutions d’immersion a �et�e constat�ee propor-tionnellement a l’augmentation de la concentration en fluor etde l’acidit�e. L’observation au MEB a permis de montrer l’�etat
isponible en ligne : 13 aout 2014
*Correspondence and reprints / Correspondance et tir�es a part.
e-mail address / Adresse e-mail : [email protected] (Wiam Rerhrhaye)
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Wiam RERHRHAYE et al.
� 2014 CEO. Published by Elsevier Masson SAS. All rightsreserved
Key-words
·Degradation.
·Mechanical properties.·Nickel titanium.
·Fluoride. ·Acid.IntroductionMost orthodontic forces are delivered by means of orthodonticwires available in a range of different alloys. In order toachieve optimal dental movement, good control of the ortho-dontic forces is recommended. These forces need to be con-tinuous and constant, a requirement, which can now beachieved thanks to the introduction of titanium-based alloysin orthodontics (nickel titanium and beta-titanium) [1,2]. NiTiwires (52.9% Ni and 47.1% Ti) possess excellent mechanicalproperties such as super-plasticity, shape-memory and deliv-ery of light forces. They are available as preformed archwires.The beta-titanium alloy (78% Ti, 11.5% Mo, 6% Zr and 4.5%Sn) has good plasticity and a lower elasticity modulus thanstainless steel but close to that of NiTi. In addition, it can besoldered. It also has a very rough surface and low fractureresistance during bending [3,4].
Unfortunately, these wires are subject to corrosion in anaggressive oral environment [5,6]. The alloys can exhibit lowresistance to corrosion in the presence of fluoride ions, whichdestroy the titanium oxide passivation film on the surface ofthe wire [7]. The presence of fluoride can be due to medicationor to food. Sea fish have relatively high levels of fluoride (1 to3 mg/100 g), as has tea (roughly 0.5 to 1.5 mg/L). In medicalproducts (toothpaste, mouth rinses and fluoride gels), the fluo-ride content ranges between 0.1% (1000 ppm) and 1%(10,000 ppm) and can even reach 2% (20,000 ppm) in profes-sional applications [8].
One might wonder, therefore, about the impact of this corro-sion on the mechanical properties of the alloys in question,and notably on their elasticity and fracture resistance.The aim of this study was to investigate, in vitro, the effect offluoride ions on the electrochemical behavior and the mechan-ical properties of orthodontic titanium-based alloys.
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de surface des diff�erents alliages et la pr�esence de piqures decorrosion.� 2014 CEO. Edite par Elsevier Masson SAS. Tous droitsreserves
Mots-cl�es
·D�egradation.
·Propri�et�es m�ecaniques.·Nickel titane.
·Fluor. ·Acide.IntroductionLes forces orthodontiques sont d�elivr�ees majoritairement parles fils orthodontiques qui se d�eclinent dans plusieurs alliages.Afin d’obtenir unmouvement dentaire optimal, un bon controledes forces orthodontiques est recommand�e. Celles-ci doiventetre continues et constantes ce qui est obtenu grace a l’intro-duction des alliages a base de titane en orthodontie (le nickeltitane et le beta-titane) [1,2]. Les fils en nickel titane (52,9 %deNi et 47,1 % de Ti) ont d’excellentes propri�et�es m�ecaniquestelles que la superplasticit�e, la m�emoire de forme et la lib�era-tion de forces l�eg�eres. Ils sont utilis�es sous forme d’arcspr�eform�es. L’alliage beta-titane (78 % de Ti, 11,5 % de Mo,6 % de Zr et 4,5 % de Sn) pr�esente un module �elastiqueinf�erieur a l’acier inoxydable et proche du nickel titane (NiTi),une bonne plasticit�e et de plus, il est soudable. Par ailleurs, il aune grande rugosit�e de surface et une basse r�esistance a lafracture lors du pliage [3,4].Malheureusement, ces fils se corrodent dans les conditionsorales agressives [5,6]. Ces alliages peuvent avoir une faibler�esistance a la corrosion en pr�esence d’ions fluor quid�etruisent le film de passivation d’oxyde de titane form�e a lasurface du fil [7]. Les sources de fluor peuvent etrem�edicamenteuses ou alimentaires. Les poissons de mer sontrelativement riches en fluor (1 a 3 mg/100 g), de meme que leth�e (environ 0,5 a 1,5 mg/L). Dans les produitsm�edicamenteux (dentifrices, bains de bouche et gels fluor�es)le contenu en fluor est compris entre 0,1 % (1000 ppm) et 1 %(10 000 ppm) et peut meme aller jusqu’a 2 % (20 000 ppm)pour des applications professionnelles [8].On peut alors se poser la question de l’effet de cette corrosionsur les propri�et�es m�ecaniques de ces alliages et notammentsur l’�elasticit�e et la r�esistance a la fracture.L’objectif de ce travail est d’�etudier, in vitro, l’influence des ionsfluorures sur le comportement �electrochimique et lespropri�et�es m�ecaniques des alliages orthodontiques a basede titane.
International Orthodontics 2014 ; 12 : 271-280
Degradation of the mechanical properties of orthodontic NiTi alloys in the oral environment: An in vitro studyD�egradation des propri�et�es m�ecaniques des alliages orthodontiques NiTi en milieu buccal : �etude in vitro
Materials and methods
Orthodontic titanium-based (nickel titanium and beta-tita-nium) and stainless steel (controls) wires, all commercializedby the same manufacturer (GAC� Intern USA), were used inthe study. The shapes and cross-sections of the wires areshown in Table I.Ninety 20 mm samples were cut from the straight extremitiesof the preformed archwires. The samples were cleaned for tenminutes in an ultrasonic bath, then randomly distributed in 5groups. Each group of 18 archwires was further distributedinto 3 sub-groups containing 6 archwire segments of eachalloy. The study design is shown in fig. 1.
Group 1 was kept as control (non-immersed) and the 4 othergroups (i.e. 75 samples) were immersed in 4 artificial media ata constant temperature of 37 �C:
— group 1: segments of nine control archwires;— group 2: archwire segments after immersion in saliva alone(medium 1). The artificial saliva used in our study wasSaliveze�, a therapeutic saliva with a neutral pH (6.8) closeto human saliva [9,10] (Table II);— group 3: archwire segments after immersion in a fluoridesolution with a 2.3 pH. Medium 1 + 1% (10,000 ppm) of NaF+ lactic acid;— group 4: archwire segments after immersion in a fluoridesolution with a 5.4 pH. Medium 1 + 0.5% (5000 ppm) of NaF+ lactic acid;— group 5: archwire segments after immersion in a fluoridesolution with a 2.3 pH. Medium 1 + 1% (10000) of NaF + lac-tic acid.Sodium fluoride powder was added to the saliva and the pHwas measured using an electronic pH meter (pH meter,model). The pH was obtained by addition of lactic acid andcontinuous mechanical agitation for 30 minutes.All the samples were incubated at 37 �C in individual 20 mLrecipients (Fisher Scientific, Hanover Park, Ill) with 5 mL ofone of the artificial media. Immersion time was 6 hours, equiv-alent to 6 months of daily use of fluoride products during1 minute.
Table IShape and cross-section of the samples.
Alloy/Alliage
Stainless steel/Acier inoxydable
Thermal NiTi/NiTi thermique
Titanium molybdenum/Titane molybd�ene
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Mat�eriels et m�ethodes
Des fils orthodontiques a base de titane (nickel titane et beta-titane) ainsi que des fils en acier inoxydable (t�emoins), touscommercialis�es par le meme fabriquant (GAC� Intern �Etats-Unis) ont �et�e utilis�es pour l’�etude. Les formes et les sectionsdes fils sont repr�esent�ees dans le Tableau I.Quatre-vingt-dix �echantillons de 20 mm ont �et�e coup�es dansles extr�emit�es rectilignes des arcs pr�eform�es. Les �echantillonsd’arcs ont �et�e nettoy�es dans un bac a ultrasons pendant10 minutes, puis r�epartis au hasard en 5 groupes. Chaquegroupe de 18 segments d’arcs sera r�eparti en 3 sous-groupesde 6 segments d’arcs de chaque alliage. Le sch�ema de l’�etudea �et�e repr�esent�e sur la fig. 1.Le groupe 1 a �et�e gard�e comme t�emoin (a l’air libre) et les4 autres groupes (soit 75 �echantillons) ont �et�e plac�es enimmersion dans 4 milieux artificiels (a temp�erature constantea 37 �C) :— groupe 1 : les segments d’arcs neufs t�emoins ;— groupe 2 : les segments d’arcs apr�es immersion dans lasalive seule (milieu 1). La salive artificielle utilis�ee dans notre�etude est une salive th�erapeutique a pH neutre (6,8) et qui serapproche de la salive humaine, Saliveze�[9,10] (Tableau II) ;— groupe 3 : les segments d’arcs apr�es immersion dans unesolution fluor�ee a pH neutre. Milieu 1 + 0,1 % (1000 ppm) deNaF ;— groupe 4 : les segments d’arcs apr�es immersion dans unesolution fluor�ee a pH 5,4.Milieu 1 + 0,5 % (5000 ppm) deNaF+ acide lactique ;— groupe 5 : les segments d’arcs apr�es immersion dans unesolution fluor�ee a pH 2,3.Milieu 1 + 1 % (10 000 ppm) deNaF+ acide lactique.Le fluorure de sodium (NaF) en poudre a �et�e rajout�e a la salivepuis le pH a �et�e mesur�e a l’aide d’un pH m�etre �electronique(pH meter, model). Il a �et�e obtenu par adjonction d’acide lac-tique et agitation magn�etique continue pendant 30 minutes.L’ensemble des �echantillons a �et�e plac�e en incubation a 37 �Cdans des bacs individuels de 20 mL (Fisher Scientific,Hanover Park, Ill) avec 5 mL de l’un des milieux artificiels. Ladur�ee d’immersion a �et�e de 6 heures (l’�equivalent de 6 moisd’utilisation quotidienne pendant 1 minute de produitsfluor�es).
Tableau IForme et section des �echantillons.
Shape/Forme Cross-section/Section
Round/Rond 0.016 inch
Round/Rond 0.016 inch
Square/Carr�e 0.016 � 0.016 inch
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[(Fig._1)TD$FIG]
Fig. 1: Sample distribution diagram.Fig. 1 : Diagramme de r�epartition de l’�echantillon.
Wiam RERHRHAYE et al.
Traction tests
Traction tests were performed using an Instron Machine(model – 88512).Five tests were conducted for each sub-group (G1a,b,c–G2a,b,c–G3a,b,c–G4a,b,c–G5a,b,c).
Table IIComposition of Saliveze� therapeutic saliva.
Components/Composantes
Sodium carboxymethylcellulose
Potassium chloride
Sodium chloride
Magnesium chloride
Calcium chloride
Di-potassium hydrogen orthophosphate
Potassium di-hydrogen orthophosphate
Sodium fluoride
Sorbitol
Methyl p-hydroxybenzoate
Spirit of lemon
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Tests de traction
Un test de traction a �et�e r�ealis�e a l’aide d’une Machine Instron(model – 88512).5 tests ont �et�e r�ealis�es pour chaque sous-groupe (G1a,b,c–G2a,b,c–G3a,b,c–G4a,b,c–G5a,b,c).
Tableau IIComposition de la salive th�erapeutique Saliveze�.
Quantity in g/L/Quantit�e en g/L
10
0.62
0.87
0.06
0.17
0.80
0.30
0.0044
29.95
1.00
5 mL
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Degradation of the mechanical properties of orthodontic NiTi alloys in the oral environment: An in vitro studyD�egradation des propri�et�es m�ecaniques des alliages orthodontiques NiTi en milieu buccal : �etude in vitro
For each sub-group, the stress-strain curves measured thefollowing values: Young’s modulus, the elastic limit (se), themaximum tensile load (sm) and elongation at break (E).
Analysis of the solutions by mass inductivelycoupled plasma (ICP) spectroscopy
Analysis by mass ICP (inductively coupled plasma) spectros-copy enabled quantification of the substances released intothe immersion solutions.Four main metals were targeted: chromium, iron, nickel andtitanium.
Electron scanning microscope (ESM) observationof the samples
A sample from each group was observed under an electronicscanning microscope (model FEI Quanta 200).
Statistical analysis
The data were analysed using SPSS software for Windows(version 13.0, SPSS Inc., USA). K-S test was used to test thenormal distribution of the sample, following which the quan-titative variables were expressed as means and standarddeviations. The statistical tests used were the repeated mea-sures Anova and the Bonfferoni post-hoc test. The significancethreshold was set at 5%.
Results
The results of the mechanical tests are shown in Table III andfigs. 2 and 3.Following the statistical analysis, immersion in the fluoridemedium revealed a statistically significant reduction of theYoung’s modulus (P = 0.02) compared with the controls. Theamount of released ions as measured in the different immer-sion solutions is given in Table IV. Statistical analysis showedthat more Ni was released in the solutions with the higher Fconcentration and acidity.
ESM observation showed the surface status of the differentalloys revealing the presence of corrosion pitting.Images taken with�5000 enlargement showing a sample fromeach group revealed round and oval corrosion pitting on thesamples in groups 4 and 5, with a greater pit density in thelatter group (fig. 4).
Discussion
Titanium-based alloys are widely used during the early stagesof orthodontic treatment for their super-elasticity and shape-
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Les courbes contrainte/d�eformation ont permis de mesurer,pour chaque sous-groupe, lamoyenne des valeurs suivantes :le module de Young (E), la limite �elastique (se), la chargemaximale a la rupture (sm) et l’allongement a la rupture (A).
Analyse des solutions par spectroscopie de masseinductively coupled plasma (ICP)
Une analyse par spectroscopie de masse ICP (inductivelycoupled plasma) a permis le dosage des �el�ements lib�er�esdans les solutions d’immersion.Quatre �el�ements majoritaires ont �et�e recherch�es : le chrome,le fer, le nickel et le titane.
Observation des �echantillons au MEB
Un �echantillon de chaque sous-groupe a �et�e observ�e aumicroscope �electronique a balayage (mod�ele FEI Quanta200).
Analyse statistique
Les donn�ees ont �et�e analys�ees a l’aide du logiciel SPSS pourWindows (version 13.0, SPSS Inc., �Etats-Unis). Apr�es avoirtest�e la distribution normale de l’�echantillon par le test K-S, lesvariables quantitatives ont �et�e exprim�ees enmoyenne et �ecarttype et les tests statistiquesutilis�es ont �et�e l’Annova amesuresr�ep�et�ees et le test post-hoc de Bonfferoni. Le seuil de signifi-cativit�e a �et�e fix�e a 5 %.
R�esultats
Les r�esultats des tests m�ecaniques sont report�es dans leTableau III et les fig. 2 et 3.Apr�es l’�etude statistique, l’immersion dans le milieu fluor�eacide a montr�e une diminution statistiquement significativedu module de Young (p = 0,02), par rapport aux t�emoins. Letaux de lib�eration d’ions mesur�e dans les diff�erentes solutionsd’immersion est report�e dans le Tableau IV. L’�etude statistiquea montr�e une lib�eration plus importante du Ni dans les solu-tions avec l’augmentation de la concentration en F et del’acidit�e.L’observation au MEB a permis de montrer l’�etat de surfacedes diff�erents alliages et la pr�esence de piqures de corrosion.Les images prises a un grossissement�5000 d’un �echantillonde chaque groupe ontmontr�e des piqures de corrosion rondesou ovales sur les �echantillons des groupes 4 et 5 avec unedensification dans ce dernier groupe (fig. 4).
Discussion
Les alliages a base de titane, tr�es largement utilis�es durant lespremi�eres phases du traitement orthodontique pour leurs
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Table IIIResults showing the mechanical properties of the alloysanalyzed in the different media.
Tableau IIIR�esultats des propri�et�es m�ecaniques des alliages �etudi�es dans les diff�erents milieux.
Parameter/Param�etre Alloy/Alliage
Control/Controle
Saliva alone/Salive seule
0.1% (1000 ppm)NaF
0.5% (5000 ppm)NaF pH = 5.4
1% (10,000 ppm)NaF pH = 2.3
Young’s modulus (GPa)/Module de Young (GPa)
NiTi 54.92 W 7.4 54.59 W 8.32 37.54 W 3.2 31.78 W 5.27 29.68 W 4.98a
Stainless steel/Acier inox
176 W 13.53 172.47 W 12.3 178.02 W 10.4 168.48 W 10.64 152.41 W 9.72
Elasticity limit (MPa)/Limite �elastique (MPa)
NiTi 60.83 W 20.51 59.8 W 19.6 58.09 W 12.55 55.33 W 11.34 39.82 W 17.8a
Stainless steel/Acier inox
1886.33 W 23.86 1842.5 W 20.36 1919.84 W 19.5 1952.17 W 21.23 1913.24 W 19.53
Maximum load at break (MPa)/Charge maximale a la rupture (MPa)
NiTi 225.72 W 44.1 224.72 W 54.41 229.19 W 53.5 213.03 W 44.34 154 W 50.64a
Stainless steel/Acier inox
347.67 W 50.64 335.19 W 48.54 346.15 W 46.3 320.52 W 48.4 224.11 W 49.51a
a Statistically significant values.
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[(Fig._2)TD$FIG]
Fig. 2: Stress-strain curves of NiTi.Fig. 2 : Courbes contrainte/d�eformation du NiTi.
Degradation of the mechanical properties of orthodontic NiTi alloys in the oral environment: An in vitro studyD�egradation des propri�et�es m�ecaniques des alliages orthodontiques NiTi en milieu buccal : �etude in vitro
memory properties and are claimed to be fairly resistant tocorrosion. However, recent studies have shown that, in thepresence of fluoride, these alloys corrode and suffer a drop intheir mechanical properties [1,11].
In effect, our study showed that immersion in the fluoridatedacidic medium led to a lowering of the Young’s modulus (E),the elasticity limit (se) and the maximum tensile load (sm)(P < 0.001).
[(Fig._3)TD$FIG]
Fig. 3: Stress-strain curves of stainFig. 3 : Courbes contrainte/d�eforma
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propri�et�es de super�elasticit�e et de m�emoire de forme, sontcens�es etre assez r�esistants a la corrosion. Mais des �etudesr�ecentes ont montr�e qu’en pr�esence de fluor, ces alliages secorrodent et subissent une diminution de leurs propri�et�esm�ecaniques [1,11].En effet, notre �etude a montr�e que l’immersion dans le milieufluor�e et acide entraınait une diminution du module de Young(E), de la limite �elastique (se) et de la charge maximale a larupture (sm) (p < 0,001).
less steel.tion de l’acier inoxydable.
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Table IVResults for the analysis of the amount of metal ions released inthe different media.
Tableau IVR�esultats des propri�et�es m�ecaniques des alliages �etudi�esdans les diff�erents milieux.
Parameter/Param�etre Alloy/Alliage Crmg/L
Femg/L
Nimg/L
Timg/L
Saliva alone/Salive seule NiTi � 0.02 0.02
Stainless steel/Acier inox � 0.001 0.198 � 0.02
0.1% (1000 ppm) NaF NiTi � 0.023 0.081
Stainless steel/Acier inox � 0.001 0.198 � 0.02
0.5% (5000 ppm) NaF pH = 5.4 NiTi � 0.011 0.038
Stainless steel/Acier inox � 0.009 0.292 � 0.024
1% (10,000 ppm) NaF pH = 2.3 NiTi 1.96 � 0.015
Stainless steel/Acier inox � 0.015 � 0.015 0.053
Wiam RERHRHAYE et al.
The Young’s modulus (E) characterizes the stiffness of amaterial [12]. The higher E, the stiffer the material.The results of this study confirmed that E is higher in stainlesssteel, which is therefore stiffer. However, for NiTi archwires,
[(Fig._4)TD$FIG]
Fig. 4: Electron scanning microscope (ESM) image showing developmeFig. 4 : Image du Microscope �electronique a balayage MEB montrant l’ap
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Le module de Young (E) caract�erise la rigidit�e du mat�eriau[12]. Plus il est �elev�e plus le mat�eriau est rigide.Les r�esultats de cette �etude ont confirm�e que E est plus �elev�epour l’acier inoxydable qui est donc plus rigide. Celui-ci
nt of corrosion pits.parition de piqures de corrosion.
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Degradation of the mechanical properties of orthodontic NiTi alloys in the oral environment: An in vitro studyD�egradation des propri�et�es m�ecaniques des alliages orthodontiques NiTi en milieu buccal : �etude in vitro
this value declines significantly in an acidic and fluoridatedmedium.We also demonstrated modified elasticity due to a reduction ofthe elasticity limit (se), which is higher in a fluoridated acidicmedium.Finally, the maximum tensile load (sm) characterizes a mate-rial’s ductility or fragility. This study showed a reduction inthe maximum tensile load (sm) in a fluoridated acidic medium,which points to the more fragile status of the NiTi archwires.
Likewise, we observed a greater release of Ni in the solutionswith stronger fluoride and acidity concentrations. The releaseof these ions is the result of sample corrosion in the immersionmedia [6].ESM observations revealed the presence of corrosion pitting,particularly in the acidic fluoridated media.
Orthodontists should be aware, therefore, that fluoridated pro-phylactic agents impact the mechanical and biological prop-erties of NiTi wires.Consequently, it is advisable to adapt the classic protocol forthe prevention of carious white spots in patients wearingorthodontic appliances.Good oral hygiene should be taught in order to promotemechanical elimination of bacterial plaque. Also, during theinitial treatment stages which last from 6 to 8 months andwhich involve the use of NiTi archwires, mouth rinses shouldbe prescribed with a low fluoride content or a chlorhexidinebase.Lastly, during the final treatment stages involving alloys otherthan stainless steel, it is advisable to return to the classicprescription with an appropriate fluoride dosage, notablywhen there is a high risk of caries [13].
Conclusion
Nickel titanium orthodontic alloys corrode and undergo deg-radation of their mechanical properties in a fluoridated acidicenvironment. Orthodontists need to be aware that fluoridatedprophylactic agents have an impact on the mechanical andbiological properties of NiTi archwires.Consequently, the classic protocol for the prevention of cari-ous white spots in patients wearing appliances needs to beadapted.
Disclosure of interest
The authors declare that they have no conflicts of interestconcerning this article.
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diminue significativement en milieu acide et fluor�e pour lesarcs en NiTi.Nous avons �egalement mis en �evidence une alt�eration del’�elasticit�e par la diminution de la limite �elastique (se) qui estplus importante en milieu acide fluor�e.La chargemaximale a la rupture (sm), quant a elle, caract�erisela ductilit�e ou la fragilit�e du mat�eriau. Cette �etude a permis demettre en �evidence une diminution de la charge maximale a larupture (sm) en milieu acide fluor�e, ce qui est en faveur d’unefragilisation des arcs en NiTi.De meme, on a not�e une lib�eration plus importante du Ni dansles solutions avec l’augmentation de la concentration en fluoret de l’acidit�e. Cette lib�eration d’ions est la cons�equence de lacorrosion des �echantillons dans les milieux d’immersion [6].L’observation au MEB a permis de montrer la pr�esence depiqures de corrosion avec une densification en milieu acidefluor�e.L’orthodontiste doit donc savoir que les agents prophylac-tiques fluor�es affectent les propri�et�es m�ecaniques et biologi-ques des fils en NiTi.De ce fait, une adaptation du protocole classique de pr�even-tion des taches blanches carieuses chez les porteurs d’appa-reils orthodontiques peut etre conseill�ee.Une bonne �education a l’hygi�ene buccodentaire afin de favori-ser l’�elimination m�ecanique de la plaque bact�erienne ; la pre-scription de bains de bouche a faible teneur en fluor ou a basede chlorhexidine durant les premi�eres phases du traitement(qui peuvent durer 6 a 8 mois) caract�eris�ees par l’utilisationdes fils en NiTi.Enfin un retour a la prescription classique de fluor a doseadapt�ee, surtout en pr�esence d’un risque carieux �elev�e, aucours des derni�eres phases th�erapeutiques utilisant d’autresalliages comme l’acier inoxydable [13].
Conclusion
Les alliages orthodontiques nickel titane se corrodent et sub-issent une diminution de leurs propri�et�esm�ecaniques dans unmilieu acide fluor�e. L’orthodontiste doit etre conscient que lesagents prophylactiques fluor�es affectent les propri�et�esm�ecaniques et biologiques des fils en NiTi.De ce fait, une adaptation du protocole classique de pr�even-tion des taches blanches carieuses chez les porteurs d’appa-reils est souhaitable.
D�eclaration d’int�erets
Les auteurs d�eclarent ne pas avoir de conflits d’int�erets enrelation avec cet article.
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