ORIGINAL ARTICLE

Biodegradation of orthodontic metallic bracketsand associated implications for friction

Saulo Regis Jr,a Paulo Soares,b Elisa S. Camargo,c Odilon Guariza Filho,c Orlando Tanaka,d

and Hiroshi Maruoc

Curitiba, Paran�a, Brazil

FromaPostgbAssocAssodProfeThe aucts oReprinPontiParanSubm0889-Copyrdoi:10

Introduction: This study aimed to assess the effect of clinical exposure on the surfacemorphology, dimensions,and frictional behavior of metallic orthodontic brackets.Methods: Ninety-five brackets, of 3 commercial brands,were retrieved from patients who had finished orthodontic treatment. As-received brackets, matched by type andbrand, were used for comparisons. Surface morphology and precipitated material were analyzed by optical andscanning electron microscopy and x-ray microanalysis. Bracket dimensions were measured with a measuringmicroscope. Resistance to sliding on a stainless steel wire was assessed. Results: Retrieved brackets showedsurface alterations from corrosion, wear, and plastic deformation, especially in the external slot edges. Film depo-sition over the alloy surfacewas observed to a variable extent. Themain elements in the filmwere carbon, oxygen,calcium, andphosphorus. Theas-receivedbrackets showed differences (P\0.05) in the slot sizes amongbrands,and 1 brand showed a 3% increase in the retrieved brackets’ slots. The frictional behavior differed among brands.Retrieved brackets of 2 brands showed 10% to 20% increases in resistance to sliding. Conclusions: Metallicbrackets undergo significant degradation during orthodontic treatment, possibly with increased friction. At pres-ent, it is difficult to predict the impact of these changes on the clinical performance of orthodontic components.(Am J Orthod Dentofacial Orthop 2011;140:501-9)

The degradation of metallic materials placed inpatients has long been a concern of biomaterialsscience. Laboratory experiments that simulate

in-vivo degradation of metal implants have made it pos-sible to estimate the effect of specific parameters but lackthe consistency required to represent the complexity ofthe oral environment.1-5 Variations in temperature andpH caused by diet, decomposition of foods and celldebris, and oral florae and their by-products are impor-tant factors to consider when evaluating the clinicalbehavior of dental materials and comparing them withother biomaterials. In particular, orthodontic accessoriesare under masticatory forces and multi-axial loads fromthe activation of the wire in the bracket slot.1

the Pontifical Catholic University of Paran�a, Curitiba, Paran�a, Brazil.raduate student, Graduate Dentistry Program in Orthodontics.ciate professor, Mechanical Engineering Department.ciate professor, Graduate Dentistry Program in Orthodontics.ssor, Graduate Dentistry Program in Orthodontics.uthors report no commercial, proprietary, or financial interest in the prod-r companies described in this article.t requests to: Hiroshi Maruo, Graduate Dentistry Program in Orthodontics,fical Catholic University of Paran�a, R. Imaculada Conceic~ao, 1155, Curitiba,�a, 80215-901, Brazil; e-mail, h.maruo@gmail.com.itted, May 2010; revised and accepted, January 2011.5406/$36.00ight � 2011 by the American Association of Orthodontists..1016/j.ajodo.2011.01.023

In-vivo aged orthodontic components show signs ofdegradation such as morphologic changes and surfacealterations from corrosion, wear, and formation of integu-ments.4,6-12 Concerns regarding the clinical impact ofthese alterations include (1) elements released into the oralenvironment and their implications on biocompatibility,and (2) impairment of the performance of the orthodonticappliance.13

Orthodontic metallic materials are usually composedof alloys including several base metals such as nickel,chromium, cobalt, iron, molybdenum, and titanium.Concerning biocompatibility, nickel stands out amongthe other elements. Carcinogenic,14 mutagenic,15 andcytotoxic properties16,17 have been attributed to it,although no local or systemic clinical condition isclearly related to the forms of nickel used in dentistry.13

The lone exception is hypersensitivity reactions.18 Nickelmight also elicit periodontal reactions such as gingivalhyperplasia in orthodontic patients. This condition ishardly distinguishable from microbial-induced gingivalovergrowth.19

Surface alterations in orthodontic devices might com-promise the appliance’s esthetics,20 increase microbialadhesion,11,21modify bracket-wire activations such as tor-que expression,22 cause fractures during clinical use,23 andinfluence the magnitude of friction between the bracketand the wire.1,20,24 Many situations in an orthodontic

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practice require sliding teeth through a properly contouredarchwire, and friction might cause significant loss of theapplied force.10,24 Many investigations have focused onthe frictional behavior of as-received orthodontic mate-rials.24-28 Nevertheless, the alterations that orthodonticbrackets undergo during treatment and their impact onclinical performance, especially friction, are mostlyunknown. Therefore, we aimed to evaluate the surfacemorphology, dimensional stability, and frictionalbehavior of orthodontic metallic brackets retrieved afterfull orthodontic treatment, compared with as-receivedbrackets from the manufacturers.

MATERIAL AND METHODS

The sample consisted of brackets with a slot size of0.559 3 0.711 mm (0.022 3 0.028 in), as described bythe manufacturer, retrieved from patients who hadfinished orthodontic treatment in the private practice of2 experienced orthodontists (E.S.C., O.T.). The bracketswere carefully debonded with a direct bond bracketremover pliers and kept in receptacles with distilledwater.They were brushed with an electric brush for 10 secondsand rinsed with distilled water to remove any loosely at-tached integuments. They were then kept in self-sealedsterilizing packs until analysis. The following informationwas registered: patient identification, date of placementand date of removal of the appliance. Brackets with obvi-ous distortions or calcifications that hindered the en-gagement of a 0.546 3 0.711-mm (0.0215 3 0.028 in)cross-sectional wire were discarded.

A total of 95 brackets of different types (for premolars,canines, and incisors fromboth arches) were selected. Thesample comprised 3 brands: 32 Mini Standard Edgewisestainless steel brackets (American Orthodontics, Sheboy-gan, Wis), 34 Kirium Edgewise stainless steel brackets(3M Abzil, Sumar�e, Brazil), and 29 NuEdge preadjustedRoth prescription brackets (TP Orthodontics, LaPorte,Ind) made of copper-chromium alloy. The bracketswere retrieved from 7 patients (mean age, 18 years 9months), with a mean treatment time of 41 months.Stainless steel and nickel-titanium wires used in theorthodontic treatments were ligated with elastomericand metallic ligatures.

A group of brackets as-received from the manufac-turers, matched by types and brands to the sample,was used for comparisons. They were submitted to thesame procedures as the retrieved specimens.

An optical reflected light microscope (BX60; OlympusOptical, Tokyo, Japan) was used to evaluate the surfacemorphology of the as-received and the retrieved bracketsfor areas of corrosion, signs of wear, plastic deforma-tions, and adherent materials. The images were acquiredin a bright field at variousmagnifications (50-200 times).

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Fifteen retrieved brackets, equally distributed amongbrands, and their as-received counterparts were selectedbased on reflected light images for analysis in a scanningelectron microscope (JSM-6360LV; Jeol, Tokyo, Japan)and an energy dispersive x-ray spectrometer. Secondaryelectron images and backscattered electron images wereacquired at various magnifications (20-2000 times) byusing a 20-kV accelerating voltage. The images allowedfor assessment of the micromorphologic characteristicsof the slot surfaces. Areas of interest were submittedto microanalysis for element assessment. Spectra wereacquired with the same accelerating voltage at differentmagnifications and a 100-second acquisition time.

The slot sizes (left and right, vertical dimension) andthe internal widths between the tie-wings (cervical andocclusal, horizontal dimension) of the remaining 80retrieved brackets and their as-received counterpartswere measured in a measuring microscope (MM-40;Nikon, Yokohama, Japan). Measurements were madeby 1 operator using a holder with a wire of a cross-section of 0.546 3 0.711 mm (0.0215 3 0.028 in).The wire was used to position the brackets’ slots perpen-dicular to the microscope table. To evaluate the methoderror, the measurements of 12 specimens were repeatedafter a 1-week interval. No statistically significant differ-ence was found between repeated measurements ac-cording to a paired-samples t test (P 5 0.69).

The sliding resistance analysis with stainless steelwires was conducted on both as-received and retrievedbrackets in a universal testing machine (DL-500; EMIC,S~ao Jos�e dos Pinhais, Brazil), with a device especiallydesigned for this experiment (Fig 1). Test specimenswere obtained by bonding the brackets with a cyanoacry-late adhesive to a 4 3 15 3 50-mm acrylic plate witha holder in a standardized way. This guaranteed thatthe bracket slots stayed parallel to the testing machine’svertical axis. Stainless steel wire segments (Shiny Bright,TP Orthodontics) with a cross-section of 0.48263 0.635mm (0.019 3 0.025 in) and a length of 11.5 cm wereused. The wires were cleaned with 70% alcohol. Theywere ligated in their middle portion to the bracketswith 3.0-mm elastomeric ligatures (Mini Stix noncoated,TP Orthodontics) immediately before the test to stan-dardize the ligation force. Each segment was used onlyonce.

Test specimens were mounted in the device andassembled in the test machine. A 300 g weight wasattached to the lower extremity of the wire to keep itunder tension. The wire was pulled along the bracket ata rate of 5 mm per minute for 1 minute. The force levelswere registered by a 10 kgf load cell. The sliding resis-tance was calculated by averaging the forces registeredbetween the first and fifth millimeters of displacement,

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Fig 2. Optical microscopy images (bright field, 200 timesmagnification) of the slots of: A, NuEdge; B, Mini Stan-dard Edgewise; and C, Kirium retrieved brackets. Theseimages illustrate the wear and plastic deformation (ar-rows) of the external slot edges.

Fig 1. Device with test specimen mounted for the slidingresistance test, seen with greater magnification in detail.

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disregarding the initial static friction. Since the retrievedbrackets were of different types, even for the same brand,it was of no use to consider the mean sliding resistancefor each brand, but only the differences betweenretrieved and as-received brackets. Formeans of compar-ison among the different brands, the percentage differ-ence in the sliding resistance between retrieved andas-received brackets was calculated with the followingequation: DifSR(%) 5 [(SRretrieved � SRas-received)/SRas-received]3 100, whereDifSR is the percentage dif-ference in sliding resistance, and SR is sliding resistance.

Statistical analysis

Kolmogorov-Smirnov and Levene tests were used toevaluate the normality of data distribution and the homo-geneity of variance, respectively. Only the tie-wings’widths and the percentage differences in the sliding resis-tance variables had nonnormal distributions. Only theas-received brackets’ sliding resistance had a homoge-neous variance. The effect of usage on the tie-wings’widths was investigated with the Mann-Whitney test.Slot dimensions were compared among brands and usage

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conditions (as-received or retrieved) with analysis of var-iance (ANOVA) and the Games-Howell post-hoc test. TheKruskall-Wallis multiple comparisons test was used toevaluate differences among brands and patients in thepercentage difference in the sliding resistance. A paired-samples t test was used to investigate the effect of usageon sliding resistance for each brand in a 2-tailed model.The level of significance for all tests was set at a5 0.05.

RESULTS

Surface modifications with signs of corrosion, plasticdeformation, and wear were seen in the retrievedbrackets (Fig 2) when compared with the as-receivedbrackets during optical microscopic evaluation. Wear

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Fig 3. Scanning electron microscope images of Kirium brackets: A, slot of an as-received bracket(backscattered electron image, 100-timesmagnification);B, slot of a retrieved bracket (secondary elec-tron image, 100-times magnification);C, in the retrieved brackets, there were smooth milling grooves inthe lateral walls of the slot (backscattered electron image, 100-times magnification); D, delaminationdebris on a retrieved bracket (secondary electron image, 300-times magnification); E, a precipitateof a retrieved bracket containing silica and barium (backscattered electron image, 300-timesmagnification).

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and deformation were especially seen on the externaledges of the slot bottom surface. The retrieved speci-mens had film deposition and islands of aggregatedmaterials to a variable extent. The variability occurredeven with brackets removed from the same patient,with no individual pattern in the amount of integumentformed. Mini Standard Edgewise retrieved brackets’ slotsvaried in areas with pits and film deposition or areas withextensive signs of wear and deformation. This last pat-tern was more frequent in retrieved Kirium brackets.NuEdge brackets had a rough surface, with pits andcrevices in a parallel arrangement, in both as-receivedand retrieved specimens. These originally roughenedsurfaces, a coating applied by the manufacturer, andthe presence of integuments made it difficult to evaluatespecific usage-related alterations in NuEdge brackets.

The same morphologic patterns were seen in greaterdetail in the scanning electron microscope images(Figs 3-5). The precipitated film exhibited a dark phasein the backscattered electron images, contrasting withthe bright phase of the brackets’ alloy. This findingsuggested the presence of elements with a low atomicnumber in the film. Carbon, oxygen, calcium, andphosphorus were the main elements detected in the filmby energy dispersive x-ray spectrometry microanalysis.

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However, nitrogen, sulfur, sodium, potassium, andaluminum were eventually found. In some areas, theprecipitates masked the topographic features of theunderlying alloy surface (Fig 6). Greater magnificationsshowed details of the arrangement of pits and crevicesin the NuEdge specimens (Fig 5, D and E). The coatingapplied by the manufacturer in the as-received NuEdgebrackets was seen in backscattered electron images asa dark-phased, noncontinuous pellicle. The same distri-bution was not seen in the retrieved specimens, in whichthe integuments were limited to slots and gaps. Theoriginal coating was visually indistinguishable from thebiofilm formed during oral exposure. Some materials ofatypical composition were found in retrieved brackets,including a 150-mm precipitate containing silica, alumi-num, barium, iron, carbon, and oxygen (Fig 3, E). Alsoincluded were silver and silica incrustations that hada brighter phase than the cobalt-chromium alloy (Fig 6).

A comparison between as-received and retrievedbrackets found no significant difference (P .0.05) inthe tie-wings’ internal widths (Table I). Table II pres-ents the slot dimensions according to usage andbrands. NuEdge brackets had significantly greaterslot dimensions than the other brands (P \0.05). Dif-ferences between retrieved and as-received brackets

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Fig 4. Scanning electron microscope images of Mini Standard Edgewise brackets: A, slot of an as-received bracket (backscattered electron image, 100-timesmagnification);B, slot of a retrieved bracket(backscattered electron image, 100-times magnification); C, slot lateral wall of a retrieved bracket, withplastic deformation (bright arrow) and film deposition (backscattered electron image, 300-times mag-nification); D, islands of precipitates (300 times magnification) and E, pitting corrosion (dark arrows;100-times magnification) after intraoral exposure.

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were seen only for the right slot of the Kirium brackets(P \0.05).

Sliding resistance, evaluated separately for each brandaccording to usage condition by t tests in a 2-tailedmodel, had P values of 0.054, 0.103, and 0.08 forNuEdge, Mini Standard Edgewise, and Kirium, respec-tively. Since various bracket types were tested (premolar,canine, and incisor for both arches), it was not viable tocalculate the sliding resistance means and comparethem among brands. However, the percentage differencein the sliding resistance, as a ratio of sliding resistance al-teration between retrieved and as-received brackets,could be compared amongbrands. The percentagediffer-ence in the sliding resistance had mean values of 17.99%(SD, 36.50%) for NuEdge brackets, 13.62% (SD, 34.26%)for Kirium, and�3.10% (SD, 31.82%) for Mini StandardEdgewise. Differences were statistically significant be-tween Mini Standard Edgewise and the other brands(P \0.05). No significant differences were found forthe percentage difference in the sliding resistance be-tween patients (P5 0.061).

DISCUSSION

Microscopic analysis suggested that the orthodonticbrackets underwent significant alterations in variable

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ways during treatment. Retrieved orthodontic acces-sories have shown diverse behaviors in the literature:some had significant alterations,4,6,9-12,29 and othersshowed no differences from as-received.7,8,29-32

These conflicting results might be attributed to thediversity of the materials analyzed (brackets, archwires,headgear components), the time of exposure to theoral environment, and the patient’s characteristics.Additionaly, corrosion susceptibility is influenced bythe composition of the alloy, its microstructure,manufacturing procedures and their impact on theinternal stress of the material, and the formation ofa surface passivation film.33 Since the brackets evaluatedin this study had different compositions and brands, dis-similarities in their surface morphology after orthodontictreatment could be related to each alloy’s tribological,physical, and electrochemical properties, and tomanufacturing procedures.

The precipitated film over the retrieved brackets’ sur-faces was also found on orthodontic wires9,30 andheadgear components.10 The overlap of the calcium andphosphorus distributions (Fig 6) is consistentwith crystal-line particle formation. This finding was also described byEliades et al.9,10 These authors additionally examined themolecular composition of the precipitated film. Theyfound amide, alcohol, and carbonate as the main

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Fig 5. Scanning electron microscope images of NuEdge brackets: A, slot of an as-received bracket(backscattered electron image, 100-times magnification); B, slot of a retrieved bracket (backscatteredelectron image, 100-times magnification); C, as-received bracket slot with deformation (secondaryelectron image, 100-times magnification); D, internal edge between the slot and the middle portionof the bracket, showing traces of the mechanical action in the slot (left) in contrast to the rough surface(right) that does not come into contact with the wires (backscattered electron image, 1000-times mag-nification); E, retrieved bracket slot surface showing the linear distribution of crevices typically seen inNuEdge brackets. Also seen is an area with signs of wear and plastic deformation (secondary electronimage, 300-times magnification).

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organic constituents. They also identified the presence ofchlorine, potassium, and sodium in a uniformdistribution. Such findings suggest the formation ofa biofilm composed of a proteinaceous matrix andscattered crystalline particles. The differing properties ofthe bracket materials and differing retrieval protocolsmight account for the dissimilarities in the integumentsfound in these studies.The cleansing procedures weused were intended to simulate a patient’s dentalhygiene and to remove loosely attached material. Theprecipitate containing silica and barium might beattributed to slot contamination by composite resins oradhesives, which have similar compositions.34 Althoughmass transfer can occur with the sliding of a metallicsurface, the silver-containing incrustations found onretrieved brackets (Fig 6) could not be attributed to anyclinical findings.26 Calcium and aluminum inclusionsfound on biomedically applied materials were stronglyrelated to corrosion. Other sources of inclusions couldbe raw-material impurities and inclusions not dissolvedin the melt.35

The new brackets’ slot dimensions varied amongbrands, with a peak of 0.065 mm (0.0026 in; 12%)

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beyond the nominal value in the NuEdge specimens.When evaluating the same dimensions, Oh et al25 alsofound differences among brands, although of smallermagnitudes (0.018 mm). The evaluated brackets had ac-ceptable dimensional stability during orthodontic treat-ment. Only the Kirium retrieved brackets had significantdifferences compared with their as-received counter-parts. Specifically, there was a 0.018-mm (0.0007 in;3%) increase in a slot size. These findings agree withthose of Fischer-Brandies et al,36 who found indenta-tions and a 0.016-mm slot widening on stainless steelbrackets tested with wire torque activations. They attrib-uted the results to the low stiffness of the bracket mate-rial compared with the wires. The samemechanism couldbe related to the wear and deformation of the slot edgesof the retrieved brackets in this study. Alterations causedby debonding were minimized by using a procedure thatapplied force in the bracket base, preserving the area ofinterest. Considering the slot variations among brands,the observed changes in used Kirium brackets mightnot have clinical significance.

These results did not allow the rejection of the nullhypothesis of no difference in sliding resistance between

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Fig 6. MEV and energy dispersive x-ray spectrometry images of a NuEdge bracket with film depositionand silver-containing particles: A, (100-times magnification) backscattered electron images of the slot,where silver particles appear as the brighter phase; B and C, backscattered electron and secondaryelectron images, respectively, of the marked area in A with greater magnification (1000 times); D, en-ergy dispersive x-ray spectrometry map of cobalt, with the bright dots indicating that element, and chro-mium had a similar distribution; E, energy dispersive x-ray spectrometry map of calcium demonstratinga greater concentration in the area corresponding to the precipitated film images inB andC,with phos-phorus having a similar distribution; F, energy dispersive x-ray spectrometry map of silver. Molybde-num and silica had more uniform distributions, although they were concentrated in the same areasas the silver.

Table I. Comparison of the internal horizontal widthsof the tie-wings of as-received and retrieved brackets

Mean SD Significance*Cervical tie-wings internal width (mm)As-received 1.139 0.288 0.166Retrieved 1.168 0.302

Occlusal tie-wings internal width (mm)As-received 1.143 0.282 0.289Retrieved 1.165 0.298

*Mann-Whitney test results.

Table II. Heights (mm) of right and left slots accordingto brand and use

n Mean SDHeight of left slotNU as-received 24 0.618a 0.023NU retrieved 24 0.631a 0.032MS as-received 27 0.568b 0.007MS retrieved 27 0.577b 0.014KR as-received 29 0.581b,c 0.017KR retrieved 29 0.595c,d 0.020

Height of right slotNU as-received 24 0.627a 0.018NU retrieved 24 0.633a 0.041MS as-received 27 0.566b 0.007MS retrieved 27 0.575b 0.013KR as-received 29 0.581b,c 0.016KR retrieved 29 0.599d 0.021

Similar letters identify groups without significant differences(P $0.05), according to the Games-Howell multiple comparisonstest.NU, NuEdge; MS, Mini Standard Edgewise; KR, Kirium.

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as-received and retrieved brackets by using a 2-tailedmodel for each brand separately. However, the percent-age difference in the sliding resistance indicates signifi-cantly different tendencies when comparing Kirium(13.62%; SD, 34.26%) and NuEdge (17.99%; SD,36.50%) brackets with Mini Standard Edgewise brackets(�3.10%; SD, 1.82%). Considering that the percentagedifferences in the sliding resistance results indicate thesliding resistance changes in each brand, a 1-tailedmodelcould be adopted for statistical testing. In this scenario, byusing a t test for paired samples to compare sliding resis-tance values between as-received and retrieved bracketsseparately for each brand, the P values were 0.027 forNuEdge, 0.04 for Kirium, and 0.0515 for Mini StandardEdgewise. Therefore, Kirium and NuEdge brackets hadgreater mean friction, whereas the Mini Standard Edge-wise brackets had no significant alteration.

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Although some authors have hypothesized that anincrease in friction occurs due to surface alter-ations,1,20,24 Berg et al37 suggested a lubricating effectfor salivary pellicles over the sliding surfaces in the oralenvironment. The percentage difference in the slidingresistance variable had great variability for each brand,with high standard devation values. Oh et al25 foundsimilar results in an in-vitro setup, where variations in

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the frictional resistances ranged up to 2.5 times amongbrackets of the same brand. Eliades and Bourauel1 re-ported preliminary results of an ongoing study showingthat retrieved nickel-titanium and stainless steel wiresmight have approximately 10% to 20% increases in fric-tional forces, with great fluctuations over the measureddisplacement. They attributed this finding to the com-plexes precipitated intraorally. Wichelhaus et al38 alsoverified a significant increase in friction of nickel-titanium wires after clinical use. We found no similarevaluations of retrieved brackets.

The frictional test used in this study does not fullyrepresent oral conditions. Second- and third-order incli-nations, dental-arch convexity, binding effects betweenbracket and archwire, and material-related frictionalimplications were not part of our goals. Nevertheless,this study provides evidence of the effects of clinicaluse and associated phenomena on orthodontic compo-nents’ classic friction.

The different behaviors of the brackets we evaluatedraises doubt aboutwhich factors determined the observedalterations. Further investigations with other commercialbrands, standardized treatment mechanics, and con-trolled durations of oral exposure might clarify the effectof specific factors on orthodontic accessories’ frictionduring clinical use. Our results suggest that care mustbe taken when extrapolating conclusions obtained inresearch with as-received materials to long-term clinicalscenarios.

CONCLUSIONS

Clinical use causes surface alterations in metallicorthodontic brackets, with distinct patterns of alterationsfor different brands. Differences in morphology after useare smaller than those found in as-received bracketsamong brands. Distinct frictional behaviors were ob-served for each bracket brand with clinical use. Therewere 10% to 20% increases between retrieved and as-received brackets in NuEdge and Kirium, whereas theMini Standard Edgewise brackets remained unaffected.

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ics October 2011 � Vol 140 � Issue 4