Acta of Bioengineering and Biomechanics Original paperVol. 14, No. 3, 2012 DOI: 10.5277/abb120311

Scanning electron microscope appearances of frettingin the fixed orthodontic appliances

LESZEK KLIMEK1,2, ALEKSANDRA PALATYŃSKA-ULATOWSKA3*

1 Institute of Materials Science and Technology, Technical University of Łódź, Poland.2 Restorative Dentistry, Division of Dental Techniques, Medical University of Łódź, Poland.

3 Conservative Dentistry and Endodontics, Department of Endodontics, Medical University of Łódź, Poland.

Fretting is one of the types of a tribologic wear. It is a process that occurs at a nominally immovable junction of elements. In den-tistry, such elements can be brackets and archwires of fixed orthodontic appliances. They meet all the criteria for fretting to occur, i.e.,they are nominally immovable, are subjected to initial loadings and they work in aggressive environment. Chrome-nickel stainless steelElite Opti-Mim brackets (Trachem) working in vivo in oral cavity environment in combination with NiTi and stainless steel archwireswere investigated. Scanning electron micrographs of bracket’s working surfaces showed the presence of fretting damaged areas. Thisresearch also confirmed that there were almost all the types of fretting wear on the elements of the orthodontic appliances under exami-nation.

Key words: fixed orthodontic appliances, fretting, scanning electron microscope

1. Introduction

Fretting is considered as a specific type of tribo-logic wear. It comprises a group of mechanical, ther-mal, chemical or electric phenomena of micro-frictionwear at loaded and nominally immovable junctions.Fretting damage was observed not only in the ele-ments of a mechanical device and buildings, but alsoin an orthopedic implants [1] and elements of artificialheart [2]. These types of wearing can often have dis-astrous characteristics. Adjacent, nominally immov-able surfaces contact only at peaks of roughness.Therefore, only a small fraction of nominal surface isa real contact surface. In contacts of roughness twotypes of areas can be distinguished: adhering areas,which are subjected to elastic deformations, and micro-displaced ones. Contact loadings with the normalforce as well as with the tangential force can be a con-sequence of small contact surface displacements as

a result of external cyclic loadings. Fretting occursalso during vibrations of the construction itself. De-pending on the various loading conditions and ex-ploitation, HOEPNER [3] differentiates between“fatigue by fretting” and “ wearing by fretting”.

The occurrence of adhering and microsliding areasis directly connected with two different mechanismsof wearing. Adhering area is subjected to crackingsimilar to a contact fatigue, while microsliding area issimilar to adhesive wearing [4]. In both mechanisms,the formation of wearing products is observed. Highhardness of those products works as an eraser andaccelerates wearing process, especially in its finaldisastrous phase. A pure form of fretting (withoutchemical changes of surfaces) occurs very rarely andonly in the case of precious metal contacts, in highvacuum or in inert atmosphere. A contact with anactive environment causes fretting corrosion, whichinduces earlier and more intensive wearing changes.Fretting corrosion most often occurs when pairs of

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* Corresponding author: Aleksandra Palatyńska-Ulatowska, Department of Endodontics, Medical University of Łódź, 92-217 Łódź,ul. Pomorska 251, Poland. Tel./fax.: +48 42 6757418, e-mail: aleksandra.palatynska-ulatowska@umed.lodz.pl

Received: January 21st, 2011Accepted for publication: June 26th, 2012

L. KLIMEK, A. PALATYŃSKA-ULATOWSKA80

elements work under some strain. Tension increasessurface energy and enhances surface chemical reac-tivity [5]. In the case of fretting corrosion many prod-ucts of wear, especially metal oxides, are formed.There can be oxides removed from the surface orformed because of the oxidation of the ground metalelements. The intensity of fretting depends on the kindand magnitude of forces directed to the junction, valueof strains as well as aggressiveness of an environment.

In dentistry, junctions exposed to fretting include,among others, orthodontic fixed appliance elements.They are exposed to initial loadings. Despite nominaljunction immovability there are some small movementsof elements, for example, during mastication. Moreover,they work in a very aggressive oral cavity environment.In the appliances discussed, the places especially ex-posed to fretting are the inner surfaces of bracket slotsbeing in contact with archwires (figure 1).

a)

b)

Fig. 1. Bracket – archwire structure: (a) general view,(b) magnification of nominally immovable contact surface area

The objective of this study was to investigate in the36 elements of four separate fixed orthodontic appli-ances the presence of wear qualified as fretting.

2. Materials and methods

The material consisted of 36 Elite Opti-Mimbrackets (Trachem) taken out of four orthodonticfixed appliances cemented in the patients’ oral cavity.

Brackets were exposed to aggressive environment fortwo months.

Before the examination, half of each bracket wascut along its longitudinal axis for better visibility ofsurfaces. A scanning electron microscope (SEM)HITACHI S-3000N was used to investigate the sam-ples. Magnifications from over a dozen times to sev-eral thousand times were used. For comparison, sev-eral new, unused brackets were investigated. Thechemical composition analysis of the bracket materialwas conducted in SEM using the EDS method. En-ergy Dispersive Spectroscopy (EDS) is a method ofquantifying the elemental constituents of a sample bymeasuring with a scanning electron microscope thenumber of X-rays produced by a solid sample whenirradiated by electrons versus the energy of thoseX-rays. Energy analysis of characteristic X-ray radia-tion induced by electron beam in sample under inves-tigation was performed at electron accelerating volt-age of around 15 kV.

3. Results

The results of our investigation comprise images ofbracket surfaces obtained in scanning electron micro-scope (figures 2 and 3), chemical analysis of the bracketmaterial (table 1) and EDS spectrogram from the brack-

a)

b)

Fig. 2. A scanning electron microscope appearancesof the working surfaces of a new, unused bracket

Scanning electron microscope appearances of fretting in the fixed orthodontic appliances 81

ets (figure 4). Images presented a significant differencebetween the new elements and the used ones (comparefigures 2 and 3). The surface of new brackets was quitesmooth and only traces of mechanical processing duringtheir formation were visible (figure 2). However, usedelements showed explicit marks of wear, which wereobserved as crumblings of various sizes and cracks initi-

ated crumbling of the material (figure 3c). In the bottomof some defects (figure 3e, f) products of corrosion wereobserved, which may give evidence of a complicatedmechanism of wear – fretting and fretting corrosion.

The results of chemical analysis indicated thatelements of orthodontic appliances investigated weremade of stainless steel.

a) b)

c) d)

e) f)

Fig. 3. Examples of wear on orthodontic bracket working surfaces.(Description in the text)

Table 1. Chemical analysis of the bracket material

Chemical composition of alloy examinedAl Si Cr Ni Fe

% mass. % weight % mass. % weight % mass. % weight % mass. % weight % mass. % weight0.47 0.23 1.54 0.8 23.32 22.27 2.40 2.58 rest rest

L. KLIMEK, A. PALATYŃSKA-ULATOWSKA82

4. Discussion

So far in the literature, only the occurrence of fric-tion forces between orthodontic brackets and archwireswas mainly discussed [6]–[8]. The majority of articlesin this field were reported by KUSY et al. [8]–[11].Researchers conducted their investigations in the ex-perimental dry, water, and human or artificial salivaenvironment [11] and did not mention wear by frettingin those elements. Only the friction movement modelin experimental environments was observed, withoutother wear types of bracket slot surfaces, working invivo in the patients’ oral cavity.

Fretting destruction on bracket working surfaces,observed in this study, is manifested by the loss ofmaterial. There are distinct differences between sur-faces of a new and working in vivo brackets (com-pare figures 2 and 3). In most cases (figure 3(a)–(d))there are material defects of significant size reachingseveral dozen micrometers. Various sizes of the de-fects observed (compare figures 3(a) and 3(f)) areprobably due to the observation of the differentphases of wear. Also various characteristics of suchdestruction seem to be different. There are mostlycracks of the material. Cracks found in this area (fig-ure 3(c)) can provide evidence of the material fatigue(contact fatigue). Some fragments of destruction seenin figure 3(e) are similar to material pulling out, be-ing an effect of adhesive wear. Lack of the cracks inthis case might be a confirmation of this kind of de-struction.

Corrosion, as an electro-chemical reaction be-tween metal and its environment, intensifies throughthe inner strains of the metal, non-homogenous alloystructure and the contacts with other metals in the oralcavity environment (bracket – archwire) with the for-mation of the corrosion products. In particular, nickel

ion releasing during stainless steel corrosion is clini-cally important. The presence of the metal oxides inthe oral cavity can lead to different allergic reactions.Figures 3(b) and 3(f) show that there are places indefect area where oxides can be observed. Bracketsare made of Fe–Cr alloy, and that is why chromiumoxides should rather appear because of their betterchemical affinity to oxygen than the ion of iron. De-spite the fact that the scanning electron microscope inthis research was equipped with the X-ray micro-analysis detector it was impossible to prove the pres-ence of these oxides on the surfaces observed. Thedifference between the values of characteristic radia-tion energy Lα for chrome and Kα for oxygen wasbelow the detector’s distribution capability. There isno possibility to differentiate an element from whichthe signal comes. The presence of these oxides con-firms fretting corrosion in the elements of orthodonticappliances being observed. No signs of abrasive wearwith corrosion products or other products of wearwere observed.

An orthodontic appliance fastened to the enamelworks through activated archwires inserted and fixedin the bracket slots. Orthodontic brackets, different foreach tooth, with a defined torque and angulation givea desirable tooth position in the dental arch. Wear byfretting on the bracket slot surfaces can probably in-fluence the magnitude and direction of the forcestransferred to teeth by brackets. This research provesthat there is fretting and there are almost all types offretting wear on the elements of the orthodontic appli-ances under examination. ARTICOLO [12] in his re-search put forward a theory of notch formation, inwhich a vertical movement from tooth or wire duringmastication caused fretting wear, and horizontalmovement during orthodontic procedures such asspace closure, tipping or bodily movement causedsliding wear. Undoubtedly, there is a necessity to

Fig. 4. EDS spectrogram from the bracket material

Scanning electron microscope appearances of fretting in the fixed orthodontic appliances 83

continue further research to establish the influencethis kind of bracket surface wear can have on the du-ration and the final result of a treatment with fixedorthodontic appliances.

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