Download Biodegradation of orthodontic metallic brackets and associated

ORIGINAL ARTICLE
Biodegradation of orthodontic metallic brackets
and 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, Parana, Brazil
Introduction: This study aimed to assess the effect of clinical exposure on the surface morphology, 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 and
brand, were used for comparisons. Surface morphology and precipitated material were analyzed by optical and
scanning electron microscopy and x-ray microanalysis. Bracket dimensions were measured with a measuring
microscope. Resistance to sliding on a stainless steel wire was assessed. Results: Retrieved brackets showed
surface alterations from corrosion, wear, and plastic deformation, especially in the external slot edges. Film deposition over the alloy surface was observed to a variable extent. The main elements in the film were carbon, oxygen,
calcium, and phosphorus. The as-received brackets showed differences (P\0.05) in the slot sizes among brands,
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: Metallic
brackets undergo significant degradation during orthodontic treatment, possibly with increased friction. At present, 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)
T
he degradation of metallic materials placed in
patients has long been a concern of biomaterials
science. Laboratory experiments that simulate
in-vivo degradation of metal implants have made it possible to estimate the effect of specific parameters but lack
the consistency required to represent the complexity of
the oral environment.1-5 Variations in temperature and
pH caused by diet, decomposition of foods and cell
debris, and oral florae and their by-products are important factors to consider when evaluating the clinical
behavior of dental materials and comparing them with
other biomaterials. In particular, orthodontic accessories
are under masticatory forces and multi-axial loads from
the activation of the wire in the bracket slot.1
From the Pontifical Catholic University of Parana, Curitiba, Parana, Brazil.
a
Postgraduate student, Graduate Dentistry Program in Orthodontics.
b
Associate professor, Mechanical Engineering Department.
c
Associate professor, Graduate Dentistry Program in Orthodontics.
d
Professor, Graduate Dentistry Program in Orthodontics.
The authors report no commercial, proprietary, or financial interest in the products or companies described in this article.
Reprint requests to: Hiroshi Maruo, Graduate Dentistry Program in Orthodontics,
Pontifical Catholic University of Parana, R. Imaculada Conceiç~ao, 1155, Curitiba,
Parana, 80215-901, Brazil; e-mail, [email protected].
Submitted, May 2010; revised and accepted, January 2011.
0889-5406/$36.00
Copyright Ó 2011 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2011.01.023
In-vivo aged orthodontic components show signs of
degradation such as morphologic changes and surface
alterations from corrosion, wear, and formation of integuments.4,6-12 Concerns regarding the clinical impact of
these alterations include (1) elements released into the oral
environment and their implications on biocompatibility,
and (2) impairment of the performance of the orthodontic
appliance.13
Orthodontic metallic materials are usually composed
of alloys including several base metals such as nickel,
chromium, cobalt, iron, molybdenum, and titanium.
Concerning biocompatibility, nickel stands out among
the other elements. Carcinogenic,14 mutagenic,15 and
cytotoxic properties16,17 have been attributed to it,
although no local or systemic clinical condition is
clearly related to the forms of nickel used in dentistry.13
The lone exception is hypersensitivity reactions.18 Nickel
might also elicit periodontal reactions such as gingival
hyperplasia in orthodontic patients. This condition is
hardly distinguishable from microbial-induced gingival
overgrowth.19
Surface alterations in orthodontic devices might compromise the appliance’s esthetics,20 increase microbial
adhesion,11,21 modify bracket-wire activations such as torque expression,22 cause fractures during clinical use,23 and
influence the magnitude of friction between the bracket
and the wire.1,20,24 Many situations in an orthodontic
501
Regis et al
502
practice require sliding teeth through a properly contoured
archwire, and friction might cause significant loss of the
applied force.10,24 Many investigations have focused on
the frictional behavior of as-received orthodontic materials.24-28 Nevertheless, the alterations that orthodontic
brackets undergo during treatment and their impact on
clinical performance, especially friction, are mostly
unknown. Therefore, we aimed to evaluate the surface
morphology, dimensional stability, and frictional
behavior of orthodontic metallic brackets retrieved after
full orthodontic treatment, compared with as-received
brackets from the manufacturers.
MATERIAL AND METHODS
The sample consisted of brackets with a slot size of
0.559 3 0.711 mm (0.022 3 0.028 in), as described by
the manufacturer, retrieved from patients who had
finished orthodontic treatment in the private practice of
2 experienced orthodontists (E.S.C., O.T.). The brackets
were carefully debonded with a direct bond bracket
remover pliers and kept in receptacles with distilled water.
They were brushed with an electric brush for 10 seconds
and rinsed with distilled water to remove any loosely attached integuments. They were then kept in self-sealed
sterilizing packs until analysis. The following information
was registered: patient identification, date of placement
and date of removal of the appliance. Brackets with obvious distortions or calcifications that hindered the engagement 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 from both arches) were selected. The
sample comprised 3 brands: 32 Mini Standard Edgewise
stainless steel brackets (American Orthodontics, Sheboygan, Wis), 34 Kirium Edgewise stainless steel brackets
(3M Abzil, Sumare, Brazil), and 29 NuEdge preadjusted
Roth prescription brackets (TP Orthodontics, LaPorte,
Ind) made of copper-chromium alloy. The brackets
were retrieved from 7 patients (mean age, 18 years 9
months), with a mean treatment time of 41 months.
Stainless steel and nickel-titanium wires used in the
orthodontic treatments were ligated with elastomeric
and metallic ligatures.
A group of brackets as-received from the manufacturers, matched by types and brands to the sample,
was used for comparisons. They were submitted to the
same procedures as the retrieved specimens.
An optical reflected light microscope (BX60; Olympus
Optical, Tokyo, Japan) was used to evaluate the surface
morphology of the as-received and the retrieved brackets
for areas of corrosion, signs of wear, plastic deformations, and adherent materials. The images were acquired
in a bright field at various magnifications (50-200 times).
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Fifteen retrieved brackets, equally distributed among
brands, and their as-received counterparts were selected
based on reflected light images for analysis in a scanning
electron microscope (JSM-6360LV; Jeol, Tokyo, Japan)
and an energy dispersive x-ray spectrometer. Secondary
electron images and backscattered electron images were
acquired at various magnifications (20-2000 times) by
using a 20-kV accelerating voltage. The images allowed
for assessment of the micromorphologic characteristics
of the slot surfaces. Areas of interest were submitted
to microanalysis for element assessment. Spectra were
acquired with the same accelerating voltage at different
magnifications and a 100-second acquisition time.
The slot sizes (left and right, vertical dimension) and
the internal widths between the tie-wings (cervical and
occlusal, horizontal dimension) of the remaining 80
retrieved brackets and their as-received counterparts
were measured in a measuring microscope (MM-40;
Nikon, Yokohama, Japan). Measurements were made
by 1 operator using a holder with a wire of a crosssection of 0.546 3 0.711 mm (0.0215 3 0.028 in).
The wire was used to position the brackets’ slots perpendicular to the microscope table. To evaluate the method
error, the measurements of 12 specimens were repeated
after a 1-week interval. No statistically significant difference was found between repeated measurements according to a paired-samples t test (P 5 0.69).
The sliding resistance analysis with stainless steel
wires was conducted on both as-received and retrieved
brackets in a universal testing machine (DL-500; EMIC,
S~ao Jose dos Pinhais, Brazil), with a device especially
designed for this experiment (Fig 1). Test specimens
were obtained by bonding the brackets with a cyanoacrylate adhesive to a 4 3 15 3 50-mm acrylic plate with
a holder in a standardized way. This guaranteed that
the bracket slots stayed parallel to the testing machine’s
vertical axis. Stainless steel wire segments (Shiny Bright,
TP Orthodontics) with a cross-section of 0.4826 3 0.635
mm (0.019 3 0.025 in) and a length of 11.5 cm were
used. The wires were cleaned with 70% alcohol. They
were ligated in their middle portion to the brackets
with 3.0-mm elastomeric ligatures (Mini Stix noncoated,
TP Orthodontics) immediately before the test to standardize the ligation force. Each segment was used only
once.
Test specimens were mounted in the device and
assembled in the test machine. A 300 g weight was
attached to the lower extremity of the wire to keep it
under tension. The wire was pulled along the bracket at
a rate of 5 mm per minute for 1 minute. The force levels
were registered by a 10 kgf load cell. The sliding resistance was calculated by averaging the forces registered
between the first and fifth millimeters of displacement,
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Regis et al
503
Fig 1. Device with test specimen mounted for the sliding
resistance test, seen with greater magnification in detail.
disregarding the initial static friction. Since the retrieved
brackets were of different types, even for the same brand,
it was of no use to consider the mean sliding resistance
for each brand, but only the differences between
retrieved and as-received brackets. For means of comparison among the different brands, the percentage difference in the sliding resistance between retrieved and
as-received brackets was calculated with the following
equation: DifSR(%) 5 [(SRretrieved
SRas-received)/
SRas-received] 3 100, where DifSR is the percentage difference in sliding resistance, and SR is sliding resistance.
Statistical analysis
Kolmogorov-Smirnov and Levene tests were used to
evaluate the normality of data distribution and the homogeneity of variance, respectively. Only the tie-wings’
widths and the percentage differences in the sliding resistance variables had nonnormal distributions. Only the
as-received brackets’ sliding resistance had a homogeneous 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
Fig 2. Optical microscopy images (bright field, 200 times
magnification) of the slots of: A, NuEdge; B, Mini Standard Edgewise; and C, Kirium retrieved brackets. These
images illustrate the wear and plastic deformation (arrows) of the external slot edges.
conditions (as-received or retrieved) with analysis of variance (ANOVA) and the Games-Howell post-hoc test. The
Kruskall-Wallis multiple comparisons test was used to
evaluate differences among brands and patients in the
percentage difference in the sliding resistance. A pairedsamples t test was used to investigate the effect of usage
on sliding resistance for each brand in a 2-tailed model.
The level of significance for all tests was set at a 5 0.05.
RESULTS
Surface modifications with signs of corrosion, plastic
deformation, and wear were seen in the retrieved
brackets (Fig 2) when compared with the as-received
brackets during optical microscopic evaluation. Wear
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Regis et al
504
Fig 3. Scanning electron microscope images of Kirium brackets: A, slot of an as-received bracket
(backscattered electron image, 100-times magnification); B, slot of a retrieved bracket (secondary electron image, 100-times magnification); C, in the retrieved brackets, there were smooth milling grooves in
the lateral walls of the slot (backscattered electron image, 100-times magnification); D, delamination
debris on a retrieved bracket (secondary electron image, 300-times magnification); E, a precipitate
of a retrieved bracket containing silica and barium (backscattered electron image, 300-times
magnification).
and deformation were especially seen on the external
edges of the slot bottom surface. The retrieved specimens had film deposition and islands of aggregated
materials to a variable extent. The variability occurred
even with brackets removed from the same patient,
with no individual pattern in the amount of integument
formed. Mini Standard Edgewise retrieved brackets’ slots
varied in areas with pits and film deposition or areas with
extensive signs of wear and deformation. This last pattern was more frequent in retrieved Kirium brackets.
NuEdge brackets had a rough surface, with pits and
crevices in a parallel arrangement, in both as-received
and retrieved specimens. These originally roughened
surfaces, a coating applied by the manufacturer, and
the presence of integuments made it difficult to evaluate
specific usage-related alterations in NuEdge brackets.
The same morphologic patterns were seen in greater
detail in the scanning electron microscope images
(Figs 3-5). The precipitated film exhibited a dark phase
in the backscattered electron images, contrasting with
the bright phase of the brackets’ alloy. This finding
suggested the presence of elements with a low atomic
number in the film. Carbon, oxygen, calcium, and
phosphorus were the main elements detected in the film
by energy dispersive x-ray spectrometry microanalysis.
October 2011 Vol 140 Issue 4
However, nitrogen, sulfur, sodium, potassium, and
aluminum were eventually found. In some areas, the
precipitates masked the topographic features of the
underlying alloy surface (Fig 6). Greater magnifications
showed details of the arrangement of pits and crevices
in the NuEdge specimens (Fig 5, D and E). The coating
applied by the manufacturer in the as-received NuEdge
brackets was seen in backscattered electron images as
a dark-phased, noncontinuous pellicle. The same distribution was not seen in the retrieved specimens, in which
the integuments were limited to slots and gaps. The
original coating was visually indistinguishable from the
biofilm formed during oral exposure. Some materials of
atypical composition were found in retrieved brackets,
including a 150-mm precipitate containing silica, aluminum, barium, iron, carbon, and oxygen (Fig 3, E). Also
included were silver and silica incrustations that had
a brighter phase than the cobalt-chromium alloy (Fig 6).
A comparison between as-received and retrieved
brackets found no significant difference (P .0.05) in
the tie-wings’ internal widths (Table I). Table II presents the slot dimensions according to usage and
brands. NuEdge brackets had significantly greater
slot dimensions than the other brands (P \0.05). Differences 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 asreceived bracket (backscattered electron image, 100-times magnification); B, slot of a retrieved bracket
(backscattered electron image, 100-times magnification); C, slot lateral wall of a retrieved bracket, with
plastic deformation (bright arrow) and film deposition (backscattered electron image, 300-times magnification); D, islands of precipitates (300 times magnification) and E, pitting corrosion (dark arrows;
100-times magnification) after intraoral exposure.
were seen only for the right slot of the Kirium brackets
(P \0.05).
Sliding resistance, evaluated separately for each brand
according to usage condition by t tests in a 2-tailed
model, had P values of 0.054, 0.103, and 0.08 for
NuEdge, Mini Standard Edgewise, and Kirium, respectively. Since various bracket types were tested (premolar,
canine, and incisor for both arches), it was not viable to
calculate the sliding resistance means and compare
them among brands. However, the percentage difference
in the sliding resistance, as a ratio of sliding resistance alteration between retrieved and as-received brackets,
could be compared among brands. The percentage difference 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 Standard
Edgewise. Differences were statistically significant between Mini Standard Edgewise and the other brands
(P \0.05). No significant differences were found for
the percentage difference in the sliding resistance between patients (P 5 0.061).
DISCUSSION
Microscopic analysis suggested that the orthodontic
brackets underwent significant alterations in variable
ways during treatment. Retrieved orthodontic accessories have shown diverse behaviors in the literature:
some had significant alterations,4,6,9-12,29 and others
showed no differences from as-received.7,8,29-32
These conflicting results might be attributed to the
diversity of the materials analyzed (brackets, archwires,
headgear components), the time of exposure to the
oral environment, and the patient’s characteristics.
Additionaly, corrosion susceptibility is influenced by
the composition of the alloy, its microstructure,
manufacturing procedures and their impact on the
internal stress of the material, and the formation of
a surface passivation film.33 Since the brackets evaluated
in this study had different compositions and brands, dissimilarities in their surface morphology after orthodontic
treatment could be related to each alloy’s tribological,
physical, and electrochemical properties, and to
manufacturing procedures.
The precipitated film over the retrieved brackets’ surfaces was also found on orthodontic wires9,30 and
headgear components.10 The overlap of the calcium and
phosphorus distributions (Fig 6) is consistent with crystalline particle formation. This finding was also described by
Eliades et al.9,10 These authors additionally examined the
molecular composition of the precipitated film. They
found amide, alcohol, and carbonate as the main
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506
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 (backscattered
electron image, 100-times magnification); C, as-received bracket slot with deformation (secondary
electron image, 100-times magnification); D, internal edge between the slot and the middle portion
of 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 magnification); E, retrieved bracket slot surface showing the linear distribution of crevices typically seen in
NuEdge brackets. Also seen is an area with signs of wear and plastic deformation (secondary electron
image, 300-times magnification).
organic constituents. They also identified the presence of
chlorine, potassium, and sodium in a uniform
distribution. Such findings suggest the formation of
a biofilm composed of a proteinaceous matrix and
scattered crystalline particles. The differing properties of
the bracket materials and differing retrieval protocols
might account for the dissimilarities in the integuments
found in these studies.The cleansing procedures we
used were intended to simulate a patient’s dental
hygiene and to remove loosely attached material. The
precipitate containing silica and barium might be
attributed to slot contamination by composite resins or
adhesives, which have similar compositions.34 Although
mass transfer can occur with the sliding of a metallic
surface, the silver-containing incrustations found on
retrieved brackets (Fig 6) could not be attributed to any
clinical findings.26 Calcium and aluminum inclusions
found on biomedically applied materials were strongly
related to corrosion. Other sources of inclusions could
be raw-material impurities and inclusions not dissolved
in the melt.35
The new brackets’ slot dimensions varied among
brands, with a peak of 0.065 mm (0.0026 in; 12%)
October 2011 Vol 140 Issue 4
beyond the nominal value in the NuEdge specimens.
When evaluating the same dimensions, Oh et al25 also
found differences among brands, although of smaller
magnitudes (0.018 mm). The evaluated brackets had acceptable dimensional stability during orthodontic treatment. Only the Kirium retrieved brackets had significant
differences compared with their as-received counterparts. Specifically, there was a 0.018-mm (0.0007 in;
3%) increase in a slot size. These findings agree with
those of Fischer-Brandies et al,36 who found indentations and a 0.016-mm slot widening on stainless steel
brackets tested with wire torque activations. They attributed the results to the low stiffness of the bracket material compared with the wires. The same mechanism could
be related to the wear and deformation of the slot edges
of the retrieved brackets in this study. Alterations caused
by debonding were minimized by using a procedure that
applied force in the bracket base, preserving the area of
interest. Considering the slot variations among brands,
the observed changes in used Kirium brackets might
not have clinical significance.
These results did not allow the rejection of the null
hypothesis of no difference in sliding resistance between
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Regis et al
507
Fig 6. MEV and energy dispersive x-ray spectrometry images of a NuEdge bracket with film deposition
and 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 secondary
electron images, respectively, of the marked area in A with greater magnification (1000 times); D, energy dispersive x-ray spectrometry map of cobalt, with the bright dots indicating that element, and chromium had a similar distribution; E, energy dispersive x-ray spectrometry map of calcium demonstrating
a greater concentration in the area corresponding to the precipitated film images in B and C, with phosphorus having a similar distribution; F, energy dispersive x-ray spectrometry map of silver. Molybdenum and silica had more uniform distributions, although they were concentrated in the same areas
as the silver.
Table I. Comparison of the internal horizontal widths
of the tie-wings of as-received and retrieved brackets
Mean
SD
Cervical tie-wings internal width (mm)
1.139
0.288
As-received
Retrieved
1.168
0.302
Occlusal tie-wings internal width (mm)
As-received
1.143
0.282
Retrieved
1.165
0.298
Table II. Heights (mm) of right and left slots according
to brand and use
Significance*
0.166
0.289
*Mann-Whitney test results.
as-received and retrieved brackets by using a 2-tailed
model for each brand separately. However, the percentage difference in the sliding resistance indicates significantly 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 percentage
differences in the sliding resistance results indicate the
sliding resistance changes in each brand, a 1-tailed model
could be adopted for statistical testing. In this scenario, by
using a t test for paired samples to compare sliding resistance values between as-received and retrieved brackets
separately for each brand, the P values were 0.027 for
NuEdge, 0.04 for Kirium, and 0.0515 for Mini Standard
Edgewise. Therefore, Kirium and NuEdge brackets had
greater mean friction, whereas the Mini Standard Edgewise brackets had no significant alteration.
Height of left slot
NU as-received
NU retrieved
MS as-received
MS retrieved
KR as-received
KR retrieved
Height of right slot
NU as-received
NU retrieved
MS as-received
MS retrieved
KR as-received
KR retrieved
n
Mean
SD
24
24
27
27
29
29
0.618a
0.631a
0.568b
0.577b
0.581b,c
0.595c,d
0.023
0.032
0.007
0.014
0.017
0.020
24
24
27
27
29
29
0.627a
0.633a
0.566b
0.575b
0.581b,c
0.599d
0.018
0.041
0.007
0.013
0.016
0.021
Similar letters identify groups without significant differences
(P $0.05), according to the Games-Howell multiple comparisons
test.
NU, NuEdge; MS, Mini Standard Edgewise; KR, Kirium.
Although some authors have hypothesized that an
increase in friction occurs due to surface alterations,1,20,24 Berg et al37 suggested a lubricating effect
for salivary pellicles over the sliding surfaces in the oral
environment. The percentage difference in the sliding
resistance variable had great variability for each brand,
with high standard devation values. Oh et al25 found
similar results in an in-vitro setup, where variations in
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Regis et al
508
the frictional resistances ranged up to 2.5 times among
brackets of the same brand. Eliades and Bourauel1 reported preliminary results of an ongoing study showing
that retrieved nickel-titanium and stainless steel wires
might have approximately 10% to 20% increases in frictional forces, with great fluctuations over the measured
displacement. They attributed this finding to the complexes precipitated intraorally. Wichelhaus et al38 also
verified a significant increase in friction of nickeltitanium wires after clinical use. We found no similar
evaluations of retrieved brackets.
The frictional test used in this study does not fully
represent oral conditions. Second- and third-order inclinations, dental-arch convexity, binding effects between
bracket and archwire, and material-related frictional
implications were not part of our goals. Nevertheless,
this study provides evidence of the effects of clinical
use and associated phenomena on orthodontic components’ classic friction.
The different behaviors of the brackets we evaluated
raises doubt about which factors determined the observed
alterations. Further investigations with other commercial
brands, standardized treatment mechanics, and controlled durations of oral exposure might clarify the effect
of specific factors on orthodontic accessories’ friction
during clinical use. Our results suggest that care must
be taken when extrapolating conclusions obtained in
research with as-received materials to long-term clinical
scenarios.
CONCLUSIONS
Clinical use causes surface alterations in metallic
orthodontic brackets, with distinct patterns of alterations
for different brands. Differences in morphology after use
are smaller than those found in as-received brackets
among brands. Distinct frictional behaviors were observed for each bracket brand with clinical use. There
were 10% to 20% increases between retrieved and asreceived brackets in NuEdge and Kirium, whereas the
Mini Standard Edgewise brackets remained unaffected.
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American Journal of Orthodontics and Dentofacial Orthopedics
October 2011 Vol 140 Issue 4