SPECIAL ARTICLE
Orthodontic materials research and applications:
Part 2. Current status and projected future
developments in materials and biocompatibility
Theodore Eliades
Thessaloniki, Greece
The purpose of this 2-part opinion article was to project the developments expected to occur in the next few
years in orthodontic materials research and applications. Part 1 reviewed developments in bonding to
enamel. Part 2 looks at other orthodontic materials applications and explores emerging research strategies
for probing the biological properties of materials. In the field of metallic brackets, expansion of the use of
titanium alloys with improved hardness and nickel-free steels with better corrosion resistance and increased
hardness is expected. Manufacturing techniques might be modified to include laser-welding methods and
metal injection molding. Esthetic bracket research will involve the synthesis of high-crystallinity biomedical
polymers with increased hardness and stiffness, decreased water sorption, and improved resistance to
degradation. New plastic brackets might incorportate ceramic wings. Fiber-reinforced composite archwires,
currently experimental, could soon be commercially available, and long-term applications of shape-memory
plastics might become viable. Advancements in elastomeric materials will result in polymers with reduced
relaxation, broader use of fluoride-releasing elastomers with decreased relaxation, and large-scale film
coating of elastomers to decrease reactivity, water sorption, and degradation. Finally, biocompatibility
assessments will incorporate testing of potential endocrinological action. New polymer formulations might be
tested in adhesive and plastic bracket manufacturing, based on benzoic ring-free monomers to avoid the
adverse effects of the estrogenic molecule bisphenol-A. (Am J Orthod Dentofacial Orthop 2007;131:253-62)
he second part of this article includes a review
of the current status of brackets, elastomerics,
and archwires along with a projection of future
developments in materials technology and clinical applications. It also gives a brief description of the novel
assessment of the biological properties of polymers,
which have already been implemented in associated
biomedical disciplines.
T
BRACKETS
The evolution of a bondable appliance equipped
with an insert to facilitate engagement of the wire onto
it has a remarkable growth curve. Less than half a
century ago, in 1962, Robert Ricketts demonstrated the
use of prefabricated bands for full banding at the
American Association of Orthodontists conference in
Los Angeles.1 In a televised demonstration, he manAssociate Professor, Department of Orthodontics, School of Dentistry, Aristotle University, Thessaloniki, Greece.
Part of this article was presented as a keynote lecture at the 6th International
Orthodontic Congress, Paris, France, September, 2005.
Reprint requests to: Dr Theodore Eliades, 57 Agnoston Hiroon St, Nea Ionia
14231, Greece; e-mail, teliades@ath.forthnet.gr.
Submitted, October 2005; revised and accepted, December 2005.
0889-5406/$32.00
Copyright © 2007 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2005.12.029
aged to band all 4 quadrants of a patient with prefabricated bands made by Rocky Mountain Orthodontics
(Denver, Colo) in less than 20 minutes. This was a
breakthrough development at that time, because banding requires lengthy appointments because of the time
needed to weld the attachments onto the bands.
After that, appliance development expanded greatly
to include all aspects of brackets; changes in size,
design, composition, manufacturing process, and engagement scheme with the archwire occurred, and new
wire engagement features were introduced—ie, active
and passive self-ligation. In addition, a totally new
concept, engineered removable appliances (Invisalign,
Align Technology Inc, Santa Clara, Calif), programmed to move teeth to a predetermined position,
was developed.
Composition of metallic brackets: stainless steel,
nonnickel steel, or titanium?
Apart from standard stainless steel, concerns on the
allergenicity of nickel have provoked the introduction
of various nonnickel, or very low-nickel content, stainless-steel types that supposedly have little allergenic
potential. However, the allergenic action of orthodontic
alloys might have been overestimated because studies
253
254 Eliades
showed that the percentage of orthodontic patients who
react to nickel-containing orthodontic alloys is only a
fraction of the general population.2,3 Some authors
have gone a step farther by suggesting that orthodontics
probably desensitizes those who receive therapy.4
Nonetheless, recent studies have shown that nickel has
genotoxic effects, and thus care should be taken to
minimize potential exposure to this element and its
compounds.5
Nonnickel and low-nickel stainless steels were
introduced in orthodontics as alternatives to conventional 316 and 318 types. These steels contain substantially less nickel relative to conventional types and the
same or even higher hardness relative to the types of
steel used for bracket manufacturing. Moreover, some
types such as the 2205 alloy demonstrated substantially
less crevice corrosion than the 316L alloy when coupled with nickel-titanium, beta-titanium,6 or stainless
steel archwires in vitro. Another steel type, the precipitation-hardening 17-4 steel, exhibited higher hardness
than the 316L steel bracket alloy, although the latter
had significantly higher corrosion resistance.7 In general, this field shows extensive activity, with novel
experimental steel compositions being introduced in
the literature,8,9 and further research is needed to find
the stainless steel alloy with an optimum combination
of strength and corrosion resistance for orthodontic
brackets.
Titanium brackets consist of titanium or a titanium
alloy (Ti-6Al-4V) and are currently available in 2
types: one with Vickers hardness (HV) close to grade II
commercially pure titanium and a wing component of
Ti-6Al-4V alloy, and another type made entirely of
grade IV commercially pure titanium.10 The difference
in hardness between the brackets tested might have
significant effects on the wear phenomena when an
archwire is engaged into the preadjusted bracket slot.
Nickel-titanium archwires have a hardness of 300 to
430 HV, which is close to that of titanium bracket
wings, whereas stainless steel archwires have a hardness of 600 HV. In contrast, the hardness of titanium
brackets has been found to be about 270 HV for the
1-piece bracket and from 160 to 350 HV for the base
and wing of the 2-piece appliance, respectively—
values much lower than those of nickel-titanium and
steel archwires. The clinical significance of this effect
relates to the formation of obstacles in transferring
torque because low hardness induces wear, which
precludes full engagement of the wire to the slot walls
and possibly causes plastic deformation of the wing.11
Also, the Ti-6Al-4V alloy with a friction coefficient of
0.28 might have different frictional variants from the
commercially pure titanium with a coefficient of 0.34,
American Journal of Orthodontics and Dentofacial Orthopedics
February 2007
Fig 1. Three-dimensional x-ray microtomographic image of stainless steel bracket shows soldering alloy
(middle phase).
whereas, from a corrosion perspective, brackets formed
from 2 components might be more susceptible to
galvanic corrosion.12
Projected short-term future developments
in metallic bracket composition
●
●
Expansion of the use of titanium alloys with improved alloys of increased hardness.
Introduction and greater use of nickel-free stainless
steels for bracket manufacturing with corrosion resistance and hardness comparable to conventional
types.
Manufacturing of metallic brackets: 2 piece with
alloy brazing, laser welded, or metal injection
molded?
As a standard manufacturing process, the industry
uses brazing alloys to join the base and wing components of brackets (Fig 1). These alloys also contain
traces of the cytotoxic cadmium, which is added to
lower the melting temperature and improve wetting.13
Moreover, silver-based brazing alloys form a galvanic
couple that can lead to ionic release, mainly copper and
zinc. Corrosion, which has been substantially minimized in current materials, is the main reason for the
progressive dissolution of brazing filler metal, leading
to detachment of the wing from the bracket base during
orthodontic therapy or at the debonding stage. To
overcome this problem, several manufacturers have
introduced gold-based brazing materials that might lead
Eliades 255
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 131, Number 2
to the dissolution of stainless steel, because of the
formation of the galvanic couple.14 Thus, although
brazing alloys can facilitate the manufacturing of
brackets with alloys of certain properties— eg, a stiffer
alloy for the wing to withstand the loads from activated
wires and a softer alloy for the base to facilitate a
peel-off effect during debonding—they have several
problems. The selection of an optimum brazing alloy
presents some difficulties, because the ideal soldering
medium should fulfill a wide range of criteria relevant
to metallurgical structure, corrosion resistance, and
biologic properties of materials.15
Laser welding was relatively recently introduced in
bracket manufacturing as an alternative to alloy soldering.
With this method, welding of the wing to the base does
not extend to the bulk material, and thus a “surface seal”
is formed that is confined to the periphery of the joint (Fig
2). This technique eliminates the intermediate phases such
as soldering alloys and shows acceptable mechanical
performance with a low risk of joint failure.
The metal injection molding (MIM) process, which
has significantly expanded during the past few years,
involves mixing metal powders with particle sizes of a
few microns with organic binders, lubricants, and
dispersants to obtain a homogeneous mixture. Injection
of the feedstock is performed by using an injection
molding machine similar to that used in the plastic
industry (Fig 3).16 MIM-manufactured products are
1-piece appliances with tolerances of the desired dimensions of approximately 0.3% and density values
more than 97% of the theoretical density of the material. A recent study showed that MIM brackets had
excessive porosity, which could be caused by the
shrinkage of manufacturing components during sintering.17 Porosity is a known defect of MIM parts, with
adverse effects on the mechanical and corrosion resistance of most MIM-manufactured products.18 The
hardness of the MIM-made brackets tested varied from
154 to 287 HV, a value much lower than the hardness
of wing components of conventional stainless steel
brackets, introducing the problems associated with soft
and compliant wing components, as noted previously.
Currently, laser welding seems to have the most
advantages, with reduced risks for corrosion or effect
on the bulk material.
Projected short-term future developments in the
manufacturing process of metallic brackets
●
●
●
Laser-welding will probably become routine.
MIM manufacturing will increase rapidly.
Alloy soldering will become obsolete.
Fig 2. Three-dimensional x-ray microtomographic image of laser-welded bracket shows gap in bulk material
of base-wing joint.
Fig 3. Three-dimensional x-ray microtomographic image of stainless steel bracket manufactured with MIM
process shows continuous phase.
Esthetic brackets: plastic or ceramic?
Esthetic bracket manufacturing involves a wide
array of raw materials including zirconia, polycrystalline or single-crystal alumina, and plastics, most often
polycarbonate-based appliances. Although transparent
brackets are more attractive than their metallic counterparts, they have several undesirable effects such as
higher incidence of bracket fracture attributed to the
lack of grain boundaries for the inhibition of crack
growth in single-crystal alumina, excessive wear because of decreased hardness as in polycarbonates, and
256 Eliades
failure to deliver sufficient torque because of their low
modulus.19
The first generation of plastic brackets had excessive creep deformation when subjected to torsional
loads generated by activated archwires to the teeth and
discoloration during clinical use.20 Ceramic- and fiberglass-reinforced and metallic insert-polycarbonate
brackets were subsequently introduced to alleviate this
deficiency, and novel syntheses were tested to overcome the esthetically unpleasing discoloration. Currently available plastic brackets still have some problems pertinent to their decreased hardness and wear
resistance, as well as intraoral plasticization and softening.21-24
In general, the key properties to assess in examining
esthetic brackets include (1) optical clarity (transparency or light transmittance), which is the main advantage of the single-crystal ceramic appliances because of
the lack of grain boundaries as in the polycrystalline
brackets or the presence of fillers, which cause light
scattering and refraction, as used in polymeric brackets
(Fig 4); (2) hardness, and consequently wear resistance,
deals with the capacity of the appliance to maintain
surface structural integrity with loads from mechanics
such as archwire sliding, formation of high torquing
moments, or masticatory forces; current ceramic brackets have greater hardness, although they might be brittle
and have higher wear resistance and low degradation
(hydrolytic or enzymatic); and (3) roughness, which is
critical for the avoidance of high friction variants and
associated obstacles in tooth movement.
The use of low elastic moduli raw materials for
manufacturing the wing and base components of plastic
brackets inevitably imposes several limitations on the
performance of the appliances. A recent study showed
that poly(oxy)methylene bracket raw materials consistently had the lowest roughness and a higher hardness
of plastic brackets25; however, this product might be
less appealing because of its milky color and opacity.
Also, a recent investigation caused some alarming
concerns about possible formaldehyde release from
poly(oxy)methylene brackets subjected to in vitro aging.26 However, laboratory aging media and various
treatments of samples including exposure to excessive
heat cannot simulate the oral environment reliably;
also, the effect of biofilm formation of appliances,
which can reduce their reactivity with the environment,
has not been assessed. On the other hand, formaldehyde
has also been shown to be eluted in vitro at minute
concentrations from composite resins.27,28 Therefore,
further research is required with samples aged in vivo
before a definitive conclusion can be drawn on this
subject.
American Journal of Orthodontics and Dentofacial Orthopedics
February 2007
Fig 4. Photograph showing difference in optical clarity
between plastic bracket and ceramic bracket assigned
to decreased light transmittance of polymeric appliance.
Ceramic brackets—the esthetic alternative to plastic brackets—provide significantly better hardness and
stiffness relative to polymeric appliances. Also, new
generations of ceramic brackets have improved
debonding characteristics, and, therefore, enamel damage risk during this stage is eliminated.29,30 These
appliances have some problems because of their brittle
nature. Particularly the single-crystal ceramics, which
are the most transparent and consequently the most
esthetic, have low fracture toughness because of their
inability to absorb energy during loading, leading to
failure.31 Because the critical stress for the start of a
crack in brittle solids depends on the elastic modulus
and the critical surface tension of the material, intraoral
aging predisposes to fracture.32 This effect arises from
the exposure to moisture and the resultant decrease in
the critical surface tension of the material, which
reduces the critical stress value.32,33 In support of the
foregoing effect, a study reported that alumina ceramics
had significantly reduced 3-point bending strength after
exposure to water.34
Currently, ceramic brackets have superior mechanical properties, increased transparency, decreased reactivity with the oral environment, and an inert biological
character. The latter, to be analyzed later here, is a
matter of dispute for plastic brackets because of the
potential action of various polymers at subtoxic levels.
Projected short-term future developments in
esthetic brackets
●
Introduction of high-crystallinity biomedical polymers with increased hardness and stiffness, decreased water sorption, and high resistance to degra-
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 131, Number 2
●
●
dation for use as the raw material in plastic bracket
manufacturing.
No significant advances are expected for ceramic
brackets.
Plastic combined with ceramic wings might become
more commom, and the availability of esthetic selfligating brackets (currently limited to 3 brands) will
be expanded.
ARCHWIRES
After the introduction of thermoelastic and niobium
nickel-titanium archwires, a breakthrough in archwires,
no major development has emerged in the past decade.
Since the mid-1990s, 2 research teams working independently in the United States and Japan presented
extensive evidence on the feasibility of esthetic polymeric wires.35-41 This new product consists of a composite polymer matrix reinforced with fibers. By varying the reinforcing fiber content of the composite
matrix, the elastic modulus of these wires can be
adjusted to the preferred range. Work by Zufall and
Kusy37 characterized fundamental properties of the
experimental material such as water sorption; they
concluded that this experimental product seems promising.
Recent research efforts in the broader polymer
science field produced shape-memory plastics, which
find many biomedical applications.42 The first plastics
that can be reformed into temporary, preprogrammed
shapes by illumination with ultraviolet light were developed in a joint project between German and US
researchers. When exposed to ultraviolet light of a
different wavelength, the bent plastic wires return to
their original shapes. The mechanism involved relates
to the grafting of photosensitive groups into the polymer network; this acts as a molecular switch.
Projected short-term future developments in
archwires
●
●
Composite wires will be commercially available
during the next several years if the industry finds that
introducing them to the market will be profitable.
Shape-memory plastics for orthodontic use might be
a viable alternative in the future.
ELASTOMERIC MODULES AND CHAINS
Although self-ligating brackets can eliminate the
need for elastomer modules by engaging the wire with
a passive or an active mechanism, and nickel-titanium
coil springs can replace elastomeric chains in retracting
teeth, chain and elastic thread are the only options to
close small diastemas in the anterior regions of arches.
Eliades 257
The issue of force relaxation of elastomeric chains
has attracted the interest of most investigators in the
field because of the apparent clinical significance of the
material’s performance.43-54 In spite of extensive evidence on this subject, there is a lack of information on
the structural changes during stretching and unloading,
including molecular conformation of the material.
In general, a stretched elastomer must possess high
tensile strength to avoid premature rupture; this, in turn,
introduced the requirement for high crystallinity.54
High molecular weight polymers can serve this purpose; however, exaggerated molecular chain length
might adversely affect the ability of the module to
extend. Polymers consisting of molecular chains with
polymerization greater than 1000 have little extensibility.33 When very long chains are deformed beyond a
critical amount, the applied load must be carried by the
primary bonds of the polymer chain, and, since there is
no slippage that will allow dissipation of stress, the
probability for breakage of those bonds is higher than
that of unraveling the chains.55 This effect is termed
“noodle analog” because it resembles the complexity of
removing a very long noodle from a large pile without
breaking it, because of the entanglement of the chains.
On the other hand, fillers in the elastomers in the
forms of color pigments, fluoride releasing beads, and
substances to increase the strength of the materials
might have a pronounced effect on the behavior of
elastomers during stretching. As a rule, filler particles
in the polymer structure have a larger modulus than the
surrounding structure, and, consequently, they fail to
extend to the same amount as the remaining material.
That means that the ends of the fibrils in contact with
the filler must be stretched more than the adjacent
nonfiller-connected polymer fibrils to counteract the
fillers’ inability to stretch. Filler content might thus be
critical for the chain strain at the microscopic level,
because closely packed fillers induce greater stretching
of the intervening polymer chains, which ultimately fail
earlier than their unbonded counterparts, reducing the
capacity of the material to withstand loads (Fig 5).52
Evidence supporting this mechanism showed greater
relaxation rates for colored specimens, whereas fluoride-releasing elastomerics could not deliver force levels comparable with those of conventional elastomerics
after a week of fixed strain.53,54
In the future, polymers with less reactivity will
become necessary to minimize water sorption, solubility, and associated degradation sequelae, which affect
the mechanical properties of the material. Researchers
reported that polymers treated with compounds have
decreased water solubility and are not prone to hydrolytic degradation when tested in vitro.54 Although this
258 Eliades
American Journal of Orthodontics and Dentofacial Orthopedics
February 2007
Fig 5. Schematic of effect of fillers (cubes) on tensile strength of filled and stretched elastomer in
which fibril has been outlined. Shorter fibrils, bonded to fillers (lower drawing), cannot extend to
same length as long ones because fillers are stiffer than matrix, and therefore adjacent fibrils
fracture. Unbonded ones (upper drawing) will probably survive longer, but tensile load is distributed
to fewer fibrils, and some cannot withstand load and ultimately break. This might have softening
effect on stretched material.
innovative process might be a viable future application,
further evidence must be available from in vivo-aged
samples to validate its effectiveness. In the oral cavity,
absorption of lipids was shown to cause potent structural alterations on polyurethanes because these complexes act as nuclei for calcification, lower the glass
transition temperature of the polymer inducing a plasticizing effect, and decrease the free energy for crack
propagation.56
Projected short-term future developments in
orthodontic elastomers
●
●
●
Introduction of new polymers with reduced and
predetermined relaxation.
Development and large-scale use of fluoride-releasing elastomers with decreased force decay.
Application of films that can decrease reactivity of
elastomers with water, resulting in less swelling and
degradation.
FUTURE ASSESSMENT OF BIOLOGICAL
PROPERTIES OF POLYMERIC BIOMATERIALS:
BEYOND CONVENTIONAL CYTOTOXICITY
In recent years, the investigation of the biological
properties of materials has departed from various
routine cytotoxicity assays, ie, MTT and DNA synthesis. The wide application of new polymers has
provoked investigation of their long-term effects at
subtoxic levels—an array of effects with little relevance to common research approaches exploring
biocompatibility.
Fig 6. Chemical structures: A, BPA and B, hormone
17- estradiol. Resemblance leads to BPA’s estrogenic
action.
A concern associated with polymeric adhesives and
plastic brackets relates to the possibility of bisphenol-A
(BPA) release.57,58 BPA is used in the production of
epoxy resins and polycarbonate plastics for food-contact surface coatings in cans, metal jar lids, and adhesives, and as a coating for polyvinyl chloride water
pipes.57-59 Most governmental standards do not consider BPA a pollutant of concern, although recent
research indicates that it can act as an estrogen in
biological systems. Estrogen analogs have the effect of
feminizing the male fetus in animals, as well as
interfering with normal estrogen production in females.60-64 This effect arises from its composition and
structure, which demonstrate a remarkable resemblance
to the female hormone estradiol (Fig 6). As a result, the
body recognizes BPA as a female hormone and adapts
its function accordingly, leading to a series of effects
that include premature puberty and ovarian cancer in
females, and disruption of the maturation of male
reproductive organs. This mimicking effect occurs at
Eliades 259
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 131, Number 2
levels far below the recommended safe concentrations
listed by various organizations such as the US Environmental Protection Agency.57
The turmoil in the dental literature was initially
provoked by a study published by a Spanish group of
researchers, Olea et al,65 who reported elevated
salivary levels of BPA in patients with dental sealants. Their results confirmed the leaching of estrogenic monomers into the environment by Bis-GMAbased composites and sealants in concentrations at
which biologic effects had been previously demonstrated in in-vivo experimental models. However, the
significance of the amount of BPA eluted from materials depends on the actual biological effects induced in
humans and not the elution per se.66
The orthodontic concerns derive from the fact that
monomers identical to those used for sealants are used
in orthodontic polymeric adhesives, and plastic brackets and other polycarbonate-made appliances might
also be sources of BPA.67-69 These studies demonstrated increases in BPA elution from polycarbonate
brackets and adhesives with time in vitro.
The literature cited by Olea et al65 in support of
their statement indicated that 3 days of exposure to
levels of BPA (60-100 g per day) promoted cellular
proliferation in rat uterus and vagina, yielding molecular and morphologic alterations nearly identical to
those induced by estradiol. In addition, the authors
claimed that some people might be sensitive to BPA,
and thus the effects of low doses might not hold true for
them, and other plastics might also expose humans to
additive risks.70
It is surprising that the results from this research
team were not confirmed by other independent laboratories. On the contrary, many others arrived at opposite
conclusions.71-74 The latter group of studies suggested
that the results of Olea et al65 were due to the use of
bulky sealants that were not polymerized properly.73,74
In addition, investigations showed that BPA release
from sealants was very low even when compared with
the threshold for long-term exposure, which is 0.05 mg
per kilogram of body weight daily. Nathanson et al74
measured BPA released from sealants in vitro and
reported that this did not exceed 0.0001 g of BPA per
gram of sealant. Although this seems extremely small,
BPA has estrogenic activity in vitro at concentrations as
low as 10⫺6 mol/L.75,76
The intense interest in the literature on this subject
has provoked the publication of guidelines by various
organizations and legislative bodies. The American
Dental Association (ADA) released a statement, referring to relevant experiments, that assessed the potential
of BPA release by 12 brands of dental sealants that
currently carry the ADA Seal of Acceptance. This
research showed that 11 of the 12 sealants included in
the study leached no detectable BPA, but 1 brand
leached BPA within the range of detection threshold of
the experimental method (5 ppb). The relevant ADA
committee also examined blood samples from 40 dentists, 30 patients who had received sealants, and 10
controls. BPA was not found in any of those blood
samples, suggesting that, if BPA is leached from dental
sealants, it is not detectable in blood tests.77-79
In Europe, the Scientific Committee on Toxicity,
Ecotoxicity, and the Environment of the European
Union’s directorate of Human Health and Consumer
Protection also produced relevant documents. This
committee concluded that, although no carcinogenic,
mutagenic, or genotoxic effects have been documented
for BPA, its potential reproductive toxicity requires
further investigation.58
The industry has been responsive to these concerns,
and at least 2 manufacturers are currently developing
sealants without BPA-forming byproducts based on
alternative monomers without benzoic aromatic rings.
These aromatic rings are part of the monomer systems
of adhesives and plastics. In adhesive technology,
monomers with those rings are usually of high molecular weight and thereby provide stability and the
necessary consistency of the paste for handling purposes. Nevertheless, these materials have lower degrees
of cure because of the stiffness of the molecule. To
alleviate this effect, high molecular weight monomers
are mixed with low molecular weight ones, which are
capable of polymerizing at much higher percentages.
The latter monomers, however, are very reactive and
usually are found at higher proportions in immersion
media.80 Thus, exclusion of the aromatic ring-containing monomer would result in some undesirable effects
in product handling and potentially in the amount of
unpolymerized monomer released. Therefore, there is a
need for replacing the backbone monomer with an
alternative high-molecular weight one that does not
contain benzoic rings and will be free of other adverse
effects.
Projected short-term future developments in
orthodontic polymeric materials
●
●
Introduction of new monomers without the undesirable potential effects of benzoic rings; modification
of manufacturing methods or synthesis of adhesives
and plastic brackets to ensure that no BPA is released
during use, including aging.
Large-scale in-vitro and animal studies focusing on
the effects of BPA released from dental and orth-
260 Eliades
odontic materials on developmental and reproductive
toxicity.
CONCLUSIONS
Advancements in orthodontic materials have had an
impact in orthodontic practice, with prominent effects
in mechanotherapy and biomechanics research.81,82
The search for efficient materials and convenient techniques to shorten treatment times has made significant
progress, and the future outlook of orthodontic practice
will change notably. However, the assessment of biocompatibility of materials must also evolve to incorporate aspects of the biologic properties of materials,
which will not be confined to in-vitro cytotoxicity
assays.
The author thanks William A. Brantley, Ohio State
University, Columbus, Ohio; Claude Matasa, Orthocycle; Spiros Zinelis, University of Athens, Athens,
Greece, for discussions on the metallurgy of orthodontic alloys; and Petros Tsakiridis, University of Athens,
for the reconstruction of the 3D x-ray microtomography
images of brackets.
REFERENCES
1. Ricketts RM. The reappearing American. 1993. Wright: Scottsdale, Ariz: p. 176.
2. Menezes LM, Campos LC, Quintao CC, Bolognese AM. Hypersensitivity to metals in orthodontics. Am J Orthod Dentofacial
Orthop 2004;126:58-64.
3. Kusy RP. Clinical response to allergies in patients. Am J Orthod
Dentofacial Orthop 2004;125:544-7.
4. Kerosuo H, Kullaa A, Kerosuo E, Kanerva L, Hensten-Pettersen
A. Nickel allergy in adolescents in relation to orthodontic
treatment and piercing of ears. Am J Orthod Dentofacial Orthop
1996;109:148-54.
5. Faccioni F, Franceschetti P, Cerpelloni M, Fracasso ME. In vivo
study on metal release from fixed orthodontic appliances and
DNA damage in oral mucosa cells. Am J Orthod Dentofacial
Orthop 2003;124:687-93.
6. Platt JA, Guzman A, Zuccari A, Thornburg DW, Rhodes BF,
Oshida Y, et al. Corrosion behavior of 2205 duplex stainless
steel. Am J Orthod Dentofacial Orthop 1997;112:69-79.
7. Matasa CG. Direct bonding metallic brackets: where are they
heading? Am J Orthod Dentofacial Orthop 1992;102:552-60.
8. Oh KT, Kim YS, Park YS, Kim KN. Properties of super stainless
steels for orthodontic applications. J Biomed Mater Res B (Appl
Biomater) 2004;69:183-94.
9. Oh KT, Choo SU, Kim KM, Kim KN. A stainless steel bracket
for orthodontic application. Eur J Orthod 2005;27:237-44.
10. Gioka C, Bourauel C, Zinelis S, Eliades T, Silikas N, Eliades G.
Titanium brackets: structure, composition, hardness and assessment of ionic release. Dent Mater 2004;20:693-700.
11. Gioka C, Eliades T. Materials-induced variation in the torque
expression of preadjusted appliances. Am J Orthod Dentofacial
Orthop 2004;125:323-8.
12. Zinelis S, Annousaki O, Eliades T, Makou M. Metallographic
structure and hardness of titanium brackets. J Orofac Orthop
2003;64:426-33.
American Journal of Orthodontics and Dentofacial Orthopedics
February 2007
13. Lucas MJ. Brazing of stainless steel. In: Olson DL, Siewert TA,
Liu S, Edwards GL, editors. Welding, brazing, and soldering.
Materials Park, Ohio: ASM International; 1993. p. 911-5.
14. Brockhurst PJ, Pham HL. Orthodontic silver brazing alloys. Aust
Orthod J 1989;11:96-9.
15. Zinelis S, 〈nnousaki O, Eliades T, Makou M. Elemental composition of bracket brazing materials. Angle Orthod 2004;74:
394-9.
16. Kinkard C. Focus: medical plant tour: metal injection molding
smiles. Injection Molding Magazine. Available at: http://www.
immnet.com/article_printable.html?article⫽1962. Accessed March
20, 2003.
17. Cohrt H. Metal injection molding. Mater World 1999;7:201-3.
18. Zinelis S, Annousaki O, Makou M, Eliades T. Metallurgical
characterization of orthodontic brackets produced by metal
injection molding (MIM). Angle Orthod 2005;75:1024-31.
19. Castro L, Merino S, Levenfeld B, Varez A, Torralba J. Mechanical properties and pitting corrosion behaviour of 316L stainless
steel parts obtained by a modified metal injection moulding
technique. J Mater Processing Technol 2003;143:397-402.
20. Eliades T, Gioka C, Zinelis S, Eliades T, Makou M. Plastic
brackets: hardness and associated clinical implications. World
J Orthod 2004;5:62-6.
21. Aird JC, Millett DT, Sharples K. Fracture of polycarbonate
brackets—a related photoelastic stress analysis. Br J Orthod
1988;15:87-92.
22. Alkire RG, Bagby MD, Gladwin MA, Kim H. Torsional creep of
polycarbonate orthodontic brackets. Dent Mater 1997;13:2-6.
23. Aird JC, Durning P. Fracture of polycarbonate edgewise brackets. A clinical and SEM study. Br J Orthod 1986;14:192-5.
24. Gmyrek H, Bourauel C, Richter G, Harzer W. Torque capacity of
metal and plastic brackets with reference to materials, application, technology and biomechanics. J Orofac Orthop 2002;63:
113-28.
25. Zinelis S, Eliades T, Eliades G, Makou M, Silikas N. Comparative assessment of the roughness, hardness, and wear resistance
of aesthetic bracket materials. Dent Mater 2005;21:890-4.
26. Kusy RP, Whitley JQ. Degradation of plastic polyoxymethylene
brackets and the subsequent release of toxic formaldehyde. Am J
Orthod Dentofacial Orthop 2005;127:420-7.
27. Bayne SC. Dental composites/glass ionomers: clinical reports.
Adv Dent Res 1992;6:65-78.
28. Øysæd H, Ruyter IE, Kleven IJS. Release of formaldehyde from
dental composites. J Dent Res 1988;7:64-8.
29. Eliades T, Viazis AD, Lekka M. Failure mode analysis of
ceramic brackets bonded to enamel. Am J Orthod Dentofacial
Orthop 1993;104:21-6.
30. Bishara SE, Olsen ME, VonWald L, Jakobsen JR. Comparison of
the debonding characteristics of two innovative ceramic bracket
designs. Am J Orthod Dentofacial Orthop 1999;116:86-92.
31. Scott GE Jr. Fracture toughness and surface cracks—the key to
understanding ceramic brackets. Angle Orthod 1988;58:5-8.
32. Suresh S. Fatigue of materials. Cambridge Solid State Science
Series. 1991. Cambridge University Press; Cambridge, United
Kingdom: p. 236, 407.
33. Darvell BW. Materials science for dentistry. 1997. University of
Hong Kong; Hong Kong: p. 43–56.
34. Drummond JL, Lenke JW. Aging of dense alumina. Adv Ceram
Mater 1988;3:159-61.
35. Kennedy KC, Chen T, Kusy RP. Behaviour of photopolymerized
silicate glass fibre-reinforced dimethacrylate composites subjected to hydrothermal ageing: part II. Hydrolytic stability of
mechanical properties. J Mater Sci Mater Med 1998;9:651-60.
Eliades 261
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 131, Number 2
36. Fallis DW, Kusy RP. Variation in flexural properties of photopultruded composite archwires: analyses of round and rectangular profiles. J Mater Sci Mater Med 2000;11:683-93.
37. Zufall SW, Kusy RP. Sliding mechanics of coated composite
wires and the development of an engineering model for binding.
Angle Orthod 2000;70:34-47.
38. Zufall SW, Kusy RP. Stress relaxation and recovery behaviour of
composite orthodontic archwires in bending. Eur J Orthod
2000;22:1-12
39. Imai T, Yamagata S, Watari F, Kobayashi M, Nagayama K,
Toyoizumi H, et al. Temperature-dependence of the mechanical properties of FRP orthodontic wire. Dent Mater J 1999;
18:167-75.
40. Imai T, Watari F, Yamagata S, Kobayashi M, Nagayama K,
Nakamura S. Effects of water immersion on mechanical properties of new esthetic orthodontic wire. Am J Orthod Dentofacial
Orthop 1999;116:533-8.
41. Imai T, Watari F, Yamagata S, Kobayashi M, Nagayama K,
Toyoizumi Y, et al. Mechanical properties and aesthetics of FRP
orthodontic wire fabricated by hot drawing. Biomaterials 1998;
19:2195-200.
42. Lendlein A, Jiang H, Junger O, Langer R. Light-induced shapememory polymers. Nature 2005;434:879-82.
43. Brantley WA, Salander S, Myers LC, Winders RV. Effect of
prestretching on force degradation characteristics of plastic
modules. Angle Orthod 1979;49:37-43.
44. Chau LT, Wang WM, Tarng TH, Chen JW. Force decay of
elastomeric chains. Am J Orthod Dentofacial Orthop 1993;104:
373-7.
45. de Genova DC, McIness-Ledoux P, Weinberg R, Shaye R. Force
degradation of orthodontic elastomeric chains. A product comparison study. Am J Orthod 1985;87:377-84.
46. Eliades T, Eliades G, Watts DC. Structural conformation of in
vitro and in vivo-aged orthodontic elastomeric modules. Eur
J Orthod 1999;6:633-42.
47. Eliades T, Eliades G, Brantley WA, Watts DC. Elastomeric
ligatures and chains. In: Brantley WA, Eliades T, editors.
Orthodontic materials: scientific and clinical aspects. Stuttgart:
Thieme; 2001. p. 173-89.
48. Ferriter JP, Meyers CE, Lorton L. The effects of hydrogen ion
concentration on the force-degradation rate of orthodontic polyurethane chain elastics. Am J Orthod Dentofacial Orthop 1990;
98:404-10.
49. Rock WP, Wilson HJ, Fisher SE. A laboratory investigation of
orthodontic elastomeric chains. Br J Orthod 1985;12:202-7.
50. Renick MR, Brantley WA, Beck FM, Vig KW, Webb CS.
Studies of orthodontic elastomeric modules. Part 1: glass transition temperatures for representative pigmented products in the
as-received condition and after orthodontic use. Am J Orthod
Dentofacial Orthop 2004;126:337-43.
51. Eliades T, Eliades G, Silikas N, Watts DC. Tensile properties of
orthodontic elastomeric chains. Eur J Orthod 2004;26:157-62.
52. Gedde UM. Polymer physics. London: Chapman and Hall; 1995.
p. 39 –75.
53. Baty DL, Volz JE, von Fraunhofer JA. Force delivery properties
of colored elastomeric modules. Am J Orthod Dentofacial
Orthop 1994;106:40-6.
54. Storie DJ, Regennitter F, von Fraunhoven JA. Characteristics of
a fluoride-releasing elastomeric chain. Angle Orthod 1994:64:
199-210.
55. McKamey RP, Whitley JQ, Kusy RP. Physical and mechanical
characteristics of a chlorine-substituted poly(para-xylylene) coat-
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
ing on orthodontic chain modules. J Mater Sci Mater Med
2000;11:407-19.
Wu Y, Sellitic C, Anderson JH, Hiltner A, Lodoeng A, Payet CR.
An FTIR-ATR investigation of in vivo poly (etherurethane)
degradation. J Appl Pol Sci 1992;46:201-7.
US Environmental protection agency (EPA). Integrated risk
information system. Bisphenol-A. Available at: http://www.epa.
gov/iris/subst/0356.htm. Accessed July 17, 2005.
European Commission. Directorate of general health and consumer protection. Scientific committee on toxicity, ecotoxicity
and the environment. Opinion on the results of the risk assessment of bisphenol-A. Human health part. Cas no. 80-05-7.
Brussels; 2003.
Brede C, Fjeldal P, Skjevrak I, Herikstad H. Increased migration
levels of bisphenol A from polycarbonate baby bottles after
dishwashing, boiling and brushing. Food Addit Contam 2003;
20:684-9.
Wetherill YB, Petre CE, Monk KR, Puga A, Knudsen KE. The
xenoestrogen bisphenol A induces inappropriate androgen receptor activation and mitogenesis in prostatic adenocarcinoma cells.
Mol Cancer Ther 2002;1:515-24.
Ramos JG, Varayoud J, Sonnenschein C, Soto AM, Munoz De
Toro M, Luque EH. Prenatal exposure to low doses of bisphenol
A alters the periductal stroma and glandular cell function in the
rat ventral prostate. Biol Reprod 2001;65:1271-7.
Gupta C. Reproductive malformation of the male offspring
following maternal exposure to estrogenic chemicals. Proc Soc
Exp Biol Med 2000;224:61-8.
Timms BG, Howdeshell KL, Barton L, Bradley S, Richter CA,
vom Saal FS. Estrogenic chemicals in plastic and oral contraceptives disrupt development of the fetal mouse prostate and
urethra. Proc Natl Acad Sci USA 2005;102:7014-9.
vom Saal FS, Timms BG, Montano MM, Palanza P, Thayer KA,
Nagel SC, et al. Prostate enlargement in mice due to fetal
exposure to low doses of estradiol or diethylstilbestrol and
opposite effects at high doses. Proc Natl Acad Sci USA 1997;
94:2056-61.
Olea N, Pulgar R, Olea-Serrano F, Rivas A, Novillo-Fertrell A,
Pedraza V, et al. Estrogenicity of resin-based composites and
sealant used in dentistry. Environ Health Perspect 1996;104:298305.
Söderholm KJ, Mariotti A. BIS-GMA-based resins in dentistry:
are they safe? J Am Dent Assoc 1999;130:201-9.
Watanabe M. Degradation and formation of bisphenol A in
polycarbonate used in dentistry. J Med Dent Sci 2004;51:1-6.
Watanabe M, Hase T, Imai Y. Change in the bisphenol A content
in a polycarbonate orthodontic bracket and its leaching characteristics in water. Dent Mater J 2001;20:353-8.
Suzuki K, Ishikawa K, Sugiyama K, Furuta H, Nishimura F.
Content and release of bisphenol A from polycarbonate dental
products. Dent Mater J 2000;19:389-95.
Schmalz G, Preiss A, Arenholt-Bindslev D. Bisphenol-A content
of resin monomers and related degradation products. Clin Oral
Invest 1999;3:114-9.
Al-Hiyasat AS, Darmani H, Elbetieha AM. Leached components
from dental composites and their effects on fertility of female
mice. Eur J Oral Sci 2004;112:267-72.
Pulgar R, Olea-Serrano MF, Novillo-Fertrell A, Rivas A, Pazos
P, Pedraza V, et al. Determination of bisphenol A and related
aromatic compounds released from bis-GMA-based composites
and sealants by high performance liquid chromatography. Environ Health Perspect 2000;108:21-7.
262 Eliades
American Journal of Orthodontics and Dentofacial Orthopedics
February 2007
73. Fung EY, Ewoldsen NO, St Germain HA Jr, Marx DB, Miaw
CL, Siew C, et al. Pharmacokinetics of bisphenol-A released
from a dental sealant. J Am Dent Assoc 2000;131:51-8.
74. Nathanson D, Lertpitayakun P, Lamkin MS, Edalatpour M, Chou
LL. In vitro elution of leachable components from dental
sealants. J Am Dent Assoc 1997;128:1517-23.
75. Arenholt-Bindslev D, Breinholt V, Preiss A, Schmalz G. Timerelated bisphenol-A content and estrogenic activity in saliva
samples collected in relation to placement of fissure sealants.
Clin Oral Invest 1999;3:120-5.
76. Atkinson JC, Diamond F, Eichmiller F, Selwitz R, Jones G. Stability
of bisphenol A, triethylene-glycol dimethacrylate, and bisphenol-A
dimethacrylate in whole saliva. Dent Mater 2002;18:128-35.
77. Schafer TE, Lapp CA, Hanes CM, Lewis JB, Wataha JC,
Schuster GS. Estrogenicity of bisphenol-A and bisphenol-A
dimethacrylate in vitro. J Biomed Mater Res 1995;45:192-7.
78. Tarumi H, Imazato S, Narimatsu M, Matsuo M, Ebisu S.
Estrogenicity of fissure sealants and adhesive resins determined
by reporter gene assay. J Dent Res 2000;79:1838-43.
79. Matasa CG. Polymers in orthodontics: a worrisome present? In:
Graber TM, Eliades T, Athanasiou AE, editors. Risk management in orthodontics: experts’ guide to malpractice. Chicago:
Quintessence; 2004. p. 113-31.
80. Gioka C, Bourauel C, Hiskia A, Kletsas D, Eliades T, Eliades
G. Light-cured or chemically cured orthodontic adhesive
resins? A selection based on the degree of cure, monomer
leaching, and cytotoxicity. Am J Orthod Dentofacial Orthop
2005;127:413-9.
81. Kusy RP. Orthodontic biomaterials: from the past to the present.
Angle Orthod 2002;72:501-12.
82. Kusy RP. Ongoing innovations in biomechanics and materials
for the new millennium. Angle Orthod 2000;70:366-76.
Editors of the International Journal of Orthodontia (1915-1918),
International Journal of Orthodontia & Oral Surgery (1919-1921),
International Journal of Orthodontia, Oral Surgery and Radiography (1922-1932),
International Journal of Orthodontia and Dentistry of Children (1933-1935),
International Journal of Orthodontics and Oral Surgery (1936-1937), American
Journal of Orthodontics and Oral Surgery (1938-1947), American Journal of
Orthodontics (1948-1986), and American Journal of Orthodontics and Dentofacial
Orthopedics (1986-present)
1915
1931
1968
1978
1985
2000
to
to
to
to
to
to
1932 Martin Dewey
1968 H. C. Pollock
1978 B. F. Dewel
1985 Wayne G. Watson
2000 Thomas M. Graber
present David L. Turpin