Jd corrosion

Corrosion of Orthodontic
Appliances – should we care?
www.indiandentalacademy.com

Contents
• Introduction
• Process and types of Corrosion
• The implications….
 Manufacturing
 Mechanical
 Health
• Cross references

Introduction
• Orthodontic treatment relies on various components :
 Bonded attachments
 Archwires
 Springs……..
• These components are made of varying materials with own
varying physical and mechanical properties

• These components are placed under many stresses in the oral
cavity:
 Immersion in saliva
 Ingested fluids
 Temperature fluctuations
 Masticatory and
 Appliance loading……..

Two Principle Issues….
Whether corrosion products, if produced are
absorbed into the body and cause localized or
systemic problems
What are the effects of corrosion on physical
properties and clinical performance of the
orthodontic appliance

Corrosion…. the process
• Corrosion is an electrochemical process that results in the loss
of the essential metallic properties of a metal
• Corrosion occurs either from :
Loss of metal ions directly into solution
or
Progressive dissolution of a surface film.

• Corrosion involves two concomitant reactions:
an oxidation reaction at the anode
and
a reduction reaction at the cathode
• The oxidation reaction is dissolution of iron as ferrous ions
occurs at the anode, and
• reduction of hydrogen ions to hydrogen gas, occurs at the
cathode.

• Commonly, the anodic reaction will continue until there is
total consumption of the metal, unless the metal can form a
protective surface film (a process known as passivation)
or
• The cathodic reactant is consumed i.e.., exhaustion without
replenishment of dissolved oxygen in solution

• Stainless steel, cobalt-chromium and titanium alloys rely on
formation of passive surface oxide film to resist corrosion.
• This protective layer is not infalliable and is susceptible to
mechanical and chemical disruption.
• Acidic conditions like consumption of carbonated drinks
provides a regular supply of corrosive agents and tend to
dissolve this oxide layer.
• Another contributor – fluoride containing products –
toothpaste, mouthwash, etc

• Schiff et al compared the corrosion resistance of 3 types of brackets
when placed in a reference solution of artificial saliva and 3
commercially available fluoride mouthwashes and measured the
corrosion potential over 24 hrs.
• All 3 fluoride mouthwashes had little effect on cobalt chromium
brackets….
• But stannous fluoride in one of the formulations caused
considerable corrosion in stainless steel and titanium brackets!
- Corrosion resistance on three orthodontic brackets:
a comparative study on three fluoride mouthwashes.
- EJO 2005;27:541-9

• An alternative and industrially important means of controlling
or preventing corrosion is to add to the solution an inhibitor, a
substance that reacts with the metal to form a protective film
on its surface.
• Although adding a corrosion inhibitor to the oral environment
is generally not possible, there are indications that certain
naturally occurring proteins in saliva are corrosion inhibitors,
such as amylase and γ-globulin, can have an inhibitory
(protective) effect.
- Corrosion inhibition by salivary proteins and
enzymes. J Dent Res 1992

Types of Corrosion
• Uniform Corrosion:
 Most common form
 Affects all metal, but at diffenring rates
 Metal undergoes Redox reaction with surrounding environment
• Pitting and Crevice Corrosion:
 Seen on surface of ‘as-received’ orthodontic wires and brackets.
 Microscopically exhibit pits and crevices.
 Which harbour plaque forming microbes and increase their
susceptibilty to corrosion.

• Galvanic Corrosion:
 Occurs when 2 metals joined together and placed in conductive
solution or electrolyte.
 The more electronegative metal becomes the anode and
electropositive becomes cathode.
 In orthodontics galvanic corrosion might occur most commonly in
solder joints
 These solder joints which are mechanically active, make it more
susceptible to corrosion, resulting in the release of zinc, copper, iron
and particularly cadmium.

• Intergranular Corrosion:
 Stainless steel particularly susceptible – during brazing and welding
and occurs at temperatures low as 350₀ C.
 Heating leads to reaction of chromium with carbon forming chromium
carbide and precipitation of this carbide causes:
• The alloy to become more brittle
• Less resistant to corrosion.
(This is because the Chromium is used to form carbide and therefore less available
to form the passive oxide layer)

• Fretting Corrosion:
 Occurs in areas of metal contact subject to sustained loads
Eg: Archwire/bracket-slot interface.
 During application of a load, the 2 metals undergoes a process of cold
welding from the pressure at the interface.
 Continued application of force at such an interface eventually causes
the junction to shear and disrupt the protective oxide layer and leave
the metal susceptible to corrosion.

• Stress Corrosion:
 When archwire ligated to brackets, reactivity of metal alloy increases,
causing stress at these loading points and therefore a electrochemical
potential can be created along the wire and facilitating corrosion.
• Microbiological Corrosion:
 Certain species of microbes absorb and metabolize metal from alloys,
leading to corrosion
 And some of the metabolic by products of microbes can increases the
local acidity levels and make the environment more conducive to
corrosion www.indiandentalacademy.com

Manufacturing Implications
Alloy substitution or addition
Coatings
Modification of production process
Corrosion inhibitors

• Alloy substitution or addition
 Corrosion resistance of Ni-Ti components is due to large amount of
titanium, usually 48-54%.
• Titanium forms several oxide configurations, of which TiO2 is the most
stable and commonly formed.
 In stainless steel, addition of chromium and nickel imparts corrosion
resistance
• Chromium contributes to surface oxide layer which spontaneously
undergoes passivation and repassivation in air and oral environment.
• Nickel aids in corrosion resistance by competing with chromium to form
salts, making more chromium available for passivation.

• Coatings
 Orthodontic wires and brackets can be coated with titanium nitride or
an epoxy resin.
• Titanium nitride improves the hardness and reduces friction and evidence
does show that corrosion resistance in improved in brackets coated with
titanium nitride. ( Hartung et al EJO 1999)
• Epoxy resin improves esthetics and Kim and Johnson in their study ( AO
1999) have shown these coatings to increase corrosion resistance.
• But the epoxy resin tends to wear off with use and this might affect the
corrosion behaviour.

• Modification of production process
 Variations in manufacturing techniques and post manufacturing
finishing and polishing operations can affect the corrosion behavior.
 Microstructure of an alloy can affect the corrosion behavior and this
microstructure itself can be affected by alloying, heat treatment and
cold working.
 Cold working for example occurs during milling and cutting of bracket
slot and this is turn might induce galvanic couples between worked
and adjacent un-worked areas.
 Also using different grades of stainless steel for mesh and bracket base
or electro polishing of brackets can induce galvanic corrosion.

Mechanical Implications
• Recent studies have focused on the alterations of NiTi wires and inner bow
of stainless steel facebows.
• It was found that the material surfaces are coated by intraorally formed
proteinaceous integuments that mask the alloy surface topography.
(microbiological corrosion)
• These environments might dissolve the protective oxide film of NiTi and
allow adsorption of hydrogen.
• Since hydrogen has a high affinity to titanium, it leads to formation of
brittle hydrides like titanium hydride, which increases likelihood of wire
fracture.

• Recent evidence also suggests that some notching occurs on the wire
surface during treatment as a result of masticatory loads which can lead to
pitting corrosion.
• Although corrosion has been implicated as cause of wire fracture, not all
authors seem to agree..
• Schwaninger et al in their study on “Immersion corrosion on flexural
properties of nitinol”, concluded that surface defects generated during
manufacturing, caused early fracture of wires. (AJODO 1982;82:45-9)
• Therefore causes of archwire fracture are multifactorial – with corrosion,
surface finish and work hardening during treatment all contributing.

Health Implication
• Although orthodontic appliances differ from other medical uses of nickel
alloys because they are not implanted and not in intimate relationship
with body tissues, the oral environment is considered hostile and
potentially corrosive
• It was suggested that nickel have carcinogenic, mutagenic, cytotoxic and
allergenic effects, but studies proved that orthodontic materials have no
such severe effects which could result in apparent DNA damage.
• The most common adverse effect that orthodontists face might be nickel
hypersensitivity which can cause gingival hyperplasia, labial
desquamation, angular chelitis, swelling and burning sensation of oral
mucosa.

• Eliades et al investigated nickel release from stainless steel and NiTi wires
after clinical use and compared them with ‘as-received’ wires and they
found no significant difference with respect to nickel content ratio
between the 2 groups. (AO 2004;74:151-4)
• Facebows which contain silver solder are thought to be capable of
inducing formation of galvanic couples, leading to release of nickel and
other metal ions. (AO 2001;71:375-9)
• Even if nickel and chromium are released during orthodontic treatment, it
is taken up by saliva and blood stream and these levels are far below
average daily dietary intake (300μ for nickel and 280μ for chromium)

Cross References
• Sensitivity of titanium brackets to the corrosive influence of
fluoride containing toothpaste and tea – AO 2001;71:318-23
18 patients undergoing fixed orthodontic treatment were bonded with
titanium brackets on left side and stainless steel brackets on the right side
of mouth and were asked to brush with a fluoride gel for 5 months. They
concluded that clinically the role of fluoride in the corrosion of
orthodontic appliances might not be as important as suggested by in-vitro
studies and if it does occur to some extent, the corresponding corrosion
product, titanium tetrafluoride is known to be an ideal medium for
remineralization of enamel.

• Effect of fluoride prophylactic agents on the mechanical
properties of nickel-titanium based orthodontic wires –
AJODO 2005;127:662-9
Authors in this study placed sections of NiTi and Copper NiTi wires in high
fluoride ion concentration gels at 370
C for 1.5 hrs and measured the elastic
modulus and yield strength. The results suggested that the fluoride gel
significantly decreased the both the measured physical properties in NiTi
wires, but no significant effect was found on Copper NiTi wires. This might
suggest that clinically topical fluoride agents can reduce the mechanical
properties of NiTi wires and could contribute to prolonged orthodontic
treatment.

• Variation in corrosion resistance of nickel titanium wires from
different manufacturers – AO 2005;75:661-5
This in vitro study investigated a number of commercially available NiTi
wires in acidic artificial saliva. The results showed that even though the
surface roughness of the wires did differ significantly, it did not necessarily
correspond to the corrosion resistance. It was suggested that surface
residual stress ( stress corrosion) produced during the manufacturing process
might be more important than surface roughness in the susceptibility of
wires to corrosion.

• Surface characterisation of retrieved NiTi orthodontic
archwires - EJO 2000;22:317-26
Authors in this study found that NiTi wires became coated by a film
comprising of amides, carbonates, soduim, pottasium and calcium. They
hypothesized that these mineralized regions of the film might act as a
protective layer, especially under acidic conditions when corrosion rates of
NiTi and stainless steel wires have otherwise been shown to be increased.
This might help to explain why the in vivo behavior of metallic appliances
is often superior to the results predicted by laboratory corrosion studies.

• Nickel hypersensitivity reaction before, during and after
orthodontic treatment – AJODO 1998;113:655-60
some 48 patients in this study exhibited clinical manifestations of nickel
hypersensitivity and the results demonstrated these clinical signs were
independent of the length of exposure time to orthodontic treatment.
And that nickel containing orthodontic appliances have little or no effect
on the oral and gingival health of these nickel sensitive patients (because
higher concentrations of contact allergen are required to elicit a reaction on oral
mucosa than on skin). They also suggested that perhaps orthodontic
treatment with nickel containing components before sensitization to nickel
might lower the incidence of nickel hypersensitivity.

Conclusion
• Although corrosion of orthodontic devices does occur, when looking in
the context of the two principle issues:
1. Whether these corrosion products affect the body locally or systemically ….
the levels are far lower than that ingested in routine daily diet. And literature
has shown no significant correlation between nickel hypersensitivity patients
and the use of nickel containing orthodontic components.
1. Whether the physical and mechanical properties of the appliance is
affected…. yes it does seem so from various studies, but corrosion acts
indirectly, causing of appliance failure in the form of improper surface finish
(pitting/crevice corrosion), manufacturing process (stress corrosion), creation of
galvanic cells (galvanic corrosion) and oral hygiene (microbiological
corrosion).
• Another aspect that could need further research is the effect of fluorides
on corrosion of orthodontic components.

Thank you

Jd corrosion

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