Kopie Master-Thesis Longo.docx - dr

Aus dem Department für Interdisziplinäre Zahnmedizin und
Technologie
der Donau-Universität Krems, Österreich
„The biological effects of Nitinol in Orthodontics
A literature review“
Masterthese
zur Erlangung
des
„Master of Science Kieferorthopädie“ (MSc)
vorgelegt:
2009
von
Dr. (Univ. Padua) Longo Astrid, München
1
Prüfer: Prof. Dr. Dr. Dieter Müßig
2
Preface
I declare that all the results of this study are based on literary research
entirely done by myself.
3
Abstract
Aim: This literature review was aimed to find out which kind of interactions result
from the use of Nitinol in the oral system. In particular the main interest was
directed to the alterations of the oral tissues as well as to the effects on distant
districts.
Conclusions: Nitinol is a mostly tolerated material, widely used in orthodontic for
its useful properties. Nevertheless negative reactions to this material have been
reported. It seems that these reactions occur mainly when Nitinol is coupled with
other metals as stainless steel, but further research about this topic is needed.
Key words: Nitinol; Ni-Ti; Nickel-Titanium alloy; biological effects;
biocompa- tibility; tissue reaction; cellular reaction; orthodontics; corrosion
4
Table of contents
Page
Preface
……………………………………………………………………………………… 2
Abstract
……………………………………………………………………………………… 3
Figures
and
tables
…………………………………………………………………………. 5
Introduction
…………………………………………………………………………………. 6
Subjects
and
Methods
……………...……………………………………………………... 7
Discussion
…………………………………………………………………………………. 11
Biomechanical
properties
……………………………………………………………… 11
Biocompatibility
…………………………………………………………………………. 14
Dietary
intake
of
nickel
……………………………………………………………….. 17
Corrosion
………………………………………………………………………………… 18
Surface
treatments
…………………………………………………………………… 24
Static
and
dynamic
condition
………………………………………………………... 25
Oral
environment
5
……………………………………………………………………... 26
Electrogalvanism
……………………………………………………………………… 28
Biological
reactions
…………………………………………………………………….. 30
Summary
and
Conclusions
…………………………………………………………….... 35
Acknowledgements
………………………………………………………………………. 37
References
………………………………………………………………………………… 38
Curriculum
vitae
…………………...……………………………………………………… 48
List of figures and tables
Fig. 1: Nitinol surface in different crystallographic phases [59]
Fig. 2: Crystallographic changes of Nitinol [84]
Fig. 3: Pseudoelasticity [56]
Fig. 4: Review of principal properties of SMMs [31]
Fig. 5: Diagrammatic summary of various types of corrosion [9]
Fig. 6: Mechanism of biocorrosion on orthodontic wires in the oral cavity [57]
Fig. 7-8: Galvanic corrosion of archwire-bracket couples [39]
Tab. 1: Exclusion and inclusion criteria
Tab. 2: Typology of the reviewed literature
Tab. 3: Summary of the topics
6
Tab. 4: Parameters controlling the behaviour of SMMs [31]
Tab. 5: Different types of corrosion commonly occurring in the oral cavity [9]
Tab. 6: Parameters affecting corrosion [23]
Introduction
Nitinol belongs to a category of Shape Memory Materials (SMM), also named
“smart materials”. It is an intermetallic compound of nickel and titanium. It was
developed in 1959 by W. F. Buehler and co-workers at the Naval Ordnance
Laboratory in Silver Spring, Maryland, USA. The metallurgist researcher was
trying to develop a nonmagnetic, salt resisting, waterproof material for the
space program.
The stoichiometric proportions are nearly equiatomic. NiTi 55 is the widely
used alloy.
Nitinol gains its special characteristics through controlled heath treatment [7]:
superelasticity (constancy of applied stress, kink resistance and low wear
rate), shape memory, fatigue resistance and electric resistivity, corrosion
resistance [3, 15, 79, 83].
Nitinol has found applications in orthopaedics and cardiovascular surgery
(mainly implants, stents and instruments), solid-state heat engines, aircraft
technology (“shrink-to-fit” pipe couplers, couplings of hydraulic tubing), safety
products and, every-day tools such as coins, jewellery buttons, eyeglass
frames, toys; computer components … [24, 42, 46]
The use of this alloy in orthodontics was introduced by Andreasen in 1972.
Nitinol trying to return to its start shape continually exerts a force on the teeth.
These archwires are able to move teeth over a long treatment time and broad
tooth positions applying a constant force.
The discoverer of this metal alloy reported about the good properties of Nitinol
under strain and stress conditions [7], but for orthodontic use mainly the
loading stress is of importance.
7
The purpose of this literature review is to give an overview about the up to the
present moment studied effects of Nitinol as an orthodontic material on the
oral biological system in widest sense.
8
Subjects and methods
To gain information about this topic, I searched for literature in the libraries at
the Dental School in Vienna, at the University library of the “Donau Universität
Krems”, and at the library of the Dental School in Munich.
Most references I found at the library in Vienna, nothing in Krems and very
little in Munich. In Munich there are no periodicals about orthodontics. These
were personal property of the former chief of the orthodontic clinic. As she
retired last year, she took all the orthodontic magazines and books of the
orthodontic library with her.
With the advice of the librarian of the dental school in Vienna I could search
directly in the Internet. For example I tried to find some issues by base
(www.base-search.net), the Bielefeld Academic Search Engine, but the offer
for dentistry was very poor. I searched on the home-page of some important
Orthodontic magazines like “Angle Orthodontist” (www.angle.org), “European
Journal of Orthodontics” (www.ejo.oxfordjournals.org), “American Journal of
Orthodontics”
(www.journals.elsevierhealth.com),
“Journal
of
Dental
Materials” (www.soc.nii.ac.jp) or Quintessence Publishing for “World Journal
of
Orthodontics”
(www.quintpub.com).
The
online
publisher
Elsevier
(www.elsevier.com) lists a huge catalogue of magazines. Most of these offer
the possibility to print a sample article free of charge, which proved very
effective.
I could also find topic-related articles directly by searching in “Google scholar”
(www.scholar.google.com).
An organized description of the possible resources of literature gave me the
article of D´Anto et al. (2007) [12] who reviewed the literature about the metal
ion release from orthodontic appliances in vivo.
Some issues I needed to order with subito (www.subito-doc.de), an online
library service where you can buy copies of articles from periodicals or
support you by lending books.
9
It was a very annoying search: Most literature about Nitinol was written in the
medical field, especially about stents. In orthodontics there are lots of studies
about the application of Nitinol archwires concerning the forces, the tooth
movement, the superelasticity and shape memory effects available, but very
little about how the characteristics of this alloy influence the biological system.
I needed to search a great amount of different word combinations. My
research stretched within three languages: English but also German and
Italian.
For my research I used the word Nitinol with the following terms in various
combinations:
Biological effects / properties / influence / response
Bone effects / reactions
Periodontal effects / destruction
Negative effects
Toxicity
Tissue response / effects
Allergy
Reactions
Allergic reactions
Decay
Orthodontics
Biocompatibility
Oral mucosa
I also tried other definitions for Nitinol and its components like:
Ni-Ti
Nickel-Titanium
NI-Ti alloy
Titanium
Nickel
Ti alloys
Ti-Ni compound
10
Shape memory alloys
Because of the sparse literature about the topic I was searching for, I had to
try a huge amount of different possibilities so that I can’t give an exact list of
how many articles I could found for each combination of words.
Exclusion criteria
Inclusion criteria
Research on animals
Nitinol for orthodontic use
Nitinol for medical-surgical applications
In vitro studies
Studies on cell culture for medical purposes
In vivo studies
Nitinol for endodontic instruments
Effects related to Nitinol in orthodontic patients
Nickel sensitivity (in general) in orthodontic Studies of Nitinol as a biomaterial
patients
General reactions during orthodontic treatment Studies of Ni-Ti alloys
I reviewed articles from the late `90 till nowadays. I included some older works
because of their relevance for a better understanding of the material
characteristics and their applications.
Tab. 1: Exclusion and inclusion criteria.
I disregarded the studies about generic reactions (such as tooth movement
and bone reactions, friction behaviour etc.) during orthodontic treatment as
well as the specifics about nickel hypersensitivity or allergy by orthodontic
appliances but not directly connected to the use of Nitinol. I also excluded
studies about experiments on animals because these can’t be compared to
the situation in the human oral cavity because of the biochemical and
metabolic species-specific conditions that are quite dissimilar [21].
Article Type
Nr
Case reports
Epidemiologic studies
Literature reviews
6
4
13
Experimental studies (in-vitro)
37
Clinical studies (in-vivo)
Clinical studies (ex-vivo)
Longitudinal study
Books
Reports
Others
11
5
1
4
2
11
Tab. 2: Typology of the reviewed literature
11
References
2, 14, 17, 51, 54, 60
48, 75, 82, 88
9, 10, 12, 21, 23, 35, 65, 67, 71, 77, 78,
79, 83
4, 5, 8, 13, 19, 22, 28, 29, 30, 34, 36, 37,
38, 39, 41, 43, 47, 49, 50, 52, 53, 55, 58,
61, 62, 65, 66, 69, 70, 72, 73, 74, 76, 81,
89, 91
6, 24, 26, 30, 44, 45, 48, 57, 63, 86, 87
1, 18, 20, 64, 85
68
25, 27, 31, 77
84, 90
3, 7, 11, 15, 16, 32, 42, 46, 51, 56, 84
I couldn’t find any metanlytical or longitudinal studies and quite little in-vivo
researches. Most articles treat the problem of corrosion analysed in in-vitro
conditions. Also very little is reported about the reaction to this alloy in
orthodontics.
In the following discussion I will summarize and explain the various
information and opinions about the interaction of Nitinol for orthodontic use
and the human biological system I could gain from the overview of the
literature about this topic.
Topic
Study type
Literature reference
About the material
Characteristics
Applications
Microbiological
interactions
Allergic reactions
Other reactions
review, history
review
review
46, 78
7, 31, 69, 83, 84
3, 11, 15, 16, 23, 25, 31, 42, 56, 83
in-vitro, ex-vivo
8, 51, 57
case report, review, survey
in-vivo, in-vitro
Corrosion
review, in-vitro
in-vitro
2, 14, 17, 48, 51, 54, 60, 67, 75, 82
30, 32, 67, 86, 88
4, 5, 9, 10, 12, 18, 19, 20, 21, 35, 36,
37, 38, 41, 47, 49, 50, 52, 55, 62, 64,
66, 89, 91
13, 19, 21, 22, 23, 24, 25, 41, 43, 58,
65, 70, 71, 72, 77, 79, 82, 85, 89
28, 29, 34, 50, 76,
in-vitro
39, 43, 53, 61, 73, 74, 81
in-vivo
1, 6, 26, 44, 45, 63, 87
Biocompatibility and
Cytotoxicity
Protective surface layer
Influence of fluoride and
Galvanism
Biological fluids
evaluation
review, in-vitro, cell culture
Tab. 3: Summary of the topics.
12
Discussion
Every time a new material for medical uses is proposed, the first concern is
about the possible reactions of the human biological system. Because of the
high nickel content of Nitinol it is important to define if it is biocompatible.
Even if nickel as a trace element is necessary for human life, it is known to be
a cytotoxic, mutagenic and allergenic metal if present in higher concentration
for a prolonged time [12]. The attention of the scientists has been directed to
the determination of the amount of leached ions from the alloy and the effects
on the tissues.
Most studies about the biological effects of orthodontic devices (braches,
bands and archwires) focus on the corrosion behaviour and the metal ions
release from the surface principally of nickel and chromium [4]. The allergic
potential of an alloy is given from his corrosion attitude [12]. Virtually all the
different components of the alloy can be leached by corrosion or abrasion
processes of the material surface. If the metal surface is not stable it may
corrode until equilibrium is reached [50]. To understand how this is possible, it
is important to know the characteristics of the metal.
The influence of the warm oral environment, that shows the qualities of an
electrolytic solution, is the most important variable for the biocompatibility of
Nitinol. In addition the high microbial presence and the enzyme content of the
saliva constantly change the properties of the biological fluids [12, 24].
Biomechanical properties
The mechanical properties of Nitinol are due to the transformation from the
austenite phase to martensite transition in the crystalline structure which
happens trespassing a critical temperature range (TTR: transformation
temperature range) or under applied stress and strain [7]. Plastic deformation
within or below this temperature are recoverable [11]. The “mechanical
memory” [7] is “the material’s capability to recover its original shape or
13
configuration after mechanical distortion, by (…) heating it above the transition
temperature”.
The austenite phase (an automatically ordered solid-state parent phase)
exists at high temperatures and low stress conditions, in contrast the
martensite (a very complex solid-to-solid “shearing” of the atomic structure)
exists at low temperatures and high stresses [46, 56, 66, 83]. This
temperature-dependent transformation happens during cooling. This is a
crystalline-to-solid phase change. At low temperature in the martensite phase
the compound is soft and can be easily deformed [7; 33] but it regains
automatically its original geometry by heating at a given temperature returning
to its austenite structure [69, 84].
The martensite has a monocyclic or orthorhombic crystal structure. By heating
the alloy it transforms into twinned martensite phase. The martensite phase
can also be induced by stress (detwinned martensite). In this phase the
crystallographic orientation is aligned with the stress direction. By heating the
metal over the transformation temperature the original shape and stiffness are
restored (austenite). The austenite is stable at high temperature and has a
body-centered cubic crystal structure [56]. The TTR expresses the
temperature at which the metal returns to its original length and shape after it
has been stretched to a maximum of 7-8% of its length below TTR [7].
Sometimes between these two crystallographic forms a so-called “R phase”
has been reported. It indicates the ability of a two-way memory effect [69].
a
c
b
Fig. 1: The surface of a NiTi alloy slowly cooled (or gradually subjected to stress) [59]
a. austenite; b. austenite + martensite; c. martensite
The transformation temperature depends on the chemical content and on the
14
special processing of the alloy and can be controlled by the alloy’s heath
treatment [3]. “When the phase transformation temperature (…) is close to
oral temperature, the yield strength of shape memory could be up to
maximum” [69].
The physical behaviour of NiTinol is explained by the stress-strain curve
above the temperature at which the formation of austenite ends (Af)
(hysteresis loop with energy dissipation), expressing the pseudoelastic effect
of the material. “Pseudoelastic” alloys show only a nonlinear unloading
behaviour; on the contrary “superelastic” alloys exhibit an inflection point that
indicates the presence of an unloading plateau [16].
Fig. 2: Crystallographic changes of Nitinol [84]
Af: temperature where material has finished transforming to austenite upon
heating
As: temperature where material starts to transform to austenite upon heating
Ms: temperature where material starts to transform to martensite upon cooling
Mf: temperature where material has finished transforming to martensite upon cooling
In ideal conditions the superelasticity exerts at best the stress-strain curve
should display a broad hysteresis with nearly horizontal plateau branches
[89]. Superelasticity is due to the forming of martensite areas were stress is
applied [15].
Tab. 4: Parameters controlling the behaviour of SMMs. [31]
Fig. 3: Pseudoelasticity. [56]
A
B
C
D
B: martensitic transformation
C: elastic recovery during the unloading process
D: reverse martensitic transformation
0: elastic discharge
As, Af and Ms and Mf = temperature at which the formation of austenite and martensite starts
and ends, respectively. σε = stress-strain curve
15
To shape the alloy in a permanent form, it has to be constrained in the desired
form under load and heath treatment at high temperatures and water quench
or fast cooled [79, 15]. The thermal treatment within a range of moderate
temperatures is also necessary to prevent a decrease in mechanical
properties such as superelasticity [33]. This property allows the compound to
undergo strain with a relatively moderate stress increase [43].
Fig. 4: Review of principal properties of SMMs [31]:
one-way memory (outer line and ); two-way memory with learning and the memory effect
itself ; pseudoelastic effect . Superelastic effect, shifted toward non-zero force values;
rubber-like effect (learning cycle loops )
Biocompatibility
At first it must be defined what biocompatibility is: “The ability to perform with
an appropriate host response in a specific application” [90]. It is determined by
the host’s reaction to the material and its degradation in the body environment
[79]. Biocompatibility is mainly surface-related because this part of the foreign
material comes in contact with the tissues, mucosae and body fluids [31].
Academics started researches about the biocompatibility of this new alloy
since the late `70s [26]. There is a wide spectrum of studies for medical
applications (surgery and general dentistry) but little in orthodontics.
16
Also important is the definition of biomaterial as “any non-drug material that
can be used to treat, enhance or replace any tissue, organ or function in an
organism” (mondofacto dictionary – www.mondofacto.com/facts/dictionary).
The biocompatibility question is related to the amount of ion release
(corrosion) of the metal in the moist oral environment. The leached alloy
elements can enter the organism through ingestion (systemic intake) or
through the oral mucosa (local intake) [70]. Because of the widely recognised
tolerability of titanium, the main problem in the event of ion release is related
to nickel leaching. “Orthodontic appliances differ from other medical uses of
nickel alloys because they are not implanted” [35]. Ni can be taken into the
human body through the lungs, the skin or orally [23].
I could only find one study about the adverse biological effects on the oral
mucosa of metal ions leaching from orthodontic appliances, a few in-vitro
studies of the effects of Nitinol on cell cultures, one about direct testing of
different nickel concentrations on cell-cultures and one literary review about in
vitro studies about the biocompatibility of Nitinol used for orthodontic
purposes.
Faccioni et al. (2003) [24] were the only ones who could demonstrate a
cytotoxic reaction in form of DNA damage and cellular death (apoptosis) using
epithelial cells retrieved from the buccal mucosa of patient wearing a fixed
orthodontic appliance in comparison to a control group without orthodontic
therapy. They applied the trypan blue exclusion viability test and the alkaline
version of the comet assay. These results aren’t in direct relation to the
specific usage of Nitinol, but are in general referred to the use of orthodontic
appliances (in which even stainless steel devices are applied).
All the cell-culture studies on Nitinol come to the same result that this alloy
isn’t cytotoxic but the testing methods were different. Rose et al. (1998) [70],
Eliades et al. (2004) [20], and Vannet et al. (2006) [85] used the Mossman’s
MTT
(3-[4,5-dimethylthiazol-2-yl]2,5-diphenyl
tetrazolium
bromide)
colorimetric viability test for bioassays. The first authors tested on mouse
fibroblast cell-cultures using samples from as-received archwires, the second
on gingival and periodontal ligament fibroblast strains with full sets of fixed
17
stainless steel orthodontic appliances and Ni-Ti archwires and the last ones
on the three-dimensional human epithelium model using specimens of
retrieved archwires.
El Medawar et al. (2002) [22] undertook the (Human epithelial embryonic and
Human –L-132, ATCC CCL 5- Embryonic Palatal Mesenchymal –HEPM,
ATCC CRL-1486-) cell-cultures to electrochemical, cell proliferation and
viability tests respectively using disk samples of the metal for the first two
tests and metal powder (grains < 10 µm) for the last. Matarese et al. (2006)
[58] evaluated the involvement of transglutaminase (TGase) associated with
interleukin-6 (IL-6) release in human gingival fibroblasts (HGF-1 CRL 2014)
cultures exposed to titanium-containing orthodontic alloys. The TGase is a
marker-enzyme of cell differentiation, responsible for cell adhesion and
important for maturation and stabilization of extracellular matrix; IL-6 is a
cytokine present in the inflammatory process. They tested for citotoxicvity with
the MTT test, the IL-6 release with ELISA test system, TGase activity for the
proliferating and differentiating activity, subcellular distribution of TGase
activity with confocal laser scanning microscopy and Western blotting for
tissue transglutamine (tTg) expression. The results of this study are
inconclusive and, apart for the demonstrated low cytotoxicity, give only a hint
that in cell-cultures exposed to Ni-Ti alloys IL-6 production and tTG
expression increases. These data do not explain what kind of effects these
results provoke on the oral tissues.
The other cell-culture study [41] used human peripheral blood mononuclear
cells (PBMC) from healthy adults with and without nickel sensitivity that
weren’t in orthodontic treatment. He used the MTT test and trypan blue
exclusion test. Both tests give the result that nickel becomes cytotoxic only
above a concentration of 29 ppm.
In their review Possonet et al. (2006) [65] could ascertain “a satisfactory
biological behaviour of [Nitinol] orthodontic archwires except in fluoride
environment”. Thus they also underline that the cellular and molecular
interactions of this alloy with the host are still quite unknown. The interaction
of a material and the biological medium is a two-way mechanism and can
induce effects even far from him, depending on the propagation or elimination
18
modes of the elements released from the material. The presence of a
superficial titanium oxide layer could be confirmed from various authors who
tested the material in neutral artificial saliva, but in acid medium and in
presence of fluoride ions titanium was susceptible to corrosion.
David & Lobner (2004) [13] did not find any cytotoxicity of Ni-Ti archwires on
mixed cerebral cortical murine cell coltures.
As I mentioned above, it has been written a lot about biocompatibility of Nitinol
for medical devices. The most productive authors are Shabalowskaya [76-80]
and Ryhänen [71, 72]. Also Es Suoni et al. delivered a wide literature review
of this topic [23].
These authors in several studies and reviews between 1996 and 2006 [23,
71, 72, 77-80] reported the good biocompatibility and no genotoxicity of the
material, even if a certain cytotoxicity was observed in in-vitro studies due to
the much higher Ni concentrations and the surface treatment of the tested
specimens. All agreed that biocompatibility was due to the corrosion
resistance given from the oxide surface layer (so called passivation layer).
On the other hand the corrosion behaviour of Ni-Ti alloys in orthodontic
archwires has been exhaustively documented in in-vitro studies. This topic is
treated in a separate paragraph.
Dietary intake of Nickel
It is well known that the fermentation processes of the microorganisms in the
oral cavity lead to accumulation of trace metals from food [57].
As a trace element nickel is indispensable for the carbohydrate biochemical
transformation
in
humans.
The
necessary
daily
intake
should
be
approximately 100 μg/day [82].
The average daily dietary intake of Nickel has been determined to be between
100-800 μg/day (International Programme on Chemical Safety -ICPS- 1991;
Environmental Health Criteria 108. Nickel. WHO, Geneva). Cocoa powder
(and chocolate), nuts, vegetables, dried beans and seeds have high nickel
content, but even stainless steel kitchenware can release this metal when
preparing food, most of all in acidulous conditions [23, 54]. It is important to
19
emphasize that nickel ions leaching from orthodontic appliances in the oral
environment are normally swallowed rather than accumulated [10].
The nickel ion release must be compared with the daily dietary intake of this
metal. Several studies show that the metal leaching reaches low rates in
comparison with the dietary intake [4, 36, 38]. The authors measured levels
raging between 4,1 μg/day and 131,6 μg/day [1, 4, 5, 36, 63]. The different
reported release rates are dependent on the testing periods and the pH of the
testing solutions. Also the differences in the tested wire-types and the
detection methods employed are of relevance for the differing reports [41]. In
any case the initial Ni ions release was much higher in the first days and
successively became lower.
Corrosion
It can be defined as “the (…) interaction between a solid material and its
chemical environment, which leads to loss of substance from the material,
change in its structural characteristics, or loss of structural integrity” [10] or as
“electrochemical reactions during which the surface of a metal is deteriorated
via ion release” [52]. Corrosion is a redox (oxidation and reduction) reaction.
For example this happens through electrochemical processes. The corrosion
behaviour of orthodontic wires is very important for the biocompatibility of
Nitinol. [91].
Shabalowskaya and Eliades (2002) [78] presented the different corrosion
morphologies. Chaturvedi (2009) [9] summarized these very effectively in the
following table.
Tab. 5: Different types of corrosion commonly occurring in the oral cavity [9]
Fig. 5: Diagrammatic summary of various types of corrosion [9]
20
Pitting corrosion occurs in brackets as well as in wires, crevice corrosion in
loci exposed to corrosive environments. Fretting corrosion develops when the
archwire slides in the bracket slot (wear oxidation) [20].
Titanium is known as a very biocompatible metal, so that the main interest in
studying the corrosion behaviour of Nitinol was directed to Nickel.
Nonetheless Ti ions release implies a deterioration of the protective oxide film
on the wire surface [36]. The alloy with nickel reduces the natural resistance
to corrosion of titanium [61].
In their researches several authors could demonstrate that nickel leaching
from Nitinol wires is lower than that of stainless steel (AIRA 761L) ones, even
if the content of this metal is about the sixth magnitude in the former than in
the latter [41, 51]. During the orthodontic treatment with fixed appliances the
friction from the sliding mechanic and consequent abrasion plays an important
role [60].
Electrolyte
pH
Material’s surface state
Roughness, due to product
processing or wear use
Temperature
Geometry factors such as
sharp-angled, edged faces
Composition, such as
inhomogeneities or
residues from product
finishing
Applied stress, leading to
higher susceptibility to H+ attack
Chemical composition.
Presence of Cl-, F-, O
Tab. 6: Parameters affecting corrosion [23]
The corrosion resistance is due to the oxide layer which has been recognised
to be granted principally from a TiO2 passivation film [79]. Nonetheless this
protective layer is susceptible to mechanical and chemical disruption [35].
Other studies show that corrosion resistance of the wire surface deteriorates
with heath treatment [66, 76].
For the study of corrosion I could find three kinds of approaches.
In-vitro studies:
21
- with immersion in different synthetic fluids
- with application of potentiostatic or potentiodynamic polarisation tests
In-vivo studies:
- on retrieved used archwires (ex-vivo)
- on salivary, plaque and serum samples
Literature reviews about corrosion of orthodontic alloys (as almost discussed
above).
Trying to recreate similar conditions as in the oral cavity, various in vitro tests
were carried out through immersion of different types of Nitinol archwire
specimens in artificial saliva (for example: modified Fusyama artificial saliva,
modified Meyer’s solution, Indiana University artificial saliva, modified Gjerdet
& Hero simulated saliva, Sali Lube saliva substitute) or 0,9% NaCl [46, 19, 65]
at different acidities (pH from 2,3 to 6,75). Two authors didn’t give any
information about the pH value of the tested solutions [19, 47]. Generally the
tests were made without any treatments of the material (as-received state of
the wires). With only one exception [47], the immersion tests were made at a
temperature of 37°C for periods ranging from 2 hours to 12 weeks. The metal
ion release was measured with mass spectrometry.
In all immersion studies ion release was detectable and graded as noncytotoxic [1, 41, 46, 52] and quite lower in comparison to average daily dietary
intake [5, 36, 38, 52]. The higher ion release was measured in the initial
testing time [5, 38, 52]. The most important variables that influence the metal
ion release seem to be the chemical composition of the archwires, the acidity
value of the immersion fluid and the time of exposure [36, 52].
Eliades et al. (2002) [21] make a harsh critic about in vitro studies with
artificial
saliva
because
these
solutions
are
“nonagitated”
and
“nonreplenished” in comparison to natural saliva, so that “the release rate is
rapidly forced to reach (…) an equilibrium (…)”. The critical view on in-vitro
cytotoxicity studies has been shared by Kasacka et al. (2006) [44, 45] who
underline that “in vitro the cells are free of (…) the influence of the organism
and adapt to the particular (…) microenvironment in which they are
22
maintained”, so that even long-term immersion does not result in higher ion
release rates because of the saturation of the used solution [20].
Also other authors underline that in-vitro results can’t be automatically applied
on the clinical situation [66, 89]
Several in-vitro studies tested Ni-Ti alloy archwires in an electrochemical cell
with
artificial
saliva
(with
various
compositions
and
acidities)
with
potentiostatic and potentiodynamic polarisation tests.
Potentiosatic polarisation. This test is electrochemical in nature and
determines when a metal or alloy passes from a passive to an active state
[62]. For this kind of testing a electrochemical corrosion cell, a data acquisition
device (a software for electrochemical tests) and a potentiostat are needed. A
corrosion cell consists of two electrodes (a polarisation cell and a reference
cell) connected by a salt bridge (an electrolyte bath). The potentiostat
monitors the potential between the working and the reference electrode
maintaining the desired potential constant. The material specimens are tested
with different potentials to determine when the passive layer breakdown
occurs [50].
Potentiodynamic polarisation. As in the former test electrodes, an electrolyte
solution and a potentiostat are needed, but the alloy samples are here
undertaken to cyclic polarization tests in which different potentials are
increased stepwise (forward direction) and decreased (reverse polarisation).
The higher the current density is, the more the tendency of a material to
corrode at a given potential [49]. The results give information about the
repassivation (see paragraph above) ability of the compound [76].
Almost all authors agree that electrochemical corrosion tests show
characteristic breakdown potentials (Eb) and a tendency to pitting corrosion at
these potentials in the passive regions [49, 55, 62, 66, 89, 91]. Additionally it
was demonstrated that corrosion is probably due to the surface morphology
[49], a rougher archwires surface does not influence negatively the corrosion
behaviour [28, 37], presence of aluminium on the wire surface lower the
corrosion potential [4] and that an increase in temperature influences
negatively the corrosion resistance (4, 66).
23
Already Edie, Andreasen & Zaytoun in 1981 [18] presented a study about the
surface corrosion of retrieved archwires. A similar study of Eliades et al.
(2004) [19] comparing the corrosion surface morphology of Ni-Ti alloy to
stainless steel ones, couldn’t find any difference between this two materials.
The second authors compared the retrieved archwires even with unused ones
also here not finding any difference in the corrosion patterns. Petoumeno et
al. (2008) [64] analysed only Ni-Ti wires but controlled samples of nonstimulated saliva. They conclude that “corrosion defects (…) occurred
extremely seldom in the clinical setting”. The inspections were done with
scanning electron microscopy (SEM). All these authors underline that it wasn’t
possible to evaluate the whole surfaces of the retrieved wires because of the
organic layer (plaque residue).
The in-vivo studies referred to the salivary, plaque or serum levels of Ni in
patients with fixed orthodontic appliances. The authors took samples of
unstimulated saliva and excluded patients with dental restorations, systemic
diseases and who took medications. The importance of taking samples of
unstimulated saliva is related to the fact that the secretion of the saliva is
regulated by the autonomic nervous system. The parasympathetic branch
provides the main stimulus (high flow rate of watery saliva) for salivation,
compared with the sympathetic stimulation (low flow rate and viscous saliva)
produced by the submandibular glands. When the salivation is stimulated by
chewing, the parotids gland is activated with a change in the protein
composition of the saliva. This is an unfavourable condition due to the
tendency of nickel to combine with proteins [63] and in this combination easily
activates the immune system.
The nickel release in the saliva (between 4,12 and 11,53 ppb) was far below
the average daily dietary intake. The relative high level of nickel in serum
(between 7,87 and 10,27 ppb) was probably due to the stainless steel
venipuncture needle contamination [1].
Fors & Persson (2006) [26] measured the nickel levels in saliva and plaque of
patient with fixed orthodontic appliances in comparison to a control group
24
without orthodontic treatment. The levels were very low in the filtered saliva
samples but high in the plaque samples.
Even if in these studies Nitinol archwires were used, the findings were not in
direct connection to this material. Only Petoumenou et al. (2009) [63] could
demonstrate an increase of the nickel salivary concentration after insertion of
the Ni-Ti archwires. The patients were tested before and after bonding of the
fixed appliance and 2 weeks later when the Ni-Ti archwires were legated.
Interesting is also the decrease of Ni concentration at lower levels than the
start measure 4 weeks before. In any case these concentrations were always
lower than the average daily dietary intake.
Two authors researched the blood levels in patients wearing fixed orthodontic
appliances. There was not found any increase in blood levels of Ni in
orthodontic patients [1, 6]. Furthermore the nickel levels in serum of patients
wearing fixed orthodontic appliances might change during the orthodontic
treatment [1].
Surface treatment
Titanium and Ni-Ti alloys are very reactive and form a spontaneous oxide
passive film (passivation) almost at room temperature [61, 76]. They also
undergo oxidation in an electrochemical environment or under heat treatment
(in a furnace, steaming at 125°C, boiling in water) at high temperatures
(raging from 450°C to 900°C) or electrolytic treatment producing a superficial
layer of titanium oxide, mostly with the stechiometry TiO2 [21, 34, 79]. To
passivate the material surface it is also possible to undertake the alloy to
electro-polishing procedures [76].
The TiO2 oxidation layer on the surface of Nitinol archwires gives a certain
protection from corrosion [23, 36, 55, 79]. Tooth brushing [38] and the
mechanical grinding forces exerted by chewing [47] partly remove this
protective layer. The passive layer can also be easily damaged because of
the bending forces at which the archwires are undertaken for orthodontic
applications [55].
The oxygen deplete in the deep layer of plaque disturbs the regeneration of
25
the protective passive oxide layer (repassivation) [20].
The nitrogen ionisation leads to a nitride surface coating of the passivation
oxide layer. Kim & Johnson 1999 [50] studied the breakdown potentials of
nitride and epoxy coated nickel-titanium archwires. They measured low
potential for the first wires and high for the last. This showed a good corrosion
resistance only for the epoxy-coated archwires. To totally different results
come Gil et al. (2004) [29]. They also tested the corrosion resistance of
nitride-coated wires, finding out an improving of corrosion resistance. These
opposite results may be due to the fact that Kim & Johnson tested the wire
specimens as-received with potentiostatic anodic polarisation in saline
solution (0,9% NaCl) and Gil et al. heat treated (at 900°C) and quenched the
archwires in water and immersed the samples in artificial saliva without
electric testing. Even the use of Ni-Ti archwires from different manufacturers
can have affected the results.
Horiuchi et al. (2007) [34] treated the surface of Ni-Ti disks 1mm thick with
photocatalysis with ultraviolet (UV) light. They electrolyte treated and heat
treated the specimens before undertaking them to UV light exposure. This
could produce a Ni-free oxide film because electrolytic treatment prevents the
formation of Ni oxidation. The authors believe that trough modifying the
amorphous surface oxide film improved corrosion resistance was achieved.
In a recent article (2009) Shabalowskaya [76] explains how the distribution of
Ni in the surface sublayers is responsible for the release of this metal. Ni-Ti
wires were tested in as-received state and after straining. The low breakdown
potentials in potentiodynamic polarisation tests of the as-received samples
showed improving in corrosion resistance after heath treatment at 700°C and
weren’t affected by successive straining. But the wires with the thickest
surface TiO2 layer showed the highest Ni release. This was attributed to the
presence of isles of essentially pure nickel on the surface and due to the
corrosion cracking associated with thicker oxides.
Static & dynamic conditions
26
Most studies have taken care of the corrosion behaviour of Nitinol for several
medical uses (principally for implantation in surgery and as orthodontic
archwire) tested in different solutions and fluids under static conditions.
But the reaction of this alloy under loading stress is different. In spite of the
predominance of the works on Nitinol for medical and surgical applications,
until 2001 systematic researches about corrosion behaviour under dynamic
loads were missing in the experimental studies for the use of this alloy in
human surgery.
In opposition the corrosion behaviour in stress conditions has been studied in
the orthodontic and dentistry fields since the early ´90s.
The straining stress increases the energy at the surface of the wires so that
the activation energy necessary for the metal ion release in electrolyte
solution gets lower [27]. The author demonstrated an increase in the corrosion
rate of NiTinol and Sentalloy® archwires under bending stress. This
happened in the passive regions showed by a higher carrion current density (I
corr)
at pH 2 in artificial saliva as stress influences the electrochemical
properties of the wires [55]. Even Jia et al. (1999) [43] found that the nickel
release in an in-vitro testing of NiTi wires in artificial saliva is higher under
cyclic straining. To same results came Kerosuo et al. (1995) [47] with
simulated fixed orthodontic appliance testing samples immersed in 0,9 %
NaCl by dynamic loading. To contrary results came Arndt et al. (2005) [4]:
they found no difference in the corrosion behaviour between static and
dynamic conditions. In this in-vitro experiment the results showed that the
application of mechanical loads in combination with increase of temperature
(to 50 °C) enhances the release of bulk Ni ions with only two exceptions
(Titanol® low force-Forestadent; Nitinol classic-Unitek). The differences were
rather given from the specific wire composition and surface processing.
Oral environment
Barret et al. in 1993 [5] wrote: “The oral environment is particularly ideal for
the biodegradation of metals because of its ionic, thermal, microbiologic and
enzymatic properties”. In an aqueous solution a metal can have the tendency
to pass from solid state to ionic form when associated with energy decrease if
27
it is thermodynamically unstable [50].
The composition of the saliva depends on plasma composition and several
factors like: desquamating oral epithelial cells, leukocytes, blood morphotic
elements, bacteria and various biologically active substances which are in
suspension [43]. All tissues and materials in the mouth are exposed to the
metabolism of around 30 species of bacteria [9]. Bacteria reside on the film of
dental plaque [53]. The biofilm itself results from food debris and the
metabolic products of the microbes [10]. Dissolved oxygen, variable pH and
temperature and nutrients affect the bacterial growth [8].
Every time we eat something the specific composition of our saliva changes.
Not only has the quality of our nutrition, but even our breathing habits
influence on the characteristics of our saliva (quantity, quality and microbial
population) [10, 57]. Some factors contribute to the acidic oral conditions: a
diet rich in sodium chloride, acidic carbonate drinks, but also fluoridecontaining products. These conditions accelerate the titanium oxide film
dissolution [35].
The natural accumulation of plaque in the oral cavity on teeth and mucosae
and the resulting growth of the bacterial colonies cause an acidification of the
environment [9] through the products (lactic, acetic and propionic acids) of the
metabolization of fermentable carbohydrates by bacteria like Streptococcus
mutans or Lactobacillus Casei. This pH fall is caused by the formation of
sulphuric acid from the combination of sulphate (originated by the Sulphate
Oxiding Bacteria) with the hydrogen present in the oral cavity, influencing
negatively the corrosion of metallic appliances [10]. The regular oral hygiene
procedures dump this acidity throw plaque removal.
Microbiological-related corrosion is the result of this acidification. This is “the
deterioration of a metal by corrosion processes that occur directly or indirectly
as a result of the activity of living microorganism” [8]. Microorganisms and
their by-products affect metal alloys in two ways: directly by metabolizing
metal from the compound (resulting in corrosion) or indirectly by acidizing the
environment with the by-products (increasing the conductivity) [35].
Chang et al. (2003) [8] has investigated the corrosion behaviour of dental
28
materials in presence of Streptococcus mutans and his by-products. The
progress of plaque formation at first sees the bacterial colonisation in a very
thin layer usually of about 10 µm to 500 µm, called biofilm. These microbes
are aerobic. The growth of the bacterial colonies and the accumulation of their
by-products (as organic acid and hydrogen sulphide) and of metal oxides
forms a tubercule. The bacteria under the tubercule affect the anodic
dissolution reaction [9]. The aerobic bacteria consume oxygen and a
favourable environment for anaerobic microorganisms is created. In an
aerobic system the reduction of oxygen is the cathodic reaction. The aerobic
microorganisms in the biofilm affect primarily the initiation of corrosion and the
ones under the tubercule have an influence on the propagation of corrosion.
“The most important part of the microbiology-related corrosion lies in the thin
biofilm under a tubercule on the metal surface.”
In his study about electrochemical behaviour of microbes on orthodontic
archwires Maruthamuthu et al. (2005) [57] investigated saliva samples
collected from rural (mostly vegetarian) and urban people of an Indian city.
After dilution, the samples were plated for bacterial culture. The bacterial
composition was analysed. The authors could find a predominance of Grampositive to Gram-negative bacteria of a magnitude of 4 to 6:1 in the saliva.
They found heterotrophic aerobic bacteria, iron and manganese oxidising
bacteria, acid producing bacteria and sulphate reducing bacteria.
In the next step of the study Ni-Ti and stainless steel wire specimens were
immersed in artificial saliva sterile or inoculated with bacteria. Polarization
measurements were undergone. The results show a higher breakdown
potential in presences of microbes and that the passivity of Ni-Ti is improved.
Therefore it can be assumed that the presence of bacteria influences the
resistance of the surface oxide-film of the archwire.
Fig. 6: Mechanism of biocorrosion on orthodontic wires in oral cavity [57]
Electrogalvanism
This phenomenon occurs when metals or alloys with different corrosion
29
potentials, bathed in an electrolytic solution, are in proximity. The
electronegative metal becomes the anode and the noble one (or the more
electropositive) functions as a cathode [35], so that there is an electric
streaming in the electrolyte solution (saliva) and a consequent leaching of
metal ions from the surface of the metal with the higher corrosion potential
[42, 62]. The difference between the corrosion potentials (I
corr)
results in a
flow of electric current between the different components [9, 10]. The
electrochemical reaction occurs at the metal/solution surface [8]. This is the
typical bracket-archwire situation [9, 10, 38, 62]. When stainless steel (SS)
brackets are used in combination with Nitinol archwires, the bracket becomes
the weak link in the galvanic couple becoming the anode [39, 81]. This leads
to the opinion that in case of known nickel allergy SS brackets coupled with
Ni-Ti archwires should be avoided [62].
The extent of the corrosion potential is controlled by the concentration of the
components, the pH, the surface tension and the buffering capacity of the
electrolyte solution [10]. Due to galvanism different metals can be found in the
saliva.
Galvanic corrosion cells can also be found at different sites of the same metal
because of the different surface finishing and work hardening due to repeated
loading. Even the ligation of the archwire to the bracket increases the
reactivity of the metal creating an electrochemical potential along the wire
[35].
Even the surface ratio of the two alloys is a very important factor in galvanic
corrosion: a large cathode and a small anode are unfavourably resulting in
greater corrosion of the anodic compound [9, 10, 40].
Fig. 7: Representative galvanic current density of archwire alloys (NiTi, b-Ti, SUS 304,
CoCrNi) coupled with bracket alloys (SUS304, Ti) with area ratio of 1:2.35 during the first 24
hours of immersionin 0.9% NaCl solution. NiTi indicates nickel-titanium; b-Ti, btitanim;and
CoCr, cobalt-chromium. [39]
Fig. 8: Representative galvanic current density of archwire alloys (NiTi, b-Ti, SUS 304,
CoCrNi) coupled with bracket alloys (SUS304, Ti) with area ratio of 1:3.64 during the first 24
hours of immersionin 0.9% NaCl solution. NiTi indicates nickel-titanium; b-Ti, btitanim;and
CoCr, cobalt-chromium. [39]
30
At this point some works must be cited about the behaviour of Ti-Ni alloys in
presence of Fluoride. Some in-vitro studies showed how fluoride solutions
(with different pH) influence the surface morphology of orthodontic Ni-Ti wires.
Shiff et al. (2002, 2006) [73, 74] in a first study with potentiostatic polarization
could demonstrate that in fluoridated and acidified solutions Ni-Ti alloys have
a breakdown of the protective passivation layer. They conclude that it would
be advisable to limit the use of fluoridated solutions for people wearing
orthodontic appliances with Nitinol. In the other study the galvanic coupling of
Ni-Ti wire with stainless steel were tested after immersion in two in Germany
commonly used fluoridated mouth rinses. The corrosion potential (E
corr)
was
measured. The Ni-Ti wire acted as the anode releasing Ni ions.
In 2008 Noguchi et al. [61] presented the discoloration and dissolution effects
of acidulated fluoride-containing and peroxide solutions. Ni-Ti alloys showed
respectively a granular profile and pitting corrosion at the SEM. In the same
year Kwon et al. investigated the effects on Ni-Ti wires of fluoride released
from dental restorative materials in acidic artificial saliva. They found out that
hydrogen fluoride (HF) derived from the interaction with bacterial acid byproducts dissolves the protective titanium oxide layer, but the HF measured
was not able to corrode the tested wires. The corrosion resistance of Ti is lost
when the concentration of 30 ppm HF is reached.
Kao et al. (2007) [43] approach the problem by using acidic Fluoride solutions
evaluating the toxicity (with the MTT Test) of extracts from orthodontic wires
corroded
in
acidified
phosphate
fluoride
on
cell
cultures
(human
osteosarcoma cell lines -U2OS-). Their results show that acidified NaF
artificial saliva can cause cytotoxicity on the cell cultures.
Biological Reactions
31
“The biological response depends to a great degree on the cleanness of the
surface from contaminants that originate in the atmosphere, polishing
mixtures, lubricants, processing equipments and chemical solutions” [78].
The ion release of the alloys used in orthodontics often cause gingival and
periodontal inflammation [58]. Even an inflammation-independent orthodontic
treatment–induced overgrowth of the gingiva was connected to the insertion
of a fixed orthodontic appliance with bands, brackets, nickel-titanium wires for
the alignment and following stainless steel wires [30]. Gingiva samples were
analysed with an atomic absorption spectrometry and different nickel
concentrations were tested on HaCaT non-malignat human skin keratinocytes
cultures. They concluded that low-dose continuing nickel release can be the
inducting factor for gingival overgrowth.
Typical reactions to a foreign material are mediated by the immune system in
form of hypersensitivity or allergy. Because of the high nickel content, despite
their excellent physical characteristics, Ni-Ti alloys are still controversial from
the point of view of their biocompatibility [71, 80].
Nickel allergy is a type IV cell-mediated (by T-lymphocytes and monocytes /
macrophages) delayed reaction. It has two phases: the sensitisation and the
elicitation phase. If with first contact, commonly through jewellery,
hypersensitivity is established, with the next contact all dermal and mucosal
tissues can be involved in form of a contact mucositis or dermatitis [60]. The
absorbed nickel binds with proteins and forms antigens that activate
specialized T-cells of the regional Lymphnodes [51].
Nickel ions combined with body proteins also act as a hapten [32] with
carcinogen and mutagen effects [10].
The antigenic potential of nickel when applied on the oral mucosa must be 5
to 12 times stronger than on the skin surface to produce an allergic reaction
[17, 52]. It is widely recognized that the incidence of nickel hypersensitivity is
higher in girls than boys because of the habit of ear piercing in young age or
using of jewellery. Nickel sensitization is increased by mechanical irritation,
skin maceration or oral mucosal injury as it often occurs during orthodontic
treatment [67]. The epicutan Patch test should not be used indiscriminately
32
because it could induce sensitivity in patients who were not sensitive before
testing [6]. Nevertheless Kerosuo et al. (1996) [48] consider the application of
Patch testing with nickel sulfate as safe and consider “the number of patients
allergic to nickel before and after patch testing the same”.
If nickel hypersensitivity is suspected because of the patient history of
allergies or atopies, a nickel leachability test should be done before inserting
Nitinol archwires. This test consists of immersion of a wire sample in a fresh
mixed up 1% dimethylglyoxime and 10% ammonium hydroxide solution. A
solution’s colour change to red of the solution shows a positive result [60].
Clinically relevant methods to estimate the nickel leaching from orthodontic
alloys are to measure the amounts in biological fluids as saliva, serum or
urine. It must be taken in consideration that 90% of the nickel we ingest with
alimentation is excreted in the faeces. When nickel is absorbed, it tends to
localize in the connective tissue, the kidneys and the lungs [51].
Until the ´80s nickel allergy hasn’t been discussed in orthodontics. There are
little epidemiologic data about nickel hypersensitivity in relation to fixed
orthodontic appliances [51]. The difficulty of diagnosing nickel hypersensitivity
is due to the fact that the manifestations of mechanical injury or poor oral
hygiene often mime that of nickel lesions [60]. A relation between nickel
allergy during orthodontic treatment and ear piercing before the orthodontic
treatment was found [81]. Children who started orthodontic treatment before
ear piercing had a significantly lower nickel hypersensitivity [48].
I could find very few articles that show a direct association between Nitinol
archwires and Nickel contact dermatitis. Dunlap et al. (1989) [17] documented
an allergic contact dermatitis (proved trough the microscopically evaluation of
a buccal mucosa biopsy) in 14-years-old female patient a few days after
insertion of a full mouth fixed orthodontic appliance with stainless steel
brackets, bands and nickel-titanium wires. Complete healing occurred 4 days
after removing the Nitinol wires.
Al Waheidi (1995) [2] wrote about a patient who suffered from burning
33
sensation and ulcerations of the oral mucosa and the lips some days after the
insertion of Nitinol archwires. The symptoms disappeared after removal of the
archwires. The second time he got Nitinol wires, he showed erythematous
macular lesions in the oral cavity and the labial mucosa. Again there was a
spontaneous remission after removing the archwires. The last time the patient
needed Ni-Ti wires, one week later he reported severe allergic reactions. The
author supposes that these reactions were due to the formation of Ni salts
from the ion release from the appliance and consequent accumulation in the
adjacent tissues. De Silva & Doherty (2000) [14] report the case of a 12years-old boy with perioral and periorbital eczema after insertion of a fixed
orthodontic appliance. He had neither piercing of the ears nor wear jewellery
and had no history of reactions to metals, but had lifelong atopic dermatitis.
Noble et al. (2008) [59] wrote about two cases in which allergic symptoms
could be connected to the use of Nitinol archwires, with complete remission
shortly after replacement of these wires with stainless steel ones. The authors
conclude that the symptoms must have been caused by the NiTi alloy.
Interesting is that in one case the nickel leachability was negative (the other
case wasn’t tested for nickel leaching). In neither case was a re-challenge or
a Patch-Test undertaken. The authors do not consider the galvanic corrosion
component of stainless steel brackets that occurs when these are used in
combination with Nitinol wires. The ignorance of this fact leads to the false
conclusion that Nitinol archwires induced the allergy symptoms. Coming to a
quite different conclusion as the authors above, Volkman et al. (2007) [88]
refer in their survey that reactions to orthodontic appliances are uncommon
and simple to manage as they are mostly intraoral.
Kolokitha (2009) [51] reported of a 27-year-old patient without history of
previous allergies of any nature but with ear piercing, who developed a rash
on her face 4 days after the surgical exposure of her impacted upper jaw
canines. Two weeks later she had a severe eczematic and urticarial reaction.
Patch testing revealed highly positive results for nickel. The fixed appliance
was removed and remission was gained. The therapy continued 7 months
later with ceramic brackets and coated Ni-Ti archwires. There were no signs
of allergic reactions. It is unclear if the allergic reaction was directly related to
34
the use of Nitinol or could be referred to the stainless steel brackets.
In his literature review about orthodontic appliances in relation to nickel
hypersensitivity Lindsten (1997) [54] concludes that “A patient who is not
nickel hypersensitive at the start of treatment is probably not at risk of
developing nickel hypersensitivity caused by the appliance”. It must be said
about this study that it does not concentrate directly on Nitinol, but reports
several studies in which also Ni-Ti alloys are investigated.
On the other hand oral antigenic contact in non-sensitized patients can induce
tolerance rather than sensitization to the metal [14, 75]. The level of nickel
detected in the saliva of patients with fixed orthodontic appliances has shown
not to be statistically different from those who didn’t undergo orthodontic
therapy [1].
From another point of view Venza et al. (2002, 2004) [86, 87] studied the
possible reaction of tissues with the use of Nitinol wires. They measured the
concentration of specific enzymes (polyamines: spermine and spermidine) in
the saliva of late pubertal patients wearing ceramic fixed appliance with Ni-Ti
archwires which led to gingivitis. These enzymes are present in tissues when
injury, inflammation or infection occurs. The patients had a good oral hygiene
and were patch-tested for nickel allergy with negative results. The high
concentration values of the enzymes observed during gingivitis were
independent of plaque accumulation. The inflammatory stimulus exerted by
Ni-Ti wires acted upon the predisposing conditions of late puberty (increased
levels of sexual hormones). The authors conclude that “very likely is the
gingivitis to be considered as aspecifically induced by the [Ni-Ti] archwire
appliance by itself” taken that none of the patients had a nickel allergy.
Even the study of Ramadan (2004) [68] was aimed to disclose the effects of
released nickel (and chromium) from fixed orthodontic appliances. Stimulated
saliva samples of 20 patients with Nitinol archwires in the first 3 months of
treatment and stainless steel wires in the next 9 months were collected. In 3
cases a hypersensitivity reaction to nickel was shown after 3 months in form
of gingival inflammation. The amount of nickel and chromium release was
higher in these three patients than in the rest. Sad to say this study gives only
35
a generic impression of the possible metal release from an orthodontic
appliance. A second critic point is that the author analyses stimulated saliva in
which the amount of leached metal ions is different to that present in normal
conditions in the oral environment.
36
Summary and Conclusions
From a comprehensive point of view it can be said that Nitinol is mostly
recognised as an appropriate material for orthodontic use due to its special
mechanical characteristics, the biocompatibility and sufficient corrosion
resistance.
In the majority of the experimental studies I reviewed, the research stretches
through a short period of time so that there is very little information about the
long term effects of Nitinol for orthodontic use in the human system. No follow
up investigation at all has been done about the late effects of Nitinol corrosion
products from orthodontic appliances.
Most articles concentrate on the study of the corrosion rates of metal ions
(mainly nickel and chromium) of commonly used orthodontic devices (braces,
bands and archwires) but only one author investigated the absorption and
effects of the released ions on the human tissues and physiology.
Very little has been reported about the effects on living tissues, then mainly in
form of in-vitro studies on cell-cultures.
Some authors found that Nickel induces hypersensitivity whereas some
others found that it induces tolerance. It seems to be related to the former
exposure to this metal. Commonly girls have ear piercings at a young age;
therefore their bodies and immune system come quite early in contact with
Nickel with a consequent sensitisation to it.
Even in patients with good oral hygiene there can be found plaque
accumulation in the bracket slots. This leads to a higher microbiological,
galvanic, frictional and stress related corrosion behaviour of the Nitinol
archwires. Particularly the problem of galvanic streams should be avoided
through usage of favourable metal couples.
Different studies show the positive effect of surface treatment or coating of the
archwires for corrosion resistance. The importance of the passivation titanium
oxide layer is given. All the influences of the microbiological environment, the
galvanic currents or acidulate fluoride solutions can lead to a loss of this
37
protective film.
There is a need for further investigation of the biological effects of Nitinol for
orthodontic use not only in archwires, but also for coils or other devices (such
as brackets) to establish what kind of alterations occur in the oral cavity
tissues and if dangerous alterations in distant district like as the excretion
organs can happen.
As Eliades (2004) [20] remarked, it is difficult to distinguish if the urinary or
blood levels of nickel in patients with fixed orthodontic appliances are a direct
consequence of corrosion processes from the band-bracket-archwire system
or if it results from diet. Additionally it must be taken in mind that there are no
specific examinations how much of the leaching nickel ions undergo an
accumulation process in the local tissues or in distant districts (mainly
connective tissue).
Apart from the question if the material “NiTinol” is biocompatible or not, the
problem is that this alloy is never used alone in the oral cavity so that the
interaction with other materials, metals and biological factors can greatly
change the otherwise well proofed biological qualities of this metal.
38
Acknowledgments
First I dearly want to thank Mrs. Adamietz Astrid for dealing with the
grammatical correction work.
I also want to thank Mag. MSc Grosshaupt Gerhard of the dental library of the
University in Vienna for supporting me in the literature research.
39
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
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Curriculum Vitae
Personal Data
Dr. (Univ. Padua) Astrid Longo
Thornerstraße 19
80993 München
Born in Verona (Italy) 29.06.1972.
Education
1991 Diploma di Maturita´ scientifica (high-school degree)
Liceo I. Nievo; Padova; Italy
1997 Laurea in Odontoiatria e protesi dentaria (university degree - 5 jears)
Universita´ degli Studi di Padova; Italy
2001 Abschluss Weiterbildung Oralchirurgie (Specialisation as Oral Surgeon)
BLZK; Bavaria; Germany
Since 2007
MSc Kieferorthopädie (MSc Orthodontics)
Donau Universität Krems; Austria
Professional Experiences
1997-1998
Dentist in Dental Office of Dr. Nikolaus Longo; Padova; Italy
1998-1999
Dentist at Klinikum rechts der Isar; TU München; Germany
1999-2001
Dentist in Oral-Surgery Office Dr. Stephan Wiens; GarmischPartenkirchen; Germany
2001-2005
Dentist in my Dental Office; Padova; Italy
2005-2007
Dentist in Orthodontic Office of Dr. Dorothea Laupheimer;
Laupheim;
Germany
Since 2008 Dentist in my Dental Office; Munich; Germany
51