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Trends Biomater. Artif. Organs, 27(1), 42-46 (2013) http://www.sbaoi.org/tibao
Titanium: A Miracle Metal in Dentistry
Sulekha Gosavi1, Siddharth Gosavi*1, Ramakrishna Alla2
1Dept of Prosthodontics, 2Dept of Dental Material, Krishna School of Dental Sciences, Malakapur, Karad [M.S]
*Corresponding author, Dr. Siddharth Gosavi (siddhu_gosavi@rediffmail.com)
Received 24 July 2012; Accepted 14 January 2013; Available online 14 January 2013
Since the introduction of titanium and titanium alloys in the early 1950s, these materials in a relatively short time have become
backbone materials for the aerospace, energy, and chemical industries. Titanium and its alloys are used in dentistry as prosthetic
appliances because of its unique combination of chemical, physical, and biological properties. Cast and wrought form of titanium
are used in dentistry. The basic properties of titanium like biocompatibility, corrosion, strength, shape memory etc are discussed
in detail. These properties of titanium make it the miracle metal in dentistry.
Review Article
Introduction
The use of titanium and titanium alloys for medical and
dental applications has increased dramatically in recent
years. Titanium (pronounced /tai’ teiniem) was
discovered in England by William Gregor in 1791 and
named by Martin Heinrich Klaproth for the Titans of
Greek mythology [1]. Titanium is a chemical element
with the symbol Ti and atomic number 22. Sometimes
called the “space age metal”. It is a light, strong, lustrous,
naturally corrosion-resistant (including to sea water and
chlorine) transition metal with a grayish color. This metal
has a high melting point at around 1,7000C; one of the
highest melting points of any known metal.
Many of titanium’s physical and mechanical properties
make it desirable as a material for implants and
prostheses. Today, titanium and titanium alloys are used
in so many biomedical applications; Cp titanium is used
for dental implants, surface coatings, and more recently
for crown, partial and complete dentures, and orthodontic
wires. Several titanium alloys are also used. Wrought
alloys of Ti with Ni and Ti with Molybdenum are used
for orthodontic wires. The term titanium is often used to
include all types of pure and alloyed titanium [2].
Commercially pure Titanium (Cp Ti)
Cp Titanium is available in four grades. The main
differences among them is the concentration of the
oxygen (0.18 to 0.40 wt%) and iron (0.20 to 0.50 wt%).
These slight differences in concentration have a
substantial effect on physical and mechanical properties
[2].
Titanium alloys
Pure titanium undergoes a transition from a hexagonal
close packed structure ( phase) to a body centred cubic
structure (phase) at 8830C. It remains in this
crystallographic structure until melting at 16720C.
Elements such as Al, Ga, and Sn, with the interstitial
elements (C, O, and N) stabilize phase, resulting in
alpha titanium alloy. On the other hand, elements such
as V, Nb, Ta, and Mo, stabilize the phase. Titanium
can be alloyed with various elements to change its
characteristics, primarily to improve the physical and
mechanical properties, such as strength, high temperature
performance, creep resistance, weldability, response to
ageing heat treatments, and formability. [3, 4, 5] Alloying
elements can be added to stabilize one or the other of
these phases by either raising or lowering the transition
temperatures. Alloying elements are added to stabilize
either and phase, by changing transformation
temperature. For example, in Ti-6Al-4V, aluminium is
as stabilizer, which expands phase field by increasing
the (+ ) to -transformation temperature, where as
vanadium, as well as copper and palladium, are
stabilizers, which expand the phase by decreasing the
(+ ) to -transformation temperature [6, 7, 8].
ASTM International (the American Society for Testing
and Materials) recognizes four grades of commercially
pure titanium (Cp Ti), or Ti, and three titanium alloys
Titanium: A Miracle Metal in Dentistry
43
(Ti-6Al-4V, Ti-6Al-4V Extra Low Interstitial [low
components] and Ti-AlNb) [7]. The most widely used
titanium alloy is the Ti-6Al-4V alpha-beta alloy.
Applied Physical Properties
Biocompatibility & Osseointegration
Among all the other properties of titanium, the excellent
biocompatibility is the most practical aspect for the
application in dentistry. This useful biological property
of titanium is based on the existence of titanium oxide
(TiO2) layers, which are naturally formed in oxygen-
containing environments. It is also possible to be produced
with various artificial techniques, e.g., anodizing [9,10].
This metals forms protective surface layers of semi- or
nonconductive oxides. Because of their isolating effect,
these oxides are able to prevent to a great extent a flow
of ions. This isolating effect is demonstrated by the
dielectric constants K of the various metal oxides. The
isolating effect of an oxide layer with a dielectric constant
is similar to that of water; implants of Ti are not recognized
by the bone or tissue as foreign body. Because of their
large surface, the primary corrosion products are
particularly responsible for organic and inorganic
reactions. These corrosion products have different
thermodynamic stability with only a low reactivity to the
proteins of the surrounding tissue. Biocompatibility
depends on mechanical and corrosion/degradation
properties of the material, tissue, and host factors.
Biomaterial surface chemistry, topography (roughness),
and type of tissue integration (osseous, fibrous, and
mixed) correlate with host response. Biocompatibility of
the implants and its associated structure is important for
proper function of the prosthesis in the mouth.
Titanium is relatively inert, corrosion resistance metal
because of its thin (approximately 4nm) surface oxide
layer. Studies have shown that Titanium readily adsorbs
protein like albumin, laminin, glycosaminoglycans,
collagenase, fibronectins, complement proteins,
fibrinogens etc from the biological fluids.
“Osseointegration” is defined as the apparent direct
attachment or connection of osseous tissue to an inert,
alloplastic material without intervening connective tissue
[11-14]. Bränemark observed the fusion of bone with
titanium chambers when he had placed them into the
femurs of rabbits [15, 16, 17]. Surface composition,
hydrophilicity and roughness are parameters that may play
a role in implant–tissue interaction and osseointegration.
Osseointegration firmly anchors the titanium dental or
medical implant into place. Titanium is one of the best
metals that allows for this integration. Because it is
absolutely inert in the human body, immune to attack from
body fluids, compatible with the bone growth and strong
and flexible, titanium is the most biocompatible of all
metals the osseointegration rate of titanium dental
implants is related to their composition and surface
roughness or application of osteoconductive coatings.
Rough- surfaced implants favor both bone anchoring and
biomechanical stability. Surface treatments, such as
titanium plasma-spraying, grit-blasting, acid-etching,
anodization or calcium phosphate coatings can be used
[18].
Toxicity
Pure titanium and Ti–6Al–4V alloy have been mainly used
as implant materials. V-free titanium alloys like Ti–6Al–
7Nb and Ti–5Al–2.5Fe have been then developed because
toxicity of V has been pointed out. Al- and V-free titanium
alloys as implant materials have been developed. Most
of them are, however, á+â type alloys. â type titanium
alloys composed of non-toxic elements like Nb, Ta, Zr,
Mo or Sn with lower moduli of elasticity and greater
strength have been developed recently [19].
Chemical Properties
The oral cavity is subjected to wide changes in the pH
and fluctuation in the temperature. The disintegration of
the metal may occur through the action of moisture,
atmospheric acid and alkaline other chemical agents.
Further it has been reported that water, oxygen, chloride,
sulphur corrodes various metal present in the dental
alloys. Corrosion can severely limit the fatigue life and
ultimate strength of the material leading to mechanical
failure of the dental materials [20].
Titanium has excellent corrosion resistance and
biocompatibility in biological fluids. The influence of
contaminants and surface treatments of titanium implants
(like alumina-blasting, acid etching, anodization, hy-
droxyapatite coating,) on osseous integration has been
extensively studied [21]. Many authors have studied the
corrosion behavior of commercially pure titanium in
artificial saliva. All the studies concluded that the titanium
corrosion resistance in these media is due to the formation
of an adherent and highly protective oxide film on its
surface which is mainly formed of TiO2. [22-28].
Titanium is a thermodynamically reactive metal as
suggested by its relatively negative reversible potential
in the electrochemical series [23]. It gets readily oxidized
during exposure to air and electrolytes to form oxides,
hydrated complexes, and aqueous cationic species. The
oxides and hydrated complexes act as barrier layers
between the titanium surface and the surrounding
environment and suppress the subsequent oxidation of
titanium across the metal/barrier layer/solution interface.
Even if the barrier layer gets disrupted, it can get reformed
very easily, leading to spontaneous re-passivation [26-
33].
Clinical significance of corrosion
Resistance to corrosion is critical important for a dental
material because corrosion can lead to roughening of
surface, weakening of the restoration and liberation of
element from the dental alloy. Liberation of elements can
produce discoloration of adjacent soft tissue and allergic
reactions in susceptible patients. The long term presence
44
S. Gosavi, S. Gosavi, R. Alla
of corrosion reaction products and ongoing corrosion lead
to fractures of the alloy-abutment interface, abutment, or
implant body [20, 21].
Strength
Many of titanium’s physical and mechanical properties
make it desirable as a material for implants and prostheses.
Certainly, an implant should be designed to be as strong
as possible. Even in everyday activities, you will place
high levels of mechanical stress on your bones and joints.
The ideal implant should be able to withstand these
stresses day to day for years without breaking or
permanently changing shape. An implant should also be
designed to withstand the fatigue effect of the
accumulation of these repeating stress cycles for an
acceptable period of time (service life).
The strength and rigidity of titanium are comparable to
those of other noble or high noble alloys commonly used
in dentistry [27, 28]. Titanium also can be alloyed with
other metals, such as aluminum, vanadium or iron, to
modify its mechanical properties.
The Cp-titanium grades are nominally all alpha () in
structure, whereas many of the titanium alloys have a two
phase alpha + beta structure. There are also titanium alloys
with high alloying additions having an entire beta phase
structure, while alpha alloys cannot be heat treated to
increase strength [34].
The mechanical properties of (+ ) titanium alloys are
dictated by the amount, size, shape, and morphology of
phase and density of / interfaces. Microstructures
with a small (< 20m) grain size, a well dispersed
phase, and a small / interface area such as in equiaxed
microstructures, resist fatigue crack initiation best and
have the best high - cycle fatigue strength (approximately
500 - 700 MPa). Lamellar microstructures have greater
/ surface area and more oriented colonies; have lower
fatigue strengths (approximately 300 – 500 MPa) than
do equiaxed microstructures. Greger et al have studied
the mechanical properties of ultra-fine grain and
concluded that the strength properties of commercially
pure titanium increased significantly as a result of grain
refinement. Ultra-fine grain has higher specific strength
properties than ordinary titanium. Strength of ultra-fine
grain varies around 1250 MPa, grain size around 300 nm
[29].
Another important characteristic of titanium- base
materials is the reversible transformation of the crystal
structure from alpha (hexagonal close-packed) structure
to beta (body-centered cubic) structure when the
temperatures exceed certain level. This allotropic
behavior, which depends on the type and amount of alloy
contents, allows complex variations in microstructure and
more diverse strengthening opportunities than those of
other nonferrous alloys such as copper or aluminum.
Titanium has a relatively high tensile strength; it takes
quite a bit of pressure to pull titanium apart. According
to Key to Metals, titanium has a tensile strength of
between 30,000 and 200,000 lbs. per square inch.
Titanium alloy contains roughly six weight percent of
aluminum and four weight percent of vanadium, which
doubles its tensile strength relative to commercially-pure
titanium, but reduces its ductility [30] The yield strength
(170 – 480 MPa) and ultimate strength (240 – 550 MPa)
varies depending on the grade of titanium [35].
Shape memory
In the early 1960s, William Buehler along with Frederick
Wang at the U.S Naval Ordnance Laboratory discovered
the shape memory effect alloy of nickel and titanium,
which can be considered a breakthrough in the field of
shape memory (Buehler et al. 1967). This alloy was named
Nitinol (Nickel-Titanium Naval Ordnance Laboratory)
The first efforts to exploit the potential of NiTi as an
implant material were made by Johnson and Alicandri in
1968 (Castleman et al. 1976). The use of NiTi for medical
applications was first reported in the 1970s [36].
Nitinol [also known as a shape memory alloy (SMA),
smart alloy, memory metal, or muscle wire] is an alloy
that “remembers” its shape. Nitinol possess a unique
combination of properties, including superelasticity or
pseudoelasticity and shape memory, which are very
attractive for biomedical applications. NiTi has been used
in orthopedic and orthodontic implants [37, 38]. Their
ability to recover large strains and dissipate mechanical
work without macroscopic permanent deformation has
generated significant interest in various field ology.
Among all SMA, titanium nickel (TiNi) is the most
important alloy [39].
The unusual properties of this smart material are derived
from the two crystal structures that can be inter-converted
by changes in temperature or pressure. At temperatures
between about 0 and 100°C, there are two important
phases or crystal structures of NiTi that can be referred
to as the high temperature and low temperature phase, or
as austenite and martensite, respectively. The austenite
phase has the symmetry of a cube and is characterized by
hardness and rigidity.
The wire sample of NiTi can be bent at room temperature,
but will return to its linear shape when heated by hot air
or water as its atoms move in a kind of “atomic ballet.”
Moreover, the wire can be heated to the much higher
temperature (approximately 500°C), where it can be
trained to “remember” a new shape. Subsequently, when
the wire is distorted at room temperature and heated by
hot air or water, it will return to this new shape [40, 41].
In dentistry, the material is used in orthodontics for
brackets and wires connecting the teeth. Once the SMA
is placed in the mouth its temperature raises to ambient
body temperature. This causes the Nitinol to contract back
to its original shape applying a constant force to move
Titanium: A Miracle Metal in Dentistry
45
the teeth. These SMA wires don’t need to be retightened
as often as they can contract as the teeth move unlike
conventional stainless steel wires. Additionally, Nitinol
can be used in endodontics, where Nitinol files are used
to clean and shape the root canals during the root canal
procedure [20].
Flexibility
While strength characteristics of implants are important,
they must also be somewhat flexible to avoid shielding
of bones from stress (“stress shielding”). When stress is
applied to a stiff dental/orthopaedic implant, the implant
will carry most of the stress and the bone may start to
resorb and may become less dense and weak. On the
other hand if stress is applied to a less stiff or more flexible
implant, some of the stress can be shared with the
surrounding bone. This will help to keep the bone active
and strong. Flexibility and elasticity rivals that of human
bone. Titanium’s modulus of elasticity and coefficient of
thermal expansion matches those of human bone,
reducing the potential for implant failure. When beta
alloyed (as with niobium and zirconium); titanium is used
for low modulus application. When alpha beta alloyed
(as with titanium) is used for applications requiring greater
modulus, such as bone plates. The modulus (100 GPa) is
also about half the value of other metals [35].
Retention of a partial denture depends on the amount of
undercut engaged on an abutment tooth and the flexibility
of the clasp. Flexibility is influenced by clasp length and
the denture base material. Titanium clasps are purported
to have greater flexibility than cobalt-chromium cast
clasps which should enable them to engage deeper
undercuts or be used where shorter clasp arms are needed
such as on premolar teeth [31, 32].
Density
The density of Cp Ti (4.5 g/cm3) is about half of the value
of many of other base metals. Titanium is lighter than the
stainless steel (approximately 56% as dense) yet has a
yield strength twice and ultimate tensile strength almost
25% higher. This gives it a highest strength –to – weight
ratio of any metal suited to medical use.
Non-magnetic
Commercially pure titanium and all the titanium alloys
are non magnetic. The physical difference between
ferromagnetic and nonferromagnetic materials lies in the
degree of magnetization. [33] Titanium is not susceptible
to outside interference and won’t trigger metal detector.
[42] Another benefit to titanium for use in medicine is its
non-ferromagnetic property, patients with titanium
implants can be safely examined with magnetic resonance
imaging (convenient for long-term implants).
Conclusion
Titanium and titanium alloys, based on their physical and
chemical properties, appear to be especially suitable for
dental implants and prostheses. Titanium also shows a
low toxicity, great stability with low corrosion rates and
favourable mechanical properties compared to other
metals make titanium as a miracle metal for the biomedical
applications. The combination of high strength-to-weight
ratio, Lightweight, excellent mechanical properties
(Strong), corrosion resistance, Biocompatible, Non-toxic,
Long-lasting, Non-ferromagnetic, Osseointegrated (the
joining of bone with artificial implant), Cost-efficient and
Long range availability makes titanium the best material
choice for many critical applications.
Based on clinical experience with wrought titanium dental
implants, wrought titanium crown and bridge applications
have been developed. With the advent of reproducible
high tolerance, machining and processing techniques,
such as spark erosion, laser welding and micromachining,
and computer aided design – computer aided
manufacturing wrought titanium crowns are possible now.
Future applications are likely to include partial denture
work, other precision work, implant – supported rests,
and orthodontic components.
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