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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 ([email protected])
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
becomebackbone materials for the aerospace, energy, and chemical
industries. Titanium and its alloys are used in dentistry as
prostheticappliances because of its unique combination of chemical,
physical, and biological properties. Cast and wrought form of
titaniumare used in dentistry. The basic properties of titanium
like biocompatibility, corrosion, strength, shape memory etc are
discussedin detail. These properties of titanium make it the
miracle metal in dentistry.
Review Article
IntroductionThe use of titanium and titanium alloys for medical
anddental applications has increased dramatically in recentyears.
Titanium (pronounced /tai teiniem) wasdiscovered in England by
William Gregor in 1791 andnamed by Martin Heinrich Klaproth for the
Titans ofGreek mythology [1]. Titanium is a chemical elementwith
the symbol Ti and atomic number 22. Sometimescalled the space age
metal. It is a light, strong, lustrous,naturally
corrosion-resistant (including to sea water andchlorine) transition
metal with a grayish color. This metalhas a high melting point at
around 1,7000C; one of thehighest melting points of any known
metal.
Many of titaniums physical and mechanical propertiesmake it
desirable as a material for implants andprostheses. Today, titanium
and titanium alloys are usedin so many biomedical applications; Cp
titanium is usedfor dental implants, surface coatings, and more
recentlyfor crown, partial and complete dentures, and
orthodonticwires. Several titanium alloys are also used.
Wroughtalloys of Ti with Ni and Ti with Molybdenum are usedfor
orthodontic wires. The term titanium is often used toinclude all
types of pure and alloyed titanium [2].
Commercially pure Titanium (Cp Ti)Cp Titanium is available in
four grades. The maindifferences among them is the concentration of
theoxygen (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 alloysPure titanium undergoes a transition from a
hexagonalclose packed structure ( phase) to a body centred
cubicstructure (phase) at 8830C. It remains in thiscrystallographic
structure until melting at 16720C.Elements such as Al, Ga, and Sn,
with the interstitialelements (C, O, and N) stabilize phase,
resulting inalpha titanium alloy. On the other hand, elements
suchas V, Nb, Ta, and Mo, stabilize the phase. Titaniumcan be
alloyed with various elements to change itscharacteristics,
primarily to improve the physical andmechanical properties, such as
strength, high temperatureperformance, creep resistance,
weldability, response toageing heat treatments, and formability.
[3, 4, 5] Alloyingelements can be added to stabilize one or the
other ofthese phases by either raising or lowering the
transitiontemperatures. Alloying elements are added to
stabilizeeither and phase, by changing transformationtemperature.
For example, in Ti-6Al-4V, aluminium isas stabilizer, which expands
phase field by increasingthe (+ ) to -transformation temperature,
where asvanadium, 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
Testingand Materials) recognizes four grades of commerciallypure
titanium (Cp Ti), or Ti, and three titanium alloys
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Titanium: A Miracle Metal in Dentistry
43
(Ti-6Al-4V, Ti-6Al-4V Extra Low Interstitial [lowcomponents] and
Ti-AlNb) [7]. The most widely usedtitanium alloy is the Ti-6Al-4V
alpha-beta alloy.
Applied Physical PropertiesBiocompatibility &
Osseointegration
Among all the other properties of titanium, the
excellentbiocompatibility is the most practical aspect for
theapplication in dentistry. This useful biological propertyof
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 producedwith various artificial techniques,
e.g., anodizing [9,10].This metals forms protective surface layers
of semi- ornonconductive oxides. Because of their isolating
effect,these oxides are able to prevent to a great extent a flowof
ions. This isolating effect is demonstrated by thedielectric
constants K of the various metal oxides. Theisolating effect of an
oxide layer with a dielectric constantis similar to that of water;
implants of Ti are not recognizedby the bone or tissue as foreign
body. Because of theirlarge surface, the primary corrosion products
areparticularly responsible for organic and inorganicreactions.
These corrosion products have differentthermodynamic stability with
only a low reactivity to theproteins of the surrounding tissue.
Biocompatibilitydepends on mechanical and
corrosion/degradationproperties of the material, tissue, and host
factors.Biomaterial surface chemistry, topography (roughness),and
type of tissue integration (osseous, fibrous, andmixed) correlate
with host response. Biocompatibility ofthe implants and its
associated structure is important forproper function of the
prosthesis in the mouth.
Titanium is relatively inert, corrosion resistance metalbecause
of its thin (approximately 4nm) surface oxidelayer. Studies have
shown that Titanium readily adsorbsprotein like albumin, laminin,
glycosaminoglycans,collagenase, fibronectins, complement
proteins,fibrinogens etc from the biological fluids.
Osseointegration is defined as the apparent directattachment or
connection of osseous tissue to an inert,alloplastic material
without intervening connective tissue[11-14]. Brnemark observed the
fusion of bone withtitanium chambers when he had placed them into
thefemurs of rabbits [15, 16, 17]. Surface
composition,hydrophilicity and roughness are parameters that may
playa role in implanttissue interaction and
osseointegration.Osseointegration firmly anchors the titanium
dental ormedical implant into place. Titanium is one of the
bestmetals that allows for this integration. Because it
isabsolutely inert in the human body, immune to attack frombody
fluids, compatible with the bone growth and strongand flexible,
titanium is the most biocompatible of allmetals the
osseointegration rate of titanium dentalimplants is related to
their composition and surfaceroughness or application of
osteoconductive coatings.Rough- surfaced implants favor both bone
anchoring and
biomechanical stability. Surface treatments, such astitanium
plasma-spraying, grit-blasting, acid-etching,anodization or calcium
phosphate coatings can be used[18].
Toxicity
Pure titanium and Ti6Al4V alloy have been mainly usedas implant
materials. V-free titanium alloys like Ti6Al7Nb and Ti5Al2.5Fe have
been then developed becausetoxicity of V has been pointed out. Al-
and V-free titaniumalloys as implant materials have been developed.
Mostof them are, however, + type alloys. type titaniumalloys
composed of non-toxic elements like Nb, Ta, Zr,Mo or Sn with lower
moduli of elasticity and greaterstrength have been developed
recently [19].
Chemical Properties
The oral cavity is subjected to wide changes in the pHand
fluctuation in the temperature. The disintegration ofthe 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
dentalalloys. Corrosion can severely limit the fatigue life
andultimate strength of the material leading to mechanicalfailure
of the dental materials [20].
Titanium has excellent corrosion resistance andbiocompatibility
in biological fluids. The influence ofcontaminants and surface
treatments of titanium implants(like alumina-blasting, acid
etching, anodization, hy-droxyapatite coating,) on osseous
integration has beenextensively studied [21]. Many authors have
studied thecorrosion behavior of commercially pure titanium
inartificial saliva. All the studies concluded that the
titaniumcorrosion resistance in these media is due to the
formationof an adherent and highly protective oxide film on
itssurface which is mainly formed of TiO2. [22-28].
Titanium is a thermodynamically reactive metal assuggested by
its relatively negative reversible potentialin the electrochemical
series [23]. It gets readily oxidizedduring exposure to air and
electrolytes to form oxides,hydrated complexes, and aqueous
cationic species. Theoxides and hydrated complexes act as barrier
layersbetween the titanium surface and the surroundingenvironment
and suppress the subsequent oxidation oftitanium across the
metal/barrier layer/solution interface.Even if the barrier layer
gets disrupted, it can get reformedvery easily, leading to
spontaneous re-passivation [26-33].
Clinical significance of corrosion
Resistance to corrosion is critical important for a
dentalmaterial because corrosion can lead to roughening ofsurface,
weakening of the restoration and liberation ofelement from the
dental alloy. Liberation of elements canproduce discoloration of
adjacent soft tissue and allergicreactions in susceptible patients.
The long term presence
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S. Gosavi, S. Gosavi, R. Alla
of corrosion reaction products and ongoing corrosion leadto
fractures of the alloy-abutment interface, abutment, orimplant body
[20, 21].
Strength
Many of titaniums physical and mechanical propertiesmake it
desirable as a material for implants and prostheses.Certainly, an
implant should be designed to be as strongas possible. Even in
everyday activities, you will placehigh levels of mechanical stress
on your bones and joints.The ideal implant should be able to
withstand thesestresses day to day for years without breaking
orpermanently changing shape. An implant should also bedesigned to
withstand the fatigue effect of theaccumulation of these repeating
stress cycles for anacceptable period of time (service life).
The strength and rigidity of titanium are comparable tothose of
other noble or high noble alloys commonly usedin dentistry [27,
28]. Titanium also can be alloyed withother metals, such as
aluminum, vanadium or iron, tomodify its mechanical properties.
The Cp-titanium grades are nominally all alpha () instructure,
whereas many of the titanium alloys have a twophase alpha + beta
structure. There are also titanium alloyswith high alloying
additions having an entire beta phasestructure, while alpha alloys
cannot be heat treated toincrease strength [34].
The mechanical properties of (+ ) titanium alloys aredictated by
the amount, size, shape, and morphology of phase and density of /
interfaces. Microstructureswith a small (< 20m) grain size, a
well dispersed phase, and a small / interface area such as in
equiaxedmicrostructures, resist fatigue crack initiation best
andhave the best high - cycle fatigue strength (approximately500 -
700 MPa). Lamellar microstructures have greater/ surface area and
more oriented colonies; have lowerfatigue strengths (approximately
300 500 MPa) thando equiaxed microstructures. Greger et al have
studiedthe mechanical properties of ultra-fine grain andconcluded
that the strength properties of commerciallypure titanium increased
significantly as a result of grainrefinement. Ultra-fine grain has
higher specific strengthproperties than ordinary titanium. Strength
of ultra-finegrain varies around 1250 MPa, grain size around 300
nm[29].
Another important characteristic of titanium- basematerials is
the reversible transformation of the crystalstructure from alpha
(hexagonal close-packed) structureto beta (body-centered cubic)
structure when thetemperatures exceed certain level. This
allotropicbehavior, which depends on the type and amount of
alloycontents, allows complex variations in microstructure andmore
diverse strengthening opportunities than those ofother 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. Accordingto Key
to Metals, titanium has a tensile strength ofbetween 30,000 and
200,000 lbs. per square inch.Titanium alloy contains roughly six
weight percent ofaluminum and four weight percent of vanadium,
whichdoubles its tensile strength relative to
commercially-puretitanium, 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 FrederickWang at
the U.S Naval Ordnance Laboratory discoveredthe shape memory effect
alloy of nickel and titanium,which can be considered a breakthrough
in the field ofshape memory (Buehler et al. 1967). This alloy was
namedNitinol (Nickel-Titanium Naval Ordnance Laboratory)
The first efforts to exploit the potential of NiTi as animplant
material were made by Johnson and Alicandri in1968 (Castleman et
al. 1976). The use of NiTi for medicalapplications 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 alloythat remembers its shape.
Nitinol possess a uniquecombination of properties, including
superelasticity orpseudoelasticity and shape memory, which are
veryattractive for biomedical applications. NiTi has been usedin
orthopedic and orthodontic implants [37, 38]. Theirability to
recover large strains and dissipate mechanicalwork without
macroscopic permanent deformation hasgenerated significant interest
in various field ology.Among all SMA, titanium nickel (TiNi) is the
mostimportant alloy [39].
The unusual properties of this smart material are derivedfrom
the two crystal structures that can be inter-convertedby changes in
temperature or pressure. At temperaturesbetween about 0 and 100C,
there are two importantphases or crystal structures of NiTi that
can be referredto as the high temperature and low temperature
phase, oras austenite and martensite, respectively. The
austenitephase has the symmetry of a cube and is characterized
byhardness and rigidity.
The wire sample of NiTi can be bent at room temperature,but will
return to its linear shape when heated by hot airor water as its
atoms move in a kind of atomic ballet.Moreover, the wire can be
heated to the much highertemperature (approximately 500C), where it
can betrained to remember a new shape. Subsequently, whenthe wire
is distorted at room temperature and heated byhot air or water, it
will return to this new shape [40, 41].
In dentistry, the material is used in orthodontics forbrackets
and wires connecting the teeth. Once the SMAis placed in the mouth
its temperature raises to ambientbody temperature. This causes the
Nitinol to contract backto its original shape applying a constant
force to move
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Titanium: A Miracle Metal in Dentistry
45
the teeth. These SMA wires dont need to be retightenedas often
as they can contract as the teeth move unlikeconventional stainless
steel wires. Additionally, Nitinolcan be used in endodontics, where
Nitinol files are usedto clean and shape the root canals during the
root canalprocedure [20].
Flexibility
While strength characteristics of implants are important,they
must also be somewhat flexible to avoid shieldingof bones from
stress (stress shielding). When stress isapplied to a stiff
dental/orthopaedic implant, the implantwill carry most of the
stress and the bone may start toresorb and may become less dense
and weak. On theother hand if stress is applied to a less stiff or
more flexibleimplant, some of the stress can be shared with
thesurrounding bone. This will help to keep the bone activeand
strong. Flexibility and elasticity rivals that of humanbone.
Titaniums modulus of elasticity and coefficient ofthermal expansion
matches those of human bone,reducing the potential for implant
failure. When betaalloyed (as with niobium and zirconium); titanium
is usedfor low modulus application. When alpha beta alloyed(as with
titanium) is used for applications requiring greatermodulus, such
as bone plates. The modulus (100 GPa) isalso about half the value
of other metals [35].
Retention of a partial denture depends on the amount ofundercut
engaged on an abutment tooth and the flexibilityof the clasp.
Flexibility is influenced by clasp length andthe denture base
material. Titanium clasps are purportedto have greater flexibility
than cobalt-chromium castclasps which should enable them to engage
deeperundercuts or be used where shorter clasp arms are neededsuch
as on premolar teeth [31, 32].
Density
The density of Cp Ti (4.5 g/cm3) is about half of the valueof
many of other base metals. Titanium is lighter than thestainless
steel (approximately 56% as dense) yet has ayield strength twice
and ultimate tensile strength almost
25% higher. This gives it a highest strength to weightratio of
any metal suited to medical use.
Non-magnetic
Commercially pure titanium and all the titanium alloysare non
magnetic. The physical difference betweenferromagnetic and
nonferromagnetic materials lies in thedegree of magnetization. [33]
Titanium is not susceptibleto outside interference and wont trigger
metal detector.[42] Another benefit to titanium for use in medicine
is itsnon-ferromagnetic property, patients with titaniumimplants
can be safely examined with magnetic resonanceimaging (convenient
for long-term implants).
ConclusionTitanium and titanium alloys, based on their physical
andchemical properties, appear to be especially suitable fordental
implants and prostheses. Titanium also shows alow toxicity, great
stability with low corrosion rates andfavourable mechanical
properties compared to othermetals make titanium as a miracle metal
for the biomedicalapplications. The combination of high
strength-to-weightratio, Lightweight, excellent mechanical
properties(Strong), corrosion resistance, Biocompatible,
Non-toxic,Long-lasting, Non-ferromagnetic, Osseointegrated
(thejoining of bone with artificial implant), Cost-efficient
andLong range availability makes titanium the best materialchoice
for many critical applications.
Based on clinical experience with wrought titanium
dentalimplants, wrought titanium crown and bridge applicationshave
been developed. With the advent of reproduciblehigh tolerance,
machining and processing techniques,such as spark erosion, laser
welding and micromachining,and computer aided design computer
aidedmanufacturing wrought titanium crowns are possible now.Future
applications are likely to include partial denturework, other
precision work, implant supported rests,and orthodontic
components.
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