Dental Implant Materials: Commerciallv
Pure Tita&um and Titanium Alloys
I
Michael McCracken, DDS,MSBE
Manufacturers use six different titanium-based biomaterials to fabricate dental implants. Each
of these materials, including four grades of commercially pure titanium and two titanium alloys,
has distinct mechanical and physical properties. Clinicians should recognize these differences for
optimal treatment planning and patient care.
J Prosthod 1999;8:40-43.Copyright o 1999 by The American College of Prosthodontists.
INDEX WORDS: implant biomaterials, biomechanics, review
M
ANY CLINICIANS recognize only two types of
titanium implant biomaterials: commercially
pure (cp) titanium and titanium alloy. Among these
two general groups, however, are six distinct matcrials defined by the Americ,an Society for Testing and
Materials (ASTM). All six of these materials, which
include four grades of cp titanium and two titanium
alloys, are commercially available. The mechanical
and physical properties or these materials differ
significantly. The purpose of this article is to review
these properties and to describe their impact on
treatment decisions.
Titanium appears on the periodic table as clemcnt 22, a fourth-row transition metal with an
atomic weight of 47.88. An extremely reactive metal,
titanium forms a tenacious oxide layer that contributes to its electrochemical passivity. Titanium is the
ninth most abundant element and is plcntiful in the
Earth's crust1 (Table 1). The principal titanium ore
reserves, rutile and ilmenite, are found in abundance
in the United States, Canada, and Australia. Though
the bulk of titanium ore is mined for use in the
pigment industry, 5% to 10% of titanium ore is used
to produce cp titanium and titanium alloys.2
The element was discovcrcd by Wilhcim Gregor, a
clergyman, who found the metal in a "black magnctic
sand" in Cornwall in 1791. Three years later, Klaproth found a rutile that was the oxide of a new metal
he named titanium, after the Greek Titans. He
recogninized that this metal was identical to the
material Gregor had discovered."
Not until the refining process was devcloped did
From the Department of Restoratiue Dentisty, Unizmsi[v of Alabama
School ofDentisty, Birmingham, AL.
AcceptedDecember29,1998.
Correspondence to: Muhael McCrachn, DDS, Lkpmtment wf Restvratiue Dentis@, Chiurnsip of Alubama Schuol ufDenlislry, I919 Szuenlh
Aoenue South, Birmingham, AL 35294.
Cofiyright 0 1999 by TheArnPrican College offiusthodontish
1059-941X/9~IO801-0006~~.00/0
40
commercial production become viable. In 1925, van
Arkel refined the ore using titanium tetraiodide,
producing a metal with acceptablc propcrties and
ductility. In the 1930s, Krol developed commercial
cxtraction procedures that are still used today?
As might bc expected, the properties of titanium
change with alloyingpr~cedurcs.~
Of particular interest in the field of health care deTiiccs is Ti-6H1-4V
alloy. Titanium breaches other markets as well:
titanium tennis racquets, golf clubs, and eyeglass
frames are now available. Titanium exhibits biocompatibility, corrosion resistance, a high strength-towcight ratio, and reasonable ma~hinabiIity."-~
Physical PqfxWies of Titanium
The atomic structure of titanium is ls2, 2S', ?p6,3s2,
3pG,3d2,4s2.The lightly held 3d2and 4s' electrons are
highly reactive and rapidly form a tenacious oxidc
that is responsible for the metal's biocompatibility.
The remaining electrons are relatively stable and
tightly bound.9 At temperatures up to 882"C, pure
titanium exists as a hexagonal close-packcd atomic
structure (alpha phase). Above that temperature, the
structure is body-centered cubic (beta phase). The
metal melts at l,665°C.10
The elcmcnts oxygen, aluminum, carbon, and
nitrogen stabilize thc alpha phase of titanium because or their increased solubility in the hexagonal
close-packed structure. Oxygen occupies intcrstitial
sites. Elements that stabilize the beta phase include
manganese, chromium, iron, and vanadium. In contrast to most hexagonal metals, titanium displays
good ductility.''
ASTM Committee F-4 on Materials for Surgical
Implants recognizes four gradcs of commercially
pure titanium and two titanium alloy^.^'-^^ The twro
alloys are Ti-6Hl-4V and Ti-6Hl-4V extra low interstitial (ELI).l3>I4The commercially pure titanium
materials are commercially pure grade I titanium,
Journal afl'msthodontics, Vol8,N o 1 (March), 1999:pp 40-43
41
March 1999. Volume 8, Number 1
commercially pure grade II titanium, commercially
pure grade IT1 titanium, and commercially pure
g-radc IV titanium. Commercially pure titanium is
also referred to as unalloyed titanium. All six of these
materials are commercially available as dental implants.
Other Implant Materials
Stainless stccl, particularly 3 16L stainlcss steel, continues to be uscd as an implant material for bone
plates and screws. A typical 316L stainless steel
composition would be 18% chromium, 12% nickel,
2% molybdenum, and 0.03% carbon. Although this
alloy is stronger, cheaper, and easier to machine, its
corrosion properties are inferior to titanium. For this
reason, it has not been approved as a dental implant
material.'0,'5
Cobalt-based alloys have been used for decades to
make cast partial dcnture frameworks. Typically,
these alloys contain 62% cobalt, 3 1% chromium, 5%
molybdenum, and trace amounts of iron, magnesium, silicon; and carbon.'&These alloys cast well and
have sufficient strength to withstand the occlusal
forces applied to partial denture frameworks. Although not as resistant to corrosion as titanium,
cobalt alloys exhibit reasonable biodegradation properties when cxposcd to human tissues. This corrosion
resistance arises from an oxide layer, Cr203, that
forms on the surface of the alloy. These alloys are
frequently used to fabricate hip prosthe~es.~'
Mechunical Properties of cp Titanium
and Alloys
The mechanical properties of titanium, titanium
alloys, and other natural and implant materials are
listed inTable 2. It is important to note that while the
modulus of elasticity of cp grade I titanium to cp
grade IV titanium ranges from 102 to 104 GPa (a
change of only 2%), the yield strength incrcascs from
170 to 483 MPa (a gain of 180%). Reasons for the
changes are described below and are related chiefly
to oxygen residuals in the metal (Table 3).
The characteristic trcnd of increasing strength
with relatively constant modulus continues when
comparing cp titanium with titanium alloys. The
elastic modulus of the alloys is slightly higher (113
MFa compared with 104 MPa of cp grade IV titanium), but the yield strength increases over 60% to
795 MYa for ELI alloys and 860 MPa for Ti-GAl-4V
alloys. Typically, fatigue strength limits arc less than
50% of the ultimate tensile strendi."
v
Table 1. Weight Percent of Selected Elements Found in
Igneous Rock
Element
Percent (wt%)
46
28
8
5
4
3
3
Oxygen
Silicon
Aluminum
Iron
Calcium
Sodium
Potassium
Magnesium
Titanium
Manganese
Zirconium
Nickel
Copper
Lead
2
0.6
0.1
0.3
0.02
0.0 1
0.002
Notc. Data from Kutt.'
Compared with Co-Cr-Mo alloys, titanium alloy is
almost &ice as strong and has half the elastic
modulus. Compared with 316L stainless steel, the
Ti-6Al-4V alloy is roughly equal in strength, but
again, it has half the modulus. Strength is beneficial
because materials better resist occlusal forces without fracture or failure. Lower modulus is desirable
because the implant biomaterial better transmits
forces to the bone.'*
At the atomic level, materials differ in yield
strcngth bccause they differ in resistance to planar
slip and dislocation movement. Atoms and localized
stresses that prevent dislocation movement raise the
yield strength of such materials. In the case of
titanium, oxygen dissolves into the crystal lattice as
interstitial atoms between titanium ions. The oxygen
Table 2. Mechanical Propcrtics of Selected Materials
Material
Ultimate
T e d e Yield ElonModulus Strength StrenRth gation Denrig
(GPa) ( M a ) ( M a ) @)
('glc.)
cp grade I Ti
cp grade II Ti
cp grade Ill Ti
cp grade IVTi
Ti-6Al-4V ELI
Ti-6Al-4V
102
102
102
104
113
113
240
345
450
550
860
930
170
275
380
483
795
860
24
20
18
15
10
10
4.5
4.5
4.5
4.5
4.4
4.4
Co-Cr-Mo
3 16 L steel
Cortical Bone
Dentin
Enamel
240
200
18
18.3
84
700
965
140
52
10
450
690
8
20
1
0
0
8.5
7.9
0.7
da
da
da
2.2
3
Note. Data from Park,'" M'IM,'"'' Wataha,34 C o ~ i n , 3 ~and
3~
42
Titanium and TitaniumAlloys
il.lcCracken
Table 3. Composition ofcp Titanium and Alloys (weight percent)
Tdanium
cp grade I
cp grade 11
cp grade III
cp grade IV
Ti-6Al4V alloy
Ti-6Al4VELI all0~7
N
c
H
Ft!
0
A1
V
Ti
0.03
0.03
0.03
0.03
0.05
0.05
0.10
0.10
0.10
0.10
0.08
0.08
0.015
0.015
0.015
0.015
0.015
0.012
0.02
0.03
0.03
0.05
0.30
0.10
0.18
0.25
0.35
0.40
0.20
0.13
-
-
balance
balance
balance
balance
balance
balance
-
-
5.50-6.75
5.50-6.50
3.50-4.50
3.50-4.50
Note. Data from ASTM1*-l4
and Wataha?’
atoms take up room in the crystal lattice, effectively
squeezing the titanium atoms and creating areas of
strain within the atomic lattice. Conversely, because
the planes of atoms move less as a result of oxygen
residuals, ductility is decreased.
Vanadium stabilizes the beta phase of Ti-6Al-4V
alloy, so that it exists as a combination of alpha and
beta phases. The alpha grains are fine (3 to 10 pm),
equiaxcd rounded structures with aspect ratios near
unity.lg This combination of phases givcs the alloy
strength. Additional strength gained from dissolved
oxygen is inconsequential compared with the effect
of vanadium. Because of this, the ELI alloys are
sometimes used. “Extra low interstitial” describes
the low levels of oxygen dissolved in interstitial sites
in the metal. With lower amounts of oxygen and iron
residuals in the ELI alloy, ductility is improved
slightly.
The elastic modulus of a material, at the atomic
level, measurcs the attraction of atoms to each other.
This attraction depends on the particular atoms
involved, and to some extent the arrangement of the
atoms in the crystal structure. Because the predominant elements in a metal determine thc modulus,
small amounts of impurities or residuals in thc metal
do not greatly affcct it. In the case of cp titanium,
trace amounts of oxygen do not significantly change
the modulus. The addition of aluminum and vanadium, which make up 10% of titanium alloys, raisrs
the modulus about 10%.
of implant materials. Recognizing that some implant
materials arc stronger than others, clinicians must
treatment plan implant selection accordingly. If a
patient has a history of parafunctional habits and
implant fracture, for example, the clinician should
choose an implant made of titanium alloy, rather
than cp gradc I titanium. In addition, small-diameter
implants or implants with thin walls indicate the
need for higher-strength materials.
Some clinicians may feel more comfortable routinely using a higher strength material, such as cp
grade IV titanium or a titanium alloy, if other factors
of implant selcction are equal. These factors might
include implant dcsign, abutment availability, surface finish, and biomechanical considerations.
Selected implant manufacturers and their materials are listed in Table 4. Of note, 3i uses a grade I cp
titanium for its hcx-top implants that is cold-worked
in the manufacturing procedure to produce a material that is mechanically equivalent to a cp grade 111
titanium. Although this technique improves the yield
strength of the material without raising the elastic
modulus, the toughness is theorctically decreascd.
Independent studies should be conducted to evaluate
these effects.
Table 4. Dental Implant Manufacturers and Implant
Material Choices
cfi
Selecting an Implant
A growing body o f clinical evidcacc documents that
all six commercially available dental implant biomatcrials have exhibited excellent biocompatibility, tissue response, and predi~tability.~~~~’
1,20-33 None of the
titanium-based matcrials have proven to be more
biocompatiblc than any other group. In the absence
of long-term comparative studies, factors such as
implant design, size, and material strength should
determine implant selection for a particular patient.
This article addrcsscs one of these [actors: strength
c@
Ti-
Ti-
Ti c@ Ti Ti
Ti GAL-41, 641Grad6 Grade Grade Grade ELI
4V
I
11 III N Alloy Allay
@
3i
BioHorizons
Lifecorc
Kobe1 Biocare
Paragon
Stcri-Oss
Sulzer Calcitec
*
X
*
x
x x
x
x
X
x
x
x
X
*3i company representatives use materials that meet cp grade I
and gradc I1 sperifications, but that are cold-fiorked to improve
vield
strength (see text).
,
March 1999,Volume 8, Number I
Summary
Both elastic modulus and strength are important
considerations in choosing an implant material. The
implant must have sufficient strength to withstand
occlusal forces without permanent deformation, but
should also have a low modulus for optimum force
transfer. Clinicians will be most likely to choose the
most appropriate dental implant material when they
are knowledgeable about the material's properties.
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