Titanium for medical and
dental applications—An
introduction
1.1
F.H. (Sam) Froes
Consultant to the Titanium Industry, Tacoma, WA, United States
1.1.1
Background
Titanium was first used in orthopedic applications in the 1950s. Orthopedics is that
branch of surgery which is concerned with conditions involving the musculoskeletal
system. Orthopedic surgeons use both surgical and nonsurgical means to treat mus
culoskeletal trauma, spine diseases, sports injuries, and much more. Currently, titanium alloys are the main preference (Fig. 1.1.1) for orthopedic devices such as hip
joints, bone screws, knee joints, spinal fusion cages, shoulder and elbow joints, and
bone plates and scaffolds (Fig. 1.1.2) [1–3]. Titanium is the metal of choice for
orthopedic practitioners because of its general corrosion resistance (Fig. 1.1.3)
unlike iron (Fig. 1.1.4). It is inert in the human body and is resistant to attack
by body fluids. Additionally, it has proven to be compatible with bone density,
it is strong, and has a low modulus; hence, it is an excellent material for the
orthopedic arena.
Titanium is also used in cardiovascular applications (i.e., pertaining to the heart and
blood vessels throughout the body). Blood vessels carry nutrients and oxygen to the
tissues of the body and remove carbon dioxide and other wastes. They are subject to a
number of diseases including arteriosclerosis, coronary artery disease, heart valve dis
ease, and many more.
The human body readily accepts titanium; it has proven to be more biocompatible
than stainless steel or cobalt chrome. In addition, titanium has a higher fatigue strength
than many other metals. It is also compatible with MRI (magnetic resonance imaging)
and CT (computed technology), which also contributes to making it the material of
choice in orthopedic applications.
More than 1000 tonnes (2.2 million pounds) of titanium are implanted in patients
worldwide every year, and this is set to increase as people live longer, or are seriously
injured in road traffic or other accidents. Titanium is light, strong, and totally biocompatible, making it one of the few materials currently known that naturally matches the
requirements for implantation in the human body. Because titanium resists corrosion,
is biocompatible, and has an innate ability to join with human bone, it has become a
staple of the medical field. From surgical titanium instruments to orthopedic titanium
Titanium in Medical and Dental Applications. https://doi.org/10.1016/B978-0-12-812456-7.00001-9
© 2018 Elsevier Inc. All rights reserved.
4
Fig. 1.1.1 Characteristics of
titanium which make it
attractive for medical and dental
applications.
Titanium in Medical and Dental Applications
High corrosive
resistance
Low specific
gravity
Biocompatible
material
Nonmagnatic
property
TITANIUM
High specific
strength
Fig. 1.1.2 Ti-6Al-4V scaffold for medical implant applications fabricated using Selective Laser
Melting Additive Manufacturing technology.
rods, pins, and plates, titanium has become the fundamental material used in medicine. Titanium is also much-used outside medicine. This makes it incredibly useful
for many different industries, including the automotive, aerospace, and architectural
worlds.
Titanium for medical and dental applications—An introduction
5
Fig. 1.1.3 General corrosion behavior of commercial purity titanium and Ti-Pd alloys
compared to other metals and alloys in oxidizing and reducing acids; with and without chloride
ions. In general, each metal or alloy can be used for those environments below its respective
solid lines.
Fig. 1.1.4 Titanium is very corrosion resistant, unlike iron [4].
1.1.2
Body implants
The implant must be permanent in critical applications where it cannot readily be
maintained or replaced. There is no more challenging situation in this respect than
implants in the human body. Here, the effectiveness and reliability of both
6
Titanium in Medical and Dental Applications
implants and medical and surgical instruments are essential factors in saving lives
and in the long-term relief of suffering and pain. Implantation represents a potential assault on the chemical, physiological, and mechanical structure of the human
body. There is nothing comparable to a metallic implant in living tissue. Most
metals in body fluids and tissue are found in stable organic complexes. Corrosion
of implanted metal by body fluids results in the release of unwanted metallic ions,
with likely interference in the processes of life. Corrosion resistance is not itself
sufficient to suppress the body’s reaction to cell-toxic metals or allergenic elements such as nickel, and even in very small concentrations from a minimum level
of corrosion, these may initiate rejection reactions. Titanium is judged to be
completely inert and immune to corrosion by all body fluids and tissue, and is,
thus, wholly biocompatible.
The reasons for selecting titanium for implantation are a combination of the most
favorable characteristics, including immunity to corrosion, biocompatibility, strength,
low modulus and density, and the capacity for joining with bone and other tissue
(osseointegration). The mechanical and physical properties of titanium alloys combine to provide implants which are highly damage tolerant. The human anatomy naturally limits the shape and allowable volume of implants. The lower modulus of
titanium alloys compared to steel is a positive factor in reducing bone resorption.
Two further parameters define the usefulness of the implantable alloy, the notch sensitivity (i.e., the ratio of tensile strength in the notched to the unnotched condition) and
the resistance to crack propagation, or fracture toughness. Titanium scores well in both
cases. Typical NS/TS ratios for titanium and its alloys are 1.4–1.7 (1.1 is a minimum
for an acceptable implant material). The fracture toughness of all high-strength
implantable alloys is above 50 MPa.m-½, with critical crack lengths well above the
minimum for detection by standard methods of nondestructive testing. Examples of
titanium use in body implants are shown in Figs. 1.1.5–1.1.7, and its dental use is illustrated later in this chapter.
For dental applications, a major change in restorative dental practice worldwide has been made possible using titanium implants. A titanium “root” is introduced into the jaw bone with time subsequently allowed for osseointegration.
The superstructure of the tooth is then built onto the implant to give an effective
replacement.
Titanium is also used in surgery to repair facial damage. Use of the patient’s own
tissue cannot always obtain the desired results. Artificial parts may be required to
replace facial features lost through damage or disease and so restore the ability to
speak or eat, as well as improve cosmetic appearance. Osseointegrated titanium
implants meeting all the requirements of biocompatibility and strength have enabled
unprecedented advances in surgery, for the successful treatment of patients with large
defects and hitherto highly problematic conditions.
A comprehensive article on “Understanding Implants in Knee and Hip
Replacement” appeared in the journal of the International Titanium Association
2nd Quarter, 2016 Medical Edition [5]. It showed that titanium knee replacements
can be tailored to the patient’s size, weight, and gender (a woman has a narrower bone
structure, especially on the femur). A companion article [5, p. 20] discusses the
Titanium for medical and dental applications—An introduction
7
Fig. 1.1.5 Knee joint implant replacement x-ray showing in medical orthpodedic traumatology
scan.
Courtesy of Shutterstock.
Fig. 1.1.6 Titanium bone implants.
influence of processing (in particular, etched versus unetched conditions) on the performance of titanium middle ear prostheses, with the etched condition exhibiting
superior performance. A further article [5, p. 76] presents details on a 3D-printed titanium hip implant with a fully porous cup (Fig. 1.1.8) allowing in-growth of bone and
tissue, resulting in superior performance. An additive manufactured rib cage and sternum implant is shown in Fig. 1.1.9A and B [5, p. 82]. Fig. 1.1.10 illustrates a fecal
continence restoration system consisting of a series of titanium beads with magnetic
cores connected by titanium wires to form a ring [5, p. 83].
8
Titanium in Medical and Dental Applications
Fig. 1.1.7 A traditional total hip replacement implant.
From http://www.geripal.org/2013/01/metal-on-metalhip-replacements-tragic.html.
Fig. 1.1.8 Smith and Nephew revised acetabular fully porous cup with Conceloc technology.
(Courtesy Professor H. P. Tang, State Key Laboratory of Porous Metal Materials, Northwest
Institute for Nonferrous Metal Research).
An additive manufactured jaw bone is shown in Fig. 1.1.11 [4].
Other interesting work involved a study of Ti-Au alloys [6]. An alloy consisting of
3:1 Ti::Au (Fig. 1.1.12) was found to be four times harder than pure titanium and was
more biocompatible and exhibited superior wear resistance, making it appear to be an
attractive choice for implant applications.
Fig. 1.1.9 (A) A 3D additive manufactured rib cage and sternum implant. (B) How the
component shown in A is inserted into the human body.
Fig. 1.1.10 Titanium fecal continence restoration system.
10
Titanium in Medical and Dental Applications
Fig. 1.1.11 An additive manufactured jaw bone.
Fig. 1.1.12 Crystal structure of beta titanium-3 gold
(Courtesy E. Morosan, Rice University).
1.1.3
Dental implants
The use of titanium in dental applications has also increased dramatically over the past
20 years. The replacement of missing teeth with implant-supported prostheses, illustrated in Figs. 1.1.13–1.1.14 [4] has become widely accepted in dentistry for the rehabilitation of fully and partially edentulous patients. This breakthrough in oral
rehabilitation is based on the concept of osseointegration. This biological phenomenon is described as direct bone deposition upon a titanium implant surface. Currently,
commercially pure (CP) titanium has become the material of choice in implant dentistry, since it has excellent biological and biomechanical properties.
Titanium for medical and dental applications—An introduction
11
Fig. 1.1.13 Titanium dental implants (top) and their insertion into the mouth (bottom).
Titanium implant description
Typical ‘two-piece’ titanium implant
connection that is ready to have the
crown or bridge connected with
a little screw.
Titanium implants are placed deep
under the gum in order to hide the
metal color. This is difficult to brush
and for this reason, the connections
tend to produce halitosis. This is the
area where the corrosion of metal
implants is greater.
This connections have been proven
to accumulate bacteria in all kinds
of situations with poor or very good
oral hygiene and in any kind or brand
of titanium two-piece implants.
This is the location where the crown
or bridge is connected to the titanium
implants
Fig. 1.1.14 How the dental implant procedure works.
1.1.4
Titanium surgical instruments
A wide range of surgical instruments are made from titanium (see Figs. 1.1.15–1.1.17
[4]). One of its advantages in this respect is its lightness, which helps to reduce the
surgeon’s fatigue. Instruments are frequently anodized to provide a nonreflecting
12
Titanium in Medical and Dental Applications
Fig. 1.1.15 Titanium surgical instruments, scissors, forceps, and needle holders.
Fig. 1.1.16 Titanium instruments, surgical screws, and various implants.
surface, essential in microsurgical operations such as eye surgery. Titanium instruments can be sterilized repeatedly without compromising edge or surface quality,
corrosion resistance, or strength. Titanium is nonmagnetic, and there is, therefore,
no threat of damage to small, sensitive, implanted electronic devices.
Titanium for medical and dental applications—An introduction
13
Fig. 1.1.17 Additional
titanium instruments.
1.1.5
Titanium in wheel chairs, etc.
The same characteristics that make titanium a preferred choice for implants and instruments make it a good choice, particularly in tubular form (Fig. 1.1.18A and B [4]) for
wheel chairs (Fig. 1.1.19 [4]) and walkers (Fig. 1.1.20 [4]). The tubular products are
generally fabricated as either seamless or welded components from commercially
pure titanium or the Ti-3Al-2.5 V alloy.
1.1.6
Specifications for titanium in medical and dental
applications
Forms and material specifications are detailed in a number of international and domestic specifications, including ASTM and BS7252/ISO 5832, as shown in Table 1.1.1
[2]. Alloys such as Ti-6Al-7Nb eliminate the use of V, an element which can cause
cytotoxic outcomes (negatively influencing human cells in a similar manner to the
effect of puff adder venom).
Mechanical properties of titanium alloys suitable for use in medical and dental
applications are shown in Table 1.1.2 [7].
1.1.7
Other titanium-based materials
Nickel titanium, also known as nitinol (part of a shape memory alloy), is a metal alloy
of nickel and titanium, in which the two elements are present in roughly equal atomic
percentages, for example, nitinol 55, nitinol 60. Nitinol alloys exhibit two closely
related and unique properties: shape memory effect (SME) and superelasticity (SE;
also called pseudoelasticity, PE). Shape memory is the ability of nitinol to undergo
deformation at one temperature, then recover its original, undeformed shape upon
heating above its “transformation temperature.” Superelasticity occurs at a narrow
14
Titanium in Medical and Dental Applications
Fig. 1.1.18 (A) Titanium tubing. (B) Titanium tubing ready for use in wheel chair or walker
applications.
Titanium for medical and dental applications—An introduction
Fig. 1.1.19 (A,B) Titanium is a popular choice for wheel chairs.
Fig. 1.1.20 A titanium walker.
15
16
Titanium in Medical and Dental Applications
Table 1.1.1 Specifications for titanium in medical and dental
applications
ASTM
BS/ISO
Alloy(S) designation
F67
F136
F1472
Part 2
Part 3
Part 3
F1295
–
F1580
F1713
F1813
Part 11
Part 10
–
–
–
Unalloyed Titanium—CP grades 1–4 (ASTM F1341 specifies wire)
Ti6Al4V ELI wrought (ASTM F620 specifies ELI forgings)
Ti6Al4V standard grade (SG) wrought (F1108 specifies SG
castings)
Ti6Al7Nb wrought
Ti5Al2.5Fe wrought
CP and Ti6Al4V SG powders for coating implants
Ti13Nb13Zr wrought
Ti12Mo6Zr2Fe wrought
Table 1.1.2
Titanium alloys suitable for medical applications
ASTM grade
Property
1
2
3
4
5
Yield strength (MPa)
Ultimate tensile
strength (MPa)
Elongation (%)
Elastic modulus (GPa)
170
240
275
345
380
450
483
550
795
860
24
103–107
20
103–107
18
103–107
15
103–107
10
114–120
Adapted from ASTM F67 (Grade 1 to 4) and F136 (Grade 5).
temperature range just above its transformation temperature; in this case, no heating is
necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10–30 times that of ordinary metal.
Nitinol is highly biocompatible and has other properties that make it suitable for
use in orthopedic implants. Its unique properties have caused high demand in less
invasive medical devices. Nitinol tubing is commonly used in catheters, stents, and
superelastic needles. In colorectal surgery, the material is used in devices for reconnecting the intestine after removing the pathology. Nitinol is used for devices developed by Franz Freudenthal to treat patent ductus arteriosus, blocking a blood vessel
that bypasses the lungs and has failed to close after birth in an infant. In dentistry, this
alloy is used in orthodontics for brackets and wires connecting the teeth. Once the
SME wire is placed in the mouth, its temperature rises to ambient body temperature.
This causes the nitinol to contract back to its original shape, applying a constant force
to move the teeth. These SME wires do not need to be retightened as often as other
wires because 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.
Titanium for medical and dental applications—An introduction
17
Nitinol’s unusual properties are derived from a reversible solid-state phase transformation known as a martensitic transformation. At high temperatures, nitinol
assumes an interpenetrating, simple, cubic structure referred to as austenite. At low
temperatures, nitinol spontaneously transforms to a more complicated, body-centered,
tetragonal crystal structure known as martensite. The temperature at which austenite
transforms to martensite is generally referred to as the transformation temperature.
The mechanism of the shape memory effect involving the transformation from the
low-temperature martensite to the higher-temperature austenite causes the original
shape to be recovered (see Figs. 1.1.21 and 1.1.22 [4]).
15
nm
0
0.3
46
22
20
nm
0.3015 nm
0.
96.8
41
ᑻ
0.
Austenite
nm
Martensite
Fig. 1.1.21 Mechanism of the shape memory effect. The transformation from the lowtemperature martensite to the higher-temperature austenite causes the original shape to be
recovered.
Fig. 1.1.22 Demonstration of the shape memory effect.
18
Titanium in Medical and Dental Applications
Fig. 1.1.23 A nitinol stent.
Fig. 1.1.24 How a nitinol stent works.
A nitinol stent and how it works is shown in Figs. 1.1.23 and 1.1.24 [4].
A nitinol fixation device is shown in Fig. 1.1.25 [4].
Dental use of nitinol is shown in Fig. 1.1.26, [4] a dental brace.
Recently, alloys have been developed beyond the first-generation terminal alloys.
These include Ti-6Al-4V, Ti-6Al-7Nb, and the commercially pure grades. These
alloys are low modulus near beta and beta alloys, which match the low modulus of
human bone much better [8]. Alloys such as Ti-29Nb-13Ta-4.6Zr, Ti-13Nb-13Zr,
and Ti-10Nb-10Zr have been fabricated by metal-injection molding methods and have
good strengths and excellent biocompatibility with the human body.
Titanium for medical and dental applications—An introduction
19
Fig. 1.1.25 Nitinol fixation devices.
Fig. 1.1.26 Nitinol dental braces. (A) Schematic of braces in place on teeth and (B) dental
braces on teeth.
1.1.8
Post script
A comprehensive summary of the use of AM in dentistry is shown in Fig. 1.1.27.
1.1.9
This book
This book focuses on titanium in medical and dental applications, including implants,
instruments, and devices such as wheel chairs and walkers. The book contains
information on various fabrication techniques including conventional ingots and
20
Titanium in Medical and Dental Applications
3D Printing—Manufacturing Methods—Dental Industry
Laminated Object
Manufacturing (LOM)
3.2%
n
itio )
os
ep FDM
dD (
se ling
Fu ode 0.8%
1
M
Sele
Me ctive L
ltin
ase
g
11. (SLM r
)
3%
3D in
kjet
P
7.1% rinting
rs
he
Ot .4%
8
Se tive Laser
lec
Sintering (SL
S)
14.3%
La Dire
se ct
rS M
2. int eta
3 % er l
in
g
A)
(SL
phy
gra
o
h
lit
reo 27.5%
Ste
)
ht
ig LP
l L (D
ta ing
i
ig
D ess .5%
oc 1
Pr
c Beam
Electroni BM)
(E
Melting
2%
3.
Others Include
Laser Powder Sintering | Solid Freeform
Fabrication | Robocasting
Fig. 1.1.27 A comprehensive summary of the use of AM in dentistry.
Courtesy of Sagacious Research.
subsequent processing (forging, rolling, extrusion, etc.), and approaches such as powder metallurgy, additive manufacturing, and metal injection molding. It also contains
information on a number of alloys starting with commercially pure titanium and
Ti-6Al-4V and extends to compositions such as nitinol (with compositions close to
equiatomic Ti-Ni).
References
[1]
[2]
[3]
[4]
Titanium Industries, Inc., US office of Technical Assessment Web Page (accessed 7-07-16).
AZO Materials Web Page (accessed 7-07-1)
Wikipedia, (accessed 7-17-16).
Internet Explorer, Titanium Implants Illustrations, vol. 8 (accessed 11-25-16).
Titanium for medical and dental applications—An introduction
21
[5] Padgett, D.E., Windsor, R.E., 2016. International Titanium Association. Titan. Today (2nd
Quarter), 11.
[6] Anon, 2016. Gold boosts titanium knee strength. Adv. Mater. Process. 174 (8) 8,9.
[7] Elias, C.N., et al., 2008. Biomedical applications of titanium and its alloys. JOM (March), 46.
[8] B. Williams, Powder injection molding international, “World PM2016: PIM Technical
Sessions Review Advances in Novel Titanium Alloys for Biomedical Applications.