Oshida Y. Bioscience and Bioengineering of Titanium Materials

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Prologue

Writing a book like this one, which is based on reviewing published articles, is a

typical example of exercising an evidence-based learning (EBL). The evidence-

based literature review can provide fundamental skills in research-oriented biblio-

graphic inquiry, with an emphasis on the review and synthesis of applicable

literature. All information is obtained from numerous sources provided by schol-

ars and researchers who are involved in various disciplines. Available sources pos-

sess a variety of features, especially the degree of reliability. There is a known rank

of reliability for evidence sources, which is particularly important in the medical

and dental fields: (from the most to least reliable evidence source) (1) clinical

reports using placebo and double blind studies, (2) clinical reports not using

placebo, but conducted according to well-prepared statistical test plans, (3) study

reports on time-effect on one group of patients, (4) study reports, at one limited

time, on many groups of patients, (5) case reports on a new technique and/or idea,

and (6) retrospective reports on clinical evidence. Unfortunately, the number of

published articles increases in this descending reliability order.

The most remarkable advantage of the EBL-type reviewing of numerous pub-

lished articles is to encounter an unexpected idea and issue which might provide a

fundamental basis persisting in all related but separately and scattered manner being

presented in different sources. Accordingly, it is actually an inductive practice.

Comparing articles seen in medical/dental journals and those published in engi-

neering journals, there is a dilemma of a big difference between them. Obtained

results present in the medical/dental journals are normally further analyzed statisti-

cally; while the data presented in the engineering journals are normally not sub-

jected to statistical analysis, rather characterization of data is considered more

important to interpret results. Even though a statistical analysis is conducted for

medical/dental results, there are some cases which do not reach a conclusive point.

For example, although the mercury issue (which is used for condensation of dental

amalgam alloy to form Ag2Hg 3 and Sny_8Hg ) has been debated for more than 20

years, we still have not come to one solid consensus on its safety issue yet. Actually

mercury amalgam has been completely abandoned in Scandinavian counties and

Japan, whereas it is still used in the United States and other countries, although

tooth-colored restorations are becoming more popular for aesthetic reasons.

There is another challenge in the medical and dental field. It is a controversial

issue on the relationship between

in vivo

test results and those obtained by

in vitro

tests. To make this situation more complicated and confusing, there could be

differences even among various

in vivo

tests, due to different models (different

xiii

Prologue

Writing a book like this one, which is based on reviewing published articles, is a

typical example of exercising an evidence-based learning (EBL). The evidence-

based literature review can provide fundamental skills in research-oriented biblio-

graphic inquiry, with an emphasis on the review and synthesis of applicable

literature. All information is obtained from numerous sources provided by schol-

ars and researchers who are involved in various disciplines. Available sources pos-

sess a variety of features, especially the degree of reliability. There is a known rank

of reliability for evidence sources, which is particularly important in the medical

and dental fields: (from the most to least reliable evidence source) (1) clinical

reports using placebo and double blind studies, (2) clinical reports not using

placebo, but conducted according to well-prepared statistical test plans, (3) study

reports on time-effect on one group of patients, (4) study reports, at one limited

time, on many groups of patients, (5) case reports on a new technique and/or idea,

and (6) retrospective reports on clinical evidence. Unfortunately, the number of

published articles increases in this descending reliability order.

The most remarkable advantage of the EBL-type reviewing of numerous pub-

lished articles is to encounter an unexpected idea and issue which might provide a

fundamental basis persisting in all related but separately and scattered manner being

presented in different sources. Accordingly, it is actually an inductive practice.

Comparing articles seen in medical/dental journals and those published in engi-

neering journals, there is a dilemma of a big difference between them. Obtained

results present in the medical/dental journals are normally further analyzed statisti-

cally; while the data presented in the engineering journals are normally not sub-

jected to statistical analysis, rather characterization of data is considered more

important to interpret results. Even though a statistical analysis is conducted for

medical/dental results, there are some cases which do not reach a conclusive point.

For example, although the mercury issue (which is used for condensation of dental

amalgam alloy to form Ag2Hg 3 and Sny_8Hg ) has been debated for more than 20

years, we still have not come to one solid consensus on its safety issue yet. Actually

mercury amalgam has been completely abandoned in Scandinavian counties and

Japan, whereas it is still used in the United States and other countries, although

tooth-colored restorations are becoming more popular for aesthetic reasons.

There is another challenge in the medical and dental field. It is a controversial

issue on the relationship between

in vivo

test results and those obtained by

in vitro

tests. To make this situation more complicated and confusing, there could be

differences even among various

in vivo

tests, due to different models (different

xiii

xiv

Bioscience and Bioengineering of Titanium Materials

types of animals versus humans). Moreover, it is almost impossible to comply and

summarize to extract any conclusiveness from various

in vitro

test results, since it

is very rare to find articles where identical test methods were employed, unless

some standardized (specified) methodology was followed. For example, it is easy

to list at least ten electrolytes for electrochemical evaluation of the corrosion

behavior and resistance of a dental metallic material. In addition to this, wide vari-

ations in pH value, concentration, temperature, and other intraoral environmental

factors can easily alter the experimental outcomes. This is the reason why in the

materials science and engineering area, a phenomenon is characterized to find the

basic mechanisms as well as kinetics. Once the basics are elucidated, if any envi-

ronmental factor is changed, the possible results would not be hard to anticipate.

Although a statistical test on obtained data is considered as a powerful tool to ana-

lyze results, it should be based on postulated assumptions (which should be statis-

tically tested once data are collected), and the results are not applicable to entire

patient populations. Suppose a newly developed medicine was statistically evalu-

ated,

in vivo

test results show that it is effective to 95% of the patient population,

which indicates, on the other side of the coin that it definitely does not work for

the remaining 5% of human patients.

In this book, such differentiation and comparison in analyzing results between

medical/dental and engineering fields will be emphasized and an attempt has been

made to fill the gaps where possible. The materials and most of the technologies

employed in medical and dental fields originally developed in the

engineering/technology field. If they prove their safety and can be miniaturized,

they can be accommodated for human uses. Such successfully performed technol-

ogy transfers from engineering to medical/dental applications are also introduced

in this book. Hence, this book has a two-fold uniqueness; (1) the EBL concept has

been applied and practiced throughout the entire course of preparation, writing

and editing of this book, and (2) a bridging of the gap between medical/dental

fields and engineering/technology areas, owing to the author's unique experience

in both fields for about an equal length of 20 years each.

Chapter 1

Introduction

References 8

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3

Chapter 1

Introduction

In Greek mythology, the Giant divine being, Titan, a son of Uranos (Father

Heaven) and Gaia (Mother Earth), had lost several wars against the Olympic Gods,

which resulted in his being confined in the underground dark world. The element

titanium, confined within an ore (called rutile ore), was discovered by a German

chemist Martin Heinrich Klaproth. Klaproth confirmed it as a new element, and

in 1795 he named it for the Latin word for Earth (also the name for the Titans of

Greek myth) [1-1, 1-2].

Because of their lightweight, high specific strength (which is referred to a high

strength-to-weight ratio), low modulus of elasticity, and excellent corrosion

resistance, titanium materials (both unalloyed and alloyed) have become impor-

tant materials for the aerospace industry since the early 1950s [1-3]. It was very

hard to predict, at that time, that titanium materials would currently receive their

attention, interest and importance not only for industrial applications but also

equally for dental and medical applications. Titanium is one material which

receives equal attention and interest from both the engineering and medical/

dental fields.

Research and development in materials design, manufacturing technologies,

characterization, and evaluation methods involved in titanium materials are some

of the best examples of interdisciplinary science and technology; such disciplines

might include physics, chemistry, metallurgy, mechanics, and surface and inter-

face sciences, as well as biological science, engineering, and technology (see

Figure 12-1). One of the typical examples can be seen in dental and orthopedic

implant systems, which have been developed by integrating industrial engineering,

bioengineering, as well as supportive advanced techniques.

Although empiricism and trial and error played a major role in the early stage

of research and development on titanium materials, in the last 20 years progress

has been made toward a multidisciplinary scientific approach to the study of tita-

nium materials and the development of materials with better performance and

properties [1-4]. The research and development of titanium materials has been

heavily dependent on the aerospace industries, and this area will continue to be a

significant percentage of total consumption of titanium materials for coming years

[1-5]. Titanium usage on Boeing aircraft has continuously increased from 1% on

the 727 (1963), to 3% on the 747 (1969), 5% on the 757 (1983), and 9% on the

777 (1994). Over the last decade, the focus of titanium alloy development has

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shifted from aerospace [1-6] to industrial application [1-5]. Different industrial

sectors have been looking for different types of development in new titanium alloys;

they include Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C, Ti-6Al-2Sn-4Zr-

6Mo, and Ti-4Al-4Mo-2Sn-0.5Si for gas turbine engine materials, Ti-10V-2Fe-3Al,

Ti-15V-3Cr-3Sn-3Al, and Ti-15Mo-2.8Al-3Nb-0.2Si for airframe materials,

Ti-6Al-1.8Fe-0.2Si for ballistic armor, Ti-6.8Mo-4.5Fe-1.5Al for geothermal and

offshore tubular materials, Ti-15V-3Cr-3Sn-3Al (having higher-strength and lower-

modulus) for sporting goods [1-7], V-free Ti-6Al-4V equivalent alloys for medical

and dental applications, and NiTi-Cu alloys for medical orthopedic devices. Not

only materials but also technologies developed and successfully used in industry

can be transferred to medical and dental areas. The computer-aided design and

machining (CAD/CAM) process is an excellent example to design and fabricate

dental restorations. Investigators are also combining CAD and AI (artificial intelli-

gence) to design complex prosthetic devices such as partial dentures. Computers

have also been used to analyze the stress levels and stress distribution on dental

restorations, dental implants and orthopedic implants. They can simulate internal

stress distributions under different conditions and loading situations, and the results

can be used optimize the computer-aided design process (e.g., two- or three-dimen-

sional finite element modeling and stress analyses). The application of lasers to

dentistry is another good example of technology transfer [1-8]. Various ways of

laser application for surface modifications of metals have also been introduced, and

include cutting, welding, surface hardening, laser surface alloying, and forming

[1-9]. Superplastic forming (SPF) of denture base, and superplastic forming

with diffusion bonding (SPF/DB) for modification and roughening of implant sur-

faces are another excellent examples of the technical transfer. These transferred

technologies should not be limited to technology per se, but it should also include

technical concepts such as composites, as well as gradually functional structure,

which can be frequently seen in the surface modification of implant systems to pro-

mote the bone ingrowth.

The popularity of titanium and its alloys in the dental and medical fields can be

recognized by counting the manuscripts published in different journals, most of

which will be cited in this review. Referring to Figure 1-1, there are three straight

lines in semi-log plot for demonstrating the total accumulated number of published

articles for every 5 years. The first top line represents accumulated numbers of all

articles listed in Chemical Abstracts. Therefore, the counted articles are scattered in

wider areas in refining, metallurgy, medicine, dentistry, bioengineering, engineer-

ing, industries, pure chemistry as well as chemical engineering. The second straight

line is obtained when the search area is limited to materials and science in both the

engineering and medical/dentistry fields. If our search is furthermore defined within

only medicine/dentistry, we still have an exponential increase in publications.

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From the third straight line, only eight papers (not shown in the figure) were

published in the time period from 1960 to 1965, followed by 156 papers in

1966–1970, 352 in 1971–1975, 309 in 1976–1980, 493 in 1981–1985, 916 in

1986–1990, and it jumped to 1914 papers in the 5-year period from 1991 to 1995.

In 1996–2000, we already reached more than 3000 articles, followed by 5500 arti-

cles which have been published during a 5-year period from 2001 to 2005. These

numbers should be considered as conservative, since articles not written in English

have not been accounted for.

The exponentially increasing trend of published papers on medical and den-

tal titanium materials may be attributed to different reasons that might include

(1) needs from medical and dental sectors, (2) increased researchers and scien-

tists involved in these medical titanium materials, and (3) expanded industrial

scale being associated with the above.

About 10 years later from the previous forecasting on titanium industry and

materials developments [1-5–1-7], another remarkable change has been seen. In

addition to the ever-growing aerospace applications, precise forming technologies

Introduction 5

2

2

100,000

9

8

7

6

5

4

3

10,000

9

7

5

4

3

2

1,000

9

7

5

4

3

2

100

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 201

0

Year

Dentistry and Medicine

Material Science and Engineering

Chemical Abstract

The Number of Published Articles

Figure 1-1. Accumulated number of published articles on titanium materials for every 5 years.

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have been developed such as near-net shape (NNS) manufacturing, nanotechnol-

ogy, metal-injection molding (MIM) powder metallurgy techniques, etc. [1-10–1-13],

resulting in increasing application areas of titanium materials. As for biomaterials,

reflecting to a natural demand for allergy-free metallic materials composed of non-

toxic element(s), research and development in metallic biomaterials, dental, and

healthcare materials have been advanced remarkably [1-14, 1-15].

Owing to various characteristic properties associated with titanium and tita-

nium-based alloys, different types of dental and medical prostheses have been

developed and are currently being utilized. They include cardiac (especially,

mechanical heart valves, pacemakers), operational devices and equipment, and

orthopedic implants. Furthermore, in the dental field, the list can expand to specif-

ically orthodontic brackets and wires, dental implants, prosthetic appliances, and

endodontic files.

It is well recognized that the first reaction of a vital hard/soft tissue (i.e., host

tissue) to any type of biomaterial (ceramics, polymers, metals and alloys, com-

posites) is a rejection; accordingly, biomaterial is normally recognized as a

“foreign material” by the host tissue. The biological acceptance of these foreign

materials by the living tissues is essentially controlled by the surface and interfa-

cial reaction between the organic substance and inorganic substrate. The surface is

not just a free end of a substance, but it is a contact and boundary zone with other

substances (either in gaseous, liquid, or solid). A physical system which comprises

of a homogeneous component such as solid, liquid, or gas and is clearly distin-

guishable from each other is called a phase, and a boundary at which two or three

of these individual phases are in contact is called an interface. Surface and inter-

face reactions include reactions with organic or inorganic materials, vital or non-

vital species, hostile or friendly environments, etc. Surface activities may vary

from mechanical actions (fatigue crack initiation and propagation, stress intensifi-

cation, etc.), chemical action (discoloration, tarnishing, contamination, corrosion,

oxidation, etc.), mechano-chemical action (corrosion fatigue, stress-corrosion

cracking, etc.), thermo-mechanical action (thermal fatigue), tribological and

biotribological actions (wear and wear debris toxicity, friction, etc.) to physical

and biophysical actions (surface contact and adhesion, adsorption, absorption, dif-

fusion, cellular attachment, etc.). Accordingly, the longevity, safety, reliability, and

structural integrity of dental and medical materials are greatly governed by these

surface phenomena, which can be detected, observed, characterized, and analyzed

by virtue of various means of devices and technologies [1-5, 1-16].

Interfaces are as important as surfaces. Such interfaces can be found in various

combinations, such as dentin/resin bonding, post/lute/tooth system, implant/hard tis-

sue system, metal coping/porcelain system, etc. Common phenomenon underlying

these interfacing couples includes the fact that there is always an intermediate layer

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between two distinct bodies. Accordingly, strictly speaking, there are two bodies

with one (intermediate) interface. For example, when the porcelain is fired on the

metal coping, the resultant interface between them is normally considered as an

inter-diffusion layer in which both elements from porcelain and metal

coping are involved. The stability, bonding or adherent strength, as well as structural

integrity of these systems are strongly reliant on the interfacial behaviors under

various mechanical and/or chemical environments and actions.

To determine the compatibilities (biological, mechanical, and morphological) of a

biomaterial to receiving human hard/soft tissue, it is also important to understand the

interfacial phenomena between the biomaterial and the biological system into which

the material is to be implanted [1-16]. At such an interface, the molecular constituents

of the biological system meet and interact with the molecular constituents of the sur-

face of the biomaterials [1-17]. Since these interactions occur primarily on the molec-

ular level and in a very narrow interface zone having a width of less than 1 nm (1

nanometer⫽0.001 m⫽0.1 Å), surface properties on an atomic scale, and in partic-

ular the composition and structure of surface layers of the biomaterial, may play an

important role in interfacial phenomena [1-16]. Hence, surface and interface charac-

terizations and their biochemical and biomechanical roles in biological environments

are one of the most important portion of this book. Of interest and will be seen in later

(particularly, in Chapters 6 and 7), scientific approaches to, characterization on, and

interpretation such interfacial layers differ between materials scientists, and bio-scien-

tists and clinicians; the former tends to investigate the interface layers from material’s

side, whereas the latter observes them from the living tissue side. In most cases, these

results do not corporate each other. Actually, there is no single report which compiles

results obtained from both sides.

It has been noted that hazardous substances are used with relative safety in

dentistry when their identity is known, such as mercury in dental amalgam and

strong acids in cements and etching agents [1-17]. The following species and

elements can be added to this category; beryllium in cements and alloys, asbestos in

periodontal packs and drug solutions, lead in impression materials, and cadmium in

silver solders and temporary crowns. Alloys containing Cr, V, and Ni (which are

recognized to exhibit the heavy metal toxicity) are known to be carcinogenic in

animals and man, with local and systemic dissemination of released metallic ions in

the tissues. Such examples indicate a need for full scientific consideration of the bio-

logical effects of materials at an early stage of development when the implications

of use of certain ingredients or techniques may be recognized [1-18]. It can be

clearly recognized, currently, that most of the materials or elements mentioned in the

above are totally or partially eliminated from their original contents.

In this evidence-based overview book, accordingly, a general characterization of

titanium materials, various actions (mechanical, tribological, biotribological,

Introduction 7

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