The Chemistry of Medical and Dental Materials

John W Nicholson

• Publisher: Royal Society of Chemistry
• Year: 2020
• eBook ISBN: 9781839161322
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

With synthetic implants such as hip joints, heart valves and dental crowns now routinely used in the human body for medical purposes, study of the metals, ceramics and polymers used in these repairs is more important than ever. The Chemistry of Medical and Dental Materials examines the properties and interactions of these materials within the body at a molecular level, and includes discussion of bioengineering and cell biology, with accounts of the surgical procedures used, as well as extensive coverage of the possible biological reactions to the presence of foreign materials in the body.

Acknowledging the substantial growth of the biomaterials field since the first edition, this second edition sees each chapter comprehensively revised and updated. The new edition also includes a new chapter on ethical perspectives, covering issues from animal and human subject testing to the availability of treatments for poorer socio-economic groups.

With detailed reviews of the current literature, this book will be a key resource for researchers and practitioners in biomaterials science and dental biomaterials who are involved in the development of new and improved repair materials.

CHAPTER 1

Synthetic Materials in Medicine

1.1 Introduction

The use of synthetic materials in the body by medical and dental surgeons to repair the body and restore function has grown considerably in the last 50 years. However, the concept of using artificial materials to repair the body is very old. Plaster of Paris was pioneered as bone-substitute material towards the end of the nineteenth century, and dental fillings, including amalgam, have been around for well over 150 years. The use of engineered structures fabricated from metals and polymers in orthopaedic surgery has a more recent history, however, beginning with Dr (later Sir) John Charnley's work on the replacement of arthritic hips in the early 1960s.¹ This surgical repair technique, known as total hip arthroplasty, has grown spectacularly, and since Charnley's original cemented hip replacements there have been a variety of new materials and new designs for implantable devices. These are now available not only for hips, but also for knees, toes and fingers.

Artificial materials used in the body in this way are called biomaterials. This use of the term appears to originate in 1967 with the first ‘International Biomaterials Symposium’ at Clemson University, South Carolina, since when it has been used extensively in this way. In many ways, to apply the word biomaterials to synthetic materials is not very satisfactory because, by analogy with the word biochemistry, it might be assumed to refer to materials of biological origin. However, within the field of implantable devices, the word biomaterial has been formally defined as a non-viable material used in a biomedical device intended to interact with biological systems.² This definition was adopted at the Consensus Conference of the European Society for Biomaterials, held at Chester, UK, in March 1987, and has been widely accepted ever since. In fact, some sort of definition of this type was already implicit in the title of the organization which ran the meeting, the European Society for Biomaterials, because the Society's objective from the time it was established in 1976 was to promote the study of the science of such synthetic materials. It was never primarily concerned with the science of natural substances, such as teeth or bones. The current definition was also implicit in the title of the scientific journal Biomaterials, which was first published in 1980. Whatever the rights and wrongs of the etymology, by usage the term biomaterial has now clearly come to mean a synthetic material with a biological destination rather than a biological origin.

There is a further caveat with the term, in that it is usually applied to materials designed to stay in the body for some considerable time. Materials used for devices used only in surgery, such as sensors or catheters, are not usually regarded as biomaterials. They may interact with the body, but this interaction is usually relatively brief. Sutures, too, are not usually regarded as biomaterials for a similar reason. On the other hand, degradable polymers of the type used in sutures are used in various novel ways in medicine, for example as temporary scaffolds and supports for bone immobilization. These enable the body's own repair mechanisms time to bring about complete healing without premature loading and potential failure. Under these circumstances, the polymers become biomaterials, because they must interact with the body for a considerable time.

The field of biomaterials science involves all classes of material, i.e. polymers, ceramics, glasses and metals, and a wide range of branches of surgery: dental, ophthalmic, orthopaedic, cardiovascular and so on. The key requirement of any material or combination of materials used in the body is that, in addition to providing mechanical support or repair, it should be biocompatible. The subject of biocompatibility is covered in detail in Chapter 6, but at this stage we should note its definition. This is the ability of a material to perform with appropriate host response in a specific application.² As stated in this definition, biocompatibility is not a property of a material as such. Instead, the material needs to elicit an appropriate response, and whether such a response is appropriate will depend on the site in the body at which it has been placed. A material which shows excellent biocompatibility, for example, in contact with bone would not necessarily show good biocompatibility in contact with blood, for example as an artificial heart valve. Thus, the location within the body is as important in determining whether a material is biocompatible as the composition of the material.

The property of biocompatibility is distinct from that of inertness, which would imply a complete absence of response from the body. At one stage, it was thought that inertness was desirable, but nowadays inertness is not thought possible. Even materials that seem inert in most technical applications, such as polytetrafluoroethylene (PTFE), turn out to be highly active when placed within the body. PTFE was once used to fabricate the acetabular cups used in experimental hip replacement surgery.³ When used in conjunction with a metal femoral head, it was found to have extremely poor wear characteristics, leading to build-up of high local concentrations of particulate wear debris. This wear debris provoked extreme adverse reactions in patients, leading to severe swelling and general discomfort.⁵ Consequently, the use of PTFE for this purpose was abandoned.

Because of experiences of this type, there has been a shift in thinking and the emphasis nowadays is on materials that will elicit a response from the body that is appropriate.² In the case of titanium implants, this may be that there is no formation of fibrous capsule.⁴ Although it displays this desirable feature, titanium is not inert in the human body. It can corrode,⁵ yet the presence of titanium is well tolerated by the body,⁶ and the use of titanium for implants is found in many branches of surgery.⁷

The successful use of biomaterials presents numerous challenges. A major one is the issue of maintenance, and in particular that most devices are implanted well into the body and therefore cannot be easily inspected or repaired. An artificial hip joint, for example, is completely inaccessible, except by major surgery, and so cannot be routinely serviced. The body is a hostile environment, despite its sensitivity, and the resulting service conditions are severe. Nowhere else in technology are manufactured items expected to function without maintenance for so long in such demanding conditions.

Life expectancy in the wealthier parts of the world is now around 80 years, which means that many people now outlive the useful life of their own connective tissue.⁸ As people age, so there is a loss of cortical bone, resulting in substantial reductions in bone strength. This means that specific fractures, such as of the hip, become common in the elderly.⁹

When synthetic materials are used for repairs, they must be able to survive for considerable lengths of time without maintenance. However, it is rare to find an implant whose life expectancy exceeds 15 years, regardless of whether that implant is designed for orthopaedic, cardiovascular, dental or other application. This represents the major challenge in the field, and one that is extremely elusive. Despite large amounts of research in the field of biomaterials science, the problem of maintenance-free durability remains with us. There have been only marginal extensions in the anticipated lifetimes of implants as a result of our increased knowledge of both materials and surgical techniques. On the other hand, what has been achieved is remarkable, and there is no doubt that biomaterials alleviate suffering and add to the quality of life for a very large number of people throughout the world.

1.2 Surgical Uses of Biomaterials

Biomaterials are usually made into a medical device of some sort, and employed in the body in this form. The term device has been defined by the United States Federal Drug and Food Administration (FDA) as ‘any instrument, apparatus, implement, machine, contrivance, in vitro reagent or combination of these that is intended for diagnosis, prevention or treatment of disease’.¹⁰ This is a comprehensive definition, and includes those instances where the material itself is simply placed and takes up the shape of a prepared cavity or space, as in dental filling materials or orthopaedic bone cements. Devices defined in this way are currently used in various branches of surgery to treat different conditions. Some examples are described in detail in this chapter.

1.2.1 Orthopaedic Joint Replacement

As the body ages, it may develop osteoarthritis. This is where t...

Table of contents

• Cover
• Title
• Copyright
• Preface to the First Edition
• Preface to the Second Edition
• Contents
• Chapter 1 Synthetic Materials in Medicine
• Chapter 2 Polymers
• Chapter 3 Ceramics
• Chapter 4 Metals
• Chapter 5 Dental Materials
• Chapter 6 Biological Interactions with Materials
• Chapter 7 Tissue Engineering
• Chapter 8 Ethical Perspectives in Biomaterials Science
• Subject Index