The Engineering of Human Joint Replacements E-Book

J. A. McGeough

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 285

Veröffentlichungsjahr: 2013

Beschreibung

Since the major pioneering of joint replacement surgery more than fifty years ago, much research and progress has been made in the field of arthroplasty with new insights into better materials, types of cement and bone-cell compatible coatings, and a better understanding of the causes of implant failure. With an increasingly ageing population the requirement for arthroplastic surgery is manifest; over 800,000 hips worldwide are replaced each year, and replacement surgery is performed for almost every joint of the body.

The Engineering of Human Joint Replacements covers the design, engineering, production and manufacture of human joint replacements, as well as associated engineering concerns such as surface coatings, orthopedic bone cement, the causes and effects of wear and tear, and rapid prototyping for clinical evaluation. Materials evaluation and selection is discussed, as well as production processes and insertion methods. The author provides an overview of skeletal anatomy and the effects of pain and deterioration in order to put the engineering principles into a medical context. Examples of joint replacements for the most common regions of the body are included, and aspects of clinical studies of these cases are discussed.

Key Features:

• Provides an overview of the engineering materials and processes involved in the manufacture of    human joint replacements
• Sets the scene for engineers and clinicians embarking on research into joint replacements
• Includes clinical and industrial examples and points the way to future developments
• Provides information on medical device companies with an engineering guide to the requirements for joint replacement

The Engineering of Human Joint Replacements bridges the divide between engineering and orthopaedic surgery, offering an introductory text to young engineers entering the field, as well as a reference for medical staff who will benefit from an understanding of the materials and methods used in their design, engineering and manufacture.

Leseprobe

Contents

Cover

Title Page

Copyright

Preface

Chapter 1: Introduction

References

Chapter 2: Basic Anatomy

2.1 Terminology

2.2 Human Skeleton

2.3 Joints

2.4 Cartilage

2.5 Protein and Collagen

2.6 Human Bone

References

Chapter 3: Anatomy of Joints

3.1 Shoulder

3.2 Elbow

3.3 Wrist

3.4 Finger

3.5 Hip

3.6 Knee

3.7 Ankle

3.8 Foot

3.9 Toe

3.10 Degradation of Joints

References

Chapter 4: Methods of Inspection for Joint Replacements

4.1 Introduction

4.2 Gait Analysis

4.3 X-ray

4.4 Tomography and Computed Tomography (CT)

4.5 Radionuclide Scanning

4.6 Ultrasonography

4.7 Magnetic Resonance Imaging (MRI)

References

Chapter 5: Materials in Human Joint Replacement

5.1 Introduction

5.2 Alloy Metals

5.3 Ceramics

5.4 Polymers

5.5 Joint Replacement Materials in Service

5.6 Nanomaterials

References

Chapter 6: Methods of Manufacture of Joint Replacements

6.1 Introduction

6.2 Surface Finish

6.3 Tolerance

6.4 Wear and Friction

6.5 Machining

6.6 Forging

6.7 Casting

6.8 Manufacture of Polymer Parts

6.9 Surface Treatment

6.10 Surface Finishing of Implants

6.11 Manufacture of Joint Replacements

References

Chapter 7: Computer-Aided Engineering in Joint Replacements

7.1 Introduction

7.2 Reverse Engineering

7.3 Solid Modelling

7.4 Finite Element Analysis (FEA)

7.5 Rapid Prototyping (RP) in Joint Replacement Manufacture

7.6 Computer-Aided Manufacture

7.7 Navigation

7.8 Robotics

References

Chapter 8: Joint Replacement

8.1 Introduction

8.2 Shoulder

8.3 Elbow

8.4 Wrist

8.5 Fingers

8.6 Hip

8.7 Knee

8.8 Ankle

8.9 Foot and Toe

References

Index

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Library of Congress Cataloging-in-Publication Data

McGeough, J. A. (Joseph A.), 1940- author. The engineering of human joint replacements / by J.A. McGeough. -- 1 1 online resource. Includes bibliographical references and index. Description based on print version record and CIP data provided by publisher; resource not viewed. ISBN 978-1-118-53684-1 (ePub) -- ISBN 978-1-118-53685-8 -- ISBN 978-1-118-53690-2 (MobiPocket) -- ISBN 978-0-470-74027-9 (hardback) 1. Artificial joints. 2. Arthroplasty. 3. Biomedical engineering. I. Title. RD686 617.5′80592--dc23 2013022090

A catalogue record for this book is available from the British Library.

ISBN: 978-0-470-74027-9

Preface

I wish to thank Mr James Christie, Consultant Orthopaedic Surgeon at the Royal Infirmary of Edinburgh (RIE), who first suggested to me that I could contribute engineering solutions to clinical problems.

Our encounter happened by chance. Following an injury to my knee some weeks earlier, I was given an appointment to see him. Until we met I had been researching mainly in manufacturing processes. During a visit to a major aircraft engine manufacture to discuss cooperation in electrochemical machining (ECM) (a method of shaping tough heat-resistant alloy metals that are difficult to cut by established mechanical techniques), I stumbled on a stairway and to stop myself falling I straightened my right leg, and suffered a sharp pain in my knee. As the pain continued for some weeks I eventually saw my doctor who referred me to the RIE; that is how I met Mr Christie.

He solved the problem with my painful knee – the abrupt jerk had reactivated an old sports injury to my cartilage and associated wear and tear over the years. After asking me what I did for a living, and when I explained my professional interests, he brought me an X-ray of an intramedullary nail, the design for which he felt needed changing. Over the next few months, with a colleague, James Holmes, I designed and had manufactured a prototype in titanium alloy, which was the material I had been discussing with the aircraft engine manufacturer when my injury had occurred. The prototype was put into the hands of a medical device company for evaluation. My cooperation with orthopaedic surgeons had begun.

Other collaborative projects followed with Mr (now Professor) Charles Court-Brown and Dr Richard McCalden. We investigated the effects of age on the mechanical properties of cortical and cancellous human bone. With Professor Dugald Gardner, and later Mr John Keating, there began a longstanding cooperation on engineering studies of the meniscus.

Following a long stint of the headship of the Department of Mechanical Engineering at the University of Edinburgh, in 1991 I was granted a sabbatical period of six months. It was arranged by Mr Christie that I could be based in the academic unit of the Orthopaedic Surgery Department in the Edinburgh Royal Infirmary. In addition to meeting other surgeons in their daily work, including Miss (now Professor) Margaret McQueen, it provided me with the privilege of attending operations in order to understand the role that engineering can have in orthopaedic surgery.

On return to full-time university duties, the collaborative programmes continued, firstly in the Princess Margaret Rose Orthopaedic Surgery Hospital and then in the new Royal Infirmary of Edinburgh. While the Crichton Laboratory, devoted to orthopaedic engineering, had been opened in the Department of Mechanical Engineering in 1989, it had become increasingly evident that close geographical proximity of engineering research students and assistants to clinicians is essential for research work of this nature to flourish. To that end, with cooperation from academic colleagues in orthopaedic surgery, we were fortunate to be able to establish the then-named ‘Edinburgh Orthopaedic Engineering Centre’ (EOEC) within the College of Medicine and Veterinary Medicine's Chancellor's Building, with its own experimental and office facilities.

This book is an account of engineering's place in that part of orthopaedic surgery that deals with human joint replacement. It is based on impressions gathered in consultations with orthopaedic surgeons over many years, observations made during the attendance at surgical operations described above, and the insight provided from innumerable discussions with research clinicians and engineering researchers, both post-graduate and undergraduate, whom I have introduced to this field through their projects for PhD, MPhil, MSc, BEng and MEng degrees.

The book is aimed primarily at the last-mentioned group. They receive little, if any, formal introduction to the field in their teaching courses, which are so heavily constrained by accreditation requirements. Yet I have never known one who has not risen enthusiastically to the task of investigating some aspect of engineering relevant to orthopaedic surgery. This enthusiasm has constantly been encouraged by their meetings with clinicians, who have always made time to see them.

The book is therefore not an account of the latest research findings in this field. Instead it lays the foundations from which research can arise, although in the course of its writing some new relevant developments have been noted and introduced. For some of these I am grateful to the referees who vetted the outline plan at the outset. For others, I have drawn on the series of event proceedings organized by the Engineering in Medicine and Health division of the Institution of Mechanical Engineers.

The subject is clearly cross-disciplinary. Before the work proper began, the question of a co-author from orthopaedic surgery was raised with the publisher and clinicians. The latter are busy people in their daily duties and the added burden of writing such a tome with the time needed would have added too much to their already heavy workload. Another engineer as a co-author might have helped, but consultation revealed that this would have been possible in specialist areas and not across the entire spectrum of its contents. Instead the book has been written by me, as one author. It is therefore one engineer's perspective on the subject. I hope I can be excused if I have not given sufficient scope to topics that might be of direct interest to specialists in areas discussed in the book. Nonetheless I am grateful to colleagues who have read each of its draft chapters and made many suggestions for improvement. They include Mr C. Howie, Professors D.L. Gardner and S. Hinduja, Dr E. Keane, Drs J. Atkinson, R. Heinemann and G. McGuinness, and Messrs A. Room and M. Wright.

The staff of the library of the Royal College of Surgeons of Edinburgh, especially Mrs Marianne Smith and Mr Steven Kerr, have been most helpful in arranging access for me in my researches for the book. Special thanks are due to Mr Colin Howie, senior consultant orthopedic surgeon, who arranged for me to attend operations at the RIE in order to deepen my understanding of joint-replacement surgery.

The book has been typed by Ms Diane Reid. Miss Jiayi Shu prepared the diagrams, with useful contributions from L. Delimata, C. Fraser and C. Macmillan. Ms L. Delimata provided sterling support during the final stages of preparation of the book. I am grateful to authors, and others acknowledged in the text for permission to reproduce photographs and figures.

The staff at Wiley Engineering Publishing, notably Ms Debbie Cox, the late Nicky Skinner, Ms Liz Wingett, and Mr Tom Carter provided professional and patient encouragement from the outset to completion, for which I am much appreciative. Within the School of Engineering at the University of Edinburgh, Professors Alan Murray and Ian Underwood are thanked for their support.

Books demand time for writing. In this task patience is needed by those closest to the author. My wife Brenda possesses this virtue, for which I am continually grateful. The other members of my family, Andrew and Karen McGeough, Elizabeth and Barry Keane with Patrick and Thomas, and Simon and Louise McGeough with William and Amelia, remain my steadfast supporters, always interested in what I do.

J.A. McGeough Edinburgh

1

Introduction

Movements of the human body are controlled by its skeletal structure, which is made up of bones, joints, muscles, tendons and nerves. Even in healthy bodies, these parts of the skeletal structure can develop disorders, the symptoms of which can be pain, stiffness, swellings, deformity, loss of function and changes in sensitivity. Of these symptoms, pain is the most common.

The outcome of the sensation of pain can be impaired movement of the body; yet reduced movement could also be due to stiffness, localized to a particular joint, or more generally at more than one joint. A common form of stiffness is ‘locking’, the inability to complete a movement of the body. This condition can arise from mechanical effects, such as torn or damaged parts. For example, in the knee, vigorous athletic activity can cause a tear in the ‘meniscus’ (cartilage), causing both pain and locking.

Variations from normality (‘deformity’) in the skeletal system can take forms such as wide hips, short limbs, round shoulders and curvature of the spine; their magnitude can change with time. When joints do not function properly, muscle weakness and joint instability can be a consequence.

Damage to bones and joints, and their associated nerve supplies, can cause a change in sensitivity, which may present itself as numbness, tingling or pain. A common example is a collapsed intervertebral disc that causes pressure on a neighbouring part of the skeletal structure, leading to back ache; another is a trapped nerve in the skeleton, which usually causes pain, and loss or reduction in function or movement of the body. A frequent cause of pain stems from inflammation of a joint caused by arthritis; swelling and stiffness may come from injury, infection or degeneration due to wear and tear. When arthritis is the cause, key joints in the body, notably the knee, become unable to support the upper body weight, and everyday physical movements become impaired.

Osteoarthritis (OA) is one of the main types of arthritis causing these difficulties. This disease leads to progressive degeneration in the cartilage of the knee, lessening the ability of the cartilage to cushion the joint from impact and provide articulation of the knee. The rate of advancement of OA is influenced by body weight, the extent of physical activity, age and genetic and orthopaedic abnormalities. When pain, stiffness and knee movement reach unbearable levels, joint replacement has to be considered. The incidence and prevalence of OA increases with age and it is also associated with obesity. The World Health Organisation (WHO) defines obesity in terms of body mass index (BMI), calculated by dividing weight (kg) by square of height (m2). A BMI of respectively 25–30, 30–40, and more than 40 kg/m2 categorizes the conditions of overweight, obese, and morbidly obese. Changulani et al. (4) quote UK government statistics that between the ages of 55 and 64, about 20% of men and 33% of women are obese. Studies by Busija et al. (3) indicate that pain due to OA is more prevalent in adults who are obese or overweight. The meniscus (cartilaginous tissue) acts as the shock absorber of the knee joint, bearing 50 to 70% of load on the knee. Ordinary movements such as walking downstairs entail forces four times that of the body weight being imposed on the meniscus. When the knee joint has to bear additional bodyweight due to obesity, meniscal degradation can arise. This will be explored more fully in Chapter 3.

Other joints not directly involved in load bearing can also be affected by obesity. The accumulation of body fat may promote inflammatory effects in tissue, causing joints to deteriorate.

The World Health Organisation (10) reports that the combination of energy-dense foods and physical inactivity have contributed to a threefold increase in obesity since 1980, in countries ranging from North America to Australia and China. In the United States, more than 70% of people over 60 years of age are considered to be obese. By 2015, 700 million people worldwide are expected to be obese, according to the BBC (2).

Obese people with a BMI of more than 30 kg/m2 are encouraged to lose weight. A reduction in weight by 5 kg in obese people has been claimed to reduce by 24% their need for surgery associated with OA of the knee (Williams and Fruhbeck 9). Others have suggested that a BMI greater than 40 kg/m2, or morbid obesity, might contraindicate surgery (Horan 7).

People who undergo knee replacement surgery are less likely to have a less successful outcome if they have a BMI of more than 40 kg/m2, compared to those with a BMI below 30 kg/m2 (Amin et al. 1).

Total knee replacement is more common in women than men, and this has led to these implants being specifically adapted to fit the geometrical features of the female knee.

Rheumatoid arthritis (RA) is another common cause of joint replacement. It affects about 1 in 100 people at some stage, usually between the ages of 40 and 60, and the condition is three times more likely to occur in women than in men. It is more common in smokers and in those who consume high quantities of red meat and caffeine.

In RA, the body's immune system dysfunctions. The inflammation that ensues causes damage to tissue, bone and neighbouring ligaments, thinning out the cartilage and adversely affecting the synovial fluid that serves to lubricate the joint spaces. The joints become swollen, painful and deformed. They can no longer perform their proper function.

Every joint in the body can be affected by degenerative conditions like OA and RA. The need for joint replacements can also be caused by trauma, such as car accidents, falls, or by athletic injury, causing broken bones or fractures, or other damage to the joint structure. A fracture or break in a bone can disrupt the supply of blood to bone, which adversely affects its healing. This can lead to ‘avascular necrosis’ or ‘osteonecrosis’, that is, death of the bone. In the hip, this complication is another leading cause of joint replacement and is most often observed in patients between the ages of 30 and 50.

The lifespan of a primary hip prosthesis is estimated to be 10 to 15 years (although new designs are extending this period), after which revision may be required. The need for revision can stem from many causes including weakening of the original femur bone owing to age or disease, or ‘aseptic’ loosening of the prosthesis within the parent bone. The effects of age and gender on primary and revision hip replacement surgery are presented in Table 1.1. See also the National Health Service (NHS) National Services Scotland (8). The worldwide need for hip joint replacement is evident from Table 1.2.

Table 1.1 Effects of age and gender on number (N) of primary and revision hip replacements per 100 000 of population.

Table 1.2 Number of hip replacements in Europe, North America and Australasia.

Many attempts have been made to replace human joints with manmade substitutes. In the 1860s, knee replacements were undertaken aimed at restoring the normal functions of this joint. A platinum and rubber replacement for the shoulder joint was produced in 1893. In the early twentieth-century, work on hip replacements began, the head of the femur being replaced by ivory and then later acrylic, the prosthesis being fitted with a stem that was positioned in the femoral neck. These early devices proved of limited efficacy and were duly abandoned, although some, albeit limited, progress continued to be made. Nonetheless an increasing insight was gained into the requirements for effective joint replacement.

Throughout the twentieth and into the twenty-first century, a deeper understanding of joint biomechanics has developed. New engineering materials have been produced. Methods for their manufacture into useable products continue to evolve, often aided by computer technology. While these latter trends have been aimed primarily at manufacturing industry, they can also be applied in the engineering of joint replacements. This realization has spurred the major advancements needed to deal with the problems encountered mainly by an ageing population, to the extent that virtually every one of the parts of the human skeleton can now have an industrially produced substitute.

The steps leading to the decision to replace a joint are clear and logical. A main indication is pain, together with impaired movement. An understanding of human anatomy, in particular that of the joints, is needed to evaluate movement, or gait, and select methods of internal inspection. These topics form the substance of the next three chapters.

References

Amin, A.K., Clayton, R.A., Patton, J.T. et al. (2006) Total knee replacement in morbidly obese patients. Journal of Bone and Joint Surgery88 (10), 1321–1326.

BBC (2008) Obesity: In Statistics,news.bbc.co.uk/1/hi/health/7151813.stm (accessed 23 April 2013).

Busija, L., Hollingsworth, B., Buchbinder, R. and Osborne, R.H. (2007) Role of age, sex, and obesity in the higher prevalence of arthritis among lower socioeconomic groups: a population-based survey. Arthritis Care and Research57 (4), 553–561.

Changulani, M., Kalairajah, Y., Peel, T. and Field, R.E. (2008) The relationship between obesity and the age at which hip and knee replacement is undertaken. Journal of Bone and Joint Surgery90-B (3), 360–363.

European Federation of National Associations of Orthopeadic and Traumatology (EFORT) (2010) European Arthroplasty Register (EAR). European Arthroplasty Register (EAR) Publications. Innsbruck, Germany.

Horan, F. (2006) Obesity and joint replacement. Journal of Bone and Joint Surgery88-B (10), 1269–1271.

National Health Service (NHS) (2000) Information Centre for Health and Social Care, Office for National Statistics, London.

National Health Service (NHS) National Services Scotland (2012). Scottish Arthroplasty Project-Biennial Report, Information Services Division (ISD) Scotland Publications, Edinburgh.

Williams, G. and Fruhbeck, G. (2009) Obesity: Science to Practice, Wiley-Blackwell, Chichester, p. 230.

World Health Organisation (WHO) (2003) Global Strategy on Diet, Physical Activity and Health. Obesity and Overweight Fact Sheet, World Health Organisation Press, Geneva, Switzerland.

2

Basic Anatomy

2.1 Terminology

The human body consists of three main components: head, trunk and limbs. The trunk is made up of the neck, chest (or thorax) and abdomen (belly). The lower region of the abdomen is the pelvis. The ‘perineum’ is the lowest part of the pelvis and of the trunk. The central axis of the ‘vertebral column’, or spine, and the upper (cervical) part of the spine support the head.

The upper and lower regions of the limbs are made up of respectively (i) the arm, forearm and hand, and (ii) thigh, leg and foot.

This structure of the human body and the standard anatomical position are shown in Figure 2.1. The body is standing upright; the feet are together; the head and eyes look to the front. The arms are by the sides of the body. The palms of the hands face forward.

FIGURE 2.1 Terminology for human body.

Source: Adapted from McMinn, C.M.H., Hutchings, R.T., Pegington, J. and Abrahams, P.H. (1993) A Colour Atlas of Human Anatomy, 3rd edn, Wolfe Publishing, London.

As indicated in Figure 2.1, an imaginary plane is drawn vertically through the middle of the body, from front to back. This is termed the ‘median sagittal plane’, thus dividing it into right and left halves. The terms ‘medial’ and ‘lateral’ are used to describe respectively parts of the body closer to, and further from, the median plane. Alternative expressions are sometimes used. For example, ‘ulnar’ and ‘radial’ can replace respectively ‘medial’ and ‘lateral’. In descriptions of the forearm, which has two bones, the radial term can describe the radius on the ‘thumb’ side or lateral region. The term ulnar can be used to indicate the body part which is medial. For the two bones of the lower leg, the terms ‘fibular’ and ‘tibial’ are often used to describe the fibula on the lateral, and tibia on the medial, sides.

Regions nearer to the front of the body are termed ‘anterior’ (or ventral). Those nearer to the back are denoted ‘posterior’ (or dorsal). However, for the hand, the anterior surface is described as the ‘palm’. Its posterior surface is called the ‘dorsum’. The upper surface of the foot is its dorsum, or dorsal surface. The sole of the foot is the plantar surface.

The upper regions of the body are called ‘superior’ whereas the lower areas are ‘inferior’. The terms ‘proximal’ and ‘distal’ are used to describe positions respectively nearer to, and further from, the root of the skeletal structure.

A ‘sagittal plane’ is any plane that is parallel to the median sagittal plane. A ‘coronal’ (or ‘frontal’) plane is any plane which is both vertical and perpendicular to the median sagittal plane (McMinn et al. 1993).

2.2 Human Skeleton

The structure of the skeleton of the human body is shown in Figure 2.2. The key that accompanies this figure indicates by number the principal parts of the skeleton, and in particular those encountered in joint replacement.

FIGURE 2.2 Human female skeleton.

Source: Adapted from McMinn, C.M.H., Hutchings, R.T., Pegington, J. and Abrahams, P.H. (1993) A Colour Atlas of Human Anatomy, 3rd edn, Wolfe Publishing, London.

2.3 Joints

The junction of two bones is termed an articulation or joint. A common purpose of all joints is to hold together the relevant parts of the body. Joints are either fixed or moveable. Moveable or ‘diarthrodial’ joints enable various motions of the skeleton. Three types of joint are present in the human body: fibrous, cartilaginous and synovial. Synovial joint replacements are the main type discussed in this book.

Synovial joints depend on the properties of articular cartilage, a load-bearing and wear-resistant connective tissue that covers the bones associated with the joint, and on synovial fluid, a nutrient and lubricant within the region of the joint. Both of these substances will be discussed more fully below. Figure 2.3 illustrates the structure of a synovial joint.

FIGURE 2.3 Structure of the synovial joint.

2.4 Cartilage

Cartilage contains a type of biological cell called a ‘chondroblast’. Chondroblasts produce a cartilaginous matrix composed of collagen, elastin fibres, and a ground substance rich in proteoglycans. Chondroblasts that become caught in spaces in the extracellular matrix termed ‘lacunae’ are called ‘chondrocytes’. Cartilage is avascular (that is, it does not have a blood supply), with the exception of epiphyseal cartilage which is present in the growth plates of long bones.

2.5 Protein and Collagen

Proteins are biochemical substances made of polypeptides in a fibrous or globular form. They enable a biological function to proceed. A polypeptide is a single linear polymeric chain of amino acids. The peptide bonds within a polypeptide are formed by the condensation reaction between the amino group of one amino acid and the carboxyl groups of the next amino acid.

Amino acids are differentiated by their side group, denoted R. In the case of glycine – the simplest amino acid – R is a hydrogen atom. Figure 2.4 shows the arrangement of a polypeptide.

FIGURE 2.4 Generic arrangement of a polypeptide.

Collagen is a structural protein, the main constituents of which are given in Table 2.1. It has the general amino acid sequence of -glycine-proline-hydroxyproline-glycine-X- where X can be any other amino acid. It is arranged in a triple α helix glycine (Gly), shown in Figure 2.5 (a) as a flat sheet structure, the repeating distance of which is 0.72 nm.

FIGURE 2.5 (a) Flat sheet structure of protein; (b) helical protein chain (right handed).

Source: Reproduced from Park, J.B. and Lakes, R.S. (2007) Biomaterials: An Introduction, 3rd edn, Springer, New York.

For relatively larger side groups, the structure takes the form of a helix. Then the hydrogen bonds bind the helix together, as illustrated in Figure 2.5 (b).

These amino acids are the starting-point in the formation of collagen, as represented in Figure 2.6.

FIGURE 2.6 Formation of collagen.

Source: Reproduced from Gross, J. (1961) Collagen. Scientific American204, 121–130.

They link together producing a molecular chain, which coils into a left-handed helix. The intertwining of three chains in a triple-stranded helix leads to a ‘tropocollagen’ molecule. On the alignment and partial overlapping of many tropocollagen molecules, collagen fibrils of diameter 20 to 40 nm are produced, bundles of which, with a diameter 0.2–1.2 μm, are the basis of connective tissue.

Tissues containing significant amounts of collagen, such as human cartilage, may have significant load-bearing capacity and tensile strength. Table 2.2 shows the mechanical properties of collagen in relation to elastic fibres. The most common type in the body is the smooth, glossy hyaline cartilage, which contains chondrocytes and type II collagen. It is strong and compressible. The term ‘articular’ refers to hyaline cartilage, which covers and protects adjacent ends of bones.

Table 2.1 Constituents of collagen.

Content (mol/100 molAmino acidsamino acids)Glycine31.4–33.8Proline11.7–13.8Hydroxyproline 9.4–10.2 Acid polar (Aspartic, Glutamic, Asparagine)11.5–12.5Basic polar (Lysine, Arginine, Histidine) 8.5–8.9 OtherResidue

Source: Adapted from Park, J.B. and Lakes, R.S. (2007) Biomaterials: An Introduction, 3rd edn, Springer, New York.

Table 2.2 Mechanical properties of collagenous and elastin fibres.

Articular cartilage resembles a viscoelastic bearing surface. The structure of the collagen fibril orientation in the cartilage can convert shear forces on the articular surface to compressive forces at the interface between bone and cartilage. The coefficient of friction of cartilage is much less than that of man-made materials, such as Teflon on Teflon. This low coefficient of friction becomes relevant when the kinematics of the knee are considered, as discussed later.

Synovial fluid is a lubricant that bathes the articular cartilage and is described more fully by Balazs (1974). It is a non-Newtonian fluid: the viscosity of synovial fluid changes with shear rates. For example, in a movement like a heel strike (that is impulse loading) the synovial fluid shows high viscosity. On the other hand, the synovial fluid can provide the low viscosity needed to lubricate the sliding surfaces during flexion of the knee joint.

Another fibrocartilaginous substance is the meniscus. In the knee it acts as a load-bearing ‘washer’. Its effect is reduction in stress on the load-bearing surfaces by increasing the contact area between femoral and tibial condyles.

2.6 Human Bone

2.6.1 Structure of Bone

Bone is a composite material. It is anisotropic (that is, its properties are directionally dependent), non-homogeneous, and visco-elastic. It comprises mainly:

hydroxyapatite – a type of calcium phosphate. Its general formula is Ca10 (PO4)6 (OH)2 (about 43% by weight);collagen (type 1) (about 36%);water (about 14%);a small amount of mucopolysaccarides.

Mucopolysaccharides, or glycosaminoglycans, are an unbranched linear chain of repeating subunits. The subunit is hexose, which is a six-carbon sugar or hexuronic acid that is linked to hexosamine (six-carbon sugar that contains nitrogen). Types of glycosaminoglycan can be found in synovial fluid and in connective tissue, cartilage and tendons.

Bone also contains organic material including blood and lymph vessels, nerves, and cells termed osteoblasts and osteoclasts. The latter respectively produce and resorb bone material.

The two main types of bone are cortical (or ‘compact’) and cancellous (or ‘trabecular’). Cortical bone is a hard and compact tissue. The constituents of wet (not dehydrated) cortical bone are given in Table 2.3. (Note that dry bone will exhibit different qualities: for example their densities vary from about 1990 (wet) to 140–1110 (dry) kg/m3.) Apatite in cortical bone has a similar crystal structure to hydroxyapatite, and is formed in needle-like shapes, 20–40 nm in length, 1.5–3.0 nm in thickness, in the matrix of collagen fibres.

Table 2.3 Constituents of wet cortical bone.

ConstituentPercentage by weightMineral (apatite)69Organic matrix22Collagen(90–96% of organic matrix)Others(4–10% of organic matrix)Water9Source: Adapted from Tiffit, J.T. (1980) The organic matrix of bone tissue, in Fundamental and Clinical Bone Physiology (ed. M.R. Urist), J.B. Lippincott, Philadelphia, PA, Ch. 3.

The tissue of cortical bone is composed of secondary osteons, of diameter 100 to 200 μm, each of which is made of concentric lamellae, of thickness about 1 to 5 μm. The lamellae of osteons surround concentrically ‘Haversian’ canals. The collagen fibres take the shape of a lamellar sheet about 3 to 7 μm thick, which runs in a helical direction with respect to the long axis of the cylindrical system of osteons, or ‘Haversians’. The interconnected pores of the Haversian canals, lacunae (cavities which contains osteocytes), or canaliculi (small channels connecting the lacunae), are connected in turn to the bone marrow cavity and enable the transport of metabolic substances. This canal system is filled with body fluid (which may be regarded as essentially water containing solutes such as proteins), of volume about 18 to 19%.

Cancellous bone is more porous than cortical bone. It consists of a continuous three-dimensional network of interconnected rods and plates, known as trabeculae, with cavities that are filled with a viscous fluid (viscosity of approximately 0.04 to 0.4 Pa).

Bone can be classified by its shape. ‘Long’ bones predominate in the extremities of the human skeleton, and provide levers for movement. In the upper limbs these bones are mainly used for movement such as reaching, grasping and throwing. They are lighter and smaller than those in the lower limbs. The latter bones have to be larger and stronger in order to bear the weight of the body during movement, and associated repeated stress. Figures 2.7 and 2.8 show further features of a typical long bone. Its shaft is the ‘diaphysis’, with an outer shell, the ‘cortex’, which is made of cortical bone. The outer surface is called the periosteum (and the inner, endosteum). The cortex envelopes bone marrow contained within the medullary cavity. An ‘epiphyseal’ plate of cartilage separates the metaphysis (wider portion of long bone which grows and lengthens during childhood) from the epiphysis. The epiphysis also contains trabecular bone, and marks the proximal and distal bounds of the long bone.

FIGURE 2.7 Microstructure of long bone.

Source: Reproduced from Park, J.B. and Lakes. R.S. (2007) Biomaterials: An Introduction, 3rd edn, Springer, New York.

FIGURE 2.8 Structure of long bone.

'Shorter' bones are present in the hands and feet. Like their longer counterparts they also provide for movement. In addition they supply elasticity, flexibility and shock absorption. Other ‘flat’ bones, for example in the pelvis and scapula, protect underlying structures and provide locations for attachment of muscles.

The vertebral column is made up of irregular bones, part of whose function is to provide for muscular attachment. The vertebral bone also absorbs the impact forces associated with walking, running and jumping.

From Figure 2.9, key features of bone needed in understanding joint replacement anatomy can now be identified. A ‘condyle’ is a rounded part of bone that articulates with another bone, an ‘epicondyle’ being a smaller version. A facet is a small, flat, smooth bone where articulation occurs. The fossa part of a bone is shaped like a shallow dish. It gives space for the articulation of bone or for attachment of muscles. A ‘process’ is a prominence in bone that is often complementary to a depression in another bone. Muscles and tendons can attach to raised sections of bones called ‘tuberosities’ or ‘apophyses’.

FIGURE 2.9 Main features of long bone.

Both muscle and tendon tissue play a major part in enabling the range of motion of a joint. Muscle use enables new bone cells to be set down to create or raise the apophysis.

Muscles are made of bundles of cellular fibres, ‘myofibrils’. Human muscle is of two kinds: voluntary (striated) regulated by the central nervous system, and involuntary, regulated by the autonomic nervous system, not by the brain. Joint surgery demands a careful consideration of the function of the muscles relating to an articulation so that as much normal movement is possible after a joint defect has been corrected.