The Engineering of Human Joint Replacements

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 Surgery 88 (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 Research 57 (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 Surgery 90-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 Surgery 88-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