Knee Replacement Surgery

About Knee Replacement Surgery

Millions of people around the world are affected by arthritis. Osteoarthritis, the most common type, is ranked the fifth among all forms of disability worldwide and the majority suffer from hip and/or knee osteoarthritis. A number of treatments are available to help manage the symptoms of arthritis, but there is no cure.

Knee replacement surgery is a common procedure and is available worldwide. While joint replacement surgery can be a daunting prospect, it offers a welcome relief from pain and, for many, the ability to return to a normal life. This series of short articles provides an overview of arthritis, including how to determine whether or not you have it, what treatment options may be available to you and how to go about getting the right treatment.

• What is Arthritis?
• What is Hip Replacement Surgery
• Benefits of Hip Replacement Surgery
• Do I Need A New Hip?
• What Are My Options?
• Questions to Ask Your GP
• Knee Replacement Designs
• Types of Modern TKR
• Cost of Knee Replacement
• What to Expect from a Knee Replacement
• How Success is Measured
• Longevity and Registry data
• Cohort Studies
• Comparative Studies
• Orthopaedic Data Evaluation Panel (ODEP)
• What Should I Look for When Choosing?

What is Arthritis?

Knee Pain and Arthritis

Knee pain can be short-lived (acute) or persistent (chronic). Pain results from tissue damage and when it is persistent, it could be a sign of long-term damage sustained in an injury or by an underlying problem within the joint.

Arthritis, which means ‘inflammation within a joint’, is the most common cause of chronic knee pain. Arthritis can affect people of all ages, sexes and races but occurs more frequently as people get older. Over a third of the population over the age of 50 has arthritis pain that interferes with their normal activities¹. Although the word is widely used to describe pain, swelling or stiffness in a joint, there are many types of arthritis which can be caused by inflammatory, mechanical or degenerative conditions.

Osteoarthritis

Osteoarthritis is the most common type of arthritis. It is a condition where the smooth and lubricated cartilage that protects the ends of the bones becomes worn or damaged. Cartilage can become thin and rough and in response to the damage the joint can develop bony spurs. This causes pain, swelling and can restrict knee joint movement. It is not known what causes osteoarthritis, but several factors are thought to increase the risk of developing it:

• previous injury,
• getting older,
• being overweight (obese),
• certain hereditary factors,
• other health conditions such as rheumatoid arthritis or gout.

Along with hip osteoarthritis, knee osteoarthritis is one of the leading causes of global disability².

Rheumatoid Arthritis

Rheumatoid arthritis is a disease where the body’s immune system attacks the joints causing pain, swelling and stiffness. In addition to joint problems, rheumatoid arthritis can affect other parts of the body such as the lungs and kidneys and cause symptoms such as:

• tiredness,
• fever,
• weight loss,
• loss of appetite.

Other Types of Arthritis

Chronic knee pain can also be due to:

• haemophilia,
• gout,
• bone growth disorders (dysplasia),
• blood supply problems (avascular necrosis),
• knee injury,
• knee deformity.

Treatments for Arthritis

Unfortunately for many types of arthritis, including the most common types osteoarthritis and rheumatoid arthritis, there is no known cure. But if you are diagnosed with knee arthritis, there are a number of treatment options depending on its severity, impact and type.

Arthritis differs from person to person but a lot can be done to enable you maintain a normal active life. Depending on your circumstances, the right treatment options may help relieve the symptoms of your arthritis or help minimise its progression. It is common to be prescribed or recommended to follow more than one option.

Drug therapies may be suitable to treat the symptoms of your arthritis such as pain (analgesics or painkillers), to suppress inflammation (anti-inflammatory drugs) or to inhibit some forms of inflammatory arthritis (disease-modifying anti-rheumatic drugs).

Physical therapies may include physiotherapy or light-exercise options such as hydrotherapy to restore or maintain function. Occupational therapy may help you to work with your arthritis and continue to achieve the activities that you need or want to maintain.

You may also be able to discuss lifestyle options including appropriate levels of activity including leisure and work activities, weight loss or even types of food that may trigger increased symptoms or arthritis.

If your arthritis becomes severe enough that despite these treatments it becomes too difficult to get on with everyday life or your pain is no longer sufficiently managed, surgery may be recommended.

What is Knee Replacement Surgery

What Is A Joint Replacement?

A joint replacement aims to remove the pain and allow a patient to return to their normal active function. Joint replacement is the term used for surgically removing diseased or worn out bearing material in the body (cartilage) and implanting new bearing surfaces. The new bearing surfaces are usually a combination of metal, ceramic and/or plastic components. These components form an implant that works with the natural tissue around the joint to hold it together.

Types of Knee Replacement Surgery

A total knee replacement (TKR) is where both sides of your knee joint are replaced.& Your knee surgeon removes damaged bone and cartilage from your kneecap, thigh- and shinbones and replaces it with an artificial knee joint made of particular metal alloys and high-grade plastics that are proven to be tolerated by the human body.

A partial (or half) knee replacement is where only one side of your knee joint is replaced. The surgery can usually be done through small incisions and recovery tends to be shorter.

Your knee specialist will help you to decide on which type of knee replacement surgery is right for you.

Implants: What Are They?

The main job of an implant is to provide an articulating surface. Joint replacement implants are designed to work with your natural joint and designers attempt to reproduce a motion that feels normal. More innovative designs may provide patients with a more normal freedom of movement without any compromise to its stability.

Implants: What Are They Made From?

Implants are usually two or three components which work together to make an implant. The components are made of implantable grade metal alloys, ceramics or high-density polyethylene. Individual components are designed to work with the material properties for low friction and low wear, strength and fracture-resistance, biocompatibility (lowest risk of a biological reaction), function and stability, and for securing it in place (fixation). In most implants, the materials used have been tried and tested for many years and have been proven to work well in the human body.

Benefits of Knee Replacement Surgery

Living with Arthritis

Common arthritis joint symptoms include swelling, pain, stiffness and decreased range of motion. Symptoms may come and go. They can be mild, moderate or severe. They may stay about the same for years, but may progress or get worse over time. Severe arthritis can result in chronic pain, inability to do daily activities and make it difficult to walk or climb stairs. Arthritis can cause permanent joint changes which may be visible, but often the damage can only be seen on X-ray.

The Burden of Arthritis

Chronic arthritis is life limiting. The majority of people with arthritis experience pain most days, which seriously affects their quality of life. Arthritis limits movement and a quarter of people living with osteoarthritis cannot do normal activities because of their arthritis. This can have an impact on the ability to work, sleep and exercise and increases the risk of depression, cardiovascular disease, diabetes, hypertension and other health conditions.

Statistics available for individual countries all agree that the impact of arthritis costs millions of pounds: to sufferers of arthritis, due to their ability to work or through early retirement; to healthcare systems, including costs of treatment for arthritis and associated co-morbidities (additional conditions associated with symptoms and impact of the arthritis); and to the wider economy, for example costs of reduced work productivity and sickness absence from work.

The Value of Joint Replacement Surgery

Considering the lifetime direct and indirect costs of this chronic disease the true economic burden of chronic arthritis is underestimated, but the benefits of providing joint replacement surgery to the many thousands that require it far outweigh the cost of surgery.

For the individual sufferers of chronic arthritis, successful joint replacement provides a new lease of life. Following a relatively short recovery period and ongoing adherence to physiotherapy, patients can return to normal, pain-free, independent, active life, often including sports. A renewed ability to manage one’s own health and wellbeing brings life-extending benefits. The vast majority of patients do not require further treatment for the condition for many years and many do not require further treatment in their lifetime.

Do I Need A New Knee?

Diagnosing Arthritis

We commonly experience aches and pains in our muscles and joints, but when symptoms persist for more than a few days it is important to consult your doctor soon. In most cases, the sooner a treatment begins the more effective it will be.

You should seek advice from a healthcare professional if:

• you experience aches and pains in your joint(s) that are not related to an injury and do not ease after a few days, or aches and pains in your joint(s) that persist long after an injury;
• if a joint becomes swollen and stiff especially if not related to an injury;
• if you are unable to perform daily tasks because of aches and pains in your muscles or joints.

Before diagnosing arthritis, your doctor will ask you about your symptoms and how they developed. They will examine you and may arrange for tests to be done such as request a blood test or x-rays.

When Surgery is Recommended

Many people do not require surgery for many years if at all and not everyone who has knee arthritis needs a knee replacement. For many people the symptoms of arthritis subside or remain sufficiently mild with infrequent flare-ups for many years without severely impacting their quality of life. Many people do not experience arthritis until late in life or can reduce symptoms or slow the disease sufficiently that further treatment options are not required.

Knee replacement surgery is an option when alternative treatments have been exhausted. It is likely to be offered to you if:

• your knee is severely swollen, stiff and painful and it is affecting your mobility,
• knee pain has reduced your quality of life and affects your sleep,
• everyday tasks have become difficult or impossible,
• you feel depressed because of your pain and lack of mobility,
• your knee prevents you from being able to work or have a normal social life.

What Are My Options?

There Are Differences in Knee Replacement Designs and Providers

Like all common and familiar product categories, there are different makes and designs of knee implant and there are differences in how well they work. Success can also depend on the quality of service provided and how experienced a surgeon is in using a particular product.

Although the cost of an implant is relatively small compared to the cost of surgery, state funded and private healthcare systems usually select implants based on financial and logistical concerns and implants available from large companies that can offer bundled deals and one-shop supply. Even surgeons have less say in what implants they use than in the past. This makes it quite reasonable to believe that the best implant for you – the one that is most likely to give you what you want – might not be the one offered to you by the first surgeon or hospital that you are sent to.

You Have the Right to Choose

If you are diagnosed with knee arthritis and recommended to have knee replacement surgery, it is important to understand that you have the right to choose your implant, surgeon and hospital – regardless of how your treatment is funded. The implant is going to be yours for what should be a long time, so making good decisions early is important.

How To Choose

You are more likely than your surgeon or hospital to make the decision based on how well other patients have returned to normal life, how satisfied they are with their knee replacement and how long the device is proven to last. So the first thing to decide is what implant you want; then find the best surgeon or hospital to get it fitted. This does mean doing some research, but with the availability and easy access to good information it can be quite straightforward.

Many countries now have a National Register of joint replacements. Independent organisations also provide ratings for implants based on the quality and amount of clinical evidence for their performance. These sources usually provide information on how many patients require further treatment in the years after surgery – a fundamental but important measure of performance. National Registers may also report on the performance of individual hospitals or surgeons and may show how experienced they are and how often they perform hip or knee surgery.

Journal articles are often available on the internet free of charge and can provide comparative information including measures for pain, function and activity based on studying groups of patients. Even when journal articles need to be purchased, synopses (abstracts) are usually available for free and results of studies may be referred to in other articles.

Information on manufacturer’s websites can also be evaluated for factual versus marketing content. Do the marketing claims make sense to you? Do they provide evidence? How well do they relate to what you think is important?

Forums, blogs, YouTube videos and social media posts also include feedback from other patients who have received knee replacement surgery.

Don’t be put off by the wealth of information available. We discuss how to navigate these sources of information and what they mean on this website in ‘How Success is Measured’.

What to do next

https://www.gov.uk/government/publications/the-nhs-choice-framework/the-nhs-choice-framework-what-choices-are-available-to-me-in-the-nhs

Once you have decided on your knee implant, you can request to be treated by a knee surgeon of your choice. Most surgeons only operate at one or two hospitals – many work for the NHS or local state-funded hospital and also have a private practice. When deciding, look at the success rate of the surgeon and the hospital. After all, it is not just about the surgery – aftercare such as physiotherapy provided by the hospital is critical in the overall success of the procedure. It’s best to select a specialist who has good experience with your choice of knee implant. An example of how to do this in the UK is to visit the NJR Surgeon and Hospital profile website: http://www.njrsurgeonhospitalprofile.org.uk.

The Power of Informed Choice

The idea of being operated on can be a daunting prospect for most people so it is easy to understand why some people may not want to consider all aspects of it. Having a knee replacement is a major decision and one that you should be fully involved with. Rather like buying a car or phone, you may not fully understand all the technology involved, but you tend to know what you want it to do. It’s the same with new knee implants. The factors that healthcare systems prioritise when deciding what treatment options to offer may not be the same as your priorities. If you leave the decision to others, you might not hear about options that you would think are best for you and you might not even be offered one of the better performing devices – if only you had known beforehand! With some research you may be surprised at what you learn.

With the right information, you can take control of your disease and get the best treatment for you. By fully understanding your options, being able to assess them for yourself and being a part of the decision making process, you are more likely to receive a treatment option consistent with your values. As a result you are more likely to be happy with the outcome of your surgery.

Questions to Ask Your GP

The next step is to talk to your GP. It is important to have a frank and open conversation with them about what you want to achieve with your knee replacement. It may be a good idea to take a short-written list of questions with you to your GP consultation.

Ask your GP about their experience with other patients who have had knee implants. They may be able to help guide your research or, if you have done some research already, they may be able to comment on your findings including manufacturer’s marketing claims.

In some countries, you will need to request a referral from your GP before you see a knee specialist. Your GP can also assist you in finding the best available specialist knee surgeon and set up a consultation. Ask your GP which knee surgeon and hospital they would recommend and why. Does the surgeon perform knee replacement surgery regularly? Surgeons may get better results from devices that they are experienced in using.

Does the surgeon use the device that you would like to receive? Orthopaedic surgeons will usually be more experienced in using one or two devices, a small percentage of what is actively available on the market. By only speaking to one surgeon you are seriously limiting your options over the device you receive. Liken this to going to a Ford car showroom, not realising if you had done some research, you might actually be happier with a Jaguar. The ford dealer won’t offer you that!

You can also talk to your GP about whether to have your knee replacement surgery done privately or within your state healthcare system – for example the NHS.

When you meet the surgeon, tell them what is important to you and what you would like. Are they happy to discuss their results with you? Remember that you do not have to accept what one surgeon tells you and are free to talk to more than one surgeon. It may be that you need to shop around to get the implant you want.

Knee Replacement Designs

Although they differ slightly in shape and size, our knees all work in the same way. One might reasonably think that knee replacement designs are as straightforward as replacing the worn bony surfaces with anatomically correct pieces. A product range would just need to include a range of shapes and sizes to best fit our own knees. Unfortunately it is not this straightforward.

Like most products, there are many knee replacement designs. Different designs are available for different levels of disease. The type and amount of repair needed also depends on how badly damaged a knee is before surgery. Knees become damaged in different ways and arthritis often affects the stabilising tissues. So the implants need to restore some of the lost function of other parts of the knee – not just the bearing surface itself. Designs have also changed over time as newer materials and technologies became available. A great many variations have also resulted from different perceptions, priorities, and experiences of the surgeons and engineers who have worked on them over the years. Some designs prioritise or offer features that others do not. Some work well; others do not. Some last a long time; others are more prone to early failures.

At first there may appear to be a lot to take in, but don’t be put off. The basic concepts are quite straightforward.

This series of articles explains how modern knee replacements developed and what principal types of knee replacement are now available. We bring the main ideas together and explain what the key designs are trying to achieve and how they work. We also explain how much a knee replacement operation costs and what you should expect from a good joint replacement design.

The information here should be enough for you to work out what makes sense to you and what is most likely to give you what you want from your knee replacement.

How Modern TKRs Developed

The concept of inserting an artificial joint after the removal of a diseased joint has been considered for centuries. But it was not until the 19^th century that a Romanian surgeon called Themistocles Glück began practicing total knee replacement with ivory devices fixed with ‘bone cement’. The concept had merit, but was prone to failure for a variety of reasons including material choice, patient selection and infection. Other designs followed, with alternative materials and more suitable patients. But they all had a common feature: some form of mechanical hinge that imposed too much constraint for normal knee function and invariably led to high rates of mechanical failure.

Modern total knee replacement (TKR) designs were born in a particularly innovative period in the 1970s.

In 1969, the late Professor Michael Freeman implanted the first ever ‘total condylar knee replacement’ at the Royal London Hospital in Whitechapel, London. The knee was developed with Alan Swanson, an engineer at Imperial College London, and its design was based on recent success in hip replacement surgery by Sir John Charnley. The design was unique in that it replaced only the worn cartilage in the knee using two components. These components were intended to articulate as a ‘low friction’ bearing – and it did not include a hinge.

The components had just enough built-in constraint to allow the joint to flex in a normal way and remain stable using the joint capsule. So that the joint capsule could work normally around the implanted components, a stepwise technique was developed with special instruments to resect (cut) minimal bone and measure gaps between bone ends in the flexed and straight leg. Techniques were also developed to work with the ligaments to reliably correct angular deformities to obtain a straight leg that was stable in flexion and extension.

Unlike the comparatively simple ball-and-socket hip bearing, the knee has a more complex pattern of movement that simple hinged devices cannot restore. Without a hinge an implant could be designed to work with the natural knee structures. It would allow as normal a range of motion as possible while at the same time provide a normal degree of stability. That is, as long as the designers understood how the knee works.

The concept of attempting to understand the natural pattern and freedom of movement in a normal joint, and developing a low friction bearing that allowed the joint to work normally with its own guiding and stabilising structures, was a milestone in joint replacement surgery. The challenge to find the perfect bearing design is principally what led to a plethora of knee designs in the 1970s and a challenge that remains to this day.

Refs. Glück https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3824904/ ; history Ranawats (although poor errors) https://page-one.springer.com/pdf/preview/10.1007/978-2-287-99353-4_63; robinson;

What TKRs Are Trying to Achieve

A perfect knee implant should work in the same way and, crucially, provide the same freedom of movement and stability as a normal, healthy knee. It should also provide complete relief of pain, feel normal and allow patients to return to an active life for many years.

Anatomic Knee Replacement Designs

One approach to designing a knee replacement device is to make it anatomically correct. As long as the shapes of the components closely match to the shapes of the natural bones ends and are accurately placed, shouldn’t we expect the replaced knee to work normally?

Unfortunately, while the idea is attractive and seems to make sense, the reality is that a purely anatomic knee replacement design would fail to achieve the two crucial elements of a perfect knee implant: it would not provide the same controlled freedom of movement or stability as a normal healthy knee. The reason is would not account for the joint’s natural tissues – an essential part of the success of the condylar knee concept in 1969. Purely anatomic knee replacement designs are unlikely to feel normal and allow patients to return to an active life and forget about their knee.

Anatomy of the Knee: Before and After Surgery

A normal healthy knee joint is surrounded by a joint capsule. The capsule is principally made up of muscle and ligaments. Muscles contract to generate movement, and ligaments, which can only work in tension, control the safe limits of movement.

The Joint Capsule

On the inner side of the knee a thick bundle of ligamentous tissue called the medial collateral ligament stops the leg from collapsing in an inward direction. On the outer side of the knee a more slender ligament called the lateral collateral ligament prevents the knee from collapsing in an outward direction. Ligamentous structures at the back of the knee prevent the knee from buckling backwards, and the knee cap (patella), patella ligament and thigh muscles control movement and stability at the front – so the knee can flex. These are the principal tissue structures that remain after knee surgery and must be ‘balanced’ by the skilled surgeon to maintain overall stability and freedom of movement.

Inside the Knee

While the joint capsule provides overall stability and prevents the knee from collapsing, crucial structures inside the knee control its functional range of movement and keep the active joint stable.

Between the ends of the tibia (shin bone) and femur (thigh bone), there are two ligaments that cross each other (viewed from the side of the knee they form an ‘X’ shape). The anterior cruciate ligament (ACL) prevents the femur from shifting too far backwards on the tibia and from twisting too far in an outward direction. The posterior cruciate ligament (PCL) prevents the femur from shifting too far forwards on the tibia and from twisting too far inwards (the ACL also contributes to this). At least one and more commonly both of these ligaments need to be removed when replacing a knee.

The natural knee also includes pieces of cartilage that surround the bone ends: the medial meniscus and the lateral meniscus. These structures help protect the bone ends from overload and provide additional forwards and backwards stability (imagine chocks under airplane wheels).

After Surgery

If one or both of the cruciate ligaments are damaged or missing, an otherwise healthy knee will be unstable. Consider how many athletes have ended their career because of an ACL injury. A lot of investment goes into research and development of ACL reconstruction.

An anatomic knee replacement that perfectly replaces worn cartilage but at the same time removes the ACL and/or PCL will leave a patient no better off than someone with an untreated cruciate ligament injury.

Similarly, without one or both menisci, a knee becomes unstable. Unfortunately they are attached to the top part of the tibia removed during knee replacement surgery and it is not possible to retain them.

So without the menisci, with no ACL and likely no PCL, a knee replacement that precisely matches the cartilage of the removed bone ends – no matter how accurately it is placed, and regardless of how skilled the surgeon is at balancing the structures of the knee capsule – cannot provide adequate stability for a patient to return to a normal active lifestyle, let alone sport.

Functional Knee Replacement Designs

An alternative approach is to consider the function of a natural knee and the impact of necessary changes to tissue structures when performing the surgery. Then one can design a bearing that reintroduces sufficient constraint to permit a normal freedom of movement and at the same time prevent too much movement and instability. The more faithfully a knee’s function can be restored, the more normal it will feel for the patient.

The first successful modern knee replacement in 1969 was a functional design based on what was then known about how the knee worked. It consisted of a simple cylindrical bearing, which they called a roller-in-trough. The polished metal cylinder was attached to the femur using bone ‘cement’ and a high-grade plastic bearing with an anterior and posterior slope was fixed to the top of the tibia. The trough-shape of the plastic bearing stopped the femur sliding forwards and backwards, but was shallow enough to allow some freedom for a natural twisting motion. It did not fix a centre of rotation like previous hinges had done. The relatively simple design also allowed the surgeon to prepare the bone ends, position the implant and balance the ligaments of the joint capsule to achieve overall stability. It did not exactly replicate the anatomical shapes of the removed bone ends, but it did work with the remaining natural tissues and allowed the knee to work in a somewhat normal way.

Since then, functional knee designs have dominated knee replacement surgery. The principal variations have been based on different functional approaches to allow adequate freedom of movement while at the same time limit too much movement and restore stability.

Stability vs Mobility

When designing a functional knee replacement, stability and mobility become conflicting goals:

• A design with minimal constraint is likely to permit a good freedom of movement, but it is also likely to become or feel unstable during certain movements.
• A design with a lot of constraint may feel stable, but it may feel tight or stiff and noticeably limit the range of movement required for certain activities. As well as limiting patients from certain activities, instability and stiffness can also lead to pain.

Fundamentally, without the right balance between stability and mobility, patients are unlikely to be satisfied with their knee replacement. Even now, 1 in 5 patients report that they are not satisfied with their knee replacement. And compromised function, pain and expectations not being met are at the top of the reasons why.

Compromise

For many, the choice of knee replacement design comes down to finding the right compromise:

• A younger, more active patient with limited damage from their arthritis may choose, or be recommended to receive, a minimally stabilised knee. The basis for this is that as long as the surgeon can accurately fit the knee and balance the ligaments so that the healthy tissues and toned muscles can take on the extra work of stabilising the knee, a good level of function should be achieved.
• Conversely an older, perhaps less active patient, or one who has a significant amount of damage might choose or be recommended a more highly stabilised knee. For some, a feeling of stability and security underfoot is more important than being able to undertake active pursuits or sports.

In today’s world of total knee replacement, selecting and fitting the right type of knee replacement for the right patient is where most of the research and development is focussed. Where available this is making use of advanced technologies like digital surgical planning, customised implants and instruments and even robotics to help the surgeon get the best result – with as little constraint as possible. It is perhaps unsurprising that many believe that patients do not have the expertise to decide what is best for them, and that marketers are focussed on promoting the latest technologies they are working with.

Is Compromise Necessary?

Instead of find the right compromise, why should we have to compromise at all? Our natural knees all work in the same way and don’t compromise on stability or mobility. So why after so much development have modern knee replacement designs not solved this for everyone?

One problem is that until the advent of imaging technologies like MRI that could see inside living, working knees, no-one really knew how the knee worked. What parts of the knee provide stability during certain movements and where does the freedom of movement come from? If the knee does twist during some activities, by how much and how does the twisting occur?

We have known how the knee works since the start of the 21^st century. But by then earlier ideas and assumptions had become popularised and accepted normal practice. Clinically ‘acceptable’ outcomes were established and not based on measures of patient satisfaction and whether replaced knees felt normal. Manufacturers had also developed ways to provide devices in their multitude of sizes and shapes in economically efficient ways, which included modular assemblies of standard components.

If we do now understand how the knee works, can we not design a functional knee replacement to provide the same freedoms and limits to movements as a normal knee and restore normal function for everyone?

Well, we can.

How Does the Normal Knee Achieve Stability and Mobility?

There is now an abundance of information available on how the knee works.

We now know that the knee is predominantly stabilised around its medial side – the inner side of the knee. The medial collateral ligament (the thick bundle of ligamentous tissue on the inner side) does not allow much movement forwards or backwards between bone ends. On the other side of the medial condyle, at the centre of the knee, the ACL and PCL also prevent excessive forwards and backwards movement. The medial meniscus is well-fixed and relatively stiff (think of fixed chocks) and the bony surface of the tibia on the medial side has a modest dish-like shape. All these structures prevent excessive forwards and backwards movement inside the knee – they stabilise the knee.

The lateral (outer) side of the knee works differently – almost independently. It is stabilised, but by more flexible structures including a moveable lateral meniscus (think of train buffers), a more slender and often slack lateral ligament, and by the ligaments and muscles above and below the patella that act on the outer side of the knee. [Lift your knee up to about waist height and see where your knee cap is relative to your knee – you’ll notice that it is on the outer side, not in the centre]. Overall, the bone ends on the lateral side of the knee can slide forwards and backwards, still within safe limits, but more freely than the medial side of the knee. This is where the knee’s natural mobility comes from.

When we flex one of our knees right up to our chest, or kneel, the outer (lateral) side of the femur (thigh bone) shifts right to the back of the tibia (shin bone). The inner (medial) side however remains close to its original position on the tibia. Our bones have grown and developed this way. To prevent or not reproduce this motion means that our knees will feel less normal.

What Knee Designs Have Achieved Until Now

In a sense, the minimally stabilised knee designs have done a better job of mimicking the freedom of movement that the lateral side of the natural knee allows, whereas the highly stabilised knee designs have done a better job of mimicking the medial side of the natural knee. Choosing one or the other has partly been a result of the knee replacements having symmetric medial and lateral sides. This was of course attractive to manufacturers with modular supplies to cater for many sizes in left and right knees. Make the two sides of the artificial knee work differently and doubling an already large inventory* was a step that surgeons and engineers were unwilling to take.

However, this is where the secret had been hidden over the early part of the modern era. The natural twisting movement of the knee occurs about the more highly stabilised medial side. The moveable lateral side can be thought of as an ‘outrigger’ and necessary to stop the knee from collapsing sideways. Our knees have grown and developed to accommodate this combination of stability and freedom of movement – by the two sides operating differently – and if a functional knee replacement were able to accommodate this by restoring the stabilising function of the medial tissues and allowing the right amount of movement on the outer side, it should provide the perfect combination of overall stability and mobility that younger and older, active and less active people all want.

[*Consider a knee design that offers 8 bearing sizes, each in 5 thicknesses. That means 40 implant boxes are held on a hospital shelf for just one component of a knee replacement. If left and right components are different, this means 80 implant components need to be manufactured and stocked at a hospital for a single operation. There will also be 30+ boxes with size variations of the other components.]

What Should the Best Possible TKR Achieve?

A TKR aims to restore pain free, normal function for as long as possible.

An ideal TKR should provide the right amount of stability – without compromise! – as well as a normal freedom of movement, just like our natural knees. It should work for more and less active people, and it should work equally well for younger and older patients.

A commitment to rehabilitation and a careful return to activity is important following any surgery. But once a knee has completely healed, an ideal TKR should enable patients to return to a normal active lifestyle, including sport if desired – although the risk of injury and overuse exists just as it does for a healthy knee. Ideally, patients should be able to return to what they consider to be normal daily life and essentially ‘forget’ about their knee.

We now know how to do all these things. A TKR should provide a normal amount of stability on the medial side of the knee, taking into account that some natural tissues will have been diseased before, or by necessity removed during surgery. It should allow a natural freedom of movement on the lateral side, but not too much that risks any instability. It should allow a full range of flexion like the normal knee. It should also allow the knee cap to sit towards the outside of the knee when it is bent in a normal way.

An ideal TKR should be available in the right size, use well-proven materials, and be implanted by a skilled surgeon. At the moment balancing the knee within the natural tissues is beyond the capabilities of any robot.

With current materials and well-practiced techniques, failures are relatively rare (over 95% of patients’ replaced knees last more than 10 years). Most knee replacement designs go on to last for many years, but the better designs will last longer than others. Better designs are also more likely to enable patients to get back to their own version of normal life – without having to ‘cope’ with their artificial knee.

So it is understandable important to find out how well the knee you are considering achieves all these things. The best knees available can.

Types of Modern TKR

Partial Knee Replacements

The knee has 3 articulating ‘compartments’: (1) a medial articulation between the femur (thighbone) and tibia (shin bone), (2) a lateral articulation between the femur and tibia, and (3) an articulation between the end of the femur and the kneecap. Based on a guiding principal of orthopaedic surgery of saving natural bone and tissue where possible, surgeons can replace just one or two of the compartments.

Replacement of one compartment is called unicondylar knee replacement (medial or lateral compartment) or patellofemoral knee replacement. Two components can be replaced at the same time or on separate occasions. If the two parts are joined together in one component it is called a bicompartmental knee replacement.

Partial knee replacement is only suitable for people with limited disease that affects one or two of the compartments. The ligaments inside the knee need to be in very good condition. This situation is less common than arthritis affecting the whole joint. Surgery is also technically more difficult because components must be accurately placed in small spaces without disrupting any surrounding tissues.

In theory, a successful partial knee replacement will restore function sooner and feel more normal than a total knee replacement because more of the natural joint and stabilising tissues are maintained. Another advantage is that it may delay the need for total knee replacement. In case of failure of a partial knee replacement, conversion to a total knee replacement is relatively straightforward.

Minimally Stabilised Knees

Minimally stabilised knees are sometimes called cruciate retaining knees because they usually attempt to retain a functioning posterior cruciate ligament (PCL). Technically this is a more difficult operation. There is also some uncertainty over whether the PCL remains functioning: published literature shows that it is often damaged during surgery without the surgeon being aware. When the PCL is retained and functioning, characteristics of the ACL and menisci are still missing. So to achieve adequate stability, this type of surgery usually requires the surgeon to tighten the joint capsule more than normal to achieve stability. Studies have shown that even when surgery is considered successful, these knees are more likely to show symptoms of instability when tested on the clinician’s examination table. Nevertheless, this type of knee replacement has become much more common and is widely used worldwide.

Posterior Stabilised Knees

Until recently a ‘posterior stabilised knee’ was the most commonly used type of knee replacement around the world. This design stabilises the knee using an upright post at its centre. The upper (femoral) component contacts the post towards the end of flexion or extension. The post blocks the femur from moving too far backwards in the extended knee and from moving too far forwards in a flexed knee. It also provides a centre of rotation for the knee’s natural twisting action. Because the post is in the centre of the knee, it means the plastic tibial bearing can be symmetric.

This concept was first used in the 1970s and its basis was that the cruciate ligaments (ACL and PCL) work like a 4-bar link hinge. Unfortunately the idea was based on incomplete review of historical work that had already concluded that the concept is flawed. Ligaments could only work this way if they were rigid structures; they are not rigid and only work in tension.

Over the years this type of knee design has worked reasonably well for patients. But national registry data and comparative studies show that current patient expectations – for extremely low failure rates, feeling of normality, and return to activity – are not consistently being met with this type of device.

Medially Stabilised Knees

Very few designs of TKR around the world have included a medially stabilised knee design. The first design with this concept was implanted in the early 1990s. It was developed by the same surgeon and engineer team that had developed the first successful condylar knee in 1969 and was a 4^th iteration of that original design. It was unique in that it included a ball-and-socket articulation on the medial side of the knee and a roller-in-trough articulation on the lateral side. The idea for this functionally asymmetric design was that it would mimic the natural combination of freedom of movement and stability of a natural healthy knee. The asymmetry also made it possible to position the groove that guides the kneecap towards the outside of the knee – like the normal knee.

The design was based on hands-on experience and study of how the knee works. But it was a series of works in the late 1990s using MRI and other imaging methods that essentially demonstrated this to more closely replicate natural knee movement. The ball and socket articulation stabilises the medial side taking into account the removed meniscus and ligaments. The rolling lateral side allows freedom of movement, and the asymmetry allows natural movement of the patella.

The design was distinctly different to the established designs and was not quickly taken up. More recently most implant manufacturers are working towards this concept. But rather than introducing bold step change, the major companies are evolving their designs with increasing bias to stabilising the medial side and with marketing claims of more normality. A selection of smaller companies has developed versions of medially stabilised knee in the last decade, each with small differences. The original design and an evolved version from the concept originators is being used more widely than before and demonstrating excellent outcomes for patients.

Hinged Knees

Hinged knee designs are still used, but they are reserved for severely damaged knees that don’t have sufficient stability in the joint capsule. This amount of instability can be caused by trauma, progression of disease, gross deformity or difficult revision knee surgery. For patients that require a hinged knee, the welcome relief from pain and return to a modest level of mobility warrants that this concept should remain available.

Custom Knee Implants

Custom knee implants may be required for patients with particular deformities or where the amount of bone required for a standard implant is diseased or not present. Healthcare providers make special provisions for occasional use of these devices because they cost more and take longer to design and manufacture. These implants are usually based on an existing standard design with modifications to surfaces that are used to fix the implant within the joint.

With the digital age and advances in plastic and metal ‘printing’ technologies, producing implants and surgical instruments customised for individual patients has become more cost-effective. And a small number of companies are also now marketing custom knee implants for standard use.

The concept is similar to a tailor offering a semi-bespoke service where customers select an off-the-peg clothing item and are then offered certain adjustments to achieve a better fit.

Custom knee implants are based on a standard implant design that may be a ‘posterior stabilised knee’ or a ‘minimally stabilised knee’ design. Using a 3D scan of the patients’ joint, adjustments to the size and the profile of the part of the implant that is fixed to bone are made prior to manufacture. The goal of this is to achieve a near-perfect fit for the individual’s bone interface.

Marketing claims suggest that a custom knee may feel more normal to the patient. A well-fitting implant is appealing, but custom designs do not work any differently to standard knee implants. And while patients can perceive differences in movement and stability between knee designs, there is no evidence to confirm that patients with a customised fit would perceive any difference to a standard implant. Nevertheless the price for this technology is high: custom implants cost 2-3 times more than standard implants.

It is worth noting that some news articles have promoted custom knee surgery where the implant is not customised at all. It is more common nowadays that customised instruments are used. Using a 3D scan of a patients’ knee, custom plastic 3D-printed guides can be produced to assist the surgeon in positioning the implant during surgery according to a surgical plan created on a computer screen. Although this may be referred to as ‘custom’ surgery, in fact all surgery is customised whether standard or 3D-printed instruments are used: surgeons always need to fit implants correctly to individual patients’ anatomy and unique arthritic condition.

Cost of Knee Replacement

When selecting what implants to have available in their hospital, decision makers need to concentrate on cost-related factors. This includes, for example, focussing on the up-front cost of an implant and associated equipment, length of stay (it is more cost-effective for patients to go home as soon as possible), and likelihood of readmission or repeat surgery (each repeat treatment is costs more than the first surgery). Choosing when to treat patients can also be influenced by cost effectiveness considerations. Lifetime treatment for younger and more active patients is likely to cost more. Many patients have to wait until their disease is worse before they can have surgery. When they finally get their replacement, they are more likely to be satisfied with moderate function, will usually be less active and being older are less likely to seek repeat surgery in 10+ years.

In the UK, a knee implant can cost anywhere from £600-£3,000. Equivalent prices will vary around the world but also have a range. The price of an individual design will depend on how new it is, what materials and technologies it offers, and on any deals the hospital is able to negotiate with the manufacturer. Surgery itself can cost from average £6,500 for a straightforward knee replacement in an efficient specialist orthopaedic hospital to over £16,000 for complicated revision surgery. The NHS tariffs for knee replacement do not always cover the cost to the hospital so emphasising their need to focus on lower cost.

The price of a knee replacement in private hospital will vary between hospitals. Prices are influenced by factors including the perceived quality of care before, during and after the operation, and overall patient experience provided by the hospital. In the UK, straightforward knee replacement surgery in a private hospital is usually priced £10,000-£13,000.

In terms of how well knee replacement designs work, the factors that you are likely to think most important are not necessarily what hospital decision-makers are focused on. As can reasonably be expected, state-funded hospitals are focussed on a quality of care that means patients can return home with an acceptable relief from pain, be independent and able to live well enough that they will not require further treatment.

Privately funded hospitals usually use the same designs as state-funded hospitals because they are what the surgeons are familiar with. But because they compete for business with other hospitals and wish to attract patients, some are more focussed on better outcomes and more patient satisfaction. Surgeons also usually have more influence in what they use in their private practices and are keen to achieve high patient satisfaction. To do this they may have more freedom to choose newer and potentially better performing technologies.

Fortunately in recent years, acceptable norms are changing. Patients expect more and modern technologies can provide meaningful differences to them. Newer designs may be more stable, feel more normal, and allow patients to be more active. Some patients even report that they are able to forget about their knee during daily and recreational activities. The devices that do this are not always the lowest priced devices or ones provided by the biggest manufacturers that can offer attractive price bundles to hospitals. And although working against increasing cost pressures, performance and satisfaction factors are gaining in importance in both state and privately funded hospitals.

It is worth doing your research and seeking consultation with a surgeon you feel you can trust to ensure you have access to the best options.

Different options exist for funding a knee replacement, including state-funded treatment, medical insurance or direct payment for the treatment. It is worth considering what your requirements are. Does your local state-funded hospital provide the implant that you want? Are you able to select a different state-funded hospital or is it worth seeking private treatment? If you are considering private treatment, it is worth asking your surgeon if the device you want is covered by your insurer. It may be worth considering the value to you of getting the best treatment and whether it is worth paying up front to get your choice.

What to Expect from a Knee Replacement

When it comes to knee replacement surgery people’s expectations are wide and varied.

Some people simply want a joint that allows them to get about again, be independent and pain free. Others want to be more active and be able to go walking, running, swimming or cycling for example. For some people, how the knee feels is important. They want a knee that feels normal and not odd or different to their other possibly healthy knee. Most people want to forget about their knee most of the time rather than having to ‘cope’ with it.

How well will it work?

As well as pain relief, one of the factors important to all patients is that their knee feels stable. Instability might affect your confidence. Stepping off a bus or train for example can be unnerving on a knee that feels unstable. Active pursuits are also going to be more difficult with an unstable knee.

Fortunately, this is one aspect of knee replacement designs that can be demonstrated using modern techniques. And published literature is available demonstrating how stable patients’ knees are with some designs. The results can be quite revealing.

Having a high range of motion is useful but not essential for all activities. We all need enough motion to climb stairs, get up from a chair or tie our laces. There aren’t really any knee replacement designs that don’t provide this, but in some cases prior stiffness, difficult surgery, scar tissue or insufficient physiotherapy can lead to a stiff knee. Depending on activity requirements, some of us will expect more than others. Some modern knee replacements claim to allow very high range of motion. What they claim is not what the average patient achieves, but it is good to know that a device won’t inherently prevent what you think you need.

Our natural knees are really the benchmark for our expectations. Our healthy knees are stable and allow as much freedom of movement as we need. Fortunately it is now possible to select a knee that closely mimics the stability and movement of your natural healthy knee and is more likely to meet your expectations.

How long will it last?

We all want a knee to last as long as possible and if possible avoid the need to have repeat surgery. A relatively small number of people have complications, which can be for a number of reasons including infection. Overall just over 4% of patients require further surgery that includes replacing one or more implant components within 10 years. It is difficult to know just how long knee replacements last overall because initiatives like national knee replacement registers have only been running for 15-20 years. But we know that most knee replacements last for over 20 years and for many patients they last for the rest of their life.

When choosing which knee implant to have, you may prefer to have a design that has a long track record and has been used in many operations. Alternatively you may prefer a newer implant that uses the latest technology and promises better movement and stability. It might not have a long proven track record, but some new designs are stepwise evolutions of an earlier model that does and so is likely to at least achieve similar results. But this is not true for all new designs. There may be good evidence that some new designs have not experienced early problems, but radical changes in technology or material will not be proven until they have been in use for a number of years.

How Success is Measured

The success of knee replacement surgery is measured in terms of:

• How well a knee functions,
• How satisfied patients are,
• How likely or after how long further (revision) surgery is needed.

Determining the function of a knee implant is usually done in the clinic and involves a combination of measurements, like how comfortably you can bend your knee. X-rays are usually included to ensure an implant is aligned and well-fixed. If there are any issues with your knee, you will be able to discuss your symptoms and your surgeon may recommend further investigation or further treatment.

Most patients tend to judge success on their ability to get back to what they consider to be a normal life. Patient reported outcome measures (known as PROMs) are used to assess pain and a person’s ability to perform everyday tasks and whether they are aware of their replaced joint during activities. PROMs are usually measured by scoring a short series of questionnaires. Patient satisfaction, quite rightly, is becoming more common in PROMs, with questions like ‘are you satisfied with the outcome?’ and ‘are your problems better than before your operation?’

Measuring the success of knee replacement surgery is in the interest of healthcare providers, clinical professionals, designers and manufacturers and patients. Tools to measure performance and the availability of data have dramatically improved over recent years. And fortunately there is a wide range of information sources available to you.

This series of articles describes the principal types of information and what they are likely to mean to you. We also provide some of the key sources of information so you can review them for yourself.

What Is Expected Of a Modern Knee Replacement (State-Of-The-Art)

A modern knee replacement is expected to demonstrate a very low incidence of failures, relief from pain and a level of function that enables people to perform normal activities of daily living independently.

Likelihood of repeat surgery

The incidence of failure for an implant design can only be demonstrated by experience. By monitoring a group of patients with the same implant design for years after their surgery and recording if and when any need to have any part of their knee redone, a failure rate can be calculated. This is often called a ‘cumulative revision rate’ and is reported as a percentage at set periods of time (e.g. 1, 3, 5 and 10 years after surgery). For future surgeons and patients, it represents a likelihood of failure for the combination of factors applicable to the monitored group.

Many people consider 10 years to be a useful benchmark. Based on the experience of all knee replacements implanted in the UK over the last 17 years, the probability of failure within the first 10 years after surgery is a little over 3%. In Australia, based on knee replacements performed over the last 20 years, the probability of failure within 10 years is just over 5%. Values are similar in other countries. In other words, about 95% of patients have not required further surgery for at least 10 years.

Some knee replacement designs have been associated with higher rates of revision surgery and over the years some have been highlighted with ‘higher than expected’ numbers of failures and have been withdrawn from use. Equally some designs have been consistently reported with very low rates of failure.

Stability

Measuring knee stability is relatively straightforward. The simplest and most common method is done in the clinic. Your surgeon will ask you to lie on an examination bench, bend your leg slightly and grasp the top of your tibia (shin bone). By moving your tibia forwards and backwards against your knee, a small amount of forwards-backwards movement of the top of the shin relative to the femur (or kneecap which is more easily seen) can be observed. About 1cm of visible movement is as much as is usually desirable. If your knee is very stable knee it will be hard to see any relative movement at all.

Deciding what is acceptable is a little more difficult. All our knees have slightly different amounts of stability or, conversely, laxity: some people naturally have and can tolerate joints that might be called lax, whereas for others this would be uncomfortable. It is hard to replicate what any one individual would find normal. Joint laxity, however, is one of the more common reasons for patients needing repeat surgery.

While excessive joint laxity can be caused by patient factors, surgical factors and choice of implant design, it has been shown that some knee designs are more likely to be considered stable among groups of patients than others. It has also been shown that patients are more likely to appreciate a knee implant that feels stable even if it is perhaps more stable than their natural joint.

Most implant manufacturers offer different options for stability and claim designs have certain advanced features for stability. But very few provide evidence for how stable patients’ knees are or how infrequently patients are unhappy or even need repeat surgery because their knee is unstable.

To see whether a type of implant design you are interested in has a higher or lower incidence of failure for instability, you can search for ‘instability’ in the UK or Australian National Joint Registry Annual Reports. The Australian report separates types of knee replacement a little more than other registries for failure reasons.

The best sources of information on knee stability are in published reports of comparative studies where stability has been measured in groups of patients with specific knee designs. These can be found in full or with an overview (abstract) for free by searching the internet. You can use the implant name or type of implant, ‘knee’ and ‘stability’ in the search terms.

Range of motion

How much movement a knee replacement design can provide is measured in terms of the amount of possible flexion, or range of motion (ROM). It is a very common and simple measure that almost all comparative studies and cohort studies on knee replacement include. The goal of course is to provide as much movement or ROM as a normal healthy knee.

The most that a healthy knee can bend is a little over 150° – you might achieve in a full squat or sitting on your heels. Most activities do not require anywhere near this amount of flexion. For activities like walking, bending your knee to a right angle is enough. Some activities like tying your shoelaces and getting out of a low chair require a little more than right angles – about 110°.

Many manufacturers claim a very high range of movement based on theoretical limits of designs. The most common way this is done is to calculate the point that the back of the femur might impinge on the back of the tibial component assuming that the components are in a certain relative position when the knee is bent that far. But this is invariably not what limits flexion.

The amount that you can bend your knee after surgery will depend on a number of factors. With all things considered, patients achieve on average 110-120° flexion after their knee has healed. While some people desire more flexion for particular activities, achieving an average amount of flexion is adequate for most and for others only moderately limiting. Problems only really occur when joint becomes stiff and is unable to bend to a right angle (less than 75° is considered to be stiff) or to straighten fully.

Like instability, range of motion can be linked to patient-factors, surgical factors and choice of implant design. One of the main causes of poor flexion is inadequate physiotherapy while the knee heals, which allows scar tissues to become tight. Stiffness can also be caused by some sort of difficulty experienced in surgery or because a patient’s knee was stiff before surgery and the amount that the surgeon could correct it was limited. Nevertheless, some knee designs have been consistently shown to achieve high amounts of flexion in groups of patients, whether studied alone or when comparing groups of patients with different knee designs.

Stiffness is often treatable and is a less common reason for repeat surgery than others, so is less well reported in the registries. The best sources of information are in published study reports where range of motion has been measured in groups of patients with specific knee designs, whether reporting on one design of comparing different designs. These articles can be found in full or with an overview (abstract) for free by searching the internet. You can use the implant name or type of implant, ‘knee’ and ‘ROM’ or ‘range of motion’ in the search terms.

Pain, activity and satisfaction

Pain and activity are measured using patients’ own answers to questionnaires (PROMs). Questions relate to a variety of aspects including pain at night to when doing certain activities, your ability to do activities like climb stairs, get out of a car or walk to the shops. Some questions may even ask you how well your knee replacement allows you to do certain sports.

There has been a great deal of interest in recent years in patient satisfaction and what contributes to high levels of satisfaction. There are different views as to what is most important. Some aspects are subjective, like ‘expectations not being met’ and others are objective like ‘instability’ or ‘ability to do activities’.

As with functional aspects of stability and flexion, the best sources of information on pain, activity and satisfaction achieved with particular knee replacement designs are published cohort studies and comparative studies.

Independent analyses

When reading about clinical achievements of knee replacement surgery, it is worth knowing what has been independently scrutinised and what has not.

The national registries are independent bodies and so can be expected to present data in an unbiased way. But presentation of ‘big data’ is rarely conclusive and is unable to account for the many contributing factors that may cause certain groups to appear better or worse than others.

Published journal articles are reviewed by independent experts in the subject matter. In this way they can usually be expected to be well-designed studies and have sufficient evidence for their conclusions. On the whole this is true and published articles from high quality journals are relied on by all discerning professionals. However, being able to read with a critical mind can be useful: there are plenty of published articles that show conflicting results or points of view to others.

Manufacturers all market their designs in a positive way. By law companies are not allowed to make false or unsubstantiated claims. But presentations of ideas are often well designed to create a good impression whether or not the ideas can really be proven. Look for claims that are substantiated and reference real clinical data, rather than use of creative images or videos or that reliance on computer or laboratory simulations.

More recently there are organisations, such as ODEP which we discuss in a later section, that are intended to tell you and clinical professionals which devices perform adequately. Their aim is to review both the quality and quantity of data supporting individual knee replacement designs. It may be reassuring to see a design you are interested in with a positive review from such an organisation.

Longevity and Registry data

National Registries

The best sources of information on knee replacement failure rates are national registers. One of the oldest is the Swedish Knee Arthroplasty Register (myknee.se). It has been collecting data on patients in Sweden since 1975. The largest registers are in the UK (njrcentre.org.uk) with records of over 1 million total knee replacement operations since April 2003, and in Australia (aoanjrr.sahmri.com) with over 600,000 knee replacements recorded since September 1999.

All national registers provide publicly available reports both online and by publishing an annual report. They include records of surgery in state and privately funded hospitals.

Information reported

Although these reports appear to present a wide array of data on the success of knee replacement surgery, they in fact focus on one principal measure of success: the rate or probability of revision surgery according to the number of years since surgery.

Every record of an operation includes details such as the patient’s gender, age, other health issues, type and severity of arthritis and the implant design that was used. Every record of a revision operation also includes the reasons that the revision surgery was required.

With large datasets, the registries can divide the data into a wide range of comparative groups so that potentially contributing factors and reasons for failure can be better understood. It is important to understand that ‘big data’ like this does not prove cause or effect. But it does allows us to consider what factors could influence the likelihood that a design is likely to become a long-lasting success for a given patient.

Factors that are known to influence the revision rate include:

• Primary disease – osteoarthritis is less likely to need further surgery than inflammatory arthritis
• Age – younger age groups tend to have a higher rate of revision
• Year of operation – operations in the last 2 decades are associated with lower rates of revision than previous years (but this is not changing much in recent year-to-year comparisons)
• Gender – gender differences do exist but are more complex and depend on other factors
• Type of implant – partial knees and posterior stabilised knees tend to have higher rates of revision than other types, but this may be influenced by their patient demographics
• Material – new materials can provide better results in some designs but they are not always better than other designs using established materials
• Method of fixation – overall, using cement to fix a knee implant is still associated with a lower revision rate but many newer designs are being used without cement
• Brand of implant – this is the area that has generated the most interest over the years. The National Joint Register (njrcentre.org.uk) was in fact initiated after a particular design of hip was recalled for an excessive number of failures. The registry was seen as a mechanism to detect implants that were not performing in line with state-of-the-art.

What to look for

The rate or probability of revision is based on known experience. It is reported as a percentage of the total number of patients in a group at a series of time points. A good starting point is to compare the published revision rate for a type of knee replacement for your own age group and gender. You can also compare the overall rates of revision for particular designs of knee. Although designs of knee are not usually separated to gender or age groups, it is possible to see what the average age and proportion of males to females in the reported group is.

Data Confidence

By necessity the probability of revision is based on certain assumptions designed to address what are known as confounding factors. One example is how to account for the people no longer available for monitoring. Whether their knee still does or, if they have since passed away, would have continued to function is unknown. The calculation also takes into account how many patients were in the dataset to start with. How likely would the calculated value be the same if the dataset were much larger? The level of certainty or confidence in a value is displayed as a percentage range, usually in brackets next to the main value: based on known information, there is a 95% chance that the ‘real’ rate of revision is somewhere between the lower and upper values.

If the study group is small or the time point being reported does not include many patients (this could be because the device is new or not mainstream and widely available), the confidence interval will be ‘wide’ (they differ considerably from the stated probability). If a group includes many hundreds of knees the level of confidence will be high (the lower and upper percentages will be very close to each other).

It is worth appreciating that if a device that has a higher probability of failure than another device you are interested in, the confidence interval can help you understand whether or not the difference is meaningful. On the whole when comparing two designs, if one of the stated probabilities is between the lower and upper values of the other, the available data does not prove there is a statistically significant difference between the designs. If, however, the probabilities and confidence intervals are separated, it is safe to say that one has a demonstrably lower rate of failure than the other.

Patient information

The National Joint Registry for England, Wales, Northern Ireland and the Isle of Man also publishes patient information leaflets about the NJR and its data. These can be found on the registry website www.njrcentre.org.uk.

Cohort Studies

When designs are new or not widely available, data may not yet be available in the national registers. In this case smaller studies will be of interest. It is common for individual surgeons or small groups of surgeons monitor their patients and publish their results in clinical journals. These ‘cohort’ studies provide early information on new designs and whether or not they are achieving acceptable success rates compared to established implants.

It is usual for these studies to focus on the rate of revision and serve to demonstrate that a device does not have any particular issues and is not experiencing a higher than expected number of failures. The information is presented in the same way as in the registry reports – a percentage is given with a level of confidence based on the total number and ability to account for all the patients (lower and upper percent values usually in brackets and often called ‘95% confidence interval’). Usefully, these study reports usually also describe the reasons that knees within the study had failed and how they might relate to characteristics of the design.

If you are interested in a new device, search on the internet for the device name and ‘revision’. You may find that some data has been published. If not, you could contact the company directly or a surgeon known to use the design. They may not be in a position to provide a specific revision rate, but they will be able to say whether or not they are aware of any higher than expected failure rates or if there have been any issues with a new design.

As well as reporting on success rates, cohort studies will also report on a range of other measures that indicate how well a design works when compared to the state-of-the-art. This will include the results of questionnaires completed by the patients (PROMs) and clinical assessment of patients’ range of motion. Although you may not be sufficiently familiar with what scores are considered good or excellent, the authors’ conclusions usually include how well they think the device performs.

Sometimes cohort studies include further analysis on some or all patients in a group. These may be done to find out whether a particular design intent is actually achieved in patients. One example might be to measure whether the components of a knee design move in the same way as a normal healthy knee. Further analyses may also be used to test the ability of a design to perform to a higher level in patients who are capable of certain activities of interest and have not experienced any other issues during their recovery. Some newer designs that are aimed at more active or demanding people will feature in studies like this.

Cohort studies can be very useful to understand how well knee replacement designs actually work. While they are limited in their ability to confirm that there would not be any more or fewer incidences of failure in a large population, they provide far more information than population studies on patients’ experience of particular knee designs or types of knee design.

Comparative Studies

Comparative studies are often quite similar to cohort studies in what they measure and how. Key differences are: the sizes of groups with particular designs may be smaller, designs may or may not be new, and their focus is not on state-of-the-art but on how well certain devices work compared to a selected other. In the case of a new design, or a design with a feature or concept that is not well known or mainstream, it is usual to compare it against a device that is well-established and generally accepted to provide good outcomes.

Comparative studies can include functional assessments like stability, range of motion, how normally patients can walk (gait study). They may compare patient reported outcomes (PROMs) focussing on standard ‘activities of daily living’ or on higher level activities like sport. They may also compare levels of patient satisfaction between groups. Some recent studies also ask patients whether or not they can essentially forget about their knee during a range of activities.

Studies like this are usually undertaken by surgeons or hospitals that are assessing whether or not to use a new design or a design that is not previously used in that hospital. Sometimes these studies are undertaken by surgeons who were involved in developing the knee design with the manufacturer. If this is the case, the end of the article will state any association the authors have with the manufacturer of the design included in the study. Because the study article will have been reviewed by independent assessors before publication, you should be able to consider the results to be reliable. However it may be worth searching or waiting for other articles published by independent authors to further substantiate the qualities of a new design.

The more reliable comparative studies are known as a ‘prospective randomised controlled trial’ (pRCT). These studies are performed according to particular rules to avoid potential for any bias in the results. Steps include randomly selecting patients to receive one of the designs being compared and preventing anyone who is taking measurements or assessing outcomes to know which design each patient has. Studies like this are subject to strict codes of practice and must be approved by ethics committee. As a result, their outcomes can be very informative to professionals seeking to advance quality of care. They can also provide a useful to patients.

Orthopaedic Data Evaluation Panel (ODEP)

The Orthopaedic Data Evaluation Panel (ODEP; www.odep.org.uk) was set up in 2002 to provide information about the success of hip replacements. Since 2014 knees have been included in the database. The panel includes an independent group of leading UK surgeons and non-clinical experts with a deep understanding of the orthopaedic industry. The panel review submissions from manufacturers covering the available clinical evidence to support their knee replacement designs and rate the strength of the evidence provided, resulting in an ODEP rating.

The ODEP rating system rates implants after three, five seven and ten years of use with a number, a letter and optionally a star. The number denotes for how many years after surgery a device is rated and the letter demotes the strength of evidence – ‘A’ denotes strong evidence for a high number of patients actively monitored.

To achieve the highest rating after ten years (10A*) a knee implant must have:

• been used in more than 500 patients with at least 400 reaching 10 years,
• a revision rate of less than 5%,
• not changed the design during that period,
• had no reports of poor performance,
• been used by three or more clinics outside of co-developers.

Hospitals and surgeons around the world use the ODEP ratings to review whether or not to use a device.

In respect of newer designs, it is worth noting that knee designs with a 3A or 5A rating for example, do not necessarily perform worse than a device with a 10A rating. It is more likely that the device is new and does not yet have 10 years of data. In fact a 5A rating indicates that the device is on track to achieve a 10A* rating and it might actually be performing better than a 10A*-rated device.

What Should I Look for When Choosing?

Knee replacement implants are no different to any other marketed product in that the most detailed information is likely to come from the manufacturer. Most implant designs are presented on the manufacturer’s websites and in product brochures that explain the features and benefits of the design.

If you have read or heard about a particular design, or have been recommended a particular knee by your GP, the best starting place is to search for the manufacturer’s information on the internet. When doing so, it is important to keep an open mind and evaluate what you are reading. Does the information cover the things that are important to you? Does it make sense to you? If the answer is yes, then it is worth looking further. The next thing to consider is key: are all claims about the device supported by clinical evidence? It is important to know what evidence exists to back up any claim or suggestion that a design or feature actually delivers what it intends to. Some designs that have been in use for many years may still refer to theoretical reasoning for what particular features can achieve. Some others rely on results of simulations or computer-based analyses.

When a design is new – it has been in use for 5-years or less – it is unlikely to have a wealth of real clinical evidence to support its claims, or to prove that it works well in patients. This does not mean that it will not be supported by such evidence in the future, but it is worth considering the possibility that it might not be.

Are there any comparative studies to that show a design supersedes another in criteria that are important to you? Is it more or less stable? Does it score highly in PROMs studies? Are most patients satisfied?

Is the design mentioned in registry reports? Is it similar to the best. The lower the percent at many years, the more likely evidence suggests it will not fail – but remember to note the range of % in brackets if you are looking at a device that is new or not provided by one of the major corporations.