What is a hip replacement?
A hip replacement is a common type of surgery where a damaged hip joint is replaced with an artificial one (known as a prosthesis).
The hip joint is one of the largest joints in the human body and is what is known as a 'ball and socket joint'.
In a healthy hip joint, the bones are connected to each other with bands of tissue known as ligaments. These ligaments are lubricated with fluid to reduce friction.
Joints are also surrounded by a type of tissue called cartilage that is designed to help support the joints and prevent bones from rubbing against each other.
The main purpose of the hip joints is to support the upper body when a person is standing, walking and running, and to help with certain movements, such as bending and stretching.
Why do I need a hip replacement?
It might be necessary for you to have a hip replacement if one (or both) of your hip joints becomes damaged and causes you persistent pain or problems with everyday activities such as walking, driving and getting dressed.
Some common reasons why a hip joint can become damaged include:
- osteoarthritis – so-called ‘wear and tear arthritis’, where the cartilage inside a hip joint becomes worn away, leading to the bones rubbing against each other
- rheumatoid arthritis – this is caused by the immune system (the body’s defence against infection) mistakenly attacking the lining of the joint, resulting in pain and stiffness
- hip fracture – if a hip joint becomes severely damaged during a fall or similar accident it may be necessary to replace it
Many of the conditions treated with a hip replacement are age-related so hip replacements are usually carried out in older adults aged between 60 and 80.
However, in some cases a hip replacement may be necessary in children or younger adults whose hips are incorrectly formed (hip dysplasia).
The purpose of a new hip joint is to:
- relieve pain
- improve the function of your hip
- improve your ability to move around
- improve your quality of life
Read more about why a hip replacement may be necessary.
What happens during hip replacement surgery?
A hip replacement can be carried out under a general anaesthetic (where you are asleep during the procedure) or an epidural (where the lower body is numbed).
The surgeon makes an incision into the hip, removes the damaged hip joint and then replaces it with an artificial joint that is a metal alloy or, in some cases, ceramic.
The surgery usually takes around 60-90 minutes to complete.
Read more about how a hip replacement is performed.
Recovering from hip replacement surgery
For the first four to six weeks after the operation you will need a walking aid, such as crutches, to help support you.
You will also be enrolled on an exercise programme that is designed to help you regain and then improve the use of your new hip joint.
Most people are able to resume normal activities within two to three months but it can take up to a year before you experience the full benefits of your new hip.
Read more about recovering from hip replacement surgery.
What to expect after a hip replacement
Since its introduction in the 1960s, hip replacement surgery has proved to be one of the most effective types of surgery in modern medical history. Most people experience a significant reduction in pain and, to a lesser extent, improvement in their range of movement.
However, it is important to have realistic expectations about what the operation can achieve. For example, you should be able to ride a bike but it is unlikely that you would be able to play a game of rugby safely (although, as with most things, there are always exceptions to this rule).
The rehabilitation process after surgery can be a demanding time and requires commitment.
Risks of hip replacement surgery
A modern artificial hip joint is designed to last for at least 15 years, but there is always the risk that the artificial hip joint can wear out or go wrong in some way before this time, meaning that further surgery is required to repair or replace the joint.
This is known as revision surgery. It is estimated that around 1 in 10 people with an artificial hip will require revision surgery at a later date.
There have been recent cases of metal-on-metal (MoM) replacements wearing quicker than would be expected, causing deterioration in the bone and tissue around the hip. There are also concerns that they could leak traces of metal into the bloodstream.
For more information, read our metal-on-metal hip implant advice Q&A.
The risk of serious complications such as blood clots and infection at the site of the surgery is low – estimated to be less than 1 in a 100.
Read more about the risks of a hip replacement.
There is an alternative type of surgery to hip replacement, known as hip resurfacing. This involves removing the damaged surfaces of the bones inside the hip joint and replacing them with a metal surface.
An advantage to this approach is that it is less invasive and leaves you with a greater range of movement after surgery. However, it is usually only effective in younger adults who have relatively strong bones.
Read more about alternatives to hip replacements.
Hip replacement surgery is being improved in several ways:
- New, stronger materials for prosthetics are being developed that will allow longer wear and better joint mobility.
- Enhancements are being made to new 'cementless' implants. Patients can be recommended for newer types of joints, such as ceramic-on-ceramic and ceramic-on-plastic.
- Computer-assisted surgery is being used to generate an image of the hip joint to allow greater precision.
Another area of research is looking at regenerating a hip joint by transfusing stem cells into damaged tissue. Stem cells are specialised cells that have a useful ability to help replenish other types of cells.
Before being considered for a hip replacement you will probably be given a number of non-surgical treatments to see if they are effective in relieving hip pain and stiffness.
Non-surgical treatments can also be used if you are unable or unwilling to have hip replacement surgery.
These may include:
- Painkillers, including non-steroidal anti-inflammatory drugs (NSAIDs), if your hip joint is also inflamed.
- Steroid injections can help in some cases, although their results are unpredictable in the hip so not generally recommended.
- Pain-relieving creams, gels and rubs are available over the counter or on prescription. It is not known how these compare with more conventional ways of taking painkillers.
- Disease-modifying medications – these medications alter the working of the immune system to block the underlying processes involved in certain forms of inflammatory arthritis.
For more detailed information on treatment options for the most common causes of hip pain see:
A surgical alternative to a hip replacement is a hip resurfacing. This type of surgery involves removing the upper surface of the femur (thigh bone) as well as the surface of the cavity in the pelvis in which the femur sits.
Both of these surfaces are then covered with a metal surfacing (metal-on-metal). This helps correct a damaged joint into a correct position.
An advantage of hip resurfacing is that less bone is removed than in a hip replacement. This means it has a faster recovery time and people treated in this way tend to have a greater range of movement in the resurfaced hip.
Hip resurfacing requires that a person has relatively strong bones so it is usually only suitable for younger adults and it may not be suitable for:
- adults over the age of 65 years – bones tend to weaken as a person becomes older
- women who have gone through the menopause – one of the side effects of the menopause is that the bones can become weakened and brittle (osteoporosis)
Your surgeon should be able to tell you if you could be a suitable candidate for hip resurfacing.
Hip replacement complications
The most common complication of hip replacement is that something goes wrong with the joint, which occurs in around 1 in 10 cases.
Some common types of joint malfunctions are listed below.
Loosening of the joint
The most common problem that can arise as a result of a hip replacement is loosening of the joint. This can be caused by the shaft of the prosthesis becoming loose in the hollow of the thigh bone, or due to thinning of the bone around the implant.
Loosening of the joint can occur at any time, but it normally occurs 10-15 years after the original surgery was performed.
Signs that the joint has become loose include pain and feeling that the joint is unstable.
Another operation (revision surgery) may be necessary, although this cannot be performed on all patients.
In around 1 in 20 cases the hip joint can come out of its socket. This is most likely to occur in the first few months after surgery when the hip is still healing.
Further surgery will be required to put the joint back into place.
Wear and tear
Another common complication of hip replacement surgery is wear and tear of the artificial sockets. Particles that have worn off the artificial joint surfaces can be absorbed by surrounding tissue, causing redness and swelling (inflammation) in and around the joint.
Anti-inflammatory medication such as ibuprofen may stop the problem. If not, you may be advised to have further surgery.
There have been reports about metal-on-metal implants wearing sooner than expected and causing complications.
Read our metal-on-metal implant advice Q&A for more information.
The soft tissues can harden around the implant, causing reduced mobility. This is not usually painful and can be prevented using medication or radiation therapy (a quick and painless procedure during which controlled doses of radiation are directed at your hip joint).
Serious complications of a hip replacement are uncommon, occurring in fewer than one in a 100 cases.
These are described below.
There is a small risk of developing a blood clot in the first few weeks after surgery.
There are two main places a blood clot can develop:
- inside one of your legs – which is known as a deep vein thrombosis (DVT)
- inside your lungs – which is known as a pulmonary embolism
Symptoms of a DVT include:
- pain, swelling and tenderness in one of your legs (usually your calf)
- a heavy ache in the affected area
- warm skin in the area of the clot
Symptoms of a pulmonary embolism include:
- breathlessness, which may come on suddenly or gradually
- chest pain, which may be worse when you breathe in
If you suspect that you have either of these types of blood clots you should seek immediate medical advice from your doctor in charge of your care.
In order to reduce your risk of blood clots you may be given blood thinning medication such as warfarin, or asked to wear compression stockings.
There is always a small risk that some bacteria could work its way into the tissue around the artificial hip joint, triggering an infection.
Symptoms of an infection include:
- a high temperature (fever) of 38°C (100.4°F) or above
- shaking and chills
- redness and swelling at the site of the surgery
- a discharge of liquid from the site of the surgery
- hip pain that can persist even when resting
Seek immediate medical advice, as detailed above, if you think you may have an infection.
A couple of weeks before the operation, you will usually be asked to attend a pre-operative assessment clinic to meet your surgeon and members of the surgical team.
They will take your medical history, examine you and organise any tests needed, to make sure you are healthy enough for an anaesthetic and surgery.
They will give you advice on anything you can do to prepare for surgery and ask you about your home circumstances so your discharge from hospital can be planned. If you live alone, have a carer or feel you need extra support, tell the team so that help or support can be arranged before you go into hospital.
Take a list or the packaging of any medication you are taking. Some rheumatoid arthritis medications suppress the immune system, which can affect healing. For this reason you may be asked to stop taking them before surgery. Your surgeon can advise you about alternative medications.
Exercising before the operation
You can prepare for the operation by staying as active as you can. Strengthening the muscles around your hip will aid your recovery. You may be referred to a physiotherapist, who can give you helpful exercises. If you can, keep up any gentle exercise, such as walking and swimming, in the weeks and months before your operation.
The following exercises can help maintain your muscle strength and movement before surgery:
- Stand at the bottom of the stairs and put one foot on the second stair. Alternatively, put one foot on a kitchen stool. Hold on to the banister or another firm support. Lean forward to bend the top leg while stretching the front of the standing leg. Hold this for about 30 seconds, then repeat with the other leg.
- Stand on the affected leg for short periods, lifting the good leg off the floor. Concentrate on holding the pelvis level. Use a support, if necessary, for balance.
- Lie on your back on a bed. Pull one leg up to your chest, keeping the other leg flat down on the bed. Repeat the exercise with the other leg. (Only do this exercise if you have not already had a hip replacement on one side.)
- Lie on your back on a bed. Bend your knee up so that your foot rests flat on the bed and allow the bent knee to fall out to the side as much as is comfortable.
- Lie on your stomach and then flat on your back for approximately 20 minutes once or twice a day (early morning or late at night while in bed is often a good time), to stretch the front and back of your hip.
Planning for your recovery after surgery
You will be unable to walk unaided for at least four weeks after surgery, and other types of movement – such as stretching or picking things up – may also be severely restricted.
You may want to consider making some changes to your home to make life easier while you recover from the operation, such as:
- adding a shower seat to your bathroom
- placing any useful objects at hand level so you do not have to bend down to pick them up
- stocking up on food that is easy to prepare, such as frozen ready meals, or prepare and freeze your own dishes to reheat during your recovery
Many people find it useful to buy a ‘reacher grabber’ – a handheld device that allows you to pick up objects that are slightly out of reach. These devices are easily available through the internet as well as from shops that sell mobility products.
Preparing for surgery
For more general advice about going into hospital and preparing for surgery, see our surgery planner.
How it is performed
You may be able to choose the type of anaesthetic you are given during surgery.
There are two options:
- general anaesthetic – where you are asleep during the operation
- an epidural anaesthesia – where an injection is given into your spine that numbs the lower half of your body. This is often combined with sedation so you will be unaware of your surroundings and have no memory of the surgery taking place
Depending on the general state of your health your surgeon may recommend you have an epidural as this has less chance of causing complications in people with an underlying health condition.
Once you have been anaesthetised, the surgeon removes the existing hip joint completely. The upper part of the thigh bone (femur) is removed and the natural socket for the head of the femur is hollowed out.
A socket is fitted into the hollow in the pelvis. A short, angled metal shaft with a smooth ball on its upper end (to fit into the socket) is placed into the hollow of the thigh bone. The cup and the artificial bone head may be pressed into place or fixed with acrylic cement.
Metal-on-metal (MoM) hip resurfacing is carried out in a similar way. The main difference is that much less of the bone is removed as only the joint surfaces are replaced with metal inserts.
The prosthetic parts can be cemented or uncemented:
- Cemented parts are secured to healthy bone using a special glue.
- Uncemented parts are made from permeable material that has many tiny holes. This allows the bone to grow into it, holding it in place.
Most prosthetic parts are produced using high-density polythene for the socket, titanium alloys for the shaft and sometimes a separate ball made of an alloy of cobalt, chromium and molybdenum.
Some surgeons use a metal ball and socket and in some cases ceramic parts are used, which do not wear as quickly as plastic.
There have been recent reports about metal-on-metal hip replacements causing complications. Read our metal-on-metal implant advice Q&A for more information.
The hip replacement operation has become a routine procedure. However, as with all surgery, it carries a degree of risk. Read more about the risks of hip replacement surgery.
Choosing your prosthesis
There are more than 60 different types of implant or prosthesis. In practice, however, the options are usually limited to around four or five. Your surgeon can advise you on the type they think would suit you best.
NICE only recommends prostheses known to have a 90% chance of lasting at least 10 years. Ask your doctor if you will be getting one of these and, if not, why not. Your surgeon will also be able to discuss any concerns you have regarding metal-on-metal (MoM) replacements.
Choose a specialist who regularly performs hip replacements and can discuss their results with you. This is even more important if you are having a second or subsequent hip replacement, known as revision hip transplant, which is trickier to perform.
Look for a specialist who will work with you to find the best treatment for you.
Metal on metal implants
Patients with a common type of metal hip implant should have annual health checks for life. The all-metal devices have been found to wear down at an accelerated rate in some patients, potentially causing damage and deterioration in the bone and tissue around the hip. There are also concerns that they could leak traces of metal into the bloodstream, which the annual medical checks will monitor.
Advice on smaller metal devices or those featuring ceramic heads has not changed. Previously, guidelines suggested larger MoM implants should only be checked annually for five years after surgery. The agency now says the annual check-ups should be continued for the life of the implant. Check-ups, they say, are a precautionary measure to reduce the 'small risk' of complications and the need for further surgery.
Together with controversy over PIP breast implants, the news caused some medical quarters to call for tighter regulation of medical devices, perhaps bringing the approval process into line with that of medicines, which must undergo several years of laboratory, animal and human testing before being approved for wider use.
What should I do if I have a hip implant?
The guidance only applies to large head metal-on-metal implants, which have been used in only a minority of hip replacement surgeries. However, you can consult your doctor for further advice if you have any concerns about your hip replacement or do not know which type you have.
Patients with hip implants should also be aware of the warning signs that could signal a problem.
What are the warning signs?
Patients who have MoM implants should ensure they attend any follow-up appointments as usual.
You should see your doctor if you have:
- pain in the groin, hip or leg
- swelling at or near the hip joint
- a limp or problems walking
These symptoms do not necessarily mean that your device is failing, but they do need investigating.
Any changes in general health should also be reported, including:
- chest pain or shortness of breath
- numbness or weakness
- changes in vision or hearing
- feeling cold
- weight gain
What types of implants are involved?
There are numerous designs and materials used to make hip implants. As the name implies, MoM implants feature a joint made of two metal surfaces – a metal ‘ball’ that replaces the ball found at the top of the thigh bone (femur) and a metal ‘cup’ that acts like the socket found in the pelvis.
The advice concerns the type of MoM implant in which the head of the femur is 36mm or greater. This is often referred to as a ‘large head’ implant. The agency now says that patients fitted with this type of implant should be monitored annually for the life of the implant, and that they should also have tests to measure levels of metal particles (ions) in their blood. Patients with these implants who have symptoms should also have MRI or ultrasound scans, and patients without symptoms should have a scan if their blood levels of metal ions are rising. The previous guidance on this type of hip implant advised that patients should be monitored annually for no fewer than five years.
Advice on following up patients with other types of MoM implants remains the same, and guidance has not changed on:
- MoM hip resurfacing implants – where the socket and ball of the hip bone has a metal surface applied to it rather than being totally replaced.
- Total MoM implants where the replacement ball is less than 36mm wide.
- Implants featuring ceramic heads.
What exactly is the problem with MoM implants?
All hip implants will wear down over time as the ball and cup slide against each other during walking and running. Although many people live the rest of their lives without needing their implant to be replaced, any implant may eventually need surgery to remove or replace its components. Surgery to remove or replace part of the implant is known as ‘revision’.
However, data now suggest that large head MoM hip implants (those with a width of 36mm or greater) wear down at a faster rate than other types of implants. As friction acts upon their surfaces it can cause tiny metal particles (medically referred to as ‘debris’) to break off and enter the space around the implant. Individuals are thought to react differently to the presence of these metal particles, but, in some people, they can trigger inflammation and discomfort in the area around the implant. Over time this can cause damage and deterioration in the bone and tissue surrounding the implant and joint. This, in turn, may cause the implant to become loose and cause painful symptoms, meaning that further surgery is required.
News coverage focused on the MHRA’s recommendation to check for the presence of metal ions in the bloodstream, potentially released either from debris or the implant itself. Ions are electrically charged molecules. Levels of ions in the bloodstream, particularly of the cobalt and chromium used in the surface of the implants, may, therefore, indicate how much wear there is to the artificial hip.
There has been no definitive link between ions from MoM implants and illness, although there has been a small number of cases in which high levels of metal ions in the bloodstream have been associated with symptoms or illnesses elsewhere in the body, including effects on the heart, nervous system and thyroid gland.
Most patients with MoM implants have well functioning hips and are thought to be at low risk of developing serious problems. However, a small number of patients with these hip implants develop soft tissue reactions to the 'wear debris' associated with some MoM implants.
What actions have critics called for?
In light of the PIP breast implant controversy and this new information on hip implants, there is currently intense scrutiny on the way medical devices are regulated in the UK and Europe, with patient groups and the media arguing that medical devices should be regulated in a similar way to medicines.
Medical devices are not required to go through human trials before entering use, and can currently be approved on the basis of mechanical tests and animal research. While certain devices, such as hip implants, have been monitored through systems such as the National Joint Registry, in light of health concerns over PIP breast implants patient groups are calling for more testing before devices are allowed into clinical use, and closer, mandatory monitoring schemes to ensure their safety once they enter the market.
D Cohen How safe are metal-on-metal hip implants? BMJ 2012
Builder Norman Lane, 63, hasn’t stopped running since he had a double hip replacement
“I used to be a keen footballer and ran around 80 miles a week until I started to have problems with my hips when I was around 40. The doctor diagnosed osteoarthritis.
“At first it wasn’t too bad, but gradually things got so painful that I couldn’t turn over in bed at night, let alone run. The surgeon said both my hips were ‘shot’ and suggested a double hip replacement, which I had done in 1998.
“The operation lasted eight hours. The day after, it took me 20 minutes to walk to the end of the bed and back. It seemed impossible that I would ever run again, but I was determined. I didn’t want to die with my new hips unused!
“I was in hospital for a week. It was painful at first but I stopped taking painkillers after two days and the pain gradually went away over the course of about a month. My attitude was, 'It's only pain and it will get better'.
“After a month, I was riding a bike. After six months, I started to do some gentle running and very gradually built it up over the course of a year. After 18 months, I ran the Majorca marathon in 3 hours and 14 minutes, winning the international over-50 category. I did the New York and London marathons the next year and, three years ago, I ran from John O'Groats to Land’s End, raising more than £25,000. It took me 28 days and my wife had to pull me off the road at the end. I just got fitter and fitter over the course of it. It’s amazing what your body can do.
“There are some things I still can’t do. I don’t play football now and I would never jump off a scaffold. I run an average of 40 to 50 miles a week. I’m really pleased I had the operation and would advise anyone to go for it.”
After the operation, you will be lying flat on your back and may have a pillow between your legs to keep your hip in the correct position. The nursing staff will monitor your condition and you will have a large dressing on your leg to protect the wound.
You may be allowed to have a drink about an hour after you return to the ward and, depending on your condition, you may be allowed to have something to eat.
How soon will I be up and about?
The staff will help you to get up and walk about as quickly as possible after surgery. Some patients are able to get up and walk the same day as their surgery.
Initially, you will feel discomfort while walking and exercising, and your legs and feet may be swollen. You may be given an injection into your abdomen to help prevent blood clots forming in your legs, and possibly a short course of antibiotics to help prevent infection.
A physiotherapist may teach you exercises to help strengthen the hip and explain what should and should not be done after the operation. They will teach you how to bend and sit to avoid damaging your new hip.
Looking after your new hip
With care, your new hip should last well. The following tips will help you care for your new hip:
- avoid bending your hip more than 90° (a right angle) during any activity
- avoid twisting your hip
- do not swivel on the ball of your foot
- when you turn around, take small steps
- do not apply pressure to the wound in the early stages (so try to avoid lying on your side)
- do not cross your legs over each other
- do not force the hip or do anything that makes your hip feel uncomfortable
- avoid low chairs and toilet seats (raised toilet seats are available)
When can I go home?
You will usually be in hospital for around three to five days. If you are generally fit and well, the surgeon may suggest an enhanced recovery programme, where you start walking on the day of the operation and are discharged within one to three days.
How will I feel when I get home?
Do not be surprised if you feel very tired at first. You have had a major operation and muscles and tissues surrounding your new hip will take time to heal.
You may be eligible for home help and there may be aids that can help you. You may want to arrange to have someone to help you for a week or so.
An occupational therapist should be available to help you. They will assess how physically capable you are and, when you are about to leave hospital, they will assess your circumstances at home.
Your occupational therapist will be able to advise you on how to do daily activities, such as washing yourself. They will also advise about any equipment you may need to help you to be independent in your daily activities. This may include a raised toilet seat and aids to help you dress.
How soon will the pain go away?
The pain that you may have experienced before the operation should go immediately, although you can expect to feel some pain as a result of the operation itself, but this will not last for long.
Is there anything I should look out for or worry about?
After hip replacement surgery, contact your doctor if you notice redness, fluid or an increase in pain in the new joint.
Will I have to go back to hospital?
You will be given an appointment to check up on your progress, usually six to 12 weeks after your hip replacement. The surgeon will want to see you a year later to check that everything is OK, and every five years after that to X-ray your hip and make sure it is not beginning to loosen.
How long will it be before I feel back to normal?
Generally, you should be able to stop using your crutches within four to six weeks and feel more or less normal after three months, by which time you should be able to perform all your normal activities.
It is best to avoid extreme movements or sports where there is a risk of falling, such as skiing or riding. Your doctor or a physiotherapist can advise you about this.
When can I drive again?
You can usually drive a car after about six weeks, subject to advice from your surgeon. It can be tricky getting in and out of your car at first. It is best to ease yourself in backwards and swing both legs round together.
When can I go back to work?
This depends on your job, but you can usually return to work between six and 12 weeks after your operation.
How will it affect my sex life?
If you were finding sex difficult before because of pain, you may find that having the operation gives your sex life a boost. Your surgeon can advise when it is OK to have sex again. As long as you are careful, you should be able to have sex after six to eight weeks. Avoid vigorous sex and more extreme positions.
Will I need another new hip?
Nowadays, most hip implants last for 20 years or more. If you are older, your new hip may last your lifetime. If you are younger, you may need another new hip at some point.
Revision surgery is more complicated and time-consuming for the surgeon to perform than a first hip replacement and complication rates are usually higher. It cannot be performed in every patient. However, it is much more successful than it used to be and most people who can have it report success for 10 years or more.
Why it should be done
A hip replacement will usually be recommended if one or both of your hip joints is damaged to such an extent that:
- you are experiencing persistent hip pain that is not responding to other treatments
- the range of movement in the joint is so restricted that day-to-day tasks such as walking are very difficult or, in some cases, impossible
Common causes of hip damage
Some of the most common causes of hip damage are described below.
Osteoarthritis is the most common type of arthritis. It occurs when the joints become damaged over time and causes the surrounding cartilage to wear away. This causes the bones of the joint to rub together leading to hip pain, stiffness and loss of movement.
In cases of rheumatoid arthritis, the immune system – which usually fights infection – attacks the cells that line the joints, making them swollen, stiff and painful. Over time, this can damage the joint itself, the cartilage and nearby bone.
Hip fractures are one of the most common causes of bone injury in older people. Most cases of hip fracture occur as a result of a fall.
It is possible to repair a fractured hip, but in some circumstances a hip replacement is recommended.
Less common causes of hip damage
Less common causes of hip damage include:
- septic arthritis – this form of arthritis occurs when the joint becomes infected
- crumbling of the bone (avascular necrosis) – due to alcohol misuse
- Paget's disease of bone – this affects bone growth and can make bones weak and deformed
- bone tumours – abnormal cancerous growths that develop inside the bone
- hip dysplasia – which is where a baby is born with incorrectly formed bones in the hips. Over time this misalignment can become worse, which sometimes means that it becomes necessary to replace the affected joint