Part of Total knee replacement rehab
When is a knee replacement necessary, and who decides?
Warning signs, and when to get help
When is a knee replacement necessary?
A knee replacement becomes necessary when the knee has taken over enough of your life that arranging your days around it has stopped working, and when the treatments that usually hold an arthritic knee have stopped holding yours. The NHS puts it as a replacement that may be recommended when knee pain is having a big effect on your life and the alternatives to a knee replacement have not worked. That sentence turns on your life, and not on your imaging.
There is no threshold you cross. No grade on a report, no pain score, no amount of narrowing turns a knee into one that must be replaced, and no clinician can hand you a number and tell you that is the moment. NICE tells clinicians to use clinical assessment when deciding whether to refer someone for a joint replacement, rather than systems that score the severity of the disease numerically, and to consider referral when joint symptoms such as pain, stiffness, reduced function or a joint that is becoming more deformed are substantially affecting someone's quality of life and non-surgical treatment is ineffective or unsuitable (NICE NG226).
Read that second half twice, because the word people miss is unsuitable. Treatment you cannot have counts the same as treatment that did not work. A program your knee or another health problem will not let you do, pain relief you cannot take, weight loss advice that does not apply to you: none of those is a reason to keep waiting for permission. The decision is then made with a surgeon, on your examination and your account of what the knee stops you doing. Individual surgeons and health systems may still have their own referral criteria, so ask yours what applies to you.
NICE also says people with osteoarthritis should not be excluded from referral for a joint replacement because of their age, their sex or gender, smoking, other health conditions, or being overweight or obese as measured by BMI. Those things are not nothing. They change the risks, and the same guideline asks clinicians to explain how, which is a conversation to have with the surgeon rather than a reason to be turned away before it starts. If you have quietly decided not to raise it because you are too old, too heavy or still smoking, that is the sentence to take in with you.
If what you want is help weighing the options themselves, another page covers that. When to consider surgery for knee arthritis covers what to try first, the different operations, the risks, how long a new knee lasts and what the evidence says about waiting. This page stays with the one question in its title.
What does a surgeon actually weigh?
Mostly the things you can tell them without any equipment. How much the knee hurts and when. What you have stopped doing because of it, and what you have rearranged your week around. Whether it wakes you. How far you can walk before you plan a rest, and what stairs have become.
OrthoInfo lists severe knee pain or stiffness that limits everyday activities such as walking and stairs, moderate or severe knee pain while resting, by day or at night, long-standing swelling that does not settle with rest or medication, and a knee that has bowed in or out, among the reasons a surgeon may recommend the operation.
Then what has been tried, and whether it was genuinely tried. A few weeks of gentle leg lifts is not the same as months of progressive strengthening, and the difference matters to the conversation. The knee osteoarthritis program is what a fair trial looks like, and exercises to avoid with knee arthritis covers the loading mistakes that make a program look like a failure when it was really a dosing problem. If weight is part of your picture, weight loss and knee arthritis has that half.
Your general health comes into it, because this is major surgery with real risks, and those risks are not spread evenly between people. So does what you are hoping to get back. Write that down before you go in, because it is a surprisingly hard thing to answer on the spot. And the risks are not a footnote to this decision, so read them with their numbers on the knee arthritis surgery page linked above. Before the appointment, not after it. OrthoInfo is explicit that there are no absolute age or weight restrictions for the operation and that recommendations are based on a patient's pain and disability, not age.
What a surgeon cannot weigh is the thing you did not mention. People leave out the nights, the stairs at work and the trips they have quietly stopped making, because those feel like complaining rather than symptoms. They are the symptoms.
Does an X-ray decide it, and what does bone on bone mean?
No. Bone on bone is the phrase people are given for a film where the gap between the bone ends has narrowed so far that the bones look as though they are touching. It tells you what the joint looks like. What it cannot tell you is how much that joint is costing you, and the operation is offered for the cost, not for the picture.
The mismatch between films and symptoms is wide and it runs in both directions. A systematic summary of population studies found X-ray changes of osteoarthritis in 15% to 76% of adults who had knee pain, and knee pain in 15% to 81% of adults whose X-rays showed osteoarthritis, with the estimates shifting according to which X-ray views were taken, how pain was defined and how the X-rays were graded. The authors concluded that a knee X-ray should not be used on its own to assess an individual with knee pain (Bedson and Croft, 2008). Those figures cover radiographic osteoarthritis as the included studies defined it, which is a much wider group than the severe, bone on bone films, so read them as a measure of how loose the link is in general and not as a number attached to a bone on bone knee.
None of that makes the film pointless once an operation is genuinely on the table, and this is the part that gets lost when the X-ray is waved away. It is a worn joint that is being replaced, and on average the people who gain the most are the ones whose joints are worn the most. In 478 knees replaced for osteoarthritis at one Melbourne center, the knees with the least damage on the pre-operative films had roughly five times the odds of still having moderate or severe pain a year after surgery, compared with the knees whose films were the most severe (Dowsey and colleagues, 2012).
That estimate is imprecise, it comes from one center, and it is an average, not a forecast about you. The direction of it is the useful part. Bone on bone is not an instruction to operate, and a mild film is a reason for a surgeon to look harder for another cause of your pain before offering you a new knee, because a replacement is less likely to settle pain in a joint that is not very worn.
In clinic it cuts both ways. Some people arrive convinced the operation is overdue because of a sentence on a report, and walk out with a program instead. Others have been told their film is not bad enough, while the knee has taken over their life, and they are the ones who most need a proper assessment rather than a grade read out over the phone. Knee pain on stairs is often where the loss of function shows up first.
Why would someone need one other than for arthritis?
Osteoarthritis is the usual reason, and the NHS says most people who have a knee replacement have pain and movement problems caused by it. The other routes are less common, and people on them often cannot find themselves in the usual advice, so they get named here.
Rheumatoid arthritis and the other inflammatory arthritis conditions are one route. Here the joint damage comes from the disease process rather than from wear, the knee is one joint among several, and the medical treatment of the condition runs alongside the surgical decision. Rheumatoid arthritis covers the exercise side of living with it.
Arthritis after an injury is the other route the NHS names, and OrthoInfo calls it posttraumatic arthritis, following a serious knee injury. A knee that was broken, or that lost a ligament or a lot of meniscus decades ago, can wear unevenly and arrive at the same place much earlier in life.
Some other operations use these same words and are not this operation. A knee replaced as part of treating a broken bone, and a second operation on a knee that has already been replaced, both follow their own surgical plans and their own rehabilitation. Partial replacements, which resurface only one worn part of the knee, also exist and are a separate conversation with a surgeon. Total knee replacement rehab is written for recovery after a first, whole knee replacement, and it says plainly that other operations need their team's plan instead.
Is there such a thing as too early or too late?
There is no deadline. Nobody is going to tell you that you have missed a window, and nobody can give you the single right month either. The belief that a window exists does real harm: it hurries some people and it freezes others.
Both halves of that pull are set out with their sources on when to consider surgery for knee arthritis. Going in with worse pain and function has been linked with being worse off afterwards, which argues against grinding on indefinitely. The younger a knee is replaced, the more likely it is to need redoing in that person's lifetime, which argues against rushing. Weigh them with your surgeon, using your own figures and not averages.
One thing does change as a knee gets worse, and it is movement rather than the decision. In one American center's series of 4,727 primary knee replacements, how far the knee bent before the operation was the strongest predictor of how far it bent afterwards, whichever way the leg was lined up. In the knees that had drifted a long way out of line, taking off the bone spurs at the back of the joint and releasing the tight tissue on the inner side made a good range of movement at the end of the operation, and afterwards, more likely (Ritter and colleagues, 2003). Put plainly: a replacement cannot be relied on to hand back the bend a knee has already lost, and a stiff or badly bowed knee asks more of the surgeon on the day. Whether that applies to your knee, and whether it changes your timing, is a question for a surgeon who has examined it, not something to work out from a page.
What you can do in the meantime is keep the leg strong, because the strength you take in is the strength you start rehabilitation with. If you have a date or you are on a list, prehab exercises before a knee replacement is the page for that, and waiting for physio covers what to do with a long wait.
What to ask, and what to expect from the appointment
Expect a conversation. Expect to be examined, to be asked what you can and cannot do, and to be asked what you want back. Bring the two or three activities you most want to return to, written down, because the appointment goes better when it is about your life than when it is about your scan.
Some questions earn their place more than others. What do you expect this to change for me, and what will it probably not change. What are the main risks for someone with my health. What happens if I wait. And what would you want me to do before the operation.
A fuller list of questions to take in sits on when to consider surgery for knee arthritis.
Ask about recovery too, and ask it in your own terms, not in weeks. The NHS says it may take several months or longer to fully recover from a knee replacement, and that this can vary with your age and general health, so the useful question is what the first months will ask of you at home and at work. The recovery timeline week by week sets out what the sources say about the arc.
When to get the knee looked at sooner
Some knee changes do not wait for a planned appointment, and they are not about the surgical decision at all. A knee that swells up within hours of a twist or a fall, one that looks out of shape or leaves the foot cold or numb, needs an emergency department straight away, and you should not drive yourself.
A hot, red and swollen knee, a knee with a temperature alongside it, a knee that locks or keeps giving way, and a calf that is newly swollen, warm or tender all need medical advice the same day. Pain at night or at rest that keeps getting worse is a within a few days matter, and sooner if you have had cancer.
The warning signs below give the full set with the level of urgency for each. None of them is a reason to assume you need an operation, and all of them are reasons to be seen before your next routine appointment.
Related exercise programs
Warning signs
Get emergency help now
- Emergency: the knee has moved out of place, looks misshapen or is pointing at an odd angle, or you have tingling or no feeling in the knee, leg or toes, or your foot turns cold, pale or blue. Go to an emergency department straight away. Do not drive yourself: ask someone to drive you or call an ambulance.
- Emergency: the knee became very swollen very quickly, within a few hours of a twist, fall or blow, or you felt a pop or snap. Go to an emergency department straight away. Do not drive yourself: ask someone to drive you or call an ambulance.
Get help today
- Same day: the pain started with a fall, a twist or a blow to the knee, or you cannot put weight on the leg. Get medical advice the same day.
- Same day: the knee is locked, which means it is stuck bent and you cannot fully straighten it, even after gently trying to move it, or you cannot bend or straighten the leg at all. Get assessed the same day by a doctor, at urgent care or at an emergency department. A torn piece of meniscus can get caught in the joint and block it, and when a tear like this can be repaired, it should be done as early as possible. A knee that catches for a moment and then frees itself can happen with a meniscus tear. That is not the same as a locked knee, but tell your physio or doctor about it.
- Same day: the knee keeps giving way under you, or it gave way after a recent injury, or it gives way and is also very painful or swollen. Get medical advice the same day. If it only happens now and then, with no recent injury and no swelling, it is not an emergency, but book an assessment with a physio or doctor in the next few days.
- Same day: the knee swells up with no clear reason, especially if you take medicine to thin your blood. Get medical advice the same day.
- Same day: the knee is hot, red and swollen, or you have knee pain with a high temperature or feel hot, cold or shivery. Get medical advice the same day, and go to an emergency department if you feel very unwell. This can be a sign of infection in the joint, which needs treatment quickly.
- Same day: a calf or thigh that is newly swollen or getting more swollen, warm, tender, red or darker than usual, has swollen veins that are sore to touch, or has a throbbing or cramping pain that does not fit your injury or condition, or that feels different from normal muscle soreness after exercise. This can be a sign of a blood clot (deep vein thrombosis), especially after surgery, an injury, time in a cast or boot, a long trip or a spell of being much less mobile than usual. Get medical advice the same day. If you are also short of breath or have chest pain, call emergency services.
- Same day: pain at night or at rest that keeps getting worse. If you are being treated for cancer now, contact your cancer team the same day.
Book an appointment
- Within a day or two: pain at night or at rest that keeps getting worse. If you have had cancer, now or in the past, see your doctor sooner, within a day or two, and mention it, even if the pain is not getting worse. Do the same if you have knee pain and are losing weight without trying, even if it does not hurt at night.
- Within a few days: pain at night or at rest that keeps getting worse. See your doctor within a few days.
- Within a few days: you have had a fall, even if you were not hurt, or you keep tripping, stumbling or feeling that your legs might give way. Book an appointment with your doctor within a few days and ask about a falls assessment. Tell them about every fall, even the ones where you were not hurt.
Common questions
How do you know if you need a knee replacement?
You do not know it on your own, and that is not a failure of self-assessment. The question is answered in a consultation, because it turns on things only an examination and your own account of your life can supply: how much the knee limits what you do, what has already been tried and for how long, what else is going on in your health, and what you want back. The NHS describes the operation as something that may be recommended when knee pain is having a big effect on your life and the alternatives to a knee replacement have not worked. If you recognize yourself in that sentence, the next step is an appointment rather than a decision.
Do I need a knee replacement if I am bone on bone?
Not on its own. Bone on bone is a plain description of what a film shows when the gap between the bone ends has narrowed, and it says nothing about how much the knee is costing you. The link between what a knee X-ray shows and whether a knee hurts is loose in both directions: across population studies of radiographic osteoarthritis as those studies defined it, a wider group than the severe films on their own, X-ray changes of osteoarthritis were found in 15% to 76% of adults with knee pain, and knee pain was present in 15% to 81% of adults whose X-rays showed osteoarthritis (Bedson and Croft, 2008). Plenty of people with that phrase on their report manage well for years. It does not follow that a mild film points to an operation, though. Once a replacement is actually being weighed, the knees with the least damage on the films are the ones most likely to still have moderate or severe pain a year after the operation (Dowsey and colleagues, 2012), so a mild film is a reason for a surgeon to look for another cause of your pain rather than a reason to hurry. The X-ray is not the trigger, and it is not irrelevant either.
Is there a right age for a knee replacement?
There is no age at which it becomes necessary and none at which it stops being allowed. OrthoInfo, from the American Academy of Orthopaedic Surgeons, states that there are no absolute age or weight restrictions for total knee replacement surgery, and that recommendations for surgery are based on a patient's pain and disability rather than age. Age does change the arithmetic of timing, because a knee fitted younger has more years to last, and that is part of the conversation with a surgeon.
Can anything other than osteoarthritis mean you need a knee replacement?
Yes, although osteoarthritis is far and away the commonest route. The NHS says most people who have a knee replacement have pain and movement problems caused by osteoarthritis, and names other forms of arthritis, such as rheumatoid arthritis or arthritis after an injury, as less common reasons. OrthoInfo describes the same three groups: osteoarthritis, rheumatoid arthritis, and posttraumatic arthritis, which can follow a serious knee injury.
Who decides, me or the surgeon?
Both, and neither alone. A surgeon decides whether the operation is one they would offer you, taking in your examination, your imaging and your general health. You decide whether you want it. A referral is an assessment and a discussion rather than a booking, so you can be referred, hear the whole picture and still say no or not yet. If you feel the decision is being made for you in either direction, say so in the appointment and ask what would change it.
References
- National Institute for Health and Care Excellence. Osteoarthritis in over 16s: diagnosis and management. NICE guideline NG226, recommendations 1.6.1 to 1.6.4. 2022. https://www.nice.org.uk/guidance/ng226
- Dowsey MM, Nikpour M, Dieppe P, Choong PF. Associations between pre-operative radiographic changes and outcomes after total knee joint replacement for osteoarthritis. Osteoarthritis and Cartilage. 2012;20(10):1095-1102. https://doi.org/10.1016/j.joca.2012.05.015
- Ritter MA, Harty LD, Davis KE, Meding JB, Berend ME. Predicting range of motion after total knee arthroplasty. Clustering, log-linear regression, and regression tree analysis. Journal of Bone and Joint Surgery (American). 2003;85(7):1278-1285. https://doi.org/10.2106/00004623-200307000-00014
- NHS. Why a knee replacement is done. Page last reviewed 9 September 2026. https://www.nhs.uk/tests-and-treatments/knee-replacement/why-its-done/
- NHS. Recovering from a knee replacement. Page last reviewed 9 September 2026. https://www.nhs.uk/tests-and-treatments/knee-replacement/recovery/
- American Academy of Orthopaedic Surgeons. Total knee replacement. OrthoInfo. https://www.orthoinfo.org/en/treatment/total-knee-replacement/
- Bedson J, Croft PR. The discordance between clinical and radiographic knee osteoarthritis: a systematic search and summary of the literature. BMC Musculoskeletal Disorders. 2008;9:116. https://doi.org/10.1186/1471-2474-9-116
Written and checked by the PocketPhysio editorial team. Last updated 2026-10-17.
This page is general education, not a diagnosis or a personal treatment plan. See a physiotherapist or doctor for advice about your own situation.