Swollen knee: what it means and what helps
Warning signs, and when to get help
What does a swollen knee mean?
Extra fluid, in one of two places. Inside the joint capsule it makes the whole knee look full, stops it bending the last part of the way and leaves the leg feeling oddly unreliable. Outside the capsule the fluid collects in a bursa or in the soft tissue, and that version is more of a local lump, often sitting right over the kneecap or at the back of the joint. Most people call both of them water on the knee or fluid on the knee, which describes the feeling well enough.
What you cannot tell by looking is what the fluid is made of, and that is the part that sets the urgency. Blood fills a knee fast, and ordinary joint fluid from an irritated lining fills it slowly. Pus from an infected joint arrives with heat, and a gout attack arrives with pain far out of proportion to anything you did that day. They fill a knee at very different speeds. That is why the clock is a better opening question than a list of causes.
Nothing below names your problem. Use it to describe what has happened accurately enough to get the right appointment at the right speed.
How fast did your knee swell?
Within a few hours of a twist, fall or blow
This is the group that does not wait. A knee that fills up in the first few hours after an injury suggests blood inside it rather than ordinary joint fluid, and blood points at something inside the joint being torn rather than bruised. One review of knee ligament injury takes swelling that appears within 2 hours of the injury as favoring bleeding and damage inside the joint, a torn cruciate ligament among the possibilities (Vosoughi and colleagues, 2021). This page works to a few hours rather than to that 2 hour mark on purpose. The 2 hours is a rule of thumb handed on through review articles rather than a measured cut-off, nobody times their own knee, and there is nothing lost by arriving early.
OrthoInfo puts it plainly for cruciate ligament injuries: within 24 hours, the knee will swell, often after a popping noise and a sense of the knee giving out underneath you. The NHS sends fast swelling after an injury, or an injury where you felt a pop or snap, straight to an emergency department, and tells you not to drive yourself there.
What makes people hesitate is that the pain often settles over the first day and that plenty of them can still walk. Neither is reassuring. OrthoInfo notes that the swelling and pain may go away on their own, so a knee like this is judged on how fast it filled, not on how bad it feels by the evening.
Over a day or two after an injury
Slower swelling after a twist points more toward the meniscus, the cushion inside the joint. The NHS says the swelling may not start for a few hours or days, and OrthoInfo describes the knee gradually becoming stiffer and more swollen over 2 to 3 days, and not immediately. A tear can come from sport, or from something as ordinary as getting up out of a low chair with the foot planted.
That timing is reassuring only about the speed, not about the outcome. Meniscus tear covers what the knee needs next, and swelling, locking and giving way sorts the three symptoms that most often travel together. A knee that will not fully straighten after this kind of injury is checked the same day, not next week.
Slowly, with no injury at all
Weeks rather than hours, with nothing you can point to, is the commonest story of all. Wear-related arthritis produces fluid in waves that rise after a heavy day and fall again with a quiet one. NICE lets a clinician diagnose osteoarthritis without any imaging from 45 onward, in someone whose joint pain comes on with activity and who has either no morning joint-related stiffness at all or morning stiffness lasting no longer than 30 minutes (NG226, recommendation 1.1.1). The knee osteoarthritis program is built for that pattern, and arthritis flare ups deals with the weeks when the knee is swollen and sore.
Inflammatory arthritis behaves differently. More than one joint swells, the morning stiffness runs well past half an hour, and there is often a tiredness that does not match what you have been doing. Several swollen joints at once, or morning stiffness that lasts hours, is a conversation with a doctor and not a knee problem to manage at home, and our rheumatoid arthritis page describes that pattern. Bursitis sits outside the joint instead: the NHS describes a painful, tender, warm, swollen joint, with the knee among the commonest sites, and advises seeing a doctor if self care has not helped within 1 to 2 weeks.
A swollen knee with no pain, and no injury
A knee can hold a surprising amount of fluid without hurting much. That usually means the lining is producing fluid without a lot of inflammation driving it, which is common in long-standing arthritis and in a knee that was injured years ago. It is not an all clear, and it is not a reason to do nothing.
The NHS gives you the first reason. It puts a badly swollen knee on its list for urgent advice, and in its list of causes it pairs swelling, warmth and bruising with bleeding in the joint, which it says is more likely while you are taking anticoagulants. So a knee that has swollen with no clear reason sits on the same-day route in the warning signs below, and it sits there more firmly if you take medicine to thin your blood. The second reason is mechanical. Fluid weakens the thigh whether the knee hurts or not, and that is the next section.
So track it with something other than pain. Compare the swollen knee with the other one each morning, see how far it bends against last week, and check whether it straightens fully. Those tell you more than the ache does.
When is a swollen knee an emergency?
Some go to hospital tonight and some need advice the same day, and the full wording for each one is in the warning signs at the foot of this page.
These go to an emergency department straight away, without you driving. A knee that swelled very quickly within a few hours of a twist, fall or blow, or where you felt a pop or snap. A knee that has changed shape, is pointing at an odd angle, or comes with a cold, pale or blue foot or no feeling in the leg. A sudden pop or tearing feeling just above or below the kneecap, with a dip you can see or feel there, or a sudden inability to straighten the knee or lift the leg out straight, which can be a torn tendon at the front of the knee and goes to hospital even if you can still walk. And a replaced knee that cannot take your weight after a fall or a knock, or that is very painful, or a leg that looks out of shape or shorter than the other one.
Others need medical advice the same day. A knee that is hot, red and swollen, or swollen with a temperature or shivering. A knee you cannot walk on or put weight through, with or without an injury behind it. And a fall or a knock, even a small one, that leaves a replaced knee newly painful, swollen or bruised while you can still walk on it.
Everything else on this page is slower than those, and most swollen knees are. The fast ones lose the most by waiting, and the slow ones lose almost nothing by being seen next week instead of tonight. The warning signs below carry every route in full, and there are more of them than the ones named here: the post-operative routes, the clot route and the ones written for a child or a teenager all sit there too.
A hot, red or feverish knee
A hot, red, feverish knee is the exception to everything above. A joint can become infected, and an infected joint is treated urgently.
The NHS describes septic arthritis as sudden severe pain in one joint, being unable to move it or put weight on it, hot swollen or red skin over it, and often a high temperature, while noting that not everyone has one and that redness is harder to see on black or brown skin. It says symptoms usually develop over a few days and need checking straight away. Gout can look much the same: the NHS describes sudden severe joint pain with hot, swollen, red skin, most often in the big toe but also in the knee, and it asks for urgent advice if the pain is getting worse, or there is a very high temperature or a feeling of being hot, cold or shivery, or you feel sick or cannot eat, any one of which can mean infection inside the joint.
NICE draws the same boundary in its own words. Its definition of atypical features reads: "Atypical features could include a history of recent trauma, prolonged morning joint-related stiffness, rapid worsening of symptoms or deformity, the presence of a hot swollen joint, or concerns that may suggest infection or malignancy." Those features take a knee out of the straightforward osteoarthritis route, and the guideline tells clinicians not to use imaging to diagnose osteoarthritis unless they are there or something else points to another diagnosis (NG226, recommendation 1.1.2). A hot swollen joint is on that list by name, which is why this page does not treat it as arthritis producing fluid.
Neither of those has a program on this site, on purpose. Both are diagnosed from the joint fluid and from blood tests, and both are treated with medicine, not with exercise. If your knee is hot, red and swollen, stop reading and get advice the same day. Go to an emergency department if you feel very unwell.
Swelling at the back of the knee
If the fullness sits behind the joint rather than all round it, a Baker's cyst is the usual explanation. The NHS describes a soft lump at the back of the knee, sometimes with pain, tightness or stiffness and difficulty bending, often worse after walking, and it points out that in adults the cyst is normally the result of something else damaging the joint, naming an injury, osteoarthritis, rheumatoid arthritis and gout. Treating the cyst without treating what caused it tends to disappoint.
Our Baker's cyst page covers it properly and pain behind the knee covers everything else that hurts back there. One caution travels with this one: the NHS asks for urgent advice if leg pain or swelling suddenly gets worse, if the leg has swelling, bruising and warm skin, if the skin is red or darker than usual, or if the swelling in the leg gets harder or softer, because a cyst that has burst and a clot in the leg can feel similar. The clot wording is in the warning signs below.
Why a swollen knee feels weak and gives way
Patients describe this constantly and almost nobody explains it to them. A knee full of fluid does not just bend less. The thigh muscle above it stops working properly.
This has been tested, not assumed. Researchers injected saline into the knees of nine healthy volunteers, 4 women and 5 men, in two volumes, 30 mL and 60 mL, and measured a single leg landing. Activity in the muscles on the inner and outer side of the thigh dropped at both volumes, and the landing itself changed at the larger one, with higher forces through the leg and less knee bend to absorb them (Palmieri-Smith and colleagues, 2007). Nine healthy knees with fluid put into them are not the same as a knee with a disease in it, so that is the mechanism and not a measurement of your own leg.
The wider version of this has a name of its own. A review of quadriceps arthrogenic muscle inhibition describes a lasting difficulty switching the thigh on after arthritis, injury or surgery, driven by altered signals coming out of the joint itself, with swelling and inflammation among the triggers (Rice and McNair, 2010). It is why the exercise list on this page looks so undemanding. The first job in a swollen knee is to get the thigh switching on again, not to load it heavily. If the knee is actually letting you down under your weight, read knee giving way, and get it checked the same day if it keeps happening.
A knee that swells after exercise, and how to use that
Most people with a swollen knee have the everyday version: normal in the morning, puffed up by the evening after a long day on it. It is miserable to live with. It is also the most useful dial you have, once you start using it as information instead of worrying at it.
So use it. Pick a walk, a session or a number of repetitions, do it, then look at the knee the next morning instead of an hour later. A knee that is the same size as usual means you can repeat that amount. A knee that is clearly fuller, stiffer or harder to straighten means the last step was bigger than the joint could absorb, so take roughly a quarter off and build again from there. Our guide to the pain monitoring model sets out the same logic for pain.
That rule has limits. It is for a knee that has not been operated on, and for swelling that settles overnight, not swelling that climbs day after day. If each morning is worse than the one before, stop adjusting the amount and get advice.
Swelling in a knee that has been operated on
If the swollen knee has had an operation, the whole page above changes meaning. New swelling is not a signal to train more carefully. It is information your surgical team needs. A repair can come apart, a replaced joint can become infected, the bone around a new joint can break in a fall that looks minor, and a leg that has been operated on carries a raised risk of a clot for weeks afterward. None of those is managed by doing a little less.
Some swelling is expected after knee surgery, and swelling after a knee replacement sets out how long that normally runs for. What matters is the direction. The NHS asks for urgent advice after a knee replacement if there is throbbing or cramping leg pain, a high temperature or feeling hot, cold or shivery, oozing or pus from the wound, or redness, tenderness, swelling or pain in the knee that is not improving or is getting worse, because those can mean infection or a clot. OrthoInfo lists new or increasing swelling in the calf, ankle and foot, increasing calf pain, and tenderness or redness above or below the knee among the warning signs of a clot in the leg, and asks you to tell your doctor immediately.
One detail catches people out more than any other. A normal temperature does not rule out an infected joint replacement: OrthoInfo says fever is no longer considered a sign of an infection after a joint replacement, that the signs can be as small as mild redness and swelling, that sometimes there are none at all, and that increased pain or stiffness in a joint which had been working well is one of them. It also notes that any infection elsewhere in your body can reach a replacement through the bloodstream, and that wound drainage more than 10 days after surgery is one of the things it lists. So report a new or increasing swelling in a replaced knee without waiting to see whether you develop a temperature.
The route depends on the operation, and the ones that have a line of their own are written out in full in the warning signs below. After a replacement, swelling that is increasing instead of settling, or a knee whose redness, warmth or pain is not improving, goes to your surgical team or medical advice the same day.
A fall or a knock, even a small one, that leaves a replaced knee newly painful, swollen or bruised is its own same-day route, and it applies even if you can still walk on the leg, because a break in the bone around a new knee can be missed while you are still getting about on it. If that knee cannot take your weight afterward, or is very painful, or the leg looks out of shape or shorter than the other one, it is an emergency department rather than a phone call, and the emergency line in the warning signs below is the one to read.
After a meniscus operation, a knee that swells up or becomes much more painful after a twist, a kneel or a deep bend means keeping your weight off it and contacting your surgical team the same day. A reconstructed ligament takes the same instruction: a graft can be damaged like any other repair, so keep your weight off a knee that has swelled after a twist, a fall or an episode of giving way, and contact your surgical team the same day, which knee giving way sets out in full.
Whichever operation you had, a knee that swelled very quickly after a pop or a snap goes to an emergency department first, under the emergency line in the warning signs below, and a call to your own team is not a substitute for it. After any knee operation, a wound that is leaking or opening, or a knee that is getting hotter and more painful, is the same-day infection route, and warning signs to call your surgeon about covers the rest of them in order of speed. Your own team's instructions come before anything here.
What helps at home
For a knee that does not need a same-day appointment, the NHS advice is modest and it is the right place to start. Put as little weight as possible through the knee for a few days, which means cutting out long spells on your feet, not going to bed. Hold an ice pack, or a bag of frozen peas, wrapped in a tea towel against the knee for up to 20 minutes every 2 to 3 hours. Paracetamol, or ibuprofen as a gel or as tablets, suits many people, and a pharmacist can tell you whether either fits with your other medicines.
Its meniscus tear page adds a few more for a swollen knee after a twist or a fall. Keep the leg raised on a pillow when you can. A bandage or knee support during the day is reasonable, taken off at night. It advises against heat packs, hot baths, alcohol, massaging the knee and carrying heavy loads in the first days, and it is clear that the knee should not be kept completely still: after a few days of rest, start moving it gently.
What does not help is waiting for the swelling to disappear before you move. A knee that is not moved stiffens and the thigh switches off further, which leaves you with two problems instead of one.
Exercises for a swollen knee
The aim here is narrow. Get the thigh switching on again, keep the knee moving through whatever range it currently has, and keep the calf pump working, which matters for both swelling and clot risk. None of that needs load, and a swollen knee does not tolerate load well anyway.
Start with the ankle and with a gentle thigh contraction, add the small-range knee work, and leave the leg-holding-you-up exercises until the swelling has settled and someone has examined the knee. A working ache in the front of the thigh is expected. Sharp pain inside the joint, or a knee that is fuller the next morning, means the amount was wrong rather than the exercise.
Check these before you start. If the knee has been operated on, follow the program from your surgeon and physio instead of this one. Check with your doctor first if you have a heart or lung condition, or blood pressure that is not under control. If you have high blood pressure, breathe steadily through every hold and never hold your breath.
If you are pregnant and past the first three months, ask your physio before exercising flat on your back, because four of these five are done lying down. If an exercise is only done lying on your back, RANZCOG says it can be done sitting or standing instead, and UK pelvic health physiotherapy guidance (POGP) suggests an upright position or lying on your left side, with a change of position if you feel dizzy or unwell. If you have had a hip replacement, get the go-ahead from your surgeon or physio before the heel slides and the straight leg raise, because sliding the heel toward you bends the hip and many surgeons set a limit on that for the first months. A child or teenager with a swollen knee should be assessed before starting any exercise program. Do the four lying exercises on a bed rather than on the floor if getting down to the floor and back up is hard, if you have fallen in the past year, or if you are on your own in the house.
One rule about the order. Add the straight leg raise only when you can tighten the thigh and keep the knee straight as the heel leaves the bed: if the knee bends as you lift, go back to the quad sets for a few days and try again. A knee full of fluid often will not hold itself straight at first, and lifting it while it sags trains the sag.
Exercises that can help
Read the checks just above before you start any of these: they cover an operated knee, pregnancy, a replaced hip, a heart or lung condition, blood pressure, and anyone still growing. All five are done lying or sitting, so a knee that is full of fluid never has to hold you up on one leg while it is doing it. Ankle pumps are usually built up to 10 to 20 at a time, several times a day, especially if you are sitting or resting more than usual. Quad sets are often 10 repetitions with a 5 to 10 second hold, several times a day, and short arc quads often 2 to 3 sets of 10 to 15 with a 3 to 5 second hold at the top, once or twice a day. Heel slides are often 10 to 15 slow repetitions, 2 or 3 times a day, and the straight leg raise 2 to 3 sets of 10 on each leg, once a day. Your physio will adjust this, and after an operation the plan from your surgeon and physio comes first.
Ankle pumps
Quad sets (towel under knee)
Heel slides
Short arc quads
Straight leg raiseWhen to see a physio (physical therapist)
Book an assessment if the knee is still swollen after 2 weeks of sensible self care, if it swells repeatedly and you cannot work out what sets it off, if it is stopping you working, walking or sleeping, or if it has never been examined since the injury that started it. A swollen knee is also one of the more rewarding things to have looked at, because a clinician can feel how much fluid is in there, test what is loose, check how far the knee straightens and compare it with the other side in a couple of minutes.
Go sooner than that if you cannot put weight on the leg, if the knee locks or gives way, or if there is any numbness or weakness in the leg or foot. Anyone still growing needs the knee examined before starting exercises, and after an operation your surgeon's plan comes first. Knee clicking and cracking covers the noise question that often arrives with a swollen knee, and knee pain by body area lists the other knee pages on this site. Every warning sign below carries its own timing, from calling an ambulance to an appointment in the next few days.
Related exercise programs
Warning signs
Get emergency help now
- 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.
- 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: you felt a sudden pop or tearing feeling just above or below your kneecap, you can see or feel a dip there, the kneecap seems to have moved up toward the thigh or to sag lower than usual, or you suddenly cannot straighten your knee or lift your leg out straight. Stop exercising and go to an emergency department straight away, even if you can still walk, as this can be a torn tendon at the front of the knee, either below the kneecap (patellar tendon) or above it (quadriceps tendon). Do not drive yourself. A repair usually does better if it is done soon after the injury.
- Emergency: you have had a knee replacement and, after a fall or a knock, you cannot put weight on the leg, your knee or thigh is very painful, or the leg looks out of shape or shorter than the other. Go to an emergency department straight away, and do not drive yourself. If you cannot get up, call emergency services. A broken bone around a knee replacement is rare, but it needs treatment in hospital. If a smaller fall or knock leaves the knee newly painful, swollen or bruised, even if you can still walk on it and the leg looks normal, stop the exercises, go back to your crutches or sticks and keep your full weight off that leg, and get it checked by your surgical team the same day. A break in the bone around a new knee is not always obvious at first, and walking normally on one can move the broken ends apart.
- Emergency: you are still growing and have pain that wakes you at night, pain at rest that keeps getting worse, or a lump or swelling around the knee or thigh that keeps growing. If you also have a rash that does not fade when you press it, call emergency services or go to an emergency department.
Get help today
- 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: 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: you cannot walk or put weight on the leg, whether or not there was a fall or injury, or you have very bad knee pain after a fall or injury, or very bad knee pain that came on suddenly with no injury at all. 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. Until it has been checked, leave out the exercises you do standing on one leg, hold the rail on stairs and take care on uneven ground, because a knee that gives way without warning is how people fall.
- Same day: since your operation, your wound is oozing, leaking or has pus coming out of it, or its edges start to come apart, or your knee is tender, swollen, painful or red and it is not getting better or is getting worse, or you have a high temperature or feel hot and cold or shivery. Redness can be harder to see on brown or black skin. Contact your surgical team the same day, or get urgent medical advice if you cannot reach them. These can be signs of an infection in the wound or the knee joint. If you feel very unwell, go to an emergency department. If you have a very high or very low temperature, shivering you cannot control, you are breathing very fast or finding it hard to breathe, you are confused or your speech is slurred, your skin, lips or tongue look blue, gray, pale or blotchy (on brown or black skin this can be easier to see on the palms of the hands or soles of the feet), or you have a rash that does not fade when you press it, call emergency services, as this can be sepsis.
- Same day: since your operation, the redness, warmth, swelling or pain in your knee is not getting better or is getting worse, for example the knee pain keeps building both when you move and when you rest. Contact your surgical team or get medical advice the same day, as this can be a sign of infection around the new joint. If you feel very unwell, go to an emergency department.
- Same day: swelling in the operated leg that is increasing from day to day, or from week to week, instead of settling, including swelling spreading into your calf, ankle or foot, pain in your leg or calf away from the wound, increasing pain in your calf, or tenderness or redness above or below your knee. Get medical advice the same day. Swelling is expected after this operation: OrthoInfo describes moderate to severe swelling in the first few days or weeks and mild to moderate swelling for about 3 to 6 months, but it asks you to tell your doctor about new or severe swelling because it can be the warning sign of a blood clot, and increasing swelling of the calf, ankle and foot, increasing calf pain, and tenderness or redness above or below the knee are all on its list of clot signs. Your clot risk does not end on the day your tablets or injections do. If you are also short of breath or have chest pain, call emergency services.
- Same day: at any time after your operation, even years later, new pain in your new knee along with a high temperature or feeling hot, cold or shivery, or a knee that becomes hot, red or swollen, which counts on its own, with or without pain and with or without a temperature. Contact your surgical team or get medical advice the same day. A new joint can become infected long after the operation, and that needs treating quickly. A normal temperature is not an all clear: OrthoInfo says fever is no longer considered a sign of an infection after a joint replacement, that the signs can be as little as mild redness and swelling and sometimes there are none at all, and that increased pain or stiffness in a joint that had been working well is one of them. So do not wait for a temperature before you report new or increasing pain in your new knee to your surgical team.
- Same day: since an operation on your meniscus, the knee locks, catches or gives way again, or you feel a pop or a shift inside it, or it swells up or becomes much more painful after a twist, a kneel or a deep bend. Keep your weight off it and contact your surgical team, or get assessed the same day at urgent care or an emergency department. A repair can come apart before the tear has healed, and a trimmed meniscus can tear again. If the knee is only a little achy after a busy day, with no swelling and no catching, it is not an emergency, but tell your physio or surgeon in the next few days.
- 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: a leg or foot that is getting weaker, for example your foot drags, catches on the ground or slaps down when you walk (foot drop). Get medical advice the same day. If the weakness is getting worse by the hour, go to an emergency department.
- Same day: you are a teenager who is still growing and you have a limp, or pain in the hip, groin or thigh as well as the knee. Get an urgent appointment with a doctor the same day, and stay off sport and these exercises until you have been checked. If you cannot put weight on the leg, have a high temperature or feel very unwell, go to an emergency department. Some hip problems in growing teenagers, such as a slipped growth plate at the top of the thigh bone (slipped capital femoral epiphysis), are felt in the knee and need checking quickly.
- 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: you are still growing and have pain that wakes you at night, pain at rest that keeps getting worse, or a lump or swelling around the knee or thigh that keeps growing. See a doctor within a day or two. Ask for an urgent appointment rather than the next routine one, and say what the pain does at night and whether a lump is getting bigger. Do the same if the pain comes with looking pale, unusual tiredness, fevers with no clear cause, bruising or bleeding easily, or weight loss. Bone pain that is worse at night, or a new lump, needs checking to rule out rarer causes such as a bone tumor or a blood cancer (leukemia).
- 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.
Common questions
What causes a swollen knee?
Several things, and they sort more easily by speed than by name: an injury that tears something inside the joint, a cruciate ligament among the possibilities, bleeds into it and fills it within hours. A torn meniscus usually swells more slowly, and the NHS says the swelling may not start for a few hours or days, and OrthoInfo describes the knee becoming gradually stiffer and more swollen over 2 to 3 days. Arthritis of either kind, wear-related or inflammatory, produces fluid in waves that come and go over weeks. Infection in the joint and a gout attack both bring heat and redness with them, and both need advice the same day. And a swelling that sits behind the knee instead of all round it is often a Baker's cyst, which the NHS describes as a soft lump at the back of the knee that is usually caused by another problem inside the joint.
What does a swollen knee with no pain mean?
Usually that the lining of the joint is producing fluid without much inflammation to go with it, which is common in wear-related arthritis and after an old injury. It is not a reason to relax about it. A painless swelling still tells you the joint is irritated by something, it still weakens the thigh, and a knee that has swollen with no clear reason is on the same-day route in the warning signs below, particularly if you take medicine to thin your blood, because the NHS links swelling with warmth and bruising to bleeding in the joint and says that is likelier on anticoagulants. Pain is a poor measuring stick for a swollen knee. Use the size of the knee compared with the other one, how far it bends, and whether it is bigger or smaller than it was last week.
Why is my knee swollen after a fall?
Because something in it was loaded harder than it could take, and the clock tells you how worried to be. The NHS says to go to an emergency department if a knee becomes very swollen very quickly after an injury or you felt a pop or snap, and not to drive yourself there. Swelling that takes a day or two to appear after a fall is the commoner pattern and is the one to get assessed in the next few days, sooner if you cannot put weight on the leg. Keep the leg up on a pillow, use the knee gently instead of not at all, and get it looked at if it is not clearly improving within a week or two. A knee that will not straighten, or that gives way, moves up the queue whatever the swelling is doing.
What is water on the knee?
It is the everyday name for fluid inside the joint, which a clinician would call an effusion. Nothing has leaked in from outside: the lining of the joint makes a small amount of lubricating fluid all the time, and when the joint is irritated it makes a great deal more. That is why the term covers so many different situations, from an arthritic knee that puffs up after gardening to a knee full of blood after a tackle. Whether the fluid is worth drawing off with a needle is a decision made after someone has examined the knee, and sometimes tested the fluid, so it is not a judgement to make at home.
Why does my knee swell after exercise?
Most often because the amount of work was more than that knee currently tolerates, not because anything new has torn. A joint that is already irritated, by arthritis or by an old meniscus tear, answers a long walk or a heavy session by producing fluid for a day or so afterward. That makes next-day swelling useful: it is the clearest single sign that yesterday was slightly too much, and it beats how the knee felt at the time. Compare the knee with the other one the morning after, drop the amount by roughly a quarter and build again from there. Swelling that keeps climbing over several days without any exercise behind it is a different matter and needs advice rather than a smaller walk.
References
- NHS. Knee pain. Page last reviewed 21 December 2023. https://www.nhs.uk/symptoms/knee-pain/
- NHS. Meniscus tear (knee cartilage damage). Page last reviewed 2 September 2026. https://www.nhs.uk/conditions/meniscus-tear/
- NHS. Septic arthritis. Page last reviewed 3 September 2026. https://www.nhs.uk/conditions/septic-arthritis/
- NHS. Gout. Page last reviewed 24 August 2023. https://www.nhs.uk/conditions/gout/
- NHS. Bursitis. Page last reviewed 30 October 2023. https://www.nhs.uk/conditions/bursitis/
- NHS. Baker's cyst. Page last reviewed 26 June 2025. https://www.nhs.uk/conditions/bakers-cyst/
- NHS. Deep vein thrombosis (DVT). Page last reviewed 30 April 2026. https://www.nhs.uk/conditions/deep-vein-thrombosis-dvt/
- NHS. Dislocated kneecap. Page last reviewed 10 March 2026. https://www.nhs.uk/conditions/dislocated-kneecap/
- NHS. Complications of a knee replacement. Page last reviewed 9 September 2026. https://www.nhs.uk/tests-and-treatments/knee-replacement/complications/
- American Academy of Orthopaedic Surgeons. Meniscus Tears. OrthoInfo. https://www.orthoinfo.org/en/diseases--conditions/meniscus-tears/
- American Academy of Orthopaedic Surgeons. Anterior Cruciate Ligament (ACL) Injuries. OrthoInfo. https://www.orthoinfo.org/en/diseases--conditions/anterior-cruciate-ligament-acl-injuries/
- American Academy of Orthopaedic Surgeons. Total Knee Replacement. OrthoInfo. https://www.orthoinfo.org/en/treatment/total-knee-replacement/
- American Academy of Orthopaedic Surgeons. Activities after total knee replacement. OrthoInfo. https://www.orthoinfo.org/en/recovery/activities-after-knee-replacement/
- American Academy of Orthopaedic Surgeons. Joint Replacement Infection. OrthoInfo. https://www.orthoinfo.org/en/diseases--conditions/joint-replacement-infection/
- National Institute for Health and Care Excellence. Osteoarthritis in over 16s: diagnosis and management (NG226). 2022. https://www.nice.org.uk/guidance/ng226
- Palmieri-Smith RM, Kreinbrink J, Ashton-Miller JA, Wojtys EM. Quadriceps inhibition induced by an experimental knee joint effusion affects knee joint mechanics during a single-legged drop landing. American Journal of Sports Medicine. 2007;35(8):1269-1275. https://doi.org/10.1177/0363546506296417
- Rice DA, McNair PJ. Quadriceps arthrogenic muscle inhibition: neural mechanisms and treatment perspectives. Seminars in Arthritis and Rheumatism. 2010;40(3):250-266. https://doi.org/10.1016/j.semarthrit.2009.10.001
- Vosoughi F, Rezaei Dogahe R, Nuri A, Ayati Firoozabadi M, Mortazavi J. Medial collateral ligament injury of the knee: a review on current concept and management. Archives of Bone and Joint Surgery. 2021;9(3):255-262. https://doi.org/10.22038/abjs.2021.48458.2401
- Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG). Exercise during pregnancy (C-Obs 62). Developed July 2016, reviewed March 2020. https://ranzcog.edu.au/wp-content/uploads/2022/05/Exercise-during-pregnancy.pdf
- Pelvic, Obstetric and Gynaecological Physiotherapy (POGP). Supine lying during pregnancy. Good practice statement. Journal of Pelvic, Obstetric and Gynaecological Physiotherapy. 2018;122:77-83. https://thepogp.co.uk/_userfiles/pages/files/journals/122/gps7unb278sdc9.pdf
Written and checked by the PocketPhysio editorial team. Last updated 2026-10-21.
This page is general education, not a diagnosis or a personal treatment plan. See a physiotherapist or doctor for advice about your own situation.