Read below about swollen knees, including causes, treatment options and remedies. Or get a personalized analysis of your swollen knees from our A.I. health assistant. At Buoy, we build tools that help you know what’s wrong right now and how to get the right care.

A.I. Health Assistant

Take a quiz to find out why you’re having swollen knees

Take a quiz

Swollen Knees Symptoms

The knees are very strong weight-bearing joints, but vulnerable to illness and injury. There are no muscles within the knee joints – only bones, cartilage, and tough ligaments, which have little flexibility. That means they are relatively easy to strain or tear if you're an athlete and you overtrain.

The knees are also filled with shock absorbers called bursa. These bursae, and the lining of the knee joint that provides them with fluid, are vulnerable to infection and to autoimmune conditions which can affect both knees at the same time. Swelling of the knee is sometimes called an effusion, or "water on the knee."

Characteristics:

  • Swelling that may appear suddenly or gradually.
  • Swelling may appear with or without an injury.
  • Pain that may range from mild to moderate to severe, but worsens when you try to move the knee or bear weight on it.
  • Stiffness on trying to move.
  • Bruising, especially with trauma, although it may sometimes appear with illness-caused swelling.

Duration of symptoms:

  • Swelling that occurs a day or two after an injury is usually minor and will heal on its own.
  • Gradual swelling that occurs when there has been no injury indicates an inflammatory, autoimmune, or degenerative condition.
  • Rapid swelling with heat, redness, and pain indicates an infection and needs treatment right away.

Who is most often affected by swollen knees symptoms?

  • People past the age of about fifty, though anyone can be affected.
  • Athletes in heavy training.

When are swollen knees symptoms most likely to occur?

  • After an injury.
  • After a period of overtraining or overuse.
  • After contracting certain forms of arthritis.

Are swollen knees serious?

  • Mild bilateral knee swelling after a long period of work or exercise is usually not serious.
  • If the swelling is not properly treated and becomes chronic, it will almost certainly damage the underlying cartilage, bone, and joint structure of the knee.
  • If there is redness, pain, and fever along with the swelling, it's important to see a medical provider as soon as possible.

Swollen Knees Causes Overview

We've listed several different swollen knees causes here, in approximate order from most to least common:

Inflammation of the synovium.

  • The synovium is the lining of the knee joint and produces synovial fluid to keep the cartilage lubricated. If the joint is inflamed, synovial fluid may be overproduced and cause gradual swelling.
  • Autoimmune disorders may attack the synovium, resulting in inflammation and severe swelling of both knees along with redness and pain.
  • The inflammation may spread to the other cushioning structures within the knee joint and create pain and swelling.
  • Mineral deposits may form within the knee joint, causing intense pain with rapid swelling, warmth, and redness.

Infection of the synovium.

  • A bacterial infection of the synovium will cause rapid swelling with redness, pain, and fever.
  • A bacterial infection elsewhere in the body, such as a sexually transmitted disease, can sometimes set up an immune response throughout the body with severe swelling and pain in the joints.
  • Lyme disease can cause swollen joints as a late-stage symptom.

Degenerative disease of the cartilage. Pain and swelling result when the cartilage, and sometimes the bone, begins to wear away.

Trauma

  • Overuse/overtraining: usually from sports such as rowing, distance running, or weightlifting.
  • Acute injury:

    • Automobile accident.
    • Sports or work injury resulting in being struck across both knees.
    • Falls
    • Any of these injuries can fracture bones, and the bursae, tendons, and cartilage will also be injured.
    • The swelling is caused by the damaged tissue bleeding into the knee following the injury.

Tumors and cysts:

  • Sometimes a benign, fluid-filled cyst can form at the back of knee and create swelling, mild pain, and stiffness. It is rare, but possible, for these cysts to appear on both knees at the same time.
  • Cancerous tumors are very rare around the knee.

10 Potential Swollen Knees Causes

Disclaimer: The article does not replace an evaluation by a physician. Information on this page is provided as an information resource only, and is not to be used or relied on for any diagnostic or treatment purposes.

  1. 1.Knee Arthritis

    Arthritis is inflammation of one or more of the joints. Pain, swelling, and stiffness are the primary symptoms of arthritis. Any joint in the body may be affected by the disease, but it is particularly common in the knee.

    Knee arthritis is a chronic problem once it develops.

    Rarity:
    Common
    Top Symptoms:
    pain in both knees, knee stiffness, knee instability, swollen knee, morning joint stiffness
    Symptoms that always occur with knee arthritis:
    pain in both knees
    Urgency:
    Primary care doctor
  2. 2.Patellofemoral Pain Syndrome

    The kneecap (patella) is located directly in front of the thigh bone (femur), and should normally glide freely up and down. In this condition, called patellofemoral pain syndrome, the kneecap may rub against the thigh bone instead of gliding smoothly, causing damage and pain.

    Most people get better within weeks to months of physical therapy.

    Rarity:
    Common
    Top Symptoms:
    knee pain, pain in one knee, knee pain that gets worse when going up stairs, dull, achy knee pain, knee pain that gets worse when squatting
    Symptoms that always occur with patellofemoral pain syndrome:
    knee pain
    Urgency:
    Primary care doctor
  3. 3.Septic Arthritis

    Septic arthritis is an intensely painful joint infection.

    12-14 days with treatment

    Rarity:
    Rare
    Top Symptoms:
    fever, spontaneous shoulder pain, chills, knee pain, joint pain
    Urgency:
    Hospital emergency room
  4. 4.Iliotibial (It) Band Syndrome ('Runner's Knee')

    Iliotibial band (ITB) syndrome is the lateral knee pain that results from overuse that occurs after repetitive motion. It is the most common cause of lateral knee pain in runners and cyclists.

    Within 6 to 8 weeks with conservative nonsurgical treatment

    Rarity:
    Common
    Top Symptoms:
    knee pain, pain in one knee, dull, achy knee pain, knee pain that gets worse when going down stairs, sharp knee pain
    Symptoms that always occur with iliotibial (it) band syndrome ('runner's knee'):
    knee pain
    Urgency:
    Primary care doctor
  5. 5.Psoriatic Arthritis

    Psoriasis is a skin disease that causes itchy or sore patches of thick, red skin with silvery scales. Some people who have psoriasis also get a form of arthritis (inflammation and swelling of joints) called psoriatic arthritis.

    This type of arthritis can be managed with treatment, and permanent damage can be prevented. However, the underlying cause (psoriasis) is currently incurable.

    Rarity:
    Rare
    Top Symptoms:
    shoulder pain, lower back pain, joint pain, upper back pain, hip pain
    Urgency:
    Primary care doctor

    Swollen Knees Checker

    Take a quiz to find out why you’re having swollen knees.

    Take a quiz
  6. 6.Skin Infection of the Knee

    Cellulitis is an infection of the skin and tissues underneath it. It is most commonly caused by Group A strep and typically enters the body through a break in the skin.

    Dependent on severity of infection

    Rarity:
    Uncommon
    Top Symptoms:
    fever, knee pain, knee redness, knee injury, painful knee swelling
    Symptoms that always occur with skin infection of the knee:
    knee redness, area of skin redness
    Urgency:
    Primary care doctor
  7. 7.Meniscal Injury

    A torn meniscus is one of the most common knee injuries. Any activity that causes forceful twisting of the knee, especially when putting the pressure of one's full weight on it, can lead to a torn meniscus.

    6 to 8 weeks

    Rarity:
    Common
    Top Symptoms:
    pain in one knee, knee stiffness, knee instability, pain in the inside of the knee, swollen knee
    Urgency:
    Primary care doctor
  8. 8.Synovial Chondromatosis

    Synovial chondromatosis is a disease affecting the synovium, which is a thin flexible membrane around a joint. It can often be confused with tendinitis and/or arthritis.

    Surgery is the only effective treatment at this time, but this condition is considered benign, and spreading to other tissue is very rare.

    Rarity:
    Rare
    Top Symptoms:
    joint swelling, pain in one knee, pain in one hip, swollen knee, spontaneous knee pain
    Symptoms that always occur with synovial chondromatosis:
    joint swelling
    Symptoms that never occur with synovial chondromatosis:
    fever, night sweats, unintentional weight loss, warm red ankle swelling, warm red knee swelling, warm and red elbow swelling
    Urgency:
    Primary care doctor
  9. 9.Osteochondritis Dissecans

    At joints where bones meet, bones are covered with a layer of cartilage which provides shock absorbance and lubrication. Osteochondritis dissecans is a condition where a piece of cartilage with a thin layer of bone detach from the larger bone, causing pain.

    Conservative treatment should resolve the problem in 3-6 months.

    Rarity:
    Rare
    Top Symptoms:
    pain in one knee, knee stiffness, knee instability, knee pain that gets worse during a run, pop in the knee
    Urgency:
    Primary care doctor
  10. 10.Infrapatellar Bursitis

    Bursae are small fluid-filled sacks located around the body in strategic locations to provide a cushion and help reduce friction. There is a pair of bursae below each kneecap (patella). Infrapatellar bursitis is a condition where these bursae are inflamed. It is a common cause of knee pain in people whose work involves frequent kneeling on hard surfaces.

    Condition goes away on its own in a few weeks.

    Rarity:
    Uncommon
    Top Symptoms:
    pain in one knee, spontaneous knee pain, dull, achy knee pain, knee pain that gets worse when going up stairs, knee pain that gets worse when squatting
    Urgency:
    Self-treatment

Swollen Knees Treatments and Relief

Seek immediate treatment in the emergency room or call 911 if you have:

  • Sudden, rapid swelling within one or both knees that appears within a few minutes to a couple of hours, along with redness, pain, stiffness, and heat in the knee. You may also feel ill and feverish. These are all signs of infection, which can spread quickly.

Schedule an appointment for:

  • Chronic swelling that does not subside, especially if you also feel ill.
  • A referral for physical therapy to help strengthen injured knees.

Remedies that you can try at home:

  • Rest and keeping the knee elevated if at all possible.
  • Ice or cold packs to help reduce swelling. Avoid heat with a swollen knee in case it is caused by an infection, because heat can cause the pus within the joint to spread.
  • Use a compression wrap, as long as it is not overly tight and does not increase your pain.
  • Take over-the-counter pain relievers such as ibuprofen or other NSAIDs.

Questions Your Doctor May Ask About Swollen Knees

  • Q.Is the knee pain affecting one or both knees?
  • Q.What is your body mass?
  • Q.Where is your knee pain?
  • Q.Do you feel like your knee is unstable, weak, or giving out?

If you've answered yes to one or more of these questions, try our swollen knees symptom checker to find out more.

Take a quiz

Swollen Knees Symptom Checker Statistics

  • People who have experienced swollen knees have also experienced:

    • 7% Knee Pain
    • 3% Moderate Knee Pain
    • 2% Fatigue
  • People who have experienced swollen knees had symptoms persist for:

    • 30% Less Than a Week
    • 29% Over a Month
    • 18% Less Than a Day
  • People who have experienced swollen knees were most often matched with:

    • 14% Knee Arthritis
    • 7% Patellofemoral Pain Syndrome
    • 7% Septic Arthritis
  • Source: Aggregated and anonymized results from visits to the Buoy AI health assistant (check it out by clicking on “Take Quiz”).

A.I. Health Assistant

Take a quiz to find out why you’re having swollen knees

Take a quiz