Swollen Knee
A swollen knee occurs when excess fluid collects in or around the joint. The most common causes include injury, overuse, arthritis, and infection. Sometimes swelling develops quickly after trauma, while in other cases it appears gradually due to an ongoing condition.
This symptom can limit movement, cause discomfort, or make daily activities harder. Sports injuries, falls, or long-term wear and tear can all lead to knee swelling. Even issues like obesity or certain blood disorders can lead to knee swelling.
Definition of Symptom
A swollen knee occurs when extra fluid collects in or around the knee joint. This can affect one or both knees. Swelling may develop quickly after an injury or slowly due to an ongoing condition.
This symptom is sometimes called knee effusion or water on the knee. Fluid can collect inside the joint capsule or in surrounding soft tissues. Common signs that may appear with knee swelling include:
- Visible enlargement of the knee area.
- Stiffness that limits bending or straightening.
- Discomfort or pain when moving or bearing weight.
- Warmth over the joint in some cases.
The knee joint contains bones, cartilage, ligaments, and synovial fluid. When the body reacts to injury, irritation, or disease, it may produce more fluid than usual. This extra fluid causes the swelling that you can see and feel.
| Symptom Feature | Description |
|---|---|
| Location | In or around the knee joint |
| Onset | Sudden (injury) or gradual (chronic condition) |
| Texture | Firm or soft, depending on fluid amount |
| Associated Signs | Pain, stiffness, warmth, reduced motion |
A swollen knee is not a disease itself but a sign of another problem. Causes can range from a simple strain to arthritis or infection. Doctors often identify the source of fluid buildup through a physical exam and, in some cases, tests such as imaging or fluid analysis.
Possible Causes/Swollen Knee Causes
A swollen knee occurs when excess fluid collects in or around the joint. This can result from injury, overuse, or certain health conditions. Swelling may develop quickly after trauma or slowly over time. Common causes include:
| Cause | Description |
|---|---|
| Arthritis | Joint inflammation, including osteoarthritis and rheumatoid arthritis. |
| Bursitis | Swelling of small fluid-filled sacs that cushion the knee. |
| Hemarthrosis | Bleeding into the joint, often after a significant injury. |
| Gout | Uric acid crystal buildup causing sudden pain and swelling. |
In osteoarthritis, cartilage wears down over time, which irritates the joint and leads to fluid buildup. This happens more often in older adults but can also occur after injury. Rheumatoid arthritis makes the immune system attack joint tissue. This can cause persistent swelling, warmth, and stiffness in the knees. Bursitis often develops from repeated kneeling, pressure, or injury.
The knee may feel tender and slightly warm. Hemarthrosis causes the knee to swell rapidly and feel tight, usually after ligament tears, fractures, or other significant trauma. Gout can trigger sudden, intense pain with redness and swelling, often affecting one knee at a time. Joint infections, called septic arthritis, can also cause swelling and require urgent care to prevent damage.
Other Non-Disease Causes
Knee swelling can happen even without an underlying disease. Everyday activities, minor injuries, or environmental factors may trigger fluid buildup around the joint.
- Overuse from activity is a common reason. Repetitive movements like running, squatting, or climbing stairs can strain the tissues and cause mild inflammation. This swelling often improves with rest and simple care.
- Minor injuries like small sprains, strains, or bumps can also lead to fluid buildup. Even if pain is minimal, the body may still respond by sending extra fluid to protect the area.
- Prolonged standing or kneeling puts pressure on the knee joint and can cause temporary swelling, especially for people who work on their feet for long hours.
- Environmental factors such as heat or humidity can make swelling more noticeable. These conditions affect circulation and can increase fluid retention in the knees.
These non-disease causes often improve with simple care and usually do not require medical treatment unless swelling persists or worsens.
How It Causes the Symptom
A swollen knee occurs when excess fluid collects inside or around the joint. This fluid can build up in the joint capsule or surrounding tissues, leading to visible swelling. Injuries like ligament tears, meniscus damage, or bone fractures trigger inflammation. The body sends fluid and immune cells to the area to protect and repair tissue, which causes the knee to enlarge.
Health conditions such as osteoarthritis, rheumatoid arthritis, gout, or infections can also cause this reaction. In these cases, inflammation may be ongoing, leading to repeated or long-term swelling. Common processes that lead to swelling include:
- Inflammation: The immune system increases blood flow and fluid in response to damage or irritation.
- Bleeding in the joint: Often from trauma or surgery.
- Synovial fluid overproduction: Caused by irritation of the joint lining.
| Cause Type | How It Triggers Swelling |
|---|---|
| Injury | Tissue damage → inflammation → fluid buildup |
| Arthritis | Ongoing joint irritation → chronic swelling |
| Infection | Immune response → pus or fluid in joint |
| Gout | Crystal deposits → intense inflammation |
When swelling is severe or lasts a long time, a doctor may draw fluid from the knee to check for infection, crystals, or blood. This helps find the exact cause and guide treatment.
Possible Complications
If a swollen knee is not treated, it can lead to problems that affect movement and joint health. Some complications may develop slowly, while others can appear soon after swelling begins. Common potential complications include:
- Joint stiffness – Reduced range of motion due to prolonged swelling.
- Chronic pain – Ongoing discomfort even after the swelling decreases.
- Muscle weakness – Loss of strength from reduced activity.
- Joint damage – Wear and tear from untreated swelling.
- Infection spread – If swelling is caused by an infection, it can spread to nearby tissues.
In cases linked to injury, untreated swelling can cause scar tissue formation, which may limit flexibility. Over time, this can make walking, bending, or climbing stairs more difficult.
When swelling is due to arthritis, long-term inflammation can speed up cartilage breakdown. This may increase the risk of permanent joint changes or deformities. If the cause is an infection, there is a risk of septic arthritis, which can damage the joint quickly.
| Cause of Swelling | Possible Complication |
|---|---|
| Injury | Scar tissue, stiffness |
| Arthritis | Cartilage loss, deformity |
| Infection | Septic arthritis, tissue damage |
Some people may also develop blood clots in the leg if swelling limits movement for long periods. This risk is higher after surgery or serious injury.
When to Seek Medical Attention
You can sometimes manage a swollen knee at home, but certain symptoms need prompt care. Get help right away if the knee becomes severely swollen, red, and warm to the touch. These signs may point to an infection that needs urgent attention. See a doctor immediately if:
- A serious injury causes the swelling.
- You cannot bear weight on the leg.
- You cannot fully straighten or bend the knee.
- You notice a sudden change in skin color around the joint.
If swelling comes with severe pain or a visible deformity, you might have a fracture or ligament tear. Doctors often use imaging tests and may refer you to a specialist for these injuries. If swelling lasts more than a few days, even without severe pain, see a doctor.
Early evaluation can help find out if you have arthritis, gout, or a Baker’s cyst. Swelling with fever, chills, or feeling generally unwell should be treated as urgent, since these symptoms may mean a joint infection. If you are unsure, visit a healthcare provider who can check your knee and recommend the best next steps.
