Knee swelling typically happens when the amount of fluid in the tissues around a joint increases. Stiffness and pain or even both might also occur along with the swelling of the joints. The affected area may seem bigger or might form an irregular shape. This is a usual reaction of your body with various kinds of injuries, infections and arthritis.
Swelling in a knee joint may limit knee flexibility and function. For example, a person may find it difficult to fully bend or completely straighten a swollen knee, and the joint may naturally bend 15° to 25° while the leg is at rest.1 The swollen knee may also be painful, red, and/or difficult to put weight on.
Depending on the underlying condition, a swollen knee may be treated at home using the R.I.C.E. formula (rest, ice, compression, and elevation) or may require medical treatment. A doctor can make an accurate diagnosis
12 Potential Causes of Knee Swelling
Whether water on the knee is mildly annoying or painfully debilitating, a person will want to identify the likely cause and treat the symptoms to help reduce the likelihood of future problems. Chronic or long-standing swelling may lead to joint tissue damage, cartilage degradation, and bone softening, therefore treatment is usually recommended.
This article describes 12 conditions that frequently cause knee swelling, also called water on the knee.
Injury to the knee
Trauma to the knee’s bones, ligaments, tendons, bursae, meniscus, or articular cartilage can cause pain and swelling. Serious injury can cause blood to flood into the knee joint, leading to significant swelling, warmth, stiffness, and bruising. This condition is called hemarthrosis and warrants urgent medical care.
A patient should also seek medical attention if knee pain is severe, if the affected leg cannot bear weight, or if there is suspicion of a broken bone.
Degeneration of the cartilage of the knee joint can result in an overproduction of joint fluid, causing the knee to swell. A swollen knee due to knee osteoarthritis is typically accompanied by pain.
In fact, evidence suggests people who have severe knee pain from osteoarthritis are more likely to experience knee swelling. One study2 found that:
- People who reported mild to moderate osteoarthritic knee pain had an average of 7.0 ml of joint fluid in the affected knee—which is about the same as a healthy knee.3
- People who reported severe osteoarthritic knee pain had an average of more than 20 ml (about 4 teaspoons) of fluid in the affected knee.
Knee swelling caused by osteoarthritis is typically mild to moderate. When swelling is severe, another problem may be the cause.
There are numerous home treatments for knee osteoarthritis. A doctor can also prescribe medical treatments, such as physical therapy, and perform therapeutic injections.
Bursitis (non-septic or septic)
Throughout the body are tiny, thin, fluid-filled sacs called bursae. These slippery cushions reduce friction between bone and surrounding soft tissue, such as skin and muscle. The knee has 11 bursae, and 2 of them are especially susceptible to bursitis.
When a bursa becomes inflamed it is called bursitis. An inflamed knee bursa can fill with excess fluid, causing Knee swelling, or water on the knee. The swollen knee may feel “squishy,” like a water balloon. It may or may not be tender and painful. The most common types of knee bursitis are prepatellar bursitis and pes anserine bursitis.
While most cases of bursitis can be treated with at-home and non-urgent medical care, septic bursitis can be life-threatening. This condition occurs when a bursa has been infected with a microorganism. The bursa can become inflamed and fill with pus. The swollen knee may appear red and feel hot, and the person may have a fever and/or feel unwell.
A painful accumulation of microscopic uric acid crystals in the joint defines a gout attack. This Knee swelling often occurs at night, while lying in bed. It typically occurs rapidly and is accompanied by sudden, excruciating pain, redness, and warmth.
While gout is most likely to affect the big toe, it also commonly affects the knee as well as the heel, ankle, and foot instep.4 Gout typically affects just one joint at a time. First-time gout attacks typically occur in men in their 30s, 40s, and 50s.
While an episode of gout usually resolves on its own, medication and medical treatment can help relieve the pain and other symptoms. Many people can lower their chances of future gout attacks—and the joint damage they can cause—by making dietary changes. Oral medications may also be prescribed.
Less common but similar to gout, pseudogout is also caused by an accumulation of microscopic crystals in a joint. Calcium pyrophosphate crystals can accumulate in a knee joint and cause sudden, severe pain, and Knee swelling. The skin over the affected joint may appear discolored.
Pseudogout occurs most frequently in the knee and can also affect the shoulder, elbow, ankle, wrist, large knuckles, hip, or spine. It may affect more than one joint at a time and may be mistaken for rheumatoid arthritis or osteoarthritis.5 Doctors often refer to pseudogout as calcium pyrophosphate deposition (CPPD) or acute calcium pyrophosphate crystal arthritis (acute CPP crystal arthritis).
Old age is the most significant risk factor for pseudogout. An episode of pseudogout usually goes away in a couple of weeks.5 Medication and medical treatment can help relieve the pain.
An autoimmune disease that affects the delicate lining of the joints, rheumatoid arthritis can cause knee swelling, stiffness, pain, tenderness, and redness. The knee may feel “spongy” when pressed.
Anterior view of the upper body showing range of motion in the shoulder.Watch Rheumatoid Arthritis Videos
Although the knees can exhibit symptoms, the hands, wrists, and feet are more often affected by rheumatoid arthritis. RA tends to affect joints symmetrically, so if the right knee is affected the left knee may also be affected.
In addition to joint pain and Knee swelling, rheumatoid arthritis can cause other symptoms, such as fatigue, low-grade fever, and a general feeling of being unwell. Prompt diagnosis and treatment of rheumatoid arthritis can help minimize long-term joint damage.
Knee Swelling at the back of the knee may indicate a Baker’s cyst. The cyst may have no other symptoms or may be accompanied by pain and stiffness. The pain may get worse during certain activities, such as straightening or bending the knee or after standing for a long time.
A Baker’s cyst occurs when a bursa located at the back of the knee, called the popliteal bursa, fills with excess fluid. Another name for a Baker’s cyst is a popliteal cyst. While this condition may resolve on its own with home treatments, some people may want to go to a doctor to have the fluid drained.
Juvenile rheumatoid arthritis
Aching, swollen joints may cause a child to limp or seem clumsy and could be a sign of juvenile rheumatoid arthritis. The aching pain may be most noticeable in the morning, when joints are stiff. Juvenile rheumatoid arthritis often affects big joints, including the knee, and may affect more than one joint at a time.
Children with juvenile rheumatoid arthritis may also experience a fever or rash. Caregivers should contact a doctor if a child’s symptoms persist for a week or more.
Most common in active tweens and teens, Osgood-Schlatter disease is an inflammation of the patellar tendon in the knee. This condition can cause the knee to become swollen and painful. In addition, the very top of the shinbone (at the bottom of the knee) may be pronounced and the skin over this area may be tender to touch. The child may report tightness at the front or back of the thigh.
The diagnostic process may include getting an X-ray. After diagnosis, this condition can usually be treated at home and will resolve as the child grows.
Bacteria or other microorganisms can penetrate the delicate lining that surrounds the knee joint, infecting the joint and potentially causing it to fill with pus. Knee swelling accompanied by intense knee pain and fever can be a sign of septic arthritis. Symptoms may develop quickly, over hours or days.
If the underlying infection spreads to the bloodstream, it can be life-threatening. Patients should seek medical attention immediately if they suspect symptoms are caused by septic arthritis.
Reactive arthritis (including Reiter’s syndrome)
Certain types of bacterial infections (e.g. Chlamydia and gastrointestinal infections) can spur an inflammatory immune response in the body that may cause pain and Knee swelling in joints.
Sometimes reactive arthritis symptoms resolve on their own or with over-the-counter anti-inflammatory medications; however, serious or chronic cases require medical attention.
While relatively uncommon, a benign or malignant tumor can cause a swollen knee. The Knee swelling may be accompanied by pain that feels achy and dull. The pain may be more noticeable at night or when starting to exercise or increase activity. A tumor may also be accompanied by fever, weight loss, and night sweats.
Treatment for Knee Swelling
The treatment of Knee swelling depends on the cause of this symptom:
In case of an injury, you can try out various home remedies that can help in reducing the Knee swelling.
- Apply cold or ice pack on the affected area for about 10 minutes to relief the swelling.
- Use a wrap or elastic bandage to compress the joint.
- When resting, elevate the affected area.
- Take over the counter medication for pain and Knee swelling.
It is recommended that you avoid putting or moving weight on the affected area for a certain period of time. Try doing a few simple exercises for the affected area to ensure it doesn’t get immobilize as it can deteriorate the motion range and muscle strength of the joint. However, it is also important that you give your injury time to heal.
In case of a chronic condition, like lupus or osteoarthritis, it is best to consult your doctor for a treatment plan. They will prescribe therapy, medications and other treatments to help you in relieving the symptoms and regain your health.
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