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What Causes a Swollen Knee (Water on the Knee)?

Knees are complicated structures vulnerable to pain and swelling as they bear body weight. A buildup of fluid in or around the knee joint can lead to swelling, pain, and other symptoms. Knowing the possible causes of a swollen knee can help you prevent it from worsening and seek the best treatment for long-term relief. Consult Dr. Eric Freeman if you notice anything abnormal about your knee or experience pain during movement. The specialist at Redefine Healthcare uses advanced diagnostic methods to determine your condition and provides the highest quality, comprehensive pain treatment to ease your discomfort. He addresses the source of pain and ensures complete healing to help you live a fuller and better quality of life.

Knee swelling is often referred to as knee effusion or water on the knee. It may result from an injury, chronic use of the joint, or a disease. Swelling in the knee joint is not normal, and regardless of its causes, it can limit knee flexibility and function. It can also be painful, and make it difficult to walk and stand easily.

Potential Causes of Knee Swelling

Whether the swelling on the knee is mildly annoying or debilitating, it is essential to identify the likely causes to treat the symptoms and reduce the likelihood of further problems. Chronic or long-lasting swelling or water on the knee may lead to joint tissue damage, cartilage degradation, and softening of the bones, which can affect your ability to stand, walk and move easily.

Explained here are 12 conditions that are known for causing a swollen knee or accumulating water on the knee.

Knee Injury

Trauma to the knee bones, ligaments, tendons, bursae, meniscus, or articular cartilage can result in pain and swelling in the knee. Serious injury can increase the rush of blood into the knee joint leading to noticeable swelling, warmth, stiffness, and bruising. This condition is called hemarthrosis and needs immediate medical attention to prevent further complications.

Severe knee pain, inability to bear weight on the leg, or suspicion of a broken bone should not be ignored as they can lead to further damage and injury.

Knee Osteoarthritis

Degeneration or breakdown of the cartilage of the knee joint and overproduction of the joint fluid can lead to swelling in the knee. Swelling in the knee due to knee osteoarthritis results in pain. Healthcare experts have found that people who have severe knee pain from osteoarthritis are more likely to experience knee swelling.

Knee swelling caused by osteoarthritis is typically mild to moderate. Too much swelling may be resulting from some other problem. People who reported mild to moderate osteoarthritis knee pain had an average of 7.0 ml of joint fluid in the affected knee, about the same as a healthy knee. People with severe osteoarthritis knee pain had an average of more than 20 ml of fluid.

Knee Osteoarthritis


The human body contains tiny, thin, fluid-filled sacs called bursae. These slippery cushion-like sacs reduce friction between bone surrounding soft tissue such as skin and muscle. The knee has 11 bursae, and 2 of them are especially vulnerable to bursitis. When these bursae in the knee become inflamed or irritated, it leads to bursitis.

An inflamed knee bursa can fill with excess fluid, causing swelling or water in the knee. The swollen knee may feel squishy, just like a water balloon. There may or may not be any tenderness or pain. The most common types of knee bursitis result from frequent kneeling or blunt force trauma to the knee.

Most cases of bursitis are not serious and can be treated with home treatment and care, but septic bursitis can be life-threatening. This condition occurs if a bursa has been infected with a microorganism. The bursa can get inflamed and filled with pus, and the swollen knee may appear red and feel hot with other symptoms, like fever.


Higher levels of uric acid in the blood can lead to gout. If you produce too much uric acid and your kidneys cannot filter it right, it can cause an accumulation of microscopic uric acid crystals to form in and around the joints. The swelling often occurs at night while you are in bed. The condition develops quickly and is accompanied by sudden, excruciating pain, redness, and warmth in the affected joint.

Gout usually affects one joint at a time, and starting from the toe, it can move on to the heel, ankle, and foot instep if it is not treated timely. Most people experience their first gout attack in their 30s, 40s, and 50s.


It is a form of arthritis characterized by sudden, painful swelling in one or more joints. The episodes can last for days or weeks. It is less common but similar to gout, caused by the accumulation of microscopic crystals in a joint. Calcium pyrophosphate crystals accumulate in the knee joint and cause sudden, severe pain and swelling. The skin over the affected joint may appear discolored.

Pseudogout occurs more frequently in the knee, but, it can also affect the shoulder, elbow, ankle, large knuckles, hip, or spine. It is also known as calcium pyrophosphate deposition disease (CPPD), or acute calcium pyrophosphate crystal arthritis. Risk for pseudogout increases in old age.

Rheumatoid Arthritis

Rheumatoid arthritis is an autoimmune disease that affects the delicate lining of the joints that results in knee swelling, stiffness, pain, tenderness, and redness. The knee may also feel spongy when you press it.

RA tends to affect joints symmetrically, so if the right knee is affected, the left knee may also be affected. The severity of pain varies from mild to severe. If left untreated, chronic inflammation can damage healthy bones and cartilage in the knee. Along with joint pain and swelling, rheumatoid arthritis causes other symptoms, including fatigue, low-grade fever, and a general feeling of unwellness.

Baker’s Cyst

A baker’s cyst is a fluid-filled sac or lump that forms behind the knee. It causes a bulge and a feeling of tightness. This condition can result from a knee injury or an arthritis-like condition. A baker’s cyst occurs when a bursa located at the back of the knee, also known as the popliteal bursa, gets filled with excess fluid. A popliteal cyst is another name for baker’s cysts.

You may experience mild pain and discomfort or feel severe pain. The pain can worsen with activities, such as bending the knee or straightening it fully. This condition may resolve on its own, but in some cases, you may want to visit a doctor to have the fluid drained for relief.

Juvenile Rheumatoid Arthritis

Also known as juvenile idiopathic arthritis, this condition is a form of arthritis in children. It is the most common type of arthritis in children that causes joint inflammation, swelling, and stiffness. It may affect one or more big joints, including the knee, for at least 6 weeks in a child of 16 years and younger at a time.

Adult rheumatoid arthritis, which is ongoing or chronic, lasts a lifetime, and children often outgrow it, but the disease can affect bone development in a growing child. Aching and swollen joints may cause a child to limp or walk clumsily. Children with juvenile rheumatoid arthritis may also experience a fever or rash.

Osgood-Schlatter Disease

It is a condition that causes pain and swelling when a tendon in the knee, known as the patellar tendon, pulls against the top of the shinbone. The very top of the shinbone at the bottom of the knee may be pronounced. The area stretched over the kneecap becomes swollen and painful, and it may be tender to touch too.

Sometimes called jumper’s knee, it usually happens to adolescents and is the most common cause of knee pain in children and teenagers. Tightness at the front or back of the thigh is also reported.

Septic Arthritis

Septic arthritis is a painful infection in the joint fluid and joint tissues. Different types of bacteria, viruses, and fungi can penetrate the delicate lining, which surrounds the knee joint and infect it, filling it with pus. Septic arthritis can also occur if a penetrating injury, such as an animal bite or trauma delivers the germs directly into the joint. It can lead to swelling in the knee accompanied by intense pain and fever.

Symptoms may develop quickly, over hours or days, and result in extreme discomfort and difficulty using the affected joint. Septic arthritis is a serious condition, and if the underlying infection spreads to the bloodstream, it can be life-threatening.

Reactive Arthritis

Formerly known as Reiter’s syndrome, reactive arthritis is joint pain and swelling triggered by an infection in another part of the body, most often in the intestines, genitals, and urinary tract. The infection leads to an inflammatory immune response in the body that may cause pain and swelling in the joints.

This condition usually targets the knees, ankles, and feet. Inflammation can also affect the skin, eyes, and the urethra, the tube that carries urine out of the body. Chronic cases of reactive arthritis require medical attention.


In some cases, it may be a benign or malignant tumor that can cause a swollen knee. A tumor that occurs near or in a joint may lead to inflammation, stiffness, and tenderness, leading to a limited and painful range of movement.

Sometimes the swelling may be accompanied by pain that feels dull and achy. This pain may be more noticeable at night or when you exercise, or with an increase in activity. A tumor is also accompanied by fever, weight loss, and night sweats. If a bone with a tumor breaks or fractures a leg, it can lead to pronounced limping.

Getting Your Swollen Knee Diagnosed

Chronic swelling in the knee can lead to permanent damage to the joint tissue, cartilage, and bone. If the swelling does not go down or the pain does not ease, it may be a sign of something serious. Thus, it is essential to have your condition properly diagnosed and treated.

The doctor will examine your knee and joint carefully to look for signs of injury. They may ask for blood work and imaging tests, including x-ray, ultrasound, and MRI, to determine the exact causes behind your pain and other symptoms.

Joint Aspiration

The doctor may also arrange for you to have joint aspiration. It is a procedure in which fluid is removed from the space around the swollen joint capsule or bursa using a fine needle and syringe. Aspirating a joint can help to temporarily relieve discomfort and swelling in the knee as it drains the fluids.

This procedure also helps analyze the fluid from the knee to diagnose the reason behind knee swelling and the joint disorder causing it. Based on the test reports, the doctor can suggest the best course of treatment.

Here are some doctor’s recommended guidelines to help you determine if your knee swelling requires urgent medical attention. Consult your doctor in case of the following:

  • The knee is very swollen or has a pronounced abnormality.
  • You cannot fully straighten or bend the knee.
  • There is severe pain that does not go away with over-the-counter medication.
  • You cannot put any weight on the knee, or it feels as if the knee is going to give out.
  • The skin over the knee is hot or red.
  • The pain is accompanied by a high fever.
  • The swelling is present for 3 days or longer.

Call your doctor if the knee pain and swelling do not improve with rest. Swelling accompanied by sharp pain, redness in the calf, or a feeling of water running down the calf can be signs of a blood clot or other serious medical conditions.

You should visit the emergency room after a sudden injury, such as blunt force trauma. An open wound or pus in the knee is a sign of an infection and should not be ignored.

Swelling in the knee accompanied by pain, tenderness, and other unusual symptoms needs proper diagnosis as it could be resulting from some underlying condition or infection that may turn serious. Visit Redefine Healthcare to seek specialist help regarding your symptoms and what may be causing them. Dr. Eric Freeman is an experienced pain relief specialist and comes up with treatment options that align with your health goals. He understands the pain and limited range of motion caused by knee and joint issues and recommends advanced, noninvasive procedures that ensure you get back to a normal routine with minimum downtime.

Page Updated on Dec 12, 2022 by Dr. Freeman (Pain Management Specialist) of Redefine Healthcare


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Dr. Eric D. Freeman

Dr. Eric D. Freeman is a top-rated, best-in-class pain management doctor. He is a nationally recognized pain relief specialist and is among the top pain care doctors in New Jersey and the country. He is an award-winning expert and contributor to prominent media outlets.

Dr. Eric D. Freeman has been recognized for his thoughtful, thorough, modern approach to treating chronic pain. He has been named a “top pain management doctor in New Jersey” and one of “America’s Top Physicians” for advanced sports injury treatments.

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