What is knee effusion (water on the knee)? Causes, symptoms, and tests for water on the knee

what is knee effusionKnee effusion (water on the knee) is a condition where fluid accumulates around the knee joint. This is sometimes referred to as having a swollen knee or having “water on the knee.” Normally, our joints have a small amount of fluid surrounding them called synovial fluid. This helps to reduce friction and promote smooth joint rotation. However, there are instances where our joints accumulate extra fluid as a result of injury, infection, or due to a contributing medical condition.

What causes knee effusion (water on the knee)?


The most common form of arthritis that is due to wear and tear of the protective cartilage found on the ends of the bone. Osteoarthritis can affect any joint in your body, but the disorder commonly affects the joints of the hands, knees, hips, and spine. The disorder worsens over time and can cause joint inflammation and swelling, leading to knee effusion.

Rheumatoid arthritis


Chronic inflammation of the joint that results in pain, redness, swelling, and stiffness of the joints. The condition may also cause inflammation of ligaments, muscles, and tendons. Rheumatoid arthritis is a type of autoimmune disease with an unknown origin. A characteristic feature of the condition is inflammatory arthritis, which can present as knee effusion.


A condition caused by the accumulation of uric acid within the body with affected individuals being unable to excrete or excessively produce the substance. Uric acid is a breakdown product of food this is normally excreted in through the kidneys. Accumulation and deposit of uric acid crystals can lead to a type of arthritis pain of the joints, especially found in the big toe. Knee effusion can also be caused by gout.


Acute injuries to the knee, especially during a sporting activity, can cause pain, swelling, and stiffness. There is also a high chance of the tendons, muscles, and ligaments to become injured or be torn altogether. This may lead to conditions such as tendinitis, bursitis, and knee effusion.


Similar to gout, pseudogout presents in much the same way but is the result of the deposition of a different type of substance called calcium pyrophosphate around the joints.
Tumors: Cysts or tumors, if found in the knee, can irritate the surrounding tissue. If found in the knee, it can result in knee effusion.

Psoriatic arthritis

A form of arthritis that affects some people who have psoriasis—a skin condition that features red patches with silvery scales. Joint stiffness, pain, and swelling can affect any part of the body with symptoms ranging from relatively mild to severe.

Lyme disease

Transmitted through deer-tick bites, Lyme disease is primarily a disease that is most commonly reported in the northwestern or upper Midwest regions of the US. The early symptoms of the condition can present as fatigue, chills, fever, headache, swollen lymph nodes, and muscle and joint aches.

Related: Early sign of Lyme disease could be “water on the knee”

What are risk factors and symptoms of knee effusion (water on the knee)?

Risk factors for knee effusion depend on several factors. Generally, putting any excess stress on the knee increases the chances of developing the condition. The following are some common risk factors for the development of knee effusion:

Age: Getting older increases the likelihood of disease as well as your risk for osteoarthritis.

Sports: Wrestling, basketball, soccer, or any other type of sport that can strain or twist your knee can increase the chances of developing knee effusion.

Occupation: If your job requires you to be on your knees for extended periods of time, this can increase the likelihood of fluid buildup in the bursa, leading to a condition called prepatellar bursitis. These types of jobs include carpet layers, gardeners, and roofers.

Obesity: Being of increased weight can put excessive strain on the joints of the knee, leading to degeneration and knee effusion.

Health conditions: These include conditions such as osteoarthritis and rheumatoid arthritis. Both can lead to problems with the knee.

Symptoms of knee effusion may not always be obvious, but generally, the knee will look swollen in size. Precise signs and symptoms often depend on the particular cause of fluid buildup within the knee joint, but typically present as pain, stiffness, and reduced range of motion.

Knee effusion (water on the knee) tests

After noticing that your knee is unusually red, stiff, and painful, seeing a doctor should be your primary objective. Inflammatory material that causes knee effusion can be quite damaging to the knee joint itself and should be taken care of right away.

The first thing your doctor will do is examine the affected joint. Documentation of the presence of pain, warmth, any discoloration, the range of motion, and the size will be taken. Detailed information about how the joint becomes swollen will be noted to get a better idea of its onset.

Preliminary tests using ultrasound or MRI can help give an image of how much fluid accumulation has occurred within the joint and give an accurate diagnosis. Ultrasound, in particular, can be a speedy and convenient choice for a quick diagnosis. An x-ray may also be obtained to rule out potential fractures. Blood tests may also be taken to help diagnose any causes of bacterial infection affecting the body. However, simply knowing that there is fluid surrounding the joint is not enough information to guide treatment. This would require the use of more specific testing.

Arthrocentesis is a diagnostic tool and a therapeutic procedure. It is the most accurate test for diagnosing the cause of knee effusion. The procedure involves using a syringe to collect synovial fluid directly from the affected joint—also known as joint aspiration. Once this fluid is collected, it can be analyzed looking at color, clarity, viscosity, white blood cells, and polymorphonuclear leukocytes (PMN)—a category of white blood cells. Arthrocentesis has the added benefit of being quick and easy while inflicting minimal discomfort on the patient.
The following tables help to guide doctors by providing clues to what the origin of the knee effusion is. Having a low number of WBC would indicate non-infection causes such as trauma while having a high WBC could indicate an infection of some form.

Synovial Fluid Findings

ColorClearYellowYellow to greenYellow
Mucin clotGoodGoodGood to poorPoor
WBC per mm3< 200200 to 2,0002,000 to 150,00015,000 to 200,000
PMNs< 25%< 25%> 50%


There is a multitude of different causes of knee effusion and it is highly recommended to see a doctor right away. Depending on the level of inflammation, knee effusion can begin to degrade the knee joint further if not treated in a timely manner. By seeing your doctor, you can help mitigate any potential long term effects.

Related: What are knee effusion treatments? Home remedies and exercises for water on the knee

Author Bio

Mohan Garikiparithi got his degree in medicine from Osmania University (University of Health Sciences). He practiced clinical medicine for over a decade before he shifted his focus to the field of health communications. During his active practice he served as the head of the Dept. of Microbiology in a diagnostic centre in India. On a three-year communications program in Germany, Mohan developed a keen interest in German Medicine (Homoeopathy), and other alternative systems of medicine. He now advocates treating different medical conditions without the use of traditional drugs. An ardent squash player, Mohan believes in the importance of fitness and wellness.



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