You may develop fluid on the knee due to an injury or a health condition such as arthritis. Treatment can depend on the cause but may include physical therapy, medical procedures, and medication.

We all have a small amount of fluid in our joints. It’s our personal joint lubrication system that cuts down on friction and helps to promote smooth joint rotation.

Joint effusion happens when too much fluid accumulates around a joint. When it happens in the knee, it’s commonly referred to as swollen knee or water on the knee. It can be the result of injury, infection, or a medical condition.

Continue reading to learn more about what causes water on the knee, what you can do about it, and how you might be able to prevent it from happening.

Most of the time, water on the knee will affect only one knee. You may notice a feeling of heaviness in the joint, and it will look puffier when compared with the other knee.

Other signs and symptoms are:

  • swelling and redness of the skin surrounding your kneecap
  • joint stiffness and difficulty straightening or bending your leg
  • pain and tenderness, especially when you put weight on your knee
  • the knee will feel warmer than the opposite knee

Water on the knee can make it harder for you to walk, climb stairs, or engage in other everyday activities.

There are quite a few reasons you might have a buildup of water on the knee. Some of them are:

  • overuse of the joint (repetitive injury)
  • torn ligament or cartilage (meniscus)
  • broken bone
  • bacterial infection
  • prepatellar bursitis
  • osteoarthritis or rheumatoid arthritis
  • gout or pseudogout
  • cysts or tumors

The fluid is usually made up of white blood cells and certain chemicals they release, plus red blood cells and natural lubricants. When you have a knee injury, the body’s natural response is to produce fluids to protect it. Sometimes, there’s simply too much fluid.

Anyone can put excess stress on the knee or experience an unexpected injury. But there are some things that can increase your risk, such as:

  • Age: The likelihood of knee problems, in general, grows as you age.
  • Sports: Certain sports, such as wrestling, basketball, and soccer can strain or twist the knees.
  • Job: Carpet layers, gardeners, roofers, and other professionals that require spending time on their knees have a higher risk of a buildup of fluid in the bursa (prepatellar bursitis).
  • Weight: Surplus weight stresses your knee joints and can lead to degeneration.
  • Health conditions: Osteoarthritis and rheumatoid arthritis can cause knee problems, a risk that is magnified if you’re also overweight.

Anyone can have a sore knee for a few days. In many cases elevation, rest, ice, and over-the-counter pain relievers will improve your symptoms.

Symptoms that warrant a visit to your doctor include:

  • You know you sustained a serious injury to your knee.
  • You can’t bend or straighten the knee.
  • You can’t stand or walk comfortably for more than a few minutes.
  • Your knee is warm to the touch, red, or larger than the other.
  • Knee tenderness and pain isn’t improving.

Your doctor will want to hear about your medical history. Include any previous problems with your knees and physical activities you engage in, like sports, that can stress the knees. They’ll also conduct a thorough physical examination, including checking your range of motion.

Next, your doctor may want to withdraw fluid from the knee (joint aspiration). This is done by inserting a long, thin needle into the joint and pulling fluid out. It can be done right in the doctor’s office in a matter of minutes.

The fluid can be tested for:

  • bacteria that may be causing infection
  • crystals, protein, and glucose that may indicate gout or other conditions
  • blood cells that may indicate injury

Removing some of the fluid may also relieve pressure in your knee.

Imaging tests such as X-ray, MRI, CT scan, and ultrasound may help determine the cause of the problem.

The cause of the fluid will determine the treatment, which may include:

  • anti-inflammatories and pain meds
  • antibiotics if you have an infection
  • oral corticosteroids or those that are injected directly into the knee joint
  • joint aspiration to temporarily relieve pressure, which is sometimes followed by a corticosteroid injection
  • arthroscopy, a procedure in which a lighted tube is inserted into the knee joint to help repair damage in your knee
  • physical therapy to improve flexibility and build strength in the muscles around the joint

If your knee joint doesn’t respond well to other treatment, surgical removal of the bursa sac may be necessary. Knee replacement surgery is an option for the most severe cases.


There are many knee braces on the market. Some are intended to protect your knees from injury when you play certain sports. Some are used to support the knee or prevent the knee from moving following a medical procedure. Others are meant to provide a little relief when you’re hurting.

It’s best to talk to your doctor before purchasing a knee brace. Using the wrong type or using it incorrectly may cause further damage.

Self-care measures should include:

  • Rest your knee when you have pain and swelling and avoid weight-bearing activities. If this is a long-term problem, consider giving up strenuous athletics or repetitive movements to the knees.
  • Use over-the-counter pain relievers or anti-inflammatories as needed.
  • Put your leg up and apply ice to the knee for 15 to 20 minutes every two to four hours. Ideally, you should raise your knee higher than your heart.
  • Maintain a healthy weight to avoid unnecessary stress on your knees.
  • Exercise your legs to help build the supportive muscles around your knees. Try low-impact exercises that don’t stress the knees, such as swimming and water aerobics.

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Your outlook depends on the cause, what treatments are available, and how well your joint responds to treatment. Water on the knee can be temporary due to a minor injury or infection. With treatment, it’s possible that you’ll feel better within weeks.

After a serious injury or joint damage, your knee may improve with treatment only to have fluid build up again. Talk to your doctor about the best ways to approach treatment for the long term, which may include knee replacement surgery. Recovering from joint surgery may take weeks or months.

After reviewing your test results and evaluating treatment options, your doctor will be able to give you some idea of what to expect.

While it can’t always be prevented, there are a few things you can do to lower your chances of developing water on the knee.

For example:

  • Maintain a healthy weight.
  • Keep the muscles around your knee joints strong. Choose low-impact exercises.
  • Wear kneepads if you engage in contact sports or work on your knees.
  • Stop to stretch your legs often during the day.
  • If you believe you’ve injured your knee, see your doctor before the joint sustains further damage.

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