Fluid on the knee might form as a result of an injury or a medical condition such as arthritis. Treatment may include physical therapy, medical procedures, and medication, depending on the cause.

Each of us has a small quantity of fluid in our joints. It is our joint lubrication system that reduces friction and promotes smooth joint rotation.

Joint effusion occurs when excess fluid accumulates around a joint. When it happens in the knee, it's known as swollen knee or water on the knee. It can be caused by an accident, an infection, or a medical condition.

Read on to find out what causes water on the knee, and what you can do about it.

Signs and symptoms

Water on the knee almost always affects only one knee. You may sense heaviness in the joint, and it will appear puffier when compared to the opposite knee.

Other signs and symptoms are:

• swelling and redness of the skin around your kneecap; 
• joint stiffness and trouble straightening or bending your leg; 
• knee pain and sensitivity, particularly when you bear weight on it;
• the warmer knee will be the opposing knee;

You should also know, that water on the knee may make it difficult to walk, climb stairs, or perform other daily activities.

Causes of water on the knee

There are several possible causes for a buildup of water on the knee. Among them are:

• joint overuse (repetitive damage)
• a ripped ligament or cartilage (meniscus)
• a  broken bone
• an infection caused by bacteria
• bursitis of the prepatellar region
• rheumatoid arthritis or osteoarthritis
• gout, also known as pseudogout
• cysts or tumors.

The fluid is often composed of white blood cells and the chemicals they produce, as well as red blood cells and natural lubricants. The body's natural response to a knee injury is to create fluids to protect it. There is sometimes simply too much fluid.

Diagnosis

A sore knee might affect everyone for a few days. Elevation, rest, ice, and over-the-counter pain medicines will often alleviate your symptoms.

Your medical history will be requested by your doctor. Include any previous knee problems as well as physical activity, such as sports, that can put a strain on the knees. They will also perform a full physical examination, including a range of motion test.

Following that, your doctor may wish to remove fluid from the knee (joint aspiration). This is accomplished by introducing a long, thin needle into the joint and extracting fluid. It takes only a few minutes to complete in the doctor's office.

The fluid can be examined for:

• a bacterium that could cause infection
• gout or other disorders may be indicated by crystals, protein, and glucose.
• blood cells that may be injured

Removing some of the fluid may also help to reduce pressure in the system.

Imaging tests such as X-rays, MRIs, CT scans, and ultrasounds may be also required to find the real reason for the condition.

Is it treatable?

The treatment will be determined by the cause of the fluid, which may include:

• Anti-inflammatories and analgesics.
• Antibiotics, if you have an infection.
• Corticosteroids can be either orally or injected directly into the knee joint.
• Joint aspiration is used to reduce pressure momentarily and is occasionally followed by a corticosteroid injection.
• Arthroscopy - the surgery in which a lighted tube is introduced into the knee joint to aid in the repair of a knee injury.
• Physical therapy is used to increase flexibility and strength in the muscles surrounding the joint.

If alternative treatments fail to improve your knee joint, surgical excision of the bursa sac may be required. For the most severe situations, knee replacement surgery is an option.