Water on the knee is a kind of vague term that may be used to describe fluid that can build up around the kneecap. This fluid may be water retention but is more commonly fluid produced by the joints, called synovial fluid. Water on the knee can be a painful condition that may limit ability to move the knee fully, to bear weight on the knee or even to walk unhindered, and it has a variety of causes. Depending on cause, swelling, stiffness, and bruising may also be present.

Causes for water on the knee include knee injury, gout, certain infections, some types of arthritis, and occasionally cysts or tumors. Knee injuries that may result in fluid buildup include overuse injuries, blunt injuries, and broken bones. Usually, if you have a significant knee injury you’re likely to notice this before water on the knee occurs.

If you note water on the knee and have a painful or swollen knee you should have this checked out by a physician. Doctors may perform a variety of tests, including taking a small amount of fluid from the knee to figure out the cause. More often, they may use x-rays or other types of imaging to determine root cause, particularly if you have injured your knee recently. Injured knees frequently need medical attention.

Since causes can differ, water on the knee may be treated in a variety of ways. If gout is the cause, gout will be treated. Infection would need antibiotic treatment. Various injuries to the knee could require different steps to help the knee recover. Arthritic conditions may benefit from anti-inflammatory medications. Overuse or serious injury of the knee joint may require surgery to clean out the knee or to replace the knee joint, though knee replacement is usually not a first option.

Given differing causes, water on the knee may get better after a few weeks, or a few months. Some people will have residual swelling around both knees, but a tiny amount of swelling may not cause great problems. When treatment is not resolving the problem and it is still difficult to move around or the knees are always painful, check with your doctor to see if your treatment is adequate.

Water on the knee resulting from injury can sometimes be prevented if you take good care of your knees. Joints do wear out over time, and the knees are particularly vulnerable because they bear our weight. A person who is overweight may wear out knee joints sooner and be more vulnerable to knee injury. Getting plenty of exercise, stretching beforehand, and maintaining a healthy weight through life can help protect these valuable joints.