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A Fluid-Filled Sac on the Front of the Knee

author image Jennifer Williams
Jennifer Williams has been writing as a freelancer for local newspapers since 1999. Her work now appears on various websites. She did a five-year orthopaedic surgery residency, followed by a one-year sports medicine fellowship and has been a team physician for NCAA Division I universities and high school teams. As a former collegiate athlete, Williams continues competition at the masters level.
A Fluid-Filled Sac on the Front of the Knee
A doctor is looking at a patients knee. Photo Credit KatarzynaBialasiewicz/iStock/Getty Images

Housemaid's knee, preacher's knee and bursitis are all terms for swelling on the front of the knee, a condition that has been documented for centuries. Bursitis is the most common cause of a fluid-filled sac on the front of the knee, and it can occur for several reasons.


Bursae are fluid-filled sacs that cushion bones and tendons. Of the 11 bursae, only four typically become inflamed. The most commonly affected bursa is the prepatallar bursa, which is under the knee between the patellar tendon and the skin. The next-most-common bursa to become inflamed is the infrapatellar bursa under the patellar tendon.

Bursitis and Infection

Inflammation resulting in excess fluid in the bursa is referred to as bursitis. When prolonged pressure is applied to the bursa through kneeling, fluid may gradually accumulate. Occupations that require kneeling include roofing, carpet laying, mining, plumbing and gardening. Knee trauma is also associated with acute bursitis, as seen in wrestlers and football players. Certain diseases, including rheumatoid arthritis, gout and pseudogout, can also cause bursitis. Bursitis usually involves redness, tenderness and warmth over the bursa.


An infected bursa, much like an inflamed bursa, is tender, warm and swelling. Bursitis must be differentiated from an infected bursa, which requires prompt medical attention to prevent infection from spreading into the joint. Children are more likely to have an infected bursa than bursitis. Fever is an indication that a bursa may be infected.

Other Causes

Other causes of a fluid sac on the knee include excess fluid in the joint. As fluid accumulates the capsule around the knee joint bulges on the sides of the patellar tendon. The swollen capsule is also prominent around the kneecap. Due to pressure, a small, fluid-filled sac, or cyst, can arise from from a weak spot in the joint capsule. Cysts usually occur on the back of the knee, but they may also occur in the front. Fluid trapped between superficial and deep layers of the skin may also cause a blister to develop.

Treatment and Prevention

The initial treatment for bursitis is avoiding activity that aggravates the knee. Ice, elevation and use of nonsteroidal anti-inflammatory medicine may also alleviate symptoms. A physician may remove some fluid from your knee with a sterile needle and syringe to diagnose gout or infection. Injection of a corticosteroid may also be used to treat bursitis. If bursa fluid is infected, the bursa is opened, and antibiotics are started. When bursitis is chronic or recurrent, the bursa may need to be surgically removed.

Bursitis is less likely to develop when frequent breaks are taken from repetitive kneeling. Kneepads or a compressive sleeve are often recommended, as are ice and elevation after working or playing sports. Physical therapy may decrease the chance of bursitis recurring.

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