From Jonathan Cluett, M.D.,
Your Guide to Orthopedics.
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About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD

What is a bursa?
Every person has hundreds of bursa scattered throughout the body. The function of a bursa is to decrease friction between two surfaces that move in different directions.

• The bursa can be thought of as a Ziplock bag with a small amount of oil and no air inside. Imagine rubbing this bag between your hands; movement of your hands would be smooth and effortless. That is what a bursa functions as--a smooth, slippery surface between two moving objects.

You tend to find a bursa at points where muscles and tendons glide over bones. Without the bursa between these surfaces, movements would be painful.

What is bursitis?
Bursitis is the inflammation of a bursa. Normally, the bursa provides a slippery surface that has almost no friction. A problem arises when a bursa becomes inflamed. The bursa loses its gliding capabilities, and becomes more and more irritated when it is moved.

When the condition called bursitis occurs, the normally slippery bursa becomes swollen and inflamed. The added bulk of the swollen bursa causes more friction within an already confined space. Also, the smooth gliding bursa becomes gritty and rough. Movement of an inflamed bursa is painful and irritating.

What causes bursitis?
Bursitis usually results from a repetitive movement or due to prolonged and excessive pressure. Patients who rest on their elbows for long periods or those who bend their elbows frequently and repetitively (for example, a custodian using a vacuum for hours at a time) can develop elbow bursitis, also called olecranon bursitis. Similarly in other parts of the body, repetitive use or frequent pressure can irritate a bursa and cause inflammation.

Another cause of bursitis is a traumatic injury. Following trauma, such as a car accident or fall, a patient may develop bursitis. Usually a contusion causes swelling within the bursa. The bursa, which had functioned normally up until that point, now begins to develop inflammation, and bursitis results. Once the bursa is inflamed, normal movements and activities can become painful.

Systemic inflammatory conditions, such as rheumatoid arthritis, may also lead to bursitis. These types of conditions can make patients susceptible to developing bursitis.

Read on for more information about different types of bursitis...