A bursa sac is a thin, fluid-filled sac that sits between joints in the body. Bursae function as a smooth, gliding cushion between bones and opposing surfaces such as muscles, tendons, ligaments, and skin. The major bursae in the body are located adjacent to the tendons near our largest joints, including the hips, knees, shoulders and elbows. Their job is to reduce friction between bones and surrounding soft tissue, facilitating easy movement. You can picture bursae as tiny water balloons or pillows wedged between your joints. Here’s some more information about these helpful little joint cushions.
What Size is a Bursa?
Bursae vary in size depending on where they are located in the body. The healthiest bursae are usually very thin while larger joints have larger bursae. On average, the bursa between the kneecap and skin is just a few millimeters thick and about four inches in diameter.
How Many Bursae are in My Body?
An adult body typically contains about 160 bursae. They can be just underneath your skin, or wedged deep below muscles and tissues. While some bursae are present at birth, others develop later in parts of the body where frequent friction occurs. For example, you have probably only had the bursae in your elbows since you were about 7 years old.
What’s Inside the Sac?
Bursa sacs are made up of a synovial membrane. The membrane contains synovial fluid, which is a thick liquid comparable to an egg white in texture. When the bursa becomes irritated due to excessive friction, it produces more fluid to serve as a cushion.
The synovial membrane is semipermeable, so certain things, such as blood, can flow through it. An injury may cause the membrane to fill with blood temporarily.
What Health Issues are Related to Bursae?
A condition called bursitis occurs when the synovial membrane of a bursa becomes inflamed, usually due to excessive friction or an injury. Sometimes, however, the inflammation can be a symptom of an underlying condition like rheumatoid arthritis. Bursitis is most commonly experienced in the hip, shoulder, knee and elbow.
Septic bursitis causes a bursa to become swollen due to infection. This might cause the bursa to fill with pus. While both forms of bursitis cause localized swelling, septic bursitis may also cause fever, localized skin warmth and redness, and joint pain and tenderness. If you are experiencing symptoms of septic bursitis, seek medical attention.
