Bursae are thin, lubricated cushions located at points of friction between a bone and the surrounding soft tissue, such as skin, muscles, ligaments and tendons. A bursa lies between a bone and opposing surface (e.g. skin) like a tiny water balloon with only a few drops of fluid in it, wedged between two surfaces.
Key characteristics of bursae include:
- A healthy bursa is thin. For example, a research study of a relatively large bursa located between the kneecap and skin showed the average bursa measurement to be about four centimeters in diameter and just a few millimeters thick.1
- Bursae vary in size depending on the individual and location in the body.
- Some bursae are just beneath the skin’s surface while others are deep below muscles and other soft tissue.
- An adult body contains about 160 bursae.
- Some bursae are present at birth and others develop later as the result of frequent friction. For example, most people develop a bursa in the elbow called the olecranon bursa sometime after age 7.2
- Other bursae may develop depending on the individual. For example, a person who regularly wears constricting shoes or has abnormal foot anatomy may develop a bursa on the outside of the big toe joint.
A bursa sac is made up of a synovial membrane, or synovium, that produces and contains synovial fluid.
Synovial fluid is viscous and often compared to an egg white in looks and texture. Its primary job is to provide cushion and lubrication at and near joints. Most of the body's joints are called synovial joints, and are encapsulated with a synovial membrane containing synovial fluid.
The thin synovial membrane is semi-permeable - meaning certain materials can flow in and out of it. For example, an injury can cause a bursa to temporarily fill with blood.
Bursa and Bursitis
Bursitis occurs when the synovial membrane of a bursa becomes inflamed. The inflamed synovium will thicken and produce excess synovial fluid, causing the bursa to swell. Inflammation can be caused by excessive friction, an injury, or an underlying condition such as rheumatoid arthritis. Inflammation can also be caused by infection. This is called septic bursitis and can eventually cause the bursa to fill with pus.
Bursitis is most common in the shoulder, elbow, knee and hip but can also be present in the ankle, big toe, buttock and wrist. The hallmark symptom of bursitis is localized swelling at the joint (although hip bursitis is an exception to this).
Distinguishing between aseptic and septic bursitis can be difficult. A fever, localized skin warmth and redness, tenderness and joint pain, may be signs of septic bursitis, and medical treatment should be sought.