Tendons, ligaments, muscles, and skin must glide over bones during joint movement. Tiny, slippery sacs of fluid called bursae facilitate this gliding motion by providing a thin cushion and reducing friction between the surfaces.

Here, patellar bursae facilitate movement between the skin and the bones at the front of the knee joint.

A body has more than 140 bursae. ¹ Azar FM, Beaty JH, Canale TS, eds. Campbell’s Operative Othopaedics, 13th ed., vol 1, page 482. Philadelphia PA: Elsevier; 2017. Each one is like a miniature water balloon with only a few drops of fluid in it, wedged between bone and soft tissue. If a bursa becomes irritated and inflamed, it is called bursitis.

Bursa Membrane and Fluid

A bursal sac is made up of an outer membrane and inner fluid.

The synovial membrane forms a bursa’s enclosed sac. A healthy synovial membrane is very thin, often just a few cells thick. ² Smith MD. The normal synovium. Open Rheumatol J. 2011;5:100–106. doi:10.2174/1874312901105010100 The membrane produces the synovial fluid that is contained it the sac.
The synovial fluid is a viscous, slippery, lubricating fluid. It is often compared to an egg white in appearance and texture.

Synovial membrane and synovial fluid are also present in most of the body’s joints. These joints are called synovial joints.

See What Is a Synovial Joint?

Bursa Facts

illustration of bursa in the knee, comparing it to the size of 2 pennies stacked on top of each other, or 3mm.

A healthy bursa is thin. For example, one of the biggest bursa in the body, the prepatellar bursa in the knee, is just 3 mm thick. That's about the same depth as two US pennies.

Medical experts distinguish between three types of bursa:

Synovial bursae, which are located between bones and muscles, tendons, and ligaments.
Superficial bursa, which are located just below the skin, between skin and bone. Examples of superficial bursae include the patella bursa at the knee and the olecranon bursa at the elbow.
Adventitious or accidental bursae, which develop because of repeated irritation. 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 prominent bursa at this location is called a bunion).

All three types of bursa are susceptible to bursitis.

Other facts about bursae include:

**Infographic:**
See 4 Types of Bursitis Infographic

Healthy bursae are thin. For example, in the shoulder, a healthy bursa is typically less than 1 mm thick. ³ Soker G, Gulek B, Soker E, Kaya O, Inan I, Arslan M, Esen K, Memis D, Yilmaz C. Sonographic assessment of subacromial bursa distension during arm abduction: establishing a threshold value in the diagnosis of subacromial impingement syndrome. J Med Ultrason (2001). 2018 Apr;45(2):287-294. doi: 10.1007/s10396-017-0839-9. Epub 2017 Oct 26. PubMed PMID: 29075914. The patella bursa in the knee is about 3 mm thick. ⁴ Aguiar RO, Viegas FC, Fernandez RY, Trudell D, Haghighi P, Resnick D. The prepatellar bursa: cadaveric investigation of regional anatomy with MRI after sonographically guided bursography. AJR Am J Roentgenol. 2007 Apr;188(4):W355-8. doi: 10.2214/AJR.05.1466. PubMed PMID: 17377004. The exact thickness and width of a bursa depends on the person and its location in the body.
Some bursae are present at birth and others develop later. For example, most people develop a superficial bursa in the elbow, called the olecranon bursa, sometime after age 7. ⁵ Chen J, Alk D, Eventov I, Weintroub S: "Development of the olecranon bursa. An anatomic cadaver study." Acta Orthop Scand 1987; 58(4):408-409. PMID: 3673537
A bursa’s synovial membrane is semi-permeable, meaning certain materials can flow in and out of it. For example, it is possible for blood cells or bacteria to enter the bursal sac.

Most people are unaware of their bursae unless one becomes inflamed, causing uncomfortable symptoms, such as joint swelling.

Bursa and Bursitis

When the synovial membrane of a bursa becomes inflamed, it is called bursitis. The inflamed membrane will thicken. In addition, the membrane will produce excess synovial fluid, causing the bursa to swell.

Inflammation can be caused by an injury, repetitive irritating friction, or an underlying condition such as rheumatoid arthritis.

Bursitis and tendonitis
Bursitis is typically accompanied by inflammation of the tendon overlying the bursa. Sometimes it is difficult to distinguish between a bursitis and tendonitis as they often occur concurrently.

See What Is Bursitis?

Septic bursitis
Inflammation can also be caused by infection. When this happens, it is called septic bursitis. Septic bursitis can eventually cause the bursa to fill with pus. Septic bursitis in a superficial bursa can be caused by infectious bacteria that enter through a cut, scrape or rash on the skin.

See Septic Bursitis

Joints commonly affected by bursitis
Bursitis is common in the hip, shoulder, knee and elbow. It can also develop at the back of the back of the ankle and other joints. The hallmark symptom of bursitis is localized swelling at the joint (hip bursitis is an exception and may not produce visible swelling).

Distinguishing between aseptic and septic bursitis can be difficult. Septic bursitis requires prompt medical treatment.

See Septic Bursitis Treatment