The most common form of knee bursitis involves inflammation of a bursa at the front of the knee. Localized inflammation and swelling can produce a lime or even grapefruit sized lump under the skin. It has been called housemaid's knee because it is often caused by repeated kneeling. A trauma to the knee or a medical condition, such as gout or rheumatoid arthritis, can also cause knee bursitis.
What Is Prepatellar Bursitis?
Swelling and other bursitis symptoms occur when a bursa becomes inflamed. A bursa is a thin, slippery, fluid-filled sac that serves as both a cushion and lubricant between bone and surrounding soft tissue, such as skin and muscle. When inflamed, a bursa's normally thin lining thickens, and the bursa fills with fluid. In cases of knee bursitis, swelling may or may not be accompanied by other symptoms, such as knee tenderness and pain.
See What Is a Bursa?
An adult body contains about 160 bursae, with 11 typically found around the knee joint. Any bursa in the knee can become inflamed and cause bursitis, but two bursae, called prepatellar bursae, are especially susceptible.
The bursae that typically cause knee bursitis are called prepatellar bursae and are located at the front of the knee:
- The subcutaneous prepatellar bursa, which lies between the skin and the kneecap, and
- The superficial infrapatellar bursa, which lies between the skin and the knobby top of the shinbone (tibia), just below the kneecap.
When one or both prepatellar bursae are inflamed it is called prepatellar bursitis, or knee bursitis.
The pes anserine bursa, located on the inside of the knee, is also prone to bursitis. When this bursa is inflamed it is called pes anserine bursitis.
Watch: Pes Anserine Bursitis Video
Septic Knee Bursitis
Knee bursitis can also be caused by an infection. This is called septic bursitis. In addition to the other knee bursitis symptoms, the patient may show signs of an infection, such as feeling sick, tired, and feverish. The infected bursa may eventually fill with pus. Septic bursitis requires antibiotics to treat the infection and prevent its spread to other points in the leg or bloodstream.