Causes

People who spend a lot of time on their knees often experience swelling in the front of the knee. The constant friction irritates a small lubricating sac (bursa) located just in front of the patella. Prepatellar bursitis can be acute (sudden onset) or chronic (ongoing). In acute bursitis, a direct blow or fall on the knee tears blood vessels in the bursa. The torn vessels bleed into the bursa sac, causing swelling and triggering an inflammation reaction in the walls of the bursa. The walls may thicken, causing tenderness that remains even after the body reabsorbs the blood. Acute bursitis can also be triggered by an infection when bacteria from an injury, such as a skin wound over the kneecap, spread into the bursa causing it to fill with pus.

In chronic bursitis, previous episodes of acute bursitis or repeated injuries from overuse irritates and thickens the bursa over time. Attacks of chronic bursitis may last a few days to several weeks, and may come back frequently.

The following people are at risk for prepatellar bursitis:

• People who constantly kneel to work, such as plumbers, roofers, carpet layers, coal miners or gardeners.
• Athletes who participate in sports where direct blows or falls on the knee are common, such as football, wrestling or basketball.
• Someone who has been in a motor vehicle collision.
• People with rheumatoid arthritis or gout.