Kneecap or prepatellar bursitis is inflammation of a bursa located near the knee joint. A bursa is a small fluid-filled, pad-like sac that reduces friction and cushions pressure points between the bones and the tendons and muscles near joints. Bursitis occurs when the bursa becomes irritated and produces too much fluid, which causes it to swell and put pressure on the adjacent parts of the knee. The inflammation of a bursa leads to an increase in synovial fluid production as well.
The knee bursa is located between the front of the kneecap (called the patella) and the overlying skin. This bursa allows the kneecap to slide freely underneath the skin as we bend and straighten our knees. Knee bursitis can occur in:
- People who constantly kneel to work, such as plumbers, roofers, carpet layers, coal miners or gardeners
- Athletes who participate in sports in which 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
Causes
Bursitis of the knee usually arises when there is excessive pressure and friction on the bursa. The prepatellar bursa can become irritated and inflamed in a number of ways:
- Bursitis is usually caused by either repetitive friction on the area from movements such as jumping/kicking or muscle tightness, or from a sudden injury to the knee.
- A direct blow or a fall onto the knee can damage the bursa. This usually causes bleeding into the bursa sac, because the blood vessels in the tissues that make up the bursa are damaged and torn. In the skin, this would simply form a bruise, but in a bursa blood may actually fill the bursa sac. This causes the bursa to swell up like a rubber balloon filled with water.
- The blood in the bursa is thought to cause an inflammatory reaction. The walls of the bursa may thicken and remain thickened and tender even after the blood has been absorbed by the body. This thickening and swelling of the bursa is referred to as prepatellar bursitis.
- Prepatellar bursitis can also occur over a longer period of time. People who work on their knees, such as carpet layers and plumbers, can repeatedly injure the bursa. This repeated injury can lead to irritation and thickening of the bursa over time. The chronic irritation leads to prepatellar bursitis in the end.
- The prepatellar bursa can also become infected. This may occur without any warning, or it may be caused by a small injury and infection of the skin over the bursa that spreads down into the bursa. In this case, instead of blood or inflammatory fluid in the bursa, pus fills it. The area around the bursa becomes hot, red and very tender.
- Gout causes high levels of uric acid in the blood. Uric acid deposits can spread into the bursa.
- A difference in the length of the legs.
This can occur during surgery as the physician retracts soft tissues as part of the procedure. This can also break intercellular bridges and release substances that promote blood clotting.
Symptoms
- Pain with activity, but not usually at night
- Rapid swelling on the front of kneecap
- Tender and warm to the touch