A bursa is a fluid filled sack which allows lubrication and movement of an area of the body. It stops tendons, skin and bones from sticking to each other.
There are about 13 bursae around the knee joint but most do not create clinically relevant symptoms. The are four main bursae near the front of the knee joint, four near the outside and five towards the inside of the knee.
One of the biggest is found between the kneecap and the skin. This is very commonly injured by people who spend a lot of time kneeling. It is often called housemaids knee but the proper name for it is prepatella bursitis. The treatment for this involves immobilizing the area with an extension knee splint and avoiding direct pressure on the area. If the bursa becomes infected the antibiotics and occasionally surgical washout of the bursa with be required. Walking with the leg locked straight is still permitted.
Another commonly inflamed bursa is the pes ansernius bursa. This overlies the insertion of the hamstrings to the medial tibial condyle and rarely gets infected. A hamstring stretching programme and local modality treatment will usually fix the problem.