Bursitis at the knee
A bursa is a thin flattened fluid-containing balloon that acts as a buffer where muscles pass over bony points or it may lie between two muscles pulling in different directions to reduce friction. It can also protect bony prominences where they lie just under the skin.
Bursitis is inflammation of a bursa and it is associated with swelling and pain. Two main bursae may be involved in bursitis at the knee: the prepatellar bursa, the infrapatellar bursa and, less commonly, the pes anserine bursa.