A knee effusion refers to an accumulate of fluid within the knee joint capsule (swelling of the knee).
A knee effusion can be caused by several factors. Most often when people use the term “knee effusion” they are implying that fluid is within the joint capsule as opposed to infrapatellar, suprapatellar, or prepatellar bursitis which is an accumulation of fluid in several different bursa outside of the knee joint. Some reasons for a knee effusion include osteoarthritic flares, meniscal tears, patellar dislocations, intraarticular knee fractures, and infection.
The most obvious sign of a knee effusion is visible swelling in the knee, sometimes subtle where there is just a loss of the normal muscular and boney contours of the knee. With an effusion, pain is often present. If the effusion is due to infection, you may experience warmth in the knee and systemic fevers.
Procedural treatment will be directed toward the underlying cause of the effusion.