A joint effusion is the presence of increased intra-articular fluid.It may affect any joint. Commonly it involves the knee.
There are many causes of joint effusion. It may result from trauma, inflammation, hematologic conditions, or infections.
Septic arthritis
Septic arthritis is the purulent invasion of a joint by an infectious agent with a resultant large effusion due to inflammation.
Gout
Gout is usually present with recurrent attacks of acute inflammatory arthritis (red, tender, hot, swollen joint). It is caused by elevated levels of uric acid in the blood that crystallizes and deposits in joints, tendons, and surrounding tissues. Gout affects 1% of individuals in Western populations at some point in their lives.
Trauma
Trauma from ligamentous, osseous or meniscal injuries can result in an effusion. These are often hemarthrosis or bloody effusions