It has many common causes, including arthritis, injury to the ligaments or meniscus, or fluid collecting in the bursa, a condition known as prepatellar bursitis.

[citation needed] When the knee joint contains excess fluid, it may become difficult or painful to bend or straighten.

Diagnostic tests include: Also known as arthrocentesis, this procedure includes withdrawal of fluid from inside the knee for analysis such as cell count, culture for bacteria, and examination for crystals, such as uric acid or calcium pyrophosphate dihydrate ram crystals found in gout or pseudogout.

Magnetic Resonance Imaging detects abnormalities of the bone or knee joint, such as a tear in the ligaments, tendons or cartilage.

If blood tests reveal Lyme disease antibodies forming, the condition may be attributed to it.

General measures such as rest, ice, and analgesics such as acetaminophen (paracetamol) and NSAIDS are often recommended.