Commonly known as "water on the knee," ankle joint effusion is a fluid-filled swelling of tissue around the ankle joint. Effusions are the result of some sort of infection, trauma or injury.
According to the Mayo Clinic, "effusion" is a general medical term that describes an accumulation of excess fluid around a joint, such as the ankle or the knee. The effusion is not itself a description of the problem, but it is a symptom of an underlying issue. For example, traumatic injuries such as broken bones or ligament tears can lead to water on the knee. Gout, tumors and rheumatoid arthritis are additional conditions that might result in fluid buildup around joints, according to Mayo Clinic.