Inflammation and swelling of a movable joint because of excess amount of synovial fluid or lubricating fluid: When Hank went to the doctor to see what was causing so much pain in his knee, he was told that he had hydrarthros or "water on the knee" also known as an excessive build-up of "water in the cavity".

1. An effusion of watery liquid into the cavity of a joint: Dr. McMahon explained that typically there is an amount of hydrarthrosis in the articulating connections between the bones consisting of fluid which lubricates them.
2. Inflammation and swelling of a movable joint because of excess synovial fluid: When a person is suffering with hydrarthrosis, it is a result of a painful condition such as "water on the knee" which may require surgery to drain.