A knee effusion or “water on the knee” is always abnormal. Fluid can be broadly categorized into three categories. If the knee develops swelling and an effusion after a significant trauma, the fluid can often be blood. Blood in the joint is called a hemarthrosis. The cause of a hemarthrosis is an acute ACL tear in up to 75% of cases. With an ACL tear, the fluid often is noticeable within 1-2 hours of the injury. Other causes of a hemarthrosis include fractures, osteochondral lesions, patellar dislocation, and traumatic meniscus tears in the vascular zone of the meniscus.
The second category are the multidude of disorders that cause abnormalities of the synovium, or lining of the joint. These conditions include rheumatoid arthritis, lyme disease, gout, pseudogout, infections, and other inflammatory arthritities ( lupus, psoriasis, etc ).
The third category can be considered as problems of the knee cartilage. There are two types of cartilage in the knee. The joint surfaces of the bones are lined with articular cartilage. This can be compared to the teflon on a frying pan. Defects and breakdown of the articular cartilage can result in a release of caustic and pro-inflammatory enzymes from the cartilage that lead to inflammation and the effusion. Cartilage breakdown can be an isolated defect or can be due to wear and tear. The breakdown of articular cartilage is synonymous with osteoarthritis.
The other type of cartilage is meniscal cartilage. The menisci are cartilagenous gaskets or shock absorbers between the femur and tibia. Tears of the menisci can lead to fluid in the knee.
The exact cause of a knee effusion can be made by gathering information on the patient’s history, subjective complaints, physical examination, and imaging studies, to include x-rays and, possibly, MRI. In some cases, obtaining some joint fluid for analysis can make the diagnosis. In other cases, a patient’s blood can be analyzed for specific disease processes.