Water on the knee has long been the popular lay terminology for a condition of the knee called effusion, an abnormal and excessive buildup of joint fluid within the knee. The presence of this excess fluid is a direct response by the body to inflammation such as arthritis, cartilage tears, ligament injuries and in come cases, systemic diseases or conditions such as gout. It is the body's way of battling the offending agent causing the inflammation. Symptoms of water on the knee, or effusion, vary depending upon the cause and severity of the buildup of fluid.
Swelling
The buildup of fluid within the knee joint and swelling that occurs outside the joint often mimic each other to the untrained eye. Both manifest themselves with an enlargement of the knee area. The main difference is that water on the knee occurs within the joint capsule and literally is contained in a closed space, whereas swelling of tissues external to the joint often can spread throughout the tissues beneath the skin.
In cases of effusion, as the volume of fluid increases, the available space within the knee decreases. The membrane that surrounds the knee can stretch only so far. The knee will appear enlarged and puffy, and it will have a soft, almost boggy feel upon touching it on either side of the kneecap. Occasionally, the fluid can be observed bulging out the opposite side from where pressure is applied.
Pain
As the fluid production progresses and the volume increases, it begins to stretch the lining and capsule of the knee joint, not unlike the bladder when it becomes excessively full. Some nerves in the joint capsule and synovium are specialized to respond to increased tensions from stretching. These are called stretch receptors, and they warn the body when excess tension is being exerted in these tissues from overproduction of joint fluid.
The increased volume of fluid, together with the activation of the stretch receptors in the lining and capsule of the knee joint, can begin to transmit pain impulses to the brain, stimulating symptoms ranging from a mild discomfort to abject pain with difficulty moving the knee.
Loss of Motion
As the swelling and pain progress, the functional level of the knee joint will correspondingly decrease, mainly because of the pressure exerted by the excess fluid volume against the joint capsule, decreasing the overall room the joint has to move within. In cases of mild water on the knee, motion loss can be almost imperceptible. Not until significant loss of motion and increased difficulty in walking, stooping or sitting has occurred will some people even notice they have a deficit. As motion decreases, discomfort increases and attempts at moving the knee beyond the limits imposed by the fluid volume become increasingly uncomfortable.
Warmth
Increased warmth at the site of the knee is not an uncommon symptom. Because this excess fluid is being produced in response to an inflammatory agent, increased metabolism and resulting heat production can occur. As long as the offending inflammatory agents persist or worsen, warmth at the joint can remain and even worsen.
