Symptoms
An accumulation of fluid in the knee joint primarily leads to a visible swelling of the knee, the extent of which depends on the volume. The pressure of the fluid inside the joint capsule usually also irritates nerves inside the capsule, which can lead to pain in the knee joint. This can occur at rest, under stress or when pressure is applied to the knee.
This – as well as the space taken up by the fluid in the joint space – can lead to significant restrictions in movement in the knee joint, so that bending the leg is only possible with discomfort. A general feeling of tension in the area of the knee joint can also be noticeable, which is caused by the general swelling. If an infection of the fluid (e.g. by bacteria) occurs, in addition to the swelling and the pain, other typical signs of inflammation may also become noticeable, including redness and overheating of the knee joint.
Depending on how much fluid is in the knee joint, an effusion of the knee joint may be noticeable with the naked eye or at the latest during the clinical examination of the patient by a doctor. For example, a so-called “dancing patella (kneecap)” can be detected if more than 50 ml of fluid accumulates in the knee joint. Here the examiner can feel an elastic rebound of the patella (positive ballottement) under frontal pressure and feel that this causes the patella to float on the effusion.
A further diagnostic sign in the physical examination can be a slight bulge on the outside of the knee joint when the examiner presses both the upper part of the joint capsule as well as the part of the joint capsule located on the inside of the knee joint. This second method can be used with fluid volumes as low as 10 ml. A puncture, i.e., the collection of fluid from the knee joint using a puncture needle, can always be used when a sample of fluid is to provide information about the quality of the fluid and whether there are any pathogens in it that could possibly cause or influence this accumulation.
In addition to physical examination, imaging techniques can also objectify fluid accumulation. In addition to a simple ultrasound examination of the knee, x-rays, computed tomography and magnetic resonance imaging are also able to show the “water” in the knee. In individual cases, a blood sample can also be taken for further diagnostic clarification, so that elevated inflammation parameters can be detected in the laboratory, for example if an infection is the trigger of the effusion.
