Introduction
If fluid or water collects in the knee, this is usually referred to as a knee joint effusion. The reason for this is that the fluid is usually located within the capsule of the knee joint, although it is by no means water in the actual sense, as it is colloquially referred to as such, but rather normal or altered, endogenous joint fluid (synovia), pus or blood. A distinction can be made between an acute knee joint effusion and a chronic one that persists for a long time.
Cause
In general, the cause of fluid accumulation in the knee joint or knee joint capsule lies in the increased production of joint fluid (synovia) by specially designed cells within the capsule (synovial membrane), which accumulates because it cannot be adequately absorbed. This means that more is produced than can be removed. If this increased production occurs acutely, i.e. suddenly within a period of 3-14 days, it is usually caused by trauma.
This includes, for example, a torn cruciate ligament, a patella dislocation (patella dislocation) from its natural position or damage to the menisci. The effusion occurs as a reaction to the knee injury, and may be of normal joint fluid, but it can also be bloody, since small vessels in the knee joint can always rupture during trauma. If the water accumulates chronically in the knee than over a longer period of time, infections usually play a superordinate role, which can cause inflammation in the knee joint and stimulate the joint mucosa to produce more water.
In the worst case, an underlying infection can lead to an accumulation of pus in the knee joint (empyema), which requires prompt opening and emptying. Other causes of chronic knee joint effusion, especially in older people, can be an existing arthrosis, gout or known rheumatic diseases. In younger people, overloading the knee joint due to too much sport or incorrect loading during sporting activities can also lead to a so-called irritant effusion, just as it can also occur after operations in the knee area.
After surgical interventions on the knee joint, a reactive knee joint effusion can develop in the affected patients, which can persist for a period of 2-3 weeks. The reason for this is, on the one hand, the irritation of the joint mucosa caused by the operation, which reactively forms more joint fluid, but on the other hand, post-operative bleeding can often occur. As a rule, the effusions after surgery are therefore harmless clear joint fluid effusions or haematomas, which can be easily removed by relieving punctures in case of complaints, but in most cases disappear again by themselves.
If, however, an infectious complication of the knee joint occurs during the operation, this can manifest itself as an inflammatory effusion and, in the worst case, can turn into an accumulation of pus in the knee joint. In the case of a lesion of the inner or outer meniscus of the knee joint, in addition to pain at the edge of the knee joint, swelling and overheating of the knee, there is also a knee joint effusion, which occurs reactively due to the damage. Not all meniscus lesions are necessarily in need of surgery, small damages can also be treated conservatively with immobilization and medication.
However, if the lesion causes persistent pain and leads to knee effusion, which may even recur through repeated punctures, surgery is the treatment of choice. As a rule, the meniscus damage is repaired by means of knee arthroscopy. As with other knee operations, post-operative irritation can then occur, which can persist for 2-3 weeks and was provoked by the irritation of the joint mucosa during the surgical procedure.
During pregnancy, the knee joint can be overstressed, especially towards the end of the pregnancy, due to the fact that the knee is increasingly under pressure as the child grows up in the womb and the steady weight gain. Similar to overloading during sports activities, pregnancy can therefore also lead to a so-called irritation effusion, which is caused by a reactive surplus production of joint fluid in the joint mucosa.In addition, the increasing weight of the child puts pressure on the vena cava in the small pelvis, making it difficult for the venous blood to return to the heart, for which it is normally responsible, and causing, among other things, fluid retention in the knees (also in the hands and feet). Knee joint effusions can also occur in children, but the cause at such a young age is usually underlying inflammation of the knee joint (so-called arthritis).
These can develop on the one hand through acute infections with bacteria, but also through chronic bacterial infections, such as Lyme disease. But also in the context of a so-called rheumatic fever, a reactive arthritis/joint inflammation after an infection with streptococcus bacteria, an effusion can develop in the knee joints. The same also applies to infections with certain viruses (Parvo-B19, Rubella, EBV etc.) or gastrointestinal bacteria (shigella, salmonella, campylobacter).