Causes
Pathophysiologically, the development of a popliteal cyst is based on an irritation of the synovial membrane. As a result, the synovialis produces more synovial fluid to counteract the irritation. The result is excess pressure in the joint space and bulging of the joint capsule at its weakest point between the insertions of the calf and thigh muscles.
Triggers for such an irritation of the joint mucosa can be previous accidents (e.g. meniscus damage, cruciate ligament rupture), gonarthrosis (knee joint arthrosis), rheumatoid arthritis, bacterial inflammation, etc. If the popliteal cyst has burst due to increasing size, affected persons feel sudden, severe pain in the lower leg. The content of the cyst, mainly consisting of synovial fluid, then runs down along the calf muscles.
An inflammatory reaction with overheating, redness and swelling of the lower leg can then be observed. In the worst case, this can lead to the so-called “compartment syndrome” with severe swelling of the calf muscles and subsequent relocation of the blood vessels. Due to the lack of blood supply, parts of the musculature can die.
This clinical picture represents an absolute emergency with strongest pain and must be operated immediately. Otherwise the loss of the leg is imminent. Much more common than a “real burst” popliteal cyst are size reductions due to the underlying knee damage. The cyst can increase in size under stress and decrease in size again when the joint is subsequently relieved. This mechanism can give the impression of a burst cyst, but does not cause severe pain.
Diagnosis
Since similar symptoms can be triggered by thrombosis, pulled muscles, lipomas and other tumors or vascular aneurysms, it is important to rule them out by means of diagnostics. Typically, the patient’s medical history includes a previous injury to the knee joint, arthrosis or rheumatism. In addition to the manual examination of the knee and the hollow of the knee with an often palpable non-pulsating bump in the hollow of the knee and otherwise soft calf and thigh, the ultrasound device (sonography) is used to diagnose the popliteal cyst.
Furthermore, magnetic resonance imaging (MRI) is also suitable for diagnostic purposes. This is mainly performed to find the cause of the knee cyst, such as the knee capsule.B. Meniscus injuries.
If no cause for the development of the popliteal cyst can be found with the help of apparatus-based diagnostics, joint fluid can be obtained for microbiological examination by means of a joint puncture. With this examination a bacterial inflammation of the knee cleft can be detected. Since the knee cyst itself is only a symptom of a knee disease, it is mainly this cyst that should be treated in order to be able to treat it successfully in the long term.
If the popliteal cyst was only found by chance and there are no symptoms, a therapy is not indicated. In the case of minor complaints, cooling the knee with ice or cool packs can reduce the effusion in the joint space, shrink the popliteal cyst and thus alleviate the symptoms. As the joint effusion is reduced, for example by sparing the knee, the knee cyst can recede to such an extent that it is no longer palpable and no longer causes any symptoms.
If the popliteal cyst causes so much discomfort that it restricts the patient, it can be punctured and thus relieve the knee joint space. Surgery is generally possible, but is not recommended due to the risks and long-term poor results. An exception is the rheumatic knee joint cyst.
Here, aggressive mucous membrane tissue, so-called pannus, forms in the popliteal cyst, which can attack and damage the surrounding tendons. In the case of a rheumatic popliteal cyst that does not improve under conservative alternative treatment, surgery is usually necessary in view of the overall condition. Not every popliteal cyst requires surgery.
Only with increasing size and associated limitations, surgical removal may be appropriate. Often affected patients then report pain, numbness, circulatory disorders or reduced mobility of the knee joint. In most cases, the popliteal cyst is only the “tip of the iceberg”.
The actual problem, however, is usually inside the knee joint. For example, meniscus damage or osteoarthritis of the knee joint can be the cause. Surgery is therefore primarily aimed at eliminating the problem in the joint and not at removing the cyst itself.
If the underlying knee damage can be removed, the popliteal cyst usually disappears on its own afterwards. Very rarely, however, it may be necessary to remove the fluid-filled sac directly. In such cases, it is mostly children whose popliteal cyst causes problems due to its increasing size. As an alternative to surgery, a thin needle can be used to puncture the cyst and release the fluid from the cyst. However, there is a risk of recurrence, as the cause has not been eliminated.
