Synonyms
Popliteal cysts, knee joint ganglion cysts are tumours filled with thin or thick fluid contents. These sac-like structures are closed by a capsule and are usually formed by retention of fluid, i.e. the fluid that forms in the cyst can no longer drain away. A Baker’s cyst in the knee is a protrusion of the posterior joint capsule, it is located in the area of the hollow of the knee.
A distinction is made between a primary and a secondary Baker’s cyst. The primary form is usually present in younger patients and is often idiopathic (cause unknown). The secondary Baker’s cyst can be caused by injuries in the knee joint, which are accompanied by a joint effusion. Older patients are particularly affected by this, as they more often suffer from knee problems due to wear and tear. A Baker’s cyst in the knee is mainly caused by pain during flexion of the knee.
Cause
As already mentioned, it can be an idiopathic (unknown cause) Baker’s cyst (primary Baker’s cyst) or it can develop from a disease within the knee joint (internal knee disease) with joint effusion. This joint effusion occurs because the body reacts to inflammation in joints by producing more fluid (synovial fluid or synovia). The accumulation of fluid causes an increase in pressure in the joint, which causes the joint capsule or the inner skin of the joint to protrude backwards into the hollow of the knee.
This bulge is now called a Baker’s cyst. It is usually located on the inward-facing side of the hollow of the knee, between the tendon attachment of the flexors of the thigh and the upper part of the calf muscle. The access to the cyst – also known as the cyst stem – is narrowed due to entrapment in the muscle, so that the fluid can no longer drain from the sac.
Various previous illnesses or underlying conditions can cause a Baker’s cyst in the knee. These are mostly meniscus damages or lesions. These can be caused by accidents or trauma.
However, a Baker’s cyst can also be caused by knee arthrosis or arthritis. Elderly people often suffer from osteoarthritis of the knee, as this is the wear and tear of the knee joint structures, which are subject to considerable stress during physical activity. Arthritis is often caused by rheumatic joint inflammation, but can also have a bacterial cause.
Any other mechanical irritation of the knee joint (instability of the ligaments, wear and tear of the cartilage, overloading of the knee) can also be a cause. Overloading of the knee followed by the development of a Baker’s cyst in the knee is often the case in young people when they participate in sports that put a strain on the knee joint. Sometimes operations in the knee can also trigger a Baker’s cyst.
