Baker's cyst

A Baker's cyst, also known as a Baker's cyst or a popliteal cyst, is a non-physiologically enlarged connective tissue space that is filled with synovial fluid. It looks like a lump that can be palpated through the skin and can often be found accidentally when examining the knee. It is located in the popliteal fossa on the posterior side of the thigh between the medial head tendons of the gastrocnemius muscle and the semimembranous muscle. This ailment was first described in 1877 by Dr. William Baker, from whom it took its name.

Baker's cyst most often develops in children aged 4-7 years and in adults aged 35-70 years, and its location is in most cases constant. Cysts are common in children and are different from those in adults. Children have cystic vesicles filled with gel material that develop in the popliteal fossa, are usually asymptomatic and not associated with any intra-articular pathology. Usually, a spontaneous recovery occurs, although this process may take several years. In adults, however, Baker's cyst is often associated with other intra-articular pathologies and inflammations.

The knee joint, which carries enormous loads, is particularly prone to overloads, injuries and other pathological conditions. Cyst formation may result from damage to the joint capsule or inflammation of one of the synovial bursae. Pathology can occur in more than half of those reporting symptoms. Rapid diagnosis of cysts and other pathologies of the knee joint will allow for prompt initiation of appropriate treatment, which will protect the patient against advanced lesions and surgical treatment.

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The causes of the formation of the popliteal cyst

The causes of the popliteal cyst have not yet been clearly defined. These differences result, among others, from the anatomical structure, the age at which the disease occurs, the coexistence of systemic or intra-articular diseases. There are two basic factors that may contribute to its appearance.

The first factor that is decisive in the formation of a popliteal cyst, which is crucial for any patient, is the excess amount of exudate in the knee joint. Under physiological conditions, the production and resorption of fluid is maintained in dynamic equilibrium, and the synovium is responsible for the appropriate regulation in this respect. During proper, undisturbed functioning, the amount of synovial fluid in the joint is several milliliters, which does not increase the pressure during its movement. In cases of increased production of exudate fluid due to concomitant pathologies, excess exudate increases the pressure inside the joint (especially when the knee is bent above 30 degrees). This leads to the limitation of the knee movements and creates the possibility of increased fluid leakage through the places of weakened resistance in the joint capsule, which may result in the formation of a popliteal cyst. The fluid creates a hernia extending through the back of the articular capsule or enlarges the gastrocnemius which is naturally connected to the knee joint. Excessive production of exudate is caused by concomitant intra-articular diseases such as: degenerative changes, rheumatoid arthritis or gout.

The second factor influencing the formation and persistence of Baker's cyst is the existing valve mechanism of the articular capsule, which forces a unidirectional flow of exudate fluid. The connection of the cyst with the joint occurs at the site of the weakened resistance of the capsule tissues between the natural reinforcements, which are the arcuate ligament, the semimembranous muscle and the medial head of the gastrocnemius muscle. The joint capsule and the synovium are not able to provide anatomical strengthening in this area, which leads to the formation of a cyst.

People who are most at risk of developing a cyst under the knee joint are athletes and obese people, which results from excessive stress on the knee joints, which may result in numerous injuries of the knee joint, e.g. inflammation of the patellofemoral joint, damage to the meniscus or ligaments. All of the above-mentioned factors contribute to the excessive production of synovial fluid and, consequently, the possibility of cysts.

Types of popliteal cyst

Depending on the cause of the cyst formation, the structure of the cyst wall can vary considerably. Therefore, histopathologically, they can be divided into:

inflammatory - surrounded by a very thick wall (even 8 mm), with an uneven "shaggy" structure from the fibrin protrusions covering it. In the wall there are, among others lymphocytes, plasma cells, histiocytes and multinuclear cells. In this type of cyst, cartilage and bone-like elements can form;
fibrous - have a well-scratched and limited wall, 1-2 mm thick, which has a smooth, glossy inner surface. Its wall is made of fibrous tissue heavily saturated with hyaline, in which rice bodies can form;
synovial - less demarcated from the surrounding tissues. They have a wall 2-5 mm thick, less shiny, with villi. The walls are made of less dense fibrous connective tissue, and its surface is covered with cubic cells.

Symptoms of Baker's cyst

Symptoms associated with the popliteal cyst itself are very rare, but if they do occur, they may be related to the size and severity of the cyst and the underlying disease and inflammation of the knee joint. The most important symptoms that may indicate the presence of a Baker's cyst are:clearly palpable lumps under the skin at the back of the knee joint,
knee pain worsening with prolonged walking and exercise,
redness and warming in the vicinity of the cyst in the popliteal fossa,
swelling of the knee and a feeling of "fullness" in the knee
limitation of flexion and extension of the knee joint,
depending on the size of the cyst, there may be numbness in the calf area (also when the cyst ruptures), pain and swelling of the lower leg,
limb function limitation. 

Baker's cyst - differential diagnosis and diagnosis

The presence of a Baker's cyst can usually be confirmed on the basis of a thorough history and clinical examination (including palpation), which is usually sufficient for the correct diagnosis of the cyst. In case of doubt, an ultrasound examination (USG) of the popliteal fossa is performed. Additionally, in order to find the exact cause of the lesion, MRI examinations and, in special cases, arthroscopy are ordered.If the cyst becomes infected, it can create a painful, hard mass behind the knee that can be mistaken for a cancerous tumor. A Baker's cyst can also rupture, causing severe pain and burning in the calf, with symptoms similar to those of deep vein thrombosis. In these cases, the precise differentiation of the patient's symptoms is necessary to determine. Magnetic resonance imaging of the knee remains the gold standard for the diagnosis of a Baker's cyst and its differentiation from other disease states, as it allows the assessment of the entire spectrum of related disorders, and an ultrasound of the knee is the basis for distinguishing between cysts and thrombophlebitis.

Baker's Cyst - Treatment

Nonsurgical treatment
Depending on the type of Baker's cyst or the severity of symptoms caused by it, we can choose one of several methods of treatment. In the first place, treatment should be started with the least burdening the patient, especially when the disease affects children or does not cause pain symptoms, does not limit the functions and mobility of the lower limb. Conservative treatment may bring a good therapeutic effect in the initial stage of the disease.

In the first stage of the procedure, it is recommended to limit physical exertion, unburden the knee joint, use appropriately selected exercises and a physical activity regimen. In addition, in the case of larger cysts, periodic puncture of the cyst and removal of the accumulated synovial fluid (puncture) with injection of an anti-inflammatory drug as well as physical therapy procedures (magnetic field, ultrasound, laser therapy) are used.

If the basic symptoms of the disease are under control, in the next stage it is necessary to turn to a physiotherapist who will select appropriate rehabilitation exercises that will aim to strengthen the weakened muscles and stretch the contracted ones, improve the stabilization of the torso, improve deep sensation, which will thus reduce the risk factor and reduce possibility of recurrence of the problem.

Surgery

In cases not amenable to conservative treatment or in the presence of a large cyst, surgical treatment is indicated, which should always begin with an attempt to remove the cause of its formation.

Classic surgery to remove Baker's cyst

Classic surgical treatment consists in removing the cystic lesion of the popliteal fossa and carefully suturing the junction of the cyst with the knee joint, however, it does not remove any of the causes of its formation and is associated with the presence of a long postoperative wound, significant pain and a long return to full activity. However, this treatment does not prevent relapses, and their number can be as high as around 70%.

Arthroscopic removal of Baker's cyst

Due to the ineffectiveness of treatment with the classical method, the recommended treatment in this case is arthroscopic treatment. The procedure consists in removing the cyst, closing its connection with the joint and suturing the damaged joint capsule tightly. In the case of concomitant intra-articular diseases (or causes of secondary occurrence of the cyst), repair activities are also performed. Arthroscopy allows the least invasive removal of all known and described causes of popliteal cyst formation, which significantly improves the effects of treatment and reduces the number of recurrences. In addition, it causes much less postoperative discomfort compared to conventional treatment, ensures early return to the full range of motion in the joint and allows the patient to return to everyday activities much faster. After the surgery, rehabilitation lasting several weeks is required, appropriately selected and adjusted depending on the extent of the surgery and the performed repair procedures.

The high prevalence of the popliteal cyst and the lack of reliable, long-term results of inoperable or classic treatment, especially in young patients and children, meant that the arthroscopic method along with the treatment of comorbid knee pathologies, combined with the destruction of the valve mechanism, gives a much greater chance of permanently eliminating the problem.

Important information

Frequently asked questions about Baker's cyst: