Baker's Cyst

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Definition/Description

A popliteal cyst, better known as Baker’s cyst, is a fluid-filled swelling that is developed at the back of the knee in the popliteal fossa region. Ganglia which are benign cystic tumors, originate from synovial tissue. Common areas for cyst can occur at the wrist, hand, foot, and knee.

The cyst can exercise pressure on some anatomical structures, in most cases, the affected anatomical structure is the popliteal vein. Which can develop into thrombophlebitis.

Clinically Relevant Anatomy

A Baker’s cyst is an enlarged bursa that is normally located between the medial head of the gastrocnemius and a capsular reflection of the semimembranosus, named oblique popliteal ligament. The two requirements for a cyst formation are the anatomical communication and a chronic effusion. Knee joint effusions may replete the gastrocnemius-semimembranosus bursa with synovial fluid and if the fluid outflow is hindered by a unidirectional mechanism, the gastrocnemius-semimembranosus bursa enlarges giving rise to a pseudocystic cavity, referred to as a Baker’s cyst.

Epidemiology /Etiology

A Baker’s cyst, or a popliteal cyst, occurs if there is an underlying intraarticular problem with the knee or and inflammatory reactions, commonly as a result of losing bodies formed in conditions such as osteoarthritis, rheumatoid arthritis, gout, ACL tears, meniscal tears or because of particles following knee arthroplasty, mostly from the polyethylene liner.

• Primary Cyst: An expansion arising independently from the joint and there is no knee derangement.
• Secondary Cyst: A distension of the bursa located between the gastrocnemius and semimembranosus tendons: fluid finds its way through the channel the normal bursa communicates with the joint. This is the most common occurrence

The cyst can vary in size, from a very small (asymptomatic) to a large one but a change in size is very common. Especially in smaller cysts a septum may exist separating the semimembranosus and gastrocnemius components. This may function as a flap valve allowing fluid to enter a popliteal cyst and not to exit it.

There are differences between a popliteal cyst in children and in adults. In children, there are cystic masses filled with gelatinous material that develop in the popliteal fossa, are usually asymptomatic, and are not related to intra-articular pathology. Spontaneous resolution usually occurs, although the process can take several years. In adults, a Baker’s cyst is often found in combination with other intra-articular pathologies and inflammatory conditions.

Clinical Presentation

Symptoms can include

• Vague posterior pain
• Swelling and a mass in the popliteal space
• Limited range of motion
• Stiffness in the back of the knee sometimes increased by activity
• Tightness behind the knee.

Most cysts are found on the medial side of the popliteal space in the gastrocnemius-semimembranosus bursa, but they could be found in the popliteal bursa so the mass will be found on the lateral side of the popliteal space. Also, there are some rare cases in which popliteal cyst extend superiorly or anteriorly

Cysts may range in size from small, from clinically, asymptomatic and not palpable, to large masses causing visible swelling of the patient’s knee. The size of the cyst or pain can cause limitations in range of motion. If the cyst is large, it may result in mechanical problems in knee flexion and limiting mobility.In rare cases there will be signs and symptoms of a meniscal tear which can be tested by McMurray test.

Popliteal cysts can give pressure against other anatomic structures. Compression of the popliteal artery or vein can cause ischemia or thrombosis, respectively, whereas compression of the tibial or peroneal nerve can cause peripheral neuropathy.

A ruptured cyst can manifest as calf pain or even swelling, it can also cause an itching feeling in the calf. There are more common in patients with an inflammatory pathology than in patients with a degenerative pathology.

Differential Diagnosis

Baker’s cyst can be mistaken for several other injuries in the knee. The patient’s history, as well as the clinical investigation and imaging allow for proper differential diagnosis of the disease:

Conditions presenting as soft tissue masses in the posterior leg
Muscle strain or tear	Palpable mass/tenderness, swelling/warmth, pain on contraction and/or stretch of muscle
Muscle contusion or hematomas	Local damage to muscle (bleeding, swelling), painful muscle contraction/stretch. If the hematoma is old, organized thickening is developed
Muscle spasm or cramp	Painful palpable lump/thickening, possible range of motion restriction, pain on stretch of muscle
Fascial tear with muscle herniation	Palpable soft mass, severe muscle pain with increased activity, localized swelling post activity
Myositis ossificans	Palpable painful mass in the muscle, microtears in the muscle fibers that cause pain and swelling with muscle contraction or stretch, loss of motion from restricted muscle function
Deep vein thrombosis (DVT)5	Constant pain, pain on passive dorsiflexion (Homan’s sign), localized warmth, localized tenderness on calf palpation and possible swelling, elevated body temperature
Benign tumor	Localized pain and tenderness, may be palpably soft or hard, may cause movement dysfunction (location dependent)
Malignant tumor	Generalized malaise, possible sudden weight loss, localized pain and/or swelling various sizes and consistency
Hemangioma	Present for a long time, slow changes in size over time, palpable lump, may be painful, may/may not limit motion
Baker’s cyst5	Palpable lump with possible tenderness in posterior knee and posteromedial calf
Ruptured or enlarging Baker’s cyst (pseudothrombophlebitis)	Can mimic DVT, swelling in the calf, acute pain that worsens with compression

Popliteal Cyst may also confused with lipoma which would represent less resisting pressure when compared to a Baker’s cyst or aneurysm differentiated by Doppler evaluation (ultrasound).

Some examples of supporting and negating evidence of the possible diagnoses are:

Differential diagnoses of mass in the calf muscle.
Possible Diagnosis	Supporting evidence	Negating Evidence
Old muscle strain or trauma with scar tissue	Palpable mass	No previous history of a muscle tear or trauma, no pain with running
Fascial tear with muscle herniation	Pain walking more than 0.5 km, palpable tenderness	Nonincident related, insidious onset, No pain with running or general exercise
Localized muscle spasm	Pain walking more than 0.5 km, palpable tenderness	No pain on passive calf stretch, no pain on resisted muscle contraction, no limitation of motion in foot or knee
Deep vein thrombosis (DVT)	Palpable tenderness, pain, sitting with legs crossed, family history of increased clotting factor XII in blood, taking oral contraceptive agents	No history of DVT ,no recent immobilization, no edema, no raised body temperature, negative Homan’s sign, local pulses present
Benign tumor	Palpable mass, palpable tenderness, insidious onset	Intermittent pain
Malignant tumor	Palpable mass, palpable tenderness, insidious onset	No weight loss, no night pain or feeling unwell, intermittent pain, good general health
Hemangioma	Insidious onset, unknown cause, hormonal changes (oral contraceptive agents), present for a long time, slow changes in size over time, palpable lump, may be painfully ,may not limit motion

Precise differentiation of the patient’s symptoms can be located by ultrasonography.

If the popliteal cyst becomes infected, this can often result in a painful mass behind the knee. In such cases, it could be difficult to make a diagnosis and the infected cyst can be mistaken for a neoplasm. The cyst can rupture (split open) resulting in severe calf pain, decreased motion at the ankle and cause similar symptoms as a deep vein thrombosis (evident on ultrasound or venogram).

It is important to diagnose a ruptured Baker’s cyst early in order to determine the best treatment and avoid complications, such as compartment syndrome and to differentiate from:

• Thrombophlebitis
• Popliteal aneurysm
• Inflammatory arthritis
• Medial gastrocnemius strain
• Soft-tissue tumor or muscle tear

It must be considered, that Popliteal cyst can rarely present as a swelling on the anterior surface of the proximal tibia as part of the differential diagnoses when faced with a patient with a previous history of an ipsilateral total knee arthroplasty

Diagnostic Procedures

The imaging workup of knees with suspected popliteal cysts can include plain radiographs, arthrography, ultrasound, and MRI. Early in the evaluation it can be useful to obtain plain radiographs (as posteroanterior Rosenberg, lateral, and patellofemoral axial views) for detecting other conditions found in association with popliteal cysts, such as osteoarthritis, inflammatory arthritis, and loose bodies. In addition, loose bodies may be seen in a Baker’s cyst on plain radiographs.

At first, direct arthrography was used for detecting popliteal cysts. Direct arthrography involved intra-articular injection of the knee with gas or an iodinated contrast medium, followed by mobilization of the joint to force the contrast into the cyst. Afterwards, spot radiographs or fluoroscopy were used to detect the presence of the contrast in the cysts. The disadventages of this technique include the use of ionizing radiation and the use of invasive techniques to inject the contrast.

The advantages of ultrasound provide a replacement of arthrography by ultrasound. The advantages are its low cost, noninvasive usage and the absence of radiation. The main disadvantage is the fact that it is user dependent. Ultrasound is able to detect Baker’s cysts near 100% but lacks to differentiate from other conditions, such as meniscal cysts or myxoid tumors, it also doesn’t visualize other conditions in the knee that are often associated with these cysts.

The gold standard for diagnosis of Baker’s cysts and differentiating them from other conditions remains magnetic resonance imaging. It allows to assess soft tissue abnormalities and has the added advantage of being accurate at diagnosing associated joint disorders so the entire spectrum of related disorders is possible to assess

Conditions such as meniscal cysts are more easily differentiated from Baker’s cysts with MRI than ultrasound. This may be the gold standard but is also a high-cost technique, therefore, ultrasound should be considered as a screening modality if the evaluation of the intra-articular structures is not necessary.

Baker's cyst on axial MRI with communicating channel between the semimembranosus muscle and the medial head of the gastrocnemius muscle

Baker's cyst on MRI, sagittal image

Ultrasoud image of Baker's cyst. Fluid filled cyst noted between the medial head of the gastrocnemius and the semimembranosus tendon taking appearance of a "speech bubble".

Outcome measures

Outcome measures are used to determine and evaluate the expected results of the therapy/procedure to be compared to the results found within the patient.

1. Western Ontario and McCaster universities pain subscale (WOMAC): Is a multidimensional instrument that measures 17 functional activities. 5 pain-related activities and joint stiffness categories. It measures pain and dysfunction
2. Visual analog scale (VAS): It measures pain on a line from 0 to 10. From no pain to extreme pain. It is easy or the patient to mark how much pain he/she has.T
3. The Rauschning and Lindgren classification(RL): was used to evaluate the outcome and therapeutic efficacy. Grading is from 0 to 3
4. The Newcastle-ottawa scale(NOS): Is a simple scale to evaluate the quality of non-randomized controlled studies

Examination

The examination is more of clinical observation and ruling out other possible conditions. If infected or severe conditions, radiological investigations may be helpful.

Patients with a Baker’s cyst commonly have symptoms of meniscal or chondral pathology which can be tested by McMurray test. Symptoms related to a popliteal cyst are rare. If these symptoms are present, they may be related to the size of the cyst and include posterior or posteromedial fullness and achiness, mass and stiffness. The most common symptoms are popliteal swelling and posterior aching. Patients may also complain of pain that occurs with terminal knee extension. During the examination testing of knee flexion can be useful.

Patients with large cysts may develop loss of knee flexion because the cysts mechanically block flexion. The examination will often reveal knee meniscal or chondral pathology, there can be palpable posteromedial fullness or tenderness. A palpable cyst is often firm in full knee extension and soft in knee flexion. This finding is known as “Foucher's sign” and is due to cyst compression. With extension, the gastrocnemius and the semimembranosus muscles approximate each other and the joint capsule compressing the cyst against the deep fascia. The mechanism of Foucher’s sign is useful for distinguishing Baker’s cysts from lesions such as popliteal artery aneurysms, adventitial cysts, ganglia, and sarcomas, for which the palpation of the mass is unaffected by the knee position, therefore this is considered the test for finding a Baker's cyst.

Medical Management

Sometimes no treatment or simple supportive measures lead to spontaneous resolution or to a reduction of the symptoms. If this does not happen, invasive and surgical techniques may be an option.

The asymptomatic popliteal cyst often gets better and disappears by itself over time. If symptomatic, rest can temper the pain that bursa is causing, take non-steroidal anti-inflammatory drugs (NSAID) to alleviate the pain and restrict movement, also alternate with ice.

If the pain persists, a subsequent steroid injection with a solution of anesthetic and steroid can be advised which can relieve pain, but not prevent recurrence of the cyst. It’s only a temporary solution.By a popliteal cyst of inflammatory origin, it is enough to treat the underlying disease. When the underlying disease doesn’t get treated, the Baker’s cyst can come back. Arthroscopic examination should be performed and all pathologic conditions treated before considering the excision of a popliteal cyst.

The cyst can be removed with surgery if it becomes very large or causes symptoms such as discomfort, stiffness or painful swelling. There are three surgical techniques available to treat the cyst: common posterior approach, the posteromedial approach, and the medial intra-articular approach. The first two techniques are techniques where the cyst is going to be removed. In the last technique, they make an opening in the cyst and close it afterward. The popliteal cyst will eventually disappear.

Physical Therapy Management

An ice massage of 15 minutes every 4-7 hours will reduce the inflammation. The treatment is based on the principles of R.I.C.E (rest, ice, compression, and elevation) followed by some muscle-conditioning exercises.

A rehabilitation program can improve the control of the knee joint by range of motion exercises. It will increase the motion of the joint as well as increase flexibility. The physiotherapist will give a mobility, a hamstring stretching program and a concurrent quadriceps strengthening program that has to repeat several times a day. This will result in less pain at about 6-8 weeks.

There is an experiment comparing ultrasound guided corticosteroid injections with a horizontal therapy. 60 people were divided in three groups, only injections (group A), only horizontal therapy (group B) and (group C) they got both the injections and the horizontal therapy. The horizontal therapy was delivered through a specific commercial device, and they followed the instructions, provided by the manufacturer. Patients in groups A and C had a lower level of pain after one month. Group C had the lowest VAS scores and group C also scored best on the WOMAC. In group B and C the stiffness and disability scores were also improved the most.

Clinical bottom line

Baker’s cysts have different causes, therefore finding one solution is difficult. The most common treatments are invasive, such as corticosteroid injections and NSAID or surgery. When the pain is worse, an injection can help to relieve the pain. However, it cannot ensure that the cyst will not come back. Often the cyst disappears by itself. However, this is not always the case. A patient who has a lot of pain from the Baker’s cyst may use ice or NSAID, in order to reduce the pain. At this stage, there is a need for more research on physical therapy, applied on Baker’s cyst patients.