A popliteal cyst is a sac filled with synovial fluid that develops in the posterior knee. In adults, this is likely associated with knee trauma or underlying disease.
The clinical presentation is typically mild. For example, aches and tenderness may occur with physical activity and certain positions such as knee extension. Also, the knee may feel unsteady, stiff, and limited in motion. Since these cysts develop in the context of arthritis, trauma, or mechanical injury, symptoms of these conditions may manifest.
The fluid filled bursa is observed as a bulge on the posterior side of the knee. When the knee swells secondary to other processes, the fluid enlarges the cyst. Some cysts alternate between shrinking and growing while other spontaneously disappear. Additionally, some patients will experience recurrence.
If these sacs rupture subcutaneously, this results in pain and swelling of the calf. In this case, the clinical picture resembles that of thrombophlebitis in the lower leg. This warrants an immediate investigation to rule out a blood clot.
It is important to note that there are two types of popliteal cysts:
Primary: the cyst does not communicate with bursa or knee joint. This is more prevalent in children. There is no meniscal involvement (derangement) .
Secondary: the cyst communicates with the bursa and the joint. This affects adults and is much more common than the primary type .
A clinician should obtain a thorough history including identification of coexisting knee pathology, physical exam, and imaging to arrive at the correct diagnosis. It is crucial to rule out destructive processes as knee diseases can be devastating.
Radiography can identify any calcifications, soft tissue masses, or rare bone involvement from cysts .
Ultrasonography is useful to detect a popliteal mass. Further, it elucidates the characteristics of the mass which may be a simple or complex cyst or a solid structure. Doppler imaging with color is useful to detect blood flow in the mass. Specifically, this is helpful for ruling out an aneurysm or degeneration of the popliteal artery . Another crucial benefit of ultrasound examination is to evaluate for thromboembolism. A shortcoming of ultrasonography is its ineffectiveness in demonstrating a connection between the cyst and the joint properly. Thus, it is not helpful in distinguishing popliteal cysts from other pathologies.
If using arthrography, iodide contrast is injected directly into the joint to display a connection .
CT demonstrates a mass in the posteromedial popliteal space. Further it shows other findings such as wall thickening, bone erosion, bone fragments within the cyst, etc.
MRI provides valuabe details regarding the characteristics and composition of popliteal cysts. It offers multiple plane views and excellent contrast of the soft tissue. Furthermore, this modality can be performed for any knee complaints or presentations.
The therapeutic approach is tailored to the severity of symptoms and whether the patient has failed another mode of treatment. Of importance, patients with underlying disease should also have that addressed as well.
In asymptomatic patients, treatment is not indicated. However, imaging may be warranted to diagnose any existing pathologies.
Symptomatic: first time treatment
Mild complaints and benign presentations are usually managed conservatively through non-steroidal anti-inflammatory drugs (NSAIDs) and/or physical therapy.
Symptomatic: failed initial treatment
Studies have reported that corticosteroid intra-articular injection is effective in popliteal cyst therapy . This treatment could be employed in cases refractory to conservative measures.
Surgical intervention is uncommon. It is reserved for individuals with complicated and severe symptoms that are unresponsive to conservative treatment or knee aspiration although surgical excision is associated with increased recurrence rates.
Procedures such as arthroscopic synovectomy, capsulectomy, and capsuloplasty with grafting have demonstrated successful results. Also, good results have been observed with arthroscopic cyst resection . Another procedure utilized intra-articular injection of ethanol and sodium morrhuate. This was associated with improvement and low recurrence rates  .
The above procedures are not routinely performed but could be promising for complicated or recurrent cysts.
Popliteal cysts may resolve spontaneously or improve with surgical intervention . Recurrence has been associated with cyst excision but there are emerging procedures that may reduce further episodes.
Synovial fluid is found in the cavities of the joint. When the knee joint synthesizes excess fluid, pressure increases. This results in subsequent accumulation of fluid in the posterior knee. Hence, a bulge is formed in the popliteal fossa.
It is important to note that the knee is a complex joint and is prone to injuries and damage. The American Academy of Orthopedic Surgeons reports that approximately 19 million people presented with knee complaints in 2003. Additionally, most referrals to orthopedic specialists are due to knee signs and symptoms.
Epidemiological data regarding popliteal cysts are based on the populations investigated and the modes of imaging utilized for diagnosis. One study observes that 19% of patients who underwent MRI for knee evaluation were diagnosed with popliteal cysts . Another study evaluating patients for DVT with doppler ultrasound found popliteal cysts in 3% to 4% of these patients .
Gender: Both genders are affected.
Age: Popliteal cysts are observed more as age increases. For example, it is reported that in the 31 to 50 years old group, 26% develop popliteal cysts whereas this number is doubled in the age range of 51 to 90 years of age . Furthermore, the same study recognizes that the highest incidence occurs in two main age groups which are: 
Understanding the physiology of synovial fluid helps clarify the mechanism of popliteal cyst development. The articular joint is lined by the synovium, which consists of a complex fibrillar interstitial matrix. As part of an ongoing process, blood vessels in the synovium produce fluid secondary to an osmotic gradient between vasculature of the synovial capsule and the intra-articular space . Furthermore, when filtration of the fluid exceeds absorption, this results in effusions. Conditions that cause this excessive fluid volume include knee injuries, infection, and arthritis .
Lack of adequate anatomic support can also contribute to the development of these cysts . Moreover, the site located between the posterior cruciate ligament and the expansion of the semimembranous muscle is only supported by a segment of the medial meniscus; hence, it is a weak area. Thus any insult to the medial meniscus coupled with pressure elevation cause the articular capsule to extend outwards. Consequently, a popliteal cyst forms .
Preventing knee injuries reduces the risk of developing a popliteal cyst. Recommendations include wearing proper footwear. Also, it is advisable to exercise with appropriate warm-up and cool down periods. Applying caution with physical activity is pertinent as well. Finally, in the event of a knee injury, the individual should seek care for evaluation and possible treatment.
Since popliteal cysts occur with coexisting diseases, patients should be screened for these. Treating the underlying cause is warranted for prevention of this and also to avoid any complications and further damage to the knee joint. For example, causes such as arthritis and infection must be treated.
A popliteal cyst (Baker cyst) is a large fluid-filled sac located in the popliteal fossa. While the cyst does not usually produce symptoms, some patients may experience stiffness, discomfort, unsteadiness, and/or limited range of motion of the affected knee. Popliteal cysts are observed in children and adults, although the etiologies vary between the two groups.
The pathophysiology is attributed to processes involving the synovial fluid. Specifically, the bursa fills with fluid due to an increased gradient across the synovial cavity and the joint. Hence, this leads to sac enlargement and the subsequent development of the popliteal cyst. Once it forms, it either spontaneously regresses, or alternates between growing and shrinking.
Diagnosis is established through a history, physical exam, and imaging such as ultrasonography (US), computed tomography (CT) and/or magnetic resonace imaging (MRI). Excluding deep venous thrombosis (DVT) or other pathology during the workup is paramount.
Most patients are treated conservatively. However, patient's refractory to treatment may need corticosteroid therapy or surgical intervention, though the latter is not routinely done. It is also pertinent to treat underlying diseases that may have contributed to the cyst development.
A popliteal cyst, also called Baker's cyst, is a lump filled with fluid. This develops in the back of the knee. Most patients do not have symptoms. When this happens in adults, it is likely due to trauma, disease or infection. Rheumatoid arthritis, osteoarthritis, gout, and inflammation are common causes. These conditions change the structure of the knee. Furthermore, extra fluid builds, causing a lump to form. This lump can grow or shrink, or it can disappear altogether. Patients usually feel a ball in the back of the knee.
The cyst may resemble a blood clot which should be ruled out by your doctor.
Diagnosis is made through imaging. Your doctor will order one or more of these: x ray, ultrasound, CT scan, and MRI.
For patients without symptoms, treatment is not necessary. In patients with mild cases, therapy consists of non-steroidal anti-inflammatory drugs (NSAIDs), other analgesics, and/or physical therapy. It is also recommended for the patient to elevate the knee, avoid strenuous activity, and apply ice.
Patients who fail treatment could benefit from steroid injection directly into the affected knee. Surgery may be effective, but recurrence may occur. The need for surgery is decided on a case by case basis.
Recommendations for prevention include wearing proper shoes, being cautious with physical activity, and exercising with warm ups and cool downs. If the patient experiences knee pain or injury, s/he should seek care for evaluation and possible treatment.