Popliteal (Baker's) cyst

INTRODUCTION — Popliteal synovial cysts, also known as Baker's cysts, are common in adults and children [1,2]. They present as swelling in the popliteal fossa due to enlargement of the gastrocnemius-semimembranosus bursa, which lies between these two muscles on the medial side of the fossa, slightly distal to the center crease in the back of the knee [3,4].

Popliteal cysts in adults are often associated with osteoarthritis, inflammatory joint disease, or joint injury. They usually communicate with the adjacent knee joint space, especially in older patients with knee pathology; communicating cysts contain synovial fluid.

In children, popliteal cysts are usually a primary process, arising directly from the gastrocnemius-semimembranosus bursa; they do not communicate with the joint space.

The epidemiology, pathogenesis, clinical features, diagnosis, and management of popliteal cyst will be presented here. The clinical and radiographic evaluation of knee pain is reviewed separately:

●(See "Approach to the adult with unspecified knee pain".)

●(See "Approach to acute knee pain and injury in children and skeletally immature adolescents".)

●(See "Radiologic evaluation of the acutely painful knee in adults".)

EPIDEMIOLOGY — Most popliteal cysts are asymptomatic and are detected incidentally by an imaging study obtained when another diagnosis is suspected (eg, meniscal tear, anterior cruciate ligament tear, medial collateral ligament injury, or osteoarthritis).

The frequency of popliteal cysts varies depending on the imaging technique used, the age of the patient population, and/or whether the cyst is primary or secondary. In one study of 399 patients with knee pain, 293 patients (73.4 percent) showed sonographic features of osteoarthritis and 251 (62.9 percent) showed joint effusion. A popliteal cyst was found in 102 patients (25.8 percent) together with sonographic features of osteoarthritis and joint effusion [5].

When magnetic resonance imaging (MRI) is used to evaluate adults, the prevalence ranges from 5 to 40 percent [6-9]. With arthrography, the prevalence ranges from 23 to 32 percent [10].

Popliteal cysts tend to occur in adults from ages 35 to 70 years [11]. The prevalence increases with age, possibly because knee/bursal communications increase in prevalence with age [12]. There is no consistent difference in frequency between the sexes.

PATHOPHYSIOLOGY — There are a number of factors that contribute to the development and maintenance of most popliteal cysts:

●Production of synovial fluid is increased in response to trauma (eg, meniscal tear) or joint disease (eg, osteoarthritis or rheumatoid arthritis) [6].

●There is a valve-like effect between the knee joint space and the cyst, which is controlled by gastrocnemius-semimembranosus muscle [2].

●During knee flexion, articular pressure forces synovial fluid into the gastrocnemius-semimembranosus bursa [13].

●During knee extension, the gastrocnemius-semimembranosus muscle closes the pathway between the articular space and the popliteal fossa, which prevents fluid from escaping the bursa.

●Because of the unidirectional flow of synovial fluid, repeated knee flexion and extension increases bursal pressure.

●The connection between the gastrocnemius-semimembranosus bursa and the knee joint space gradually dilates under pressure from both sides, giving rise to a popliteal cyst.

●Popliteal cysts may enlarge, dissect, and/or rupture, resulting in compression of adjacent structures.

CLINICAL FEATURES — The clinical features of a popliteal cyst are influenced by the size of the cyst, associated joint pathology, and the presence or absence of complications, such as cyst dissection or rupture.

Asymptomatic popliteal cysts — Most cysts are small, asymptomatic, not clearly evident on physical examination, and detected by imaging studies performed because of unrelated joint symptoms [2]. (See 'Epidemiology' above.)

Symptomatic popliteal cysts — A popliteal cyst can produce posterior knee pain, knee stiffness, and swelling or a mass behind the knee (especially with the knee in extension). Patients may report discomfort with prolonged standing and with hyperflexion of the knee. Symptoms and swelling may be worsened by activity.

Patients may also present with symptoms caused by the joint disease that led to the formation of a popliteal cyst. For example, patients with a popliteal cyst may present with joint instability caused by a tear of a meniscus or anterior cruciate ligament or with joint pain caused by an inflammatory arthritis or osteoarthritis.

Complicated popliteal cyst

Pseudothrombophlebitis — A popliteal cyst may extend (ie, dissect) posterolaterally and cause compression of the popliteal neurovascular bundle. Compression of the popliteal vein can cause venous obstruction. This may result in erythema, distal edema, and a positive Homans' sign, similar to findings seen in patients with a deep vein thrombosis (DVT) of the lower extremity [14,15].

If the cyst has been noted before the development of edema, then the diagnosis of pseudothrombophlebitis rather than DVT is more likely. (See "Clinical presentation and diagnosis of the nonpregnant adult with suspected deep vein thrombosis of the lower extremity".)

The acute pain associated with this presentation generally subsides in several days, but the swelling may persist indefinitely. The enlarged cyst may result in persistent limitation of knee flexion.

Ruptured popliteal cysts — With cyst rupture, severe pain can simulate thrombosis or muscle rupture, with warmth, tenderness, and erythema of the calf.

A ruptured cyst can also produce ecchymoses, which may involve the posterior calf from the popliteal fossa down to the ankle [16]. An ecchymotic area below the medial malleolus, termed the "crescent sign," may be present. In some cases, the rupture can extend upward into the anterior thigh or down the lateral aspect of the leg [2]. A ruptured cyst that migrates anteriorly or posterolaterally can cause compression of one or more components of the popliteal neurovascular bundle [17-19].

Cyst rupture may also be more indolent, with calf and ankle swelling developing without much pain. There are rare reports of ruptured cysts occurring in patients without known joint disease following strenuous exercise [20].

Other complications — Rare complications that may be seen with cyst dissection or rupture include [14,15,21-23]:

●Posterior tibial nerve entrapment, resulting in calf pain and plantar numbness posteriorly. (See "Overview of lower extremity peripheral nerve syndromes", section on 'Tibial nerve'.)

●Anterior compartment syndrome, causing foot drop and anterolateral lower leg swelling. (See "Acute compartment syndrome of the extremities", section on 'Diagnosis'.)

●Posterior compartment syndrome, resulting in calf swelling with pain aggravated by passive toe extension with plantar dysesthesia with toe weakness. (See "Acute compartment syndrome of the extremities", section on 'Diagnosis'.)

●Popliteal artery occlusion, causing leg ischemia. (See "Clinical features and diagnosis of lower extremity peripheral artery disease", section on 'Diagnosis of lower extremity PAD'.)

DIAGNOSIS

Physical examination — A palpable popliteal mass may be identified on physical examination in a patient presenting with knee complaints or following an imaging study finding. In such patients, the diagnosis of a popliteal cyst can usually be based upon physical examination alone.

We diagnose a popliteal cyst by looking for a medial popliteal mass that is most prominent with the patient standing and the knee fully extended and that softens when the knee is bent to 45 degrees (defined as Foucher's sign) [24,25]. The knee should also be examined with the patient lying supine to look for other diagnoses that may contribute to the patient’s presentation.

Imaging studies

Indications for imaging — Imaging is usually not required to establish a diagnosis of a popliteal cyst. Imaging studies are performed in some patients, particularly when there is diagnostic uncertainty and when another condition is suspected. Imaging should be performed under the following circumstances:

●Difficulty differentiating a cystic from a solid mass on physical examination (see 'Physical examination' above)

●Lack of significant change with range of motion

●Laterally located mass

●Absence of knee pathology (eg, rheumatoid arthritis, osteoarthritis, traumatic injury) that would lead to a popliteal cyst (see 'Pathophysiology' above)

Selection of imaging modality — We suggest the use of ultrasound and plain radiography of the knee as the initial imaging modalities. They are both noninvasive and easily obtained, and they provide complementary information.

●Ultrasound – Ultrasound can readily identify cysts, usually as an anechoic mass, which may include echogenic debris or septations (image 1B) [26]. Cysts that are as small as 1 to 2 centimeters can be reliably visualized (image 1A) [27].

Advantages of ultrasound include the lack of ionizing radiation, its increasing availability in the office setting, and its ability to distinguish a popliteal cyst from other popliteal masses, such as popliteal aneurysms or ganglion cysts.

In patients with possible thrombophlebitis or cyst-related "pseudothrombophlebitis," we use ultrasound because it can image the enlarged, dissected, or ruptured cyst (image 1C), as well as the venous circulation, and the findings on ultrasound correlate well with venography [28]. (See "Clinical presentation and diagnosis of the nonpregnant adult with suspected deep vein thrombosis of the lower extremity".)

It may not be possible to distinguish cyst rupture from dissection on ultrasound [15]. In such cases, additional studies, such as an MRI, may be required (image 2).

●Radiographs – Although a plain radiograph of the knee is of limited benefit for viewing the cyst itself, it can demonstrate joint or bone abnormalities that may be associated with the cyst, such as osteonecrosis, osteoarthritis, or, rarely, a calcified popliteal aneurysm (image 3). A soft tissue mass (eg, the popliteal cyst) or a joint effusion may be seen, particularly on lateral views [1,2].

●Magnetic resonance imaging – Magnetic resonance imaging (MRI) provides better soft tissue contrast and multiplanar imaging capability than other imaging methods; thus, it is the technique of choice in patients who require further study to confirm the cystic nature of the lesion, evaluate the anatomical relationship to the joint and surrounding tissues, and identify associated intraarticular disorders [29].

On MRI, the cyst shows high signal on T2-weighted, short T1 inversion recovery (STIR) and proton density sequences (image 4).

We suggest MRI of the knee when surgery is being considered to address structural damage (eg, torn meniscus or anterior cruciate ligament) or when the diagnosis is uncertain after ultrasonography.

●Other imaging modalities – Other techniques that are rarely used to detect or characterize a popliteal cyst include arthrography and computed tomography (CT) [2,26,30]. We prefer the imaging techniques previously described.

DIFFERENTIAL DIAGNOSIS

Deep vein thrombosis — Pseudothrombophlebitis and deep vein thrombosis (DVT) have overlapping clinical presentations (see 'Pseudothrombophlebitis' above). Clinical findings favoring the diagnosis of pseudothrombophlebitis rather than DVT include [2,15]:

●Knee pain and swelling

●Inflammatory joint disease or other knee pathology

●Knee effusion

●Absence of a cord or deep venous tenderness on examination

DVT can also occur as a complication of popliteal cyst. The specific techniques for the diagnosis of DVT differ from those used for the evaluation of a popliteal cyst alone. The radiologist needs to be made aware when both diagnoses are being considered. (See 'Imaging studies' above and "Clinical presentation and diagnosis of the nonpregnant adult with suspected deep vein thrombosis of the lower extremity".)

Other cystic masses — Cysts that occur only as the result of arthrographically induced distention of the joint and a normal communicating bursa are not considered true popliteal cysts [31].

Other cystic masses may occur around the knee and extend posteriorly, mimicking a popliteal cyst, including ganglia, meniscal cysts, pes anserine bursitis, bursitis of the tibial collateral ligament, and cruciate ligament cysts [26]. These cysts can be distinguished on ultrasound imaging from popliteal cysts by their failure to extend between the gastrocnemius and semitendinosus tendons. In patients in whom a solid component is detected in a popliteal mass on ultrasound, MRI with contrast can help differentiate a tumor from a popliteal cyst with debris. (See "Ganglion cysts of the wrist and hand", section on 'Definition' and "Radiologic evaluation of the chronically painful knee in adults", section on 'Bursitis and cysts'.)

Tumors — Discrete, solid masses in the popliteal fossa, such as lipomas, liposarcomas, or other solid tumors, are rare but can often be distinguished from popliteal cysts using ultrasound or other imaging techniques. In patients with a solid component detected by ultrasound, we obtain magnetic resonance imaging (MRI) with contrast if greater definition of the mass is required and to distinguish the solid mass of a tumor from a popliteal cyst with debris. Softening of a popliteal cyst with knee flexion is consistent with, but not specific for, a Baker's cyst. (See 'Physical examination' above and "Radiologic evaluation of knee tumors in adults".)

Pigmented villonodular synovitis is a rare cause of a popliteal mass [32]. (See "Treatment for tenosynovial giant cell tumor and other benign neoplasms affecting soft tissue and bone", section on 'Tenosynovial giant cell tumor'.)

Popliteal aneurysms — A popliteal artery aneurysm may present as a mass in the popliteal fossa, but it is usually pulsatile on examination and can be identified as a vascular structure by ultrasound or other imaging. (See "Popliteal artery aneurysm".)

MANAGEMENT

Initial therapy

Asymptomatic cysts — Asymptomatic cysts do not require treatment and no precautions are required to prevent progression. We advise patients with asymptomatic cysts that there is a small risk of future cyst rupture. Patients should return if a cyst becomes symptomatic and should promptly seek further medical attention if they develop signs or symptoms of the pseudothrombophlebitis syndrome. (See 'Pseudothrombophlebitis' above.)

Symptomatic cysts

Treat the underlying joint disease — We treat any underlying joint disorder that may be present, such as osteoarthritis, rheumatoid arthritis, or meniscal injury, which is causing the increased synovial fluid and enlarged cyst. (See "Meniscal injury of the knee", section on 'Treatment' and "Management of knee osteoarthritis" and "Overview of the management of osteoarthritis" and "Management of moderate to severe knee osteoarthritis".)

Management of symptomatic popliteal cysts may also include arthrocentesis to remove excess fluid and intraarticular injection of the affected joint with glucocorticoids. This approach is based upon multiple case series and clinical experience [1,2,14,15,33-36].

Intraarticular injection — We suggest treating most adult patients with symptomatic cysts with arthrocentesis of the knee and a single intraarticular injection with glucocorticoids (eg, 40 mg triamcinolone acetonide).

A significant decrease in the size of the cyst and/or discomfort is observed in approximately two-thirds of patients within two days to a week from the time of injection in various case series, which is consistent with our experience [2,27]. Glucocorticoid injections into the joint space can also be effective in patients with cysts but without joint effusions [2]. Specific recommendations on how to perform an intraarticular injection are provided separately. (See "Joint aspiration or injection in adults: Technique and indications" and "Intraarticular and soft tissue injections: What agent(s) to inject and how frequently?".)

In patients with a torn meniscus or other structural damage to the knee, we also perform arthrocentesis and a single glucocorticoid injection, which may provide temporary relief until a more definitive procedure can be performed. (See "Meniscal injury of the knee", section on 'Treatment'.)

In cases where the cyst is so large that it prevents full knee extension, direct aspiration and injection of the popliteal cyst using imaging guidance may be preferred, if local expertise is available [37].

Control of inflammation by glucocorticoid injection can reduce the pressure gradient between the joint and the cyst, lead to symptomatic improvement, and reduce the risk of recurrence [2,14,15,33]. There are no randomized trials that have compared glucocorticoid injections with alternative treatment. In an uncontrolled series of 30 patients with osteoarthritis and a popliteal cyst that were treated with intraarticular glucocorticoid injections (ie, 40 mg triamcinolone acetonide) and evaluated at baseline and four weeks following injection by ultrasound, the treatment was associated with a reduction in cyst size in all patients and with complete disappearance of the cyst in two patients [33].

Treatment of cyst complications

●Pseudothrombophlebitis or ruptured cyst – Patients with features of pseudothrombophlebitis due to dissecting or ruptured cysts should be treated with rest, elevation, and analgesics. We also perform arthrocentesis and intraarticular glucocorticoid injection of the knee in patients with this condition. (See 'Complicated popliteal cyst' above and 'Pseudothrombophlebitis' above and 'Ruptured popliteal cysts' above.)

●Venous obstruction – The management of venous obstruction is discussed separately. (See "Clinical presentation and diagnosis of the nonpregnant adult with suspected deep vein thrombosis of the lower extremity".)

●Nerve entrapment – Patients with nerve entrapment caused by enlarged or ruptured cysts may respond to intraarticular glucocorticoid injections [34]. If available, direct aspiration and injection of the cyst aided by ultrasound may be considered. Refractory cases may require surgical management. (See 'Other complications' above and "Overview of lower extremity peripheral nerve syndromes" and 'Cysts requiring additional treatment' below and 'Popliteal cyst aspiration and injection' below and 'Surgery' below.)

●Compartment syndrome – Patients with acute compartment syndrome require immediate surgical evaluation. (See "Acute compartment syndrome of the extremities".)

Cysts requiring additional treatment

Confirm the diagnosis — In patients who do not respond to the initial arthrocentesis and injection with intraarticular glucocorticoids, imaging studies to assess the anatomy and confirm the diagnosis should be obtained. This includes ultrasonography (if not already performed) and magnetic resonance imaging (MRI) in patients in whom the diagnosis remains uncertain after ultrasound alone. (See 'Differential diagnosis' above.)

Popliteal cyst aspiration and injection — In patients who do not respond to intraarticular injection, ultrasound-guided direct aspiration of popliteal cysts, followed by injection of glucocorticoids, can be performed by clinicians experienced in this procedure (image 5) [1,2,38]. In patients found to have noncommunicating cysts, this approach can be attempted prior to surgical excision.

Substantial clinical improvement in patients with symptomatic popliteal cysts can be achieved via ultrasound-guided aspiration and glucocorticoid injection as the sole treatment. In a study of 47 patients with popliteal cysts, significant relief in pain and stiffness was noted, along with improvement in physical function. These findings were best demonstrated in patients without complex cysts or severe tricompartmental osteoarthritis. There was a recurrence rate of 12.7 percent [39].

Surgery — Surgical excision may occasionally be required if the cyst remains symptomatic with pain and/or limited mobility attributable to the cyst despite treatment of the underlying disorder and administration of intraarticular glucocorticoids. Generally, surgical excision should be reserved only for those cases where more conservative interventions have failed and where there is significant functional impairment that can be ascribed to the cysts.

Surgical excision of popliteal cysts is typically a lengthy procedure that must provide the operator with wide visual access to fully excise the cysts [1]. The procedure requires arthroscopic excision of the cyst wall and removal of intraarticular lesions that predispose to cyst formation [40]. Potential risks include difficulties with wound healing in the popliteal fossa and recurrence. Arthroscopic approaches include repair of the intraarticular abnormality with either removal of the cyst or debridement of the connecting capsular opening [35,36].

PROGNOSIS — Most popliteal cysts do not cause symptoms or complications. Some cysts resolve without any intervention, and most respond to treatment of associated disorders of the knee joint.

Cysts often recur in patients with underlying joint pathology that is not adequately treated. Surgical excision is not a definitive procedure, and cysts may recur after surgery because of the underlying joint abnormality [1]. A retrospective study of 41 patients who underwent 43 surgical procedures to remove the cyst noted a recurrence in 14 percent of cases [41].

CHILDREN — Several features of popliteal cysts in children differ from those seen in adults (picture 1). In children, popliteal cysts are usually a primary process, arising directly from the gastrocnemius-semimembranosus bursa, and they do not communicate with the joint space. By contrast, popliteal cysts that occur in children with structural damage due to injury or with inflammatory arthritis resemble those in adults [11,42-44].

The peak prevalence of popliteal cysts in children is from four to seven years of age [11]. The frequency on ultrasound examination was 2.4 percent in a study of asymptomatic children and over 50 percent in a cohort of children with juvenile idiopathic arthritis (JIA) presenting with knee effusions [42,43]. The frequency was 6.3 percent on magnetic resonance imaging (MRI) in a study of children referred primarily for knee pain [44].

Children with primary cysts usually do not require treatment, and most cysts in children resolve completely without treatment [45]. Children may be managed in collaboration with a pediatric orthopedic surgeon or pediatric rheumatologist.

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Baker's (popliteal) cyst (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Pathophysiology – In adults, popliteal cysts generally arise in association with underlying joint disease, such as osteoarthritis, inflammatory arthritis, or joint injury (eg, meniscal tear). The cyst forms as an enlargement of the gastrocnemius-semimembranosus bursa, which lies between these two muscles on the medial side of the fossa slightly distal to the center crease of the back of the knee. (See 'Pathophysiology' above.)

In children, primary cysts are the usual form (picture 1); these arise directly from the gastrocnemius-semimembranosus bursa and do not communicate with the joint space. (See 'Children' above.)

●Symptoms and physical findings – Most cysts are small, asymptomatic, and only noted incidentally. Symptoms from associated joint disease may be the major or only symptoms present.

Symptoms potentially attributable to the cyst include posterior knee pain, knee stiffness, and detection of swelling or a mass behind the knee.

The cysts are typically more prominent when the knee is in extension. (See 'Clinical features' above.)

●Complications – Infrequent but potentially significant complications include:

•Pseudothrombophlebitis, a syndrome that resembles venous thrombosis, with calf pain, erythema, distal edema, and a positive Homans' sign, results when popliteal cysts enlarge, dissect, and/or rupture, resulting in compression of adjacent structures.

•Ecchymoses involving the posterior calf from the popliteal fossa down to the ankle can occur with cyst rupture.

•Infrequent complications of popliteal cysts include leg ischemia, nerve entrapment, and compartment syndromes. (See 'Complicated popliteal cyst' above.)

●Diagnosis by physical examination – We usually diagnose popliteal cysts based upon physical examination in patients with popliteal swelling who lack signs or symptoms of cyst rupture or dissection. We examine the knee with the patient lying supine through an arc of motion ranging from full extension to at least 90 degrees of flexion. The mass, which is usually felt medially in the popliteal space and is most prominent at full knee extension, may soften or disappear on flexion to 45 degrees (Foucher's sign) as the tension on the cyst is relieved. (See 'Diagnosis' above and 'Physical examination' above.)

●Role of diagnostic imaging – In patients in whom the diagnosis is uncertain on examination alone, we use ultrasound (image 1A-C) and plain radiography (image 3) of the knee as the initial imaging modalities.

In patients with calf symptoms, we perform ultrasonography to exclude deep vein thrombosis (DVT).

We obtain magnetic resonance imaging (MRI) of the knee when surgery is being considered to address structural damage to the knee or when the diagnosis is uncertain after ultrasonography (image 4). (See 'Imaging studies' above.)

●Differential diagnosis – The differential diagnosis includes DVT if there are signs of calf pain or swelling, other cystic masses such as ganglia, solid tumors such as lipomas and liposarcomas, and popliteal artery aneurysms.

Most of these entities can be distinguished from popliteal cysts by ultrasound imaging; we perform MRI if solid elements of a cyst or a mass are detected on ultrasonography. (See 'Differential diagnosis' above.)

●Initial management – We manage popliteal cysts in adults primarily by treating the underlying process associated with the cyst.

For most patients with bothersome symptoms related to the cyst, we suggest intraarticular glucocorticoid injection (Grade 2C). Direct cyst aspiration and injection of the cyst with glucocorticoids is an appropriate alternative for patients with large, non-communicating, and/or complex cysts. This procedure generally requires imaging assistance. (See 'Management' above and 'Initial therapy' above.)

We do not treat incidentally noted asymptomatic popliteal cysts, and children with primary cysts generally do not require treatment. (See 'Management' above and 'Initial therapy' above and 'Surgery' above.)

●Additional management – Patients who do not respond to initial treatment require additional assessment with ultrasound and/or MRI if not previously performed.

For patients who have not responded to intraarticular glucocorticoid injection, we suggest ultrasound-guided aspiration of the cyst and injection with intraarticular glucocorticoids (Grade 2C).

Surgical excision is generally reserved for cases where more conservative interventions have failed and where there is significant functional impairment that can be ascribed to the cysts.

●Prognosis – Most popliteal cysts do not cause symptoms or complications. Some cysts resolve without any intervention, and most respond to treatment of associated disorders of the knee joint.

Cysts often recur in patients with underlying joint pathology that is not adequately treated. (See 'Prognosis' above.)