INTRODUCTION — Popliteal synovial cysts, also known as Baker's cysts, are a common occurrence 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 secondary to degenerative or 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. (See 'Popliteal (Baker's) cyst in children' below.)
The epidemiology, pathogenesis, clinical features, diagnosis, and management of popliteal (Baker's) 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" and "Knee bursitis" and "Radiologic evaluation of the acutely painful knee in adults" and "Radiologic evaluation of the chronically painful knee in adults" and "Approach to acute knee pain and injury in children and skeletally immature adolescents" and "Approach to chronic knee pain or injury in children or skeletally immature adolescents".)
EPIDEMIOLOGY — Most popliteal cysts are asymptomatic and are detected incidentally by an imaging study performed for some other reason. They tend to occur in adults from ages 35 to 70 [5].
In general, the prevalence of popliteal cysts varies based upon the imaging technique used, the age of the patient population, and/or whether the cyst is secondary or primary.
The prevalence ranges from 5 to 40 percent using magnetic resonance imaging (MRI) among adults with suspected internal derangement or osteoarthritis (OA) [6-9]; in a similar adult population, it ranged from 23 to 32 percent by arthrography [10]. The prevalence increases with age, possibly because knee/bursal communications increase in prevalence with age [11]. 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:
●Joint-cyst communication, which has been documented by cadaveric dissections, at the time of arthroscopy, and by injecting dye directly into these structures [2].
●A valve-like effect between the knee joint space and the cyst, which is controlled by gastrocnemius-semimembranosus muscle changes with knee flexion and extension. This contributes to sequestration of synovial fluid in the popliteal fossa [12]. The communicating space is open in flexion, but the tense gastrocnemius-semimembranosus muscles close this pathway in extension.
●Negative intraarticular knee pressure during partial knee flexion (minus 6 mmHg), compared with a positive pressure (16 mmHg) during extension. These pressure changes help direct the synovial fluid flow from the suprapatellar bursa toward the knee during flexion and toward the cyst during extension [12]. Bursal pressure exceeds joint pressure during extension, but the flow of synovial fluid is blocked by the valve-like effect just described.
●Enlargement of the gastrocnemius-semimembranosus bursa from repeated small traumas of the bursa related to muscle contraction.
●Joint capsule herniation into the popliteal fossa [2,7].
RISK FACTORS — Risk factors for the development of popliteal cysts in adults include a history of trauma, noted in about one-third of patients, or a history of coexistent joint disease, seen in about two-thirds of patients [6]. The most common knee disorders include osteoarthritis (OA), rheumatoid arthritis (RA), and meniscal tears. The presence of popliteal cysts among patients with OA was unrelated to the radiographic severity of the arthritis [9]. Popliteal cysts may also be seen in other joint disorders, such as infectious arthritis or pigmented villonodular synovitis.
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. Most cysts are small, asymptomatic, not clearly evident on physical examination, and detected only by imaging studies performed because of unrelated joint symptoms [2].
When symptomatic, common signs and symptoms include posterior knee pain, knee stiffness, and swelling or a mass behind the knee (especially with the knee in extension). Patients frequently report discomfort with prolonged standing and with hyperflexion of the knee. Symptoms and swelling may be worsened by activity.
Symptoms due primarily to an associated joint disease may also occur, such as instability due to internal derangement or joint pain from inflammatory arthritis or osteoarthritis (OA). In some patients, these are the major or only symptoms present. The presentation and clinical findings of the popliteal cyst do not differ based upon the underlying pathology or associated disease.
Complications — Popliteal cysts may enlarge, dissect, and/or rupture, resulting in compression of adjacent structures and a constellation of symptoms and signs that may resemble venous thrombosis (thrombophlebitis) [13]. Significant complications of popliteal cysts include pseudothrombophlebitis, leg ischemia, nerve entrapment, and compartment syndromes [14,15]. These events are uncommon, but there is a lack of data to accurately estimate their frequency. Though the acute pain generally subsides in a matter of days, the swelling may persist indefinitely, resulting in persistent limitation of knee flexion due to the size of the cyst.
Enlargement — A tender cyst enlarging into the calf (a dissecting cyst) can result in erythema, distal edema, and a positive Homans' sign, similar to findings in deep vein thrombosis (DVT) of the lower extremity [13,16]. (See "Clinical presentation and diagnosis of the nonpregnant adult with suspected deep vein thrombosis of the lower extremity".)
Enlargement of the cyst within the popliteal space (independent of dissection or rupture) can compress an adjacent vein, causing lower leg and ankle swelling due to venous obstruction. Historically, the cyst may be noted before the edema, suggesting that the swelling may be related to the cyst rather than thrombosis.
Rupture — With cyst rupture, severe pain simulates 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 [17]. An ecchymotic area below the medial malleolus, termed the "crescent sign," may be present. In some cases, the rupture can extend upward into the thigh or down the anterior lateral aspect of the leg [2]. Cyst rupture may also be more indolent, with calf and ankle swelling developing without much pain. Ruptured popliteal cysts can occur without known joint disease or any prior history of joint pain or swelling, sometimes after strenuous exercise [18].
Associated complications — Rare complications that may be seen with cyst dissection or rupture include posterior tibial nerve entrapment (resulting in calf pain and plantar numbness posteriorly), anterior compartment syndrome (causing foot drop and anterolateral lower leg swelling), posterior compartment syndrome (resulting in calf swelling with pain aggravated by passive toe extension with plantar dysesthesia with toe weakness), and popliteal artery occlusion (causing leg ischemia) [13,16,19].
DIAGNOSIS — In patients with a popliteal mass, the diagnosis of a popliteal (Baker's) cyst can usually be based upon physical examination alone. Imaging studies are performed in some patients, particularly when there is diagnostic uncertainty and when another condition is suspected. However, imaging is usually not required.
Physical examination — We diagnose a Baker's cyst by the finding of a medial popliteal mass that is most prominent with the patient standing and the knee fully extended. The swelling softens or disappears upon knee flexion to 45 degrees (defined as Foucher's sign), as the tension on the cyst is relieved [20]. The knee should also be examined with the patient lying supine and the knee put through an arc of motion ranging from full extension to at least 90 degrees of flexion to confirm these findings and to adequately examine the knee joint for associated pathology.
Imaging studies — Imaging, usually plain radiography and ultrasonography, should be performed if the diagnosis is uncertain on examination or if another condition is suspected (image 1). Such clinical settings may include difficulty differentiating a cystic from a solid mass on physical examination, lack of significant change with range of motion, a laterally located mass, and the absence of knee pathology. Plain radiographs may detect other associated abnormalities that can be associated with knee pain, such as osteonecrosis or significant cartilage narrowing due to osteoarthritis. Imaging is also needed if the clinical presentation suggests vascular or neurologic compromise or possible thrombophlebitis.
We take the following approach to imaging:
●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 can readily identify cysts, usually as fluid-filled spaces, and shows the size and extent of the cyst in relation to adjacent soft tissue structures. Additional advantages of ultrasound include the absence of ionizing radiation and its increasing availability in the office setting. Cysts that are as small as 1 to 2 centimeters can be reliably visualized (image 2A) [21]. Ultrasound can also distinguish popliteal (Baker's) cysts from popliteal aneurysms, ganglion cysts, or other popliteal masses. The cyst appears on ultrasound as an anechoic mass, which may include echogenic debris or septations (image 2B) [22].
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. A soft tissue mass (the cyst) or a joint effusion may be seen, particularly on lateral views [1,2].
●In patients with possible thrombophlebitis or cyst-related "pseudothrombophlebitis" syndrome, we use ultrasound because it can image the enlarged, dissected, or ruptured cyst (image 2C), as well as the venous circulation, and the findings on ultrasound correlate well with venography [23]. It may not be possible to distinguish cyst rupture from dissection, but either can usually be detected if present [13]. Infrequently, a complete cyst rupture may not be visualized ultrasonographically, and additional studies such as MRI may be required (image 3). (See "Clinical presentation and diagnosis of the nonpregnant adult with suspected deep vein thrombosis of the lower extremity".)
●We suggest MRI of the knee if an internal derangement is suspected and surgery is being considered, or when the diagnosis is uncertain after ultrasonography. For example, we perform MRI with contrast to differentiate a tumor from a popliteal cyst with debris if a solid component is detected in a popliteal mass on ultrasound or if greater definition of the mass is required. MRI provides high-quality images of the popliteal space, the cyst, and associated structures. It can also define intraarticular knee pathology, such as a meniscal or ligamentous injury, which ultrasound cannot, and it does not expose patients to ionizing radiation. On MRI, the cyst shows high signal on T2-weighted, short T1 inversion recovery (STIR), and proton density sequences (image 4). MRI may also identify synovitis in patients with previously unidentified inflammatory arthritis. (See 'Differential diagnosis' below.)
MRI provides better soft tissue contrast and multi-planar imaging capability than other imaging methods; thus, it is the technique of choice in patients who require further study following ultrasound and plain radiography to confirm the cystic nature of the lesion, evaluate the anatomical relationship to the joint and surrounding tissues, and identify associated intraarticular disorders [24].
●Other imaging techniques that are rarely used to detect or characterize a popliteal cyst include arthrography and computed tomography (CT) [2,22,25]. We prefer the imaging techniques previously described.
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 [26].
DIFFERENTIAL DIAGNOSIS — The principal disorders that mimic many of the clinical features of a Baker's cyst include:
●Deep vein thrombosis (DVT)
●Cystic masses, including synovial cysts and ganglion cysts
●Solid masses, including sarcomas and lymphoma
●Popliteal artery aneurysms
It is especially important to distinguish between the infrequently occurring symptoms of pseudothrombophlebitis caused by a popliteal cyst and symptoms resulting from a true deep vein thrombosis.
Deep vein thrombosis — Deep vein thrombosis may be difficult to distinguish on clinical grounds alone from "pseudothrombophlebitis" resulting from a markedly enlarged, dissected, or ruptured Baker's cyst. Either may cause swelling, warmth, tenderness, and a positive Homan's sign. An enlarged or ruptured cyst may also cause venous compression and an increased risk of a DVT occurring together with the cyst. (See 'Complications' above.)
Clinical findings favoring the diagnosis of pseudothrombophlebitis rather than DVT include the presence of knee pain and swelling, inflammatory joint disease or other knee pathology, a knee effusion, and the absence of a cord or deep venous tenderness on examination [2,13]. The distinction between pseudothrombophlebitis and a DVT can usually be made with ultrasound studies to identify a cyst, if present, and to evaluate for possible venous disease. The diagnostic considerations should be reviewed with the radiologist prior to the study, as the specific techniques for the diagnosis of DVT differ from those used for the evaluation of a popliteal cyst alone. (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 — Other cystic masses may occur around the knee and extend posteriorly, mimicking a Baker's cyst, including ganglia, meniscal cysts, pes anserine bursitis, bursitis of the tibial collateral ligament, and cruciate ligament cysts [22]. These cysts can be distinguished on ultrasound imaging from popliteal Baker's 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 masses in the popliteal fossa, such as lipomas, liposarcomas, or other solid tumors, are uncommon 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 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. A positive Foucher's sign of softening and/or of decreased size 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 [27]. (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 — Management of symptomatic popliteal cysts includes treatment of the underlying joint disorder (when present), arthrocentesis, and intraarticular injection of the affected joint with glucocorticoids. This approach is based upon multiple case series and clinical experience [1,2,13,16,28-31].
Asymptomatic cysts found incidentally do not require treatment. 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.
General measures — In all patients with symptomatic cysts, we treat any underlying joint disorder that may be present, such as osteoarthritis (OA), rheumatoid arthritis (RA), 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".)
Initial therapy — We suggest treating adult patients with symptomatic, painful cysts, with or without calf involvement, with arthrocentesis of the knee and intraarticular injection with glucocorticoids (eg, 40 mg triamcinolone acetonide), using the same approach as that for OA or RA. 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 [2,21]. Glucocorticoid injections into the joint space can also be effective in patients with cysts but without joint effusions [2]. (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 cases where the size of the cyst is large and the symptoms are mostly in the posterior knee, direct injection of the popliteal cyst using imaging assistance may be preferred. Direct aspiration or injection of the cyst may be performed if imaging guidance such as ultrasonography or arthrography is available [32]. Substantial clinical improvement in patients with symptomatic popliteal cysts can be achieved via ultrasound-guided intervention as the sole treatment. In 1 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 OA. There was a recurrence rate of 12.7 percent [33].
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 [2,13,16,28]. There are no randomized trials that have compared glucocorticoid injections with alternative treatment. In an uncontrolled series of 30 patients with OA and a popliteal cyst that were treated with intraarticular glucocorticoid injections (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 [28].
In patients with a torn meniscus or other internal derangement, we also perform arthrocentesis and a glucocorticoid injection, which may provide temporary relief until a more definitive procedure can be performed. (See "Meniscal injury of the knee", section on 'Treatment'.)
Treatment of cyst complications — In patients with possible cyst complications, such as pseudothrombophlebitis or syndromes related to the compression of adjacent vascular structures or nerves, we perform the appropriate studies needed to diagnose and treat such complications.
●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 'Complications' above and 'Enlargement' above and 'Rupture' above.)
●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".)
●Patients with nerve entrapment caused by enlarged or ruptured cysts may respond to intraarticular glucocorticoid injections [29]. If available, direct aspiration and injection of the cyst aided by ultrasound or arthrographic imaging may be considered next. Refractory cases may require surgical management. (See 'Associated complications' above and "Overview of lower extremity peripheral nerve syndromes" and 'Cysts resistant to initial treatment' below and 'Direct cyst aspiration and injection' below and 'Surgery' below.)
●Patients with acute compartment syndrome require immediate surgical evaluation. (See "Acute compartment syndrome of the extremities".)
Cysts resistant to initial treatment — In patients who do not respond to the initial arthrocentesis and injection with intraarticular glucocorticoids, the initial diagnosis of a Baker's cyst should be re-evaluated, and additional therapeutic measures may be necessary if the diagnosis is confirmed:
●Accuracy of the diagnosis of popliteal cyst – Imaging studies to assess the anatomy and confirm the diagnosis should be obtained. This includes ultrasonography (if not already performed) and MRI in patients in whom the diagnosis remains uncertain after ultrasound alone. (See 'Differential diagnosis' above.)
●Persistent knee pathology – The presence of a torn meniscus, other internal derangement, or persistent joint inflammation may result in failure to respond to an initial injection, persistent effusions, or recurrence of swelling after an initial response. We obtain MRI in such patients to identify underlying knee pathology that may be present. A repeat injection and, if necessary, additional interventions such as arthroscopic knee surgery or treatment with more potent medical interventions should be performed to treat such patients, depending upon the underlying knee disorder, prior to considering surgical cyst excision. (See "Meniscal injury of the knee", section on 'Treatment'.)
●Non-communicating cysts – In the infrequent adult patient who does not respond to intraarticular injection because of a lack of communication of the cyst with the joint, direct cyst aspiration and injection with glucocorticoid can be performed (see 'Direct cyst aspiration and injection' below). Symptomatic noncommunicating cysts that do not improve may require surgical excision.
●Absence of joint pathology – Surgical excision may rarely be required in the absence of demonstrable joint pathology. Thus, consultation with an orthopaedic surgeon should be obtained in patients with persistent symptoms or functional impairment who have no response to injection and in whom pathology in the knee cannot be identified. (See 'Surgery' below.)
Direct 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) [1,2,34]. In patients found to have noncommunicating cysts, this approach can be attempted prior to surgical excision.
Surgery — Surgical excision may very infrequently 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]. 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 [30,31].
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. A glucocorticoid injection is usually beneficial in patients with arthritis that can be controlled medically, but cysts often recur in patients with underlying joint pathology that is not adequately treated. Significant clinical improvement in patients with symptomatic popliteal cysts can be achieved via ultrasound-guided intervention as the sole treatment in over 87 percent of patients [33]. Surgical excision is not a definitive procedure, and cysts may recur after surgery because of the underlying joint abnormality [1]. Based upon the limited available evidence, which is limited to retrospective case series, arthroscopic excision of the cyst wall and removal, if necessary, of intraarticular lesions are the preferred surgical approaches for the enlarged popliteal cyst [35].
POPLITEAL (BAKER'S) CYST IN CHILDREN — Several features of popliteal (Baker's) 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. In contrast, popliteal cysts that occur in children with internal derangements due to injury or with inflammatory arthritis resemble those in adults [5,36-38].
The peak prevalence of popliteal cysts in children is from four to seven years of age [5]. The prevalence 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 [36,37]. The prevalence was 6.3 percent on MRI in a study of children referred primarily for knee pain [38].
Children with primary cysts usually do not require treatment, and most cysts in children resolve completely without treatment [39]. Children are often managed in collaboration with an orthopaedic surgeon.
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 5th to 6th 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 10th to 12th 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
●Characteristic pathophysiology in adults – Popliteal synovial cysts (Baker's cysts) in adults generally arise in association with underlying joint disease, such as degenerative or inflammatory arthritis or joint injury. 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.
In adults, the cysts are usually in communication with the adjacent knee joint space. The development and maintenance of the cyst is influenced by communication between the posterior joint capsule and the bursa, a pressure gradient favoring movement of synovial fluid towards the cyst, and a valve-like effect between the knee joint space and the cyst. This effect is controlled by gastrocnemius-semimembranosus muscle changes with knee flexion and extension by which fluid is sequestered in the popliteal fossa. (See 'Pathophysiology' above and 'Risk factors' above.)
●Primary cysts – Primary cysts, the usual form in children (picture 1), arise directly from the gastrocnemius-semimembranosus bursa but do not communicate with the joint space. (See 'Introduction' above and 'Epidemiology' above.)
●Symptoms and physical findings – Posterior knee pain, knee stiffness, and detection of swelling or a mass behind the knee are the most commonly noted symptoms, although most cysts are small, asymptomatic, and only noted incidentally. The cysts are typically more prominent when the knee is in extension. Symptoms from associated joint disease may be the major or only symptoms present. (See 'Clinical features' above.)
●Complications – Most symptomatic cysts do not cause significant complications. However, popliteal cysts may enlarge, dissect, and/or rupture, resulting in compression of adjacent structures and a constellation of symptoms and signs that can resemble venous thrombosis, called pseudothrombophlebitis, including calf pain, erythema, distal edema, and a positive Homans' sign. A ruptured cyst can also produce ecchymoses which may involve the posterior calf from the popliteal fossa down to the ankle. Infrequent complications of popliteal cysts include leg ischemia, nerve entrapment, and compartment syndromes. (See 'Complications' 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 or in whom the clinical presentation is consistent with pseudothrombophlebitis or vascular or neurologic compromise, we use ultrasound (image 2A-C) and plain radiography (image 1) of the knee as the initial imaging modalities. We obtain MRI of the knee when an internal derangement is suspected and surgery is being considered or when the diagnosis is uncertain after ultrasonography (image 4). (See 'Imaging studies' above.)
●Differential diagnosis – The differential diagnosis includes deep vein thrombosis (DVT) if signs of calf pain or swelling are present; 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 (Baker's) cysts by ultrasound imaging if suspected clinically; we perform MRI if solid elements of a cyst or a mass are detected on ultrasonography. (See 'Differential diagnosis' above.)
●Management – We manage popliteal cysts in adults primarily by treating the underlying intraarticular inflammatory or degenerative process associated with the cyst. In patients with painful symptomatic cysts, despite usual management of their joint disease, or those in whom temporary relief of symptoms would be of benefit prior to a more definite procedure (eg, in patients with a meniscal tear), we suggest treatment with a glucocorticoid injection of the knee joint (Grade 2C). In cases where the size of the cyst is large and the symptoms are mostly in the posterior knee, direct injection of the popliteal cyst using imaging assistance may be preferred. (See 'Management' above and 'Initial therapy' above.)
In patients with calf symptoms, we perform imaging, usually ultrasonography, to exclude DVT. We do not treat incidentally noted asymptomatic popliteal cysts, and children with primary cysts generally do not require treatment. We evaluate patients with symptomatic cysts resistant to these interventions for the accuracy of the diagnosis or the presence of persistent articular pathology or a noncommunicating cyst prior to direct cyst injection or referral for surgical excision. (See 'Management' above and 'Initial therapy' above and 'Surgery' above.)
