Cysts and Other Masses About the Knee
Identifying and Treating Common and Rare Lesions
Warren D. Yu, MD; Matthew S. Shapiro, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 27 - NO. 7 - JULY 1999
In Brief: Masses about the knee are most commonly benign cysts. The diagnosis can often be made with a history and physical exam, but radiographs and MRI are sometimes required, and histologic evaluation is occasionally necessary. Popliteal (Baker's) cysts are often indicative of arthritis; treatment for the underlying pathology may reduce swelling and permit resorption. Meniscal cysts indicate an underlying tear; treatment is resection and cyst excision. Symptomatic ganglion cysts generally require surgical excision. Treatment for bursitis is conservative. The less-common synovial chondromatosis, pigmented villonodular synovitis, synovial sarcoma, and lesions of the proximal tibiofibular joint generally require referral and surgery.
Cystic lesions in and around the knee are commonly encountered by primary care physicians during routine examinations or diagnostic workups for knee problems. The vast majority of masses are benign, but a mass or lesion can indicate an underlying condition or injury or, rarely, be malignant (table 1). Timely diagnosis and treatment are therefore important and often of particular concern to patients active in sports or exercise.
A mass about the knee has a limited differential diagnosis: Popliteal (Baker's), ganglion, and meniscal cysts are the most common, but a few other conditions must be considered. Bursal swellings surrounding the knee, for example, may be confused with cysts.
In most cases, a precise diagnosis can be made with a history and physical examination. Sometimes, ancillary studies such as radiography, ultrasonography, and magnetic resonance imaging (MRI) are necessary. Occasionally, the final diagnosis can only be made through surgery and histologic evaluation.
Popliteal, or Baker's, cysts are the most common cysts found in the knee region and the most common synovial-lined cysts in the body. An asymptomatic popliteal cyst may be an incidental finding on examination or imaging (1-6). A large cyst may impair movement, and, if a cyst ruptures, it may cause a sudden onset of calf swelling and pain, which can mimic acute thrombophlebitis.
Popliteal cysts occur when fluid from the knee overflows under pressure through a weak spot in the capsule known as the popliteal recess. This fluid buildup can occur in young, healthy, active patients because of knee inflammation resulting from abnormalities such as meniscal tears or collateral and cruciate ligament tears, but it is most commonly seen in older patients who have osteoarthritis or other inflammatory conditions. Because these cysts arise from the joint, they are filled with synovial fluid and lined by synovium.
Location. Popliteal cysts are usually located behind the medial femoral condyle and between the medial head of the gastrocnemius muscle and the semimembranosus tendon. Most commonly they can be palpated along the medial side of the popliteal fossa. They may, however, extend in any direction and may be found superficial or deep to the semimembranosus and gastrocnemius muscles. When a popliteal cyst increases in size, it often becomes a mass in the medial aspect of the proximal calf.
Clinical features. Patients typically report knee pain or mechanical symptoms such as catching, popping, or locking accompanied by a feeling of fullness or pain in the popliteal fossa. Generally, pain is caused by the underlying pathology. The cyst itself, however, may be variably or intermittently painful or may be the presenting complaint in an otherwise asymptomatic knee, even in the presence of significant pathology.
Other popliteal structures, including tendons, nerves, and vessels, may be secondarily involved by compression or inflammation, resulting in pain or numbness. As noted above, large cysts may limit flexion or extension, and a ruptured cyst may cause sudden thrombophlebitis-like calf symptoms. In addition to questions about the pattern of pain and swelling, the history should include the effectiveness of simple treatments like ice and nonsteroidal anti-inflammatory drugs (NSAIDs) for reducing pain and other symptoms.
The physical examination should be aimed at identifying a mechanical or inflammatory problem in the knee, and the popliteal fossa should be examined for a discrete mass, which may be tender or nontender. Unlike aneurysms or pseudoaneurysms, which can occur in this region, popliteal cysts do not demonstrate a rhythmic pulse. The examiner should also check for effusion, joint-line tenderness, meniscal tears, and instability.
A meniscal tear can be diagnosed using McMurray's test, the goal of which is to "catch" a torn piece of meniscus by compression and shear. Because tears differ, there is no single way to perform the test, and the results may be inconsistent. We recommend exerting varus and valgus force while moving the patient's knee from full flexion to about 90° flexion in internal and external rotation. Palpation of the lateral and medial joint surfaces during these maneuvers produces an often-painful click or crepitation. We also consider the test positive if the maneuvers reproduce the patient's pain without a click. One must be careful to differentiate joint- line (meniscal) clicks from those that occur within the patellofemoral joint.
One should also consider the patellofemoral joint as a possible source of pain, because patellofemoral pain may be referred to the popliteal fossa.
Imaging. Plain radiographs are frequently normal, although popliteal cysts are a common finding in patients who have radiographic signs of osteoarthritis. Ultrasound is a useful diagnostic tool to ensure that a mass is a fluid-filled cyst rather than a solid tumor, but MRI is a better choice, since it not only confirms the presence of the cyst but also sheds light on the underlying intra-articular pathology (figure 1) (2).
Treatment. Initial treatment of a popliteal cyst with conservative measures such as heat or ice and NSAIDs is sometimes successful, and patients may continue to participate in sports activities as tolerated. External support such as an elastic wrap around the knee may cause undue pressure on the cyst and lead to increased pain. In the case of a ruptured cyst, vascular examinations are normal, and the pain and swelling quickly resolve with symptomatic treatment. Attention to the underlying knee pathology, possibly involving surgery, may be required if the patient remains symptomatic.
Aspiration of a popliteal cyst is usually not helpful and certainly not curative. In many cases, the fluid is very viscous and impossible to draw through a needle. When the fluid can be aspirated, persistent fluid production is often present in the knee, and the cyst will most likely recur.
If cyst-related symptoms persist despite supportive measures, treatment of the underlying knee pathology is usually necessary. This can usually be accomplished arthroscopically, and it may diminish or cure the tendency for knee swelling; if so, the cyst may resorb on its own.
Operative treatment of popliteal cysts for refractory symptoms is controversial. If the cyst is secondary to intra-articular pathology, excision of the cyst will not alleviate the pain, and the cyst is likely to recur. Excision of the cyst may be indicated if the pathology within the joint has been corrected but the cyst continues to cause symptoms. Very large cysts involve intricate dissection of surrounding neurovascular structures, and such procedures may be associated with significant postoperative morbidity.
Arthroscopic treatment of cysts has been introduced recently. The popliteal recess may be identified and the cyst perforated arthroscopically, obviating the need for open surgery. Long-term results for this procedure are not available, but in our experience the procedure can, in most cases, be done with little additional morbidity as compared with routine arthroscopy.
Meniscal cysts are encapsulated-mass lesions lined with connective tissue. They contain synovial-like fluid.
Location. These cysts are always associated with a tear in a meniscus, usually the lateral, rarely the medial. As such, they arise on the joint line. Medial cysts tend to be larger and more posterior. The tear that produces a meniscal cyst is almost always a horizontal cleavage tear that extends peripherally and breaches the capsule below the meniscofemoral ligament and above the meniscotibial ligament. At this site there is a relatively weak spot in the capsule, and fluid can percolate out through the tear and form a cyst in the soft tissues surrounding the knee (7,8).
Clinical features. Meniscal cysts are firm, with a rubbery feel. They are fixed to the underlying joint structures and therefore immobile. The cysts themselves are usually painless unless they irritate surrounding soft tissues.
The patient typically notes symptoms consistent with a meniscal tear, such as catching, popping, locking, joint-line pain, and swelling. The associated tears are usually more degenerative than traumatic, even though they most commonly occur in young, athletic individuals. McMurray's test is often positive.
The joint line is generally obscured by the cyst, but fluid may at times move back inside the joint capsule, resulting in intermittent swelling. Even if the cyst comes and goes, however, the meniscal tear often remains symptomatic.
Imaging. Meniscal cysts are fluid-filled and will thus transilluminate. MRI confirms the diagnosis, although an experienced orthopedist may choose to proceed directly to surgery if confident of the diagnosis.
On MRI, meniscal cysts usually appear as well-circumscribed, smoothly marginated masses adjacent to the meniscus (8). Typically, a horizontal meniscal tear is also visualized, and a connection between tear and cyst may also be seen (figure 2).
Treatment. Treatment is primarily aimed at repairing the underlying meniscal tear. Symptomatic treatment with modalities such as ice and NSAIDs is helpful, but ultimately surgery is required. We have not found aspiration of these cysts to be helpful.
Cysts resulting from meniscal tears are often found in young athletes, for whom the timing of surgery may be an important issue. Often after discovery of the lesion, athletes are able to finish a season before undergoing surgery, because horizontal tears are not as mechanically symptomatic as other types of tears.
Arthroscopy can be used to identify and resect a tear, but the cyst should be surgically excised through an adjacent incision. Unlike popliteal cysts, which may resorb after correction of an underlying condition, meniscal cysts often persist after meniscectomy if not excised.
With a meniscal tear, a horizontal defect in the meniscus generally separates it into a superior and an inferior leaf. While one or both portions may have secondary tears or fraying, often portions of both leaves are salvageable. If the resection is relatively small and the peripheral portions of the meniscus are left in place, it is important that any defects in the peripheral part of the meniscus be surgically closed to prevent recurrence of the cyst.
Ganglion cysts are lined by connective tissue (not synovium), contain mucinous fluid, and may communicate with the knee joint. They may be the result of myxoid degeneration of periarticular connective tissue, or they may arise in the structures within the knee joint, creating intra-articular ganglia. Their cause is unknown.
Location. Ganglia have a predilection for periarticular locations and are mainly seen around the knee joint, wrist, and hand. They may be attached to a joint capsule or tendon sheath and may have a connection to the synovial cavity. They can also be found within or adjacent to muscles, ligaments, tendons, or nerves (2,9-11).
Clinical features. The clinical presentation depends on the location and size of the cyst. Most ganglion cysts are asymptomatic, palpable lesions or are seen incidentally on imaging tests. Occasionally, they can cause pain, peroneal dysfunction, chronic effusions, or cosmetic deformity secondary to mass effect.
Imaging. In our opinion, MRI is the best initial imaging study for patients who have large or painful lesions, including suspected ganglion cysts. On MRI, ganglion cysts present as well-delineated, rounded, or lobular lesions (figure 3). When located close to a joint line, ganglion cysts may resemble meniscal cysts. In such cases, absence of a meniscal tear can exclude the possibility of a meniscal cyst.
Treatment. Asymptomatic ganglion cysts require only monitoring. For symptomatic cysts, initial treatment involves conservative measures such as ice, heat, and NSAIDs. Participation in sports activities may continue as tolerated.
The fluid in ganglion cysts tends to be gelatinous, and therefore aspiration may be difficult even with a large-bore needle. If aspiration is successful, the ganglion will almost always recur as it refills with fluid.
If refractory to symptomatic treatment, ganglion cysts should be resected surgically. Intra-articular ganglia are rare but, if identified, may be amenable to arthroscopic resection (figure 4).
While not truly cystic, inflammation of the periarticular bursae may mimic cysts around the knee, but is generally less defined. Any of the numerous bursae around the knee may become irritated and swollen. Treatment may vary, depending on the location.
Patellar bursitis. Common bursae in the front of the knee include the suprapatellar, prepatellar, and infrapatellar. These bursae do not communicate with the knee joint.
Prepatellar bursitis has, in the past, been referred to as "housemaid's knee" and occurs with sports that involve kneeling, such as wrestling and trampolining (12).
Deep infrapatellar bursitis can result from striking the knee or from overuse of the knee extensor mechanism (12) and is commonly seen in runners and jumpers. On MRI, fluid accumulation is seen between the patient's patellar tendon and tibia.
Treatment for patellar bursitis includes modification of activities, NSAIDs, and icing.
Pes anserinus. The pes anserinus bursa is located between the conjoined distal tendons of the sartorius, gracilis, and semitendinosus muscles and the tibial insertion of the medial collateral ligament. It is a common site for inflammation and swelling. Clinically, pes anserinus bursitis involves focal swelling and tenderness inferior to the anteromedial portion of the proximal tibia and may, on rare occasions, look like a cyst.
Symptomatic treatment with NSAIDs and modification of activities is typically all that is necessary. In resistant cases, a cortisone injection may be indicated.
Iliotibial bursitis. The iliotibial bursa is located between the distal part of the iliotibial band near its insertion on Gerdy's tubercle and the adjacent tibial surface.
Iliotibial bursitis and tendinitis are usually due to overuse and repetitive varus stress of the knee. Classically, they occur in the downhill leg of runners who train on sloped or crowned surfaces. This condition may mimic injury to the lateral meniscus or lateral collateral ligament. MRI often shows a well-defined fluid collection near the distal iliotibial tendon insertion.
Treatment usually consists of a stretching regimen before and after activities, along with ice and NSAIDs.
The vast majority of masses about the knee fall into the category of benign cysts. The physician should, however, be aware of several rare entities that may simulate benign cystic lesions. These include synovial chondromatosis, pigmented villonodular synovitis, synovial sarcoma, and cystic lesions of the proximal tibiofibular joint (1,13,14).
Synovial chondromatosis. This is a rare, benign tumor of the synovium that generates large numbers of cartilaginous masses and occurs most commonly in the knee (1). The chondroid bodies may detach from the synovium, grow independently in the synovial fluid, and subsequently ossify. This condition may be associated with swelling of the knee and can produce outpouchings of the synovium filled with loose bodies, which may be mistaken for cysts.
Clinically, patients typically present with a swollen knee joint, pain, mechanical symptoms, and, possibly, a palpable soft-tissue mass that mimics a periarticular cyst. Calcified lesions are readily identified on routine x-rays, but x-rays may be normal with uncalcified lesions. On MRI, this condition characteristically appears as a highly lobulated, cauliflower-like configuration of loose bodies and nodular synovial thickening.
Treatment consists of open or arthroscopic removal of the loose bodies and synovectomy.
Pigmented villonodular synovitis. Pigmented villonodular synovitis (PVNS) is another benign neoplasm of synovium that primarily affects healthy young adults (14). With this condition, the synovium hypertrophies to form lesions that may be sessile or pedunculated. PVNS comes in two varieties: localized (good prognosis) or diffuse (guarded prognosis).
Patients present with chronic pain, swelling, and stiffness, as well as outpouchings of synovium that may look like periarticular cysts.
The disease may be strongly suggested by MRI, which shows a heterogeneous mass of varying signal intensity (figure 5). On MRI, the lesion in its localized form may look like an intra-articular or periarticular cyst, but its appearance is different enough that it should be readily identifiable by a musculoskeletal radiologist. Definitive diagnosis is made by biopsy and histologic evaluation.
Localized PVNS is best treated by arthroscopic removal. Treatment for the diffuse type is controversial and beyond the scope of this article.
Sarcoma. Synovial sarcoma is exceedingly rare but could potentially mimic a periarticular ganglion cyst (1). Patients with malignant lesions will often present with night pain, weight loss, or other worrisome systemic symptoms.
If this condition is suspected, MRI with contrast agent should be performed. Contrast-enhanced nodular components raise the suspicion of a neoplastic process. Biopsy is necessary for diagnosis and staging of the lesion.
Proximal tibiofibular joint lesions. Cysts that cause focal masses, pain, or neuropathy because of compression of the common peroneal nerve can arise from the tibiofibular joint (13). Large cysts may erode adjacent bone and therefore may mimic a malignant lesion. If symptomatic, these cysts can be excised through an open incision.
Cysts about the knee can often be treated symptomatically, but if they persist, surgical excision and/or treatment of concurrent pathology may be needed.
Uncommon conditions that can mimic benign cysts can often be identified by MRI when conservative treatment fails. These patients should be referred to specialists who have expertise in treating such conditions.
Dr Yu is chief resident and Dr Shapiro is an associate professor, both in the department of orthopedic surgery at the University of California School of Medicine in Los Angeles. Address correspondence to Matthew S. Shapiro, MD, UCLA Medical Center, Dept of Orthopaedic Surgery, Box 956902, Los Angeles, CA 90095-6902; address e-mail to [email protected].