Key Facts
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Cysts and ganglia demonstrate similar imaging characteristics but may be distinguished in some instances by their location.
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Cysts and ganglia may cause nerve compression. This occurs most often in the shoulder due to paralabral cysts (suprascapular nerve) or in the knee due to tibiofibular ganglia (peroneal nerve).
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Symptomatic improvement may follow drainage and injection of corticosteroids into cysts.
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Not all lesions that appear well defined and homogeneously bright on T2-weighted images are cysts. Intravenous gadolinium can differentiate solid tumors from cystic masses; vascular masses will enhance centrally, whereas cystic collections will enhance only peripherally or not at all. Ultrasound may also differentiate solid masses from cysts.
Juxtaarticular cysts and fluid collections may develop as an extension of intracapsular pathology or may present in the absence of intrinsic joint disease. Cysts may form adjacent to large and small joints both in the appendicular and axial skeleton. Commonly encountered cystic lesions include synovial cysts and ganglia. Bursal distension with fluid is common around joints; adventitial bursae may form in response to repeated localized frictional injury, particularly in the foot. Meniscal cysts develop secondary to meniscal tears, and paralabral cysts are formed in the setting of labral tears at the shoulder or hip.
Assigning the appropriate nomenclature to juxtaarticular cysts is often difficult and results in considerable confusion. A large majority of the cysts detailed below develop secondary to a process occurring within the joint capsule. Some represent distension of preexisting anatomic connections to the joint; others a de novo outpouching from the capsule. Elucidating the etiology of the cyst is far more important than assigning the correct name; in fact, these lesions share very similar imaging characteristics.
Synovial cysts are commonly encountered about joints, large and small. These synovial cell–lined cysts represent outpouching or herniation of the synovial membrane through the joint capsule. Synovial cysts may develop secondary to a variety to intracapsular processes including inflammatory arthropathy ( Figure 18-1 ), degenerative joint disease (DJD), or, rarely, crystal arthropathies.
Synovial cysts are diverticula or herniations of the synovial membrane through the joint capsule, often in response to increased intraarticular pressure. These cysts are lined by synovium.
Soft tissue ganglion cysts may arise from joints or tendon sheaths. Ganglia are considered to represent degenerative lesions. Given their similar imaging appearances to synovial cysts, differentiation of the two entities may be difficult, if not impossible. Some contend that unlike synovial cysts, ganglion cysts lack a synovial cell lining. Histologically, ganglia are unilocular or multilocular and are filled with a mucinous substance. On cytologic examination, histiocytes may be present. As will be detailed subsequently, many ganglia are filled with fluid more viscous than typical joint fluid; this may simply reflect desiccation of synovial fluid that no longer freely communicates with its source.
Ganglia are cyst-like structures filled with a mucinous fluid and lacking a synovial lining. They are difficult to differentiate on imaging studies from synovial cysts.
Bursae are synovium-lined sacs that may lie in apposition to bony prominences or between tendons and ligaments. Adventitial bursae may develop in the subcutaneous tissues adjacent to sites of repetitive frictional trauma. Some bursae may communicate with adjacent joints, including the iliopsoas bursa at the hip and semimembranosus-gastrocnemius bursa at the knee. In this regard, their behavior is analogous to synovial cysts, becoming distended secondary to a variety of intraarticular conditions.
Bursae are synovium-lined sacs that are present at sites of friction such as adjacent to bony prominences. When bursae communicate with joints (e.g., the Baker’s cyst in the knee) they behave as synovial cysts and distend in response to intraarticular increases in pressure.
Juxtaarticular cysts may be asymptomatic, although many present with localized swelling, the perception of deep pain or fullness by the patient, or as a mass confirmed on physical examination. In rare cases, nerve compression by a cyst may result in pain, motor weakness, or muscle atrophy. Some cysts, encountered in the appropriate clinical setting, may be amenable to aspiration in the clinician’s office. In most cases, however, initial imaging is warranted to define the anatomy, elucidate any related intracapsular pathology, and confirm that the palpable finding is indeed a cyst and not a solid or partially solid mass. If clinically indicated, aspiration, with or without corticosteroid injection, may be readily performed by the radiologist under imaging guidance.
IMAGING
Suspected cystic lesions around joints in the appendicular skeleton are initially evaluated with radiography followed by sonography or magnetic resonance imaging (MRI) to document the cystic nature of the mass. Deeper lesions and those in the axial skeleton may be difficult to image with ultrasound, necessitating MRI. Radiographs should be performed initially at all painful joints, even if a discrete mass is palpable, in order to evaluate for underlying arthropathies, osseous lesions, and soft tissue calcification. A cyst may be visible as a water-density soft tissue mass on radiographs, although a solid mass may be indistinguishable in its appearance. By virtue of its more limited contrast resolution, computed tomography (CT) is recommended for the evaluation of juxtaarticular cysts only in limited situations. This includes deeper lesions adjoining joint prostheses and other implants where MRI is limited by virtue of metal artifact. CT does remain a useful modality, however, for image-guided aspiration of some lesions.
On sonography, cystic lesions have a variable appearance, similar to other fluid collections, ranging from anechoic, to largely anechoic with low level echoes, to hypoechoic. Some chronic synovial cysts, for example, may have considerable avascular, solid debris along the cyst walls or layering dependently within the cyst cavity ( Figure 18-2 ). Increased through-transmission may be evident in many cases. Color or power Doppler sonography may be useful in confirming the avascular nature of the cyst, despite a heterogeneous appearance on gray-scale imaging. Synovial cysts that have developed in continuity to articulations with severe DJD may contain chondral and/or osteochondral bodies, the latter resulting in “shadowing.” Septations may be observed in many cysts.
On MRI, juxtaarticular cysts have a relatively typical appearance, being of similar signal intensity to fluid on all sequences. On T1-weighted scans, cyst fluid is of moderately low signal intensity. On T2-weighted scans, cyst fluid exhibits high signal intensity. If cysts are filled with highly proteinaceous fluid, or, in the setting of subacute hemorrhage within a cyst, modestly elevated signal on T1-weighted scans may be identified. In synovium-lined cysts with acute or chronic inflammation, wall thickening may be observed. Innumerable, small filling defects within cyst fluid may reflect fronds of hyperplastic synovium. Ossified bodies in the cyst cavity exhibit very low signal intensity (signal void) but centrally may contain high signal on T1-weighted images, indicating bone marrow elements.
Although the majority of juxtaarticular cysts are readily diagnosed on MRI by virtue of their anatomic location and imaging appearances, some cysts may exhibit atypical characteristics mimicking a solid mass, particularly a homogeneous-appearing neoplasm, such as a myxoma ( Figure 18-3 ). In equivocal cases, such as these, T1-weighted MRI should be performed before and after intravenous (IV) gadolinium administration. The avascular cyst contents will fail to exhibit contrast enhancement ( Figure 18-4 ); with solid masses, enhancement is generally visualized ( Figure 18-5 ). Rim enhancement of cysts, however, is not uncommon.
Cyst vs. solid: Masses that exhibit homogeneous fluid signal on MRI (intermediate signal on T1-weighted images and high signal on T2-weighted images) are not necessarily cysts. Tumors with high water content, such as myxomas, can exhibit similar imaging findings. In cases in which location or appearance suggest the possibility of a solid mass, IV contrast should be administered. The central part of the mass will enhance if it is a tumor but not if it is a fluid collection. Ultrasound can also distinguish cystic from solid lesions.
Knee
Cystic lesions about the knee are common and are readily detected at the time of MRI examination performed after acute knee injury or in the setting of chronic knee pain. Popliteal cysts, meniscal cysts, bursal collections, and ganglia may be encountered. Clinical manifestations are variable based on location and the presence or absence of intracapsular pathology.
Popliteal cysts are synovial cysts that extend posterior and medial to the knee, situated between the semimembranosus tendon and the proximal medial head of gastrocnemius muscle and tendon. This potential space is referred to as the semimembranosus-gastrocnemius bursa; the nomenclature itself may be a source of confusion. In most cases, there is a potential communication between joint and bursa, allowing decompression of joint fluid into the bursa ( Figure 18-6 ). When large, these cysts dissect caudad and occasionally cephalad. Patients with popliteal cysts may report a sensation of fullness posterior to the knee. Large cysts may be palpable or even manifest visible swelling or mass on physical examination.
Popliteal cysts typically form in the setting of increased intracapsular pressure, with the joint effusion decompressing posteriorly. These cysts frequently accompany both DJD and inflammatory arthropathies and can be seen with meniscal tears. Intracapsular chondroosseous bodies may extend into the cyst in the setting of DJD or with primary synovial osteochondromatosis.
On radiographs, large popliteal cysts may be visible as a soft tissue density mass in the popliteal fossa; osseous bodies within the cyst may be visualized. In the patient with posterior knee pain and fullness, sonography is a rapid means of diagnosing a popliteal cyst and excluding other conditions such as popliteal deep venous thrombosis. These cysts are typically ovoid in shape and longest in the longitudinal axis ( Figure 18-7 ). When imaged transversely, a comma shape may be observed due to the narrow neck extending anteriorly between the semimembranosus tendon and the medial head of the gastrocnemius ( Figure 18-8 ). The cyst contents are variable, from anechoic to complex, with occasional wall thickening and septation. Ultrasound is more sensitive than physical examination for the detection of popliteal cysts. On MRI in the transverse plane, the typical comma shape may also be observed with signal intensities reflecting its largely fluid content.
Popliteal cyst rupture presents as sudden severe calf pain and swelling as the cyst fluid typically dissects caudad and superficial to the medial head of the gastrocnemius muscle. MRI and/or ultrasound examination of the popliteal fossa and calf are generally indicated to exclude other conditions with a similar presentation, including gastrocnemius or soleus musculotendinous strain, plantaris rupture ( Figure 18-9 ), or deep venous thrombosis.
Acute pain and swelling may indicate popliteal cyst rupture, especially in patients with rheumatoid arthritis. Ultrasound is usually the easiest method to document cyst rupture and exclude deep venous thrombosis. MRI is most useful in documenting an underlying cause for the cyst (e.g., meniscal tear) or other conditions such as muscle strains that may simulate cyst rupture.
Another commonly encountered cyst about the knee is the meniscal cyst. By definition, these cysts develop secondary to meniscal tears and reflect joint fluid being forced out through a meniscal tear via a one-way or ball-valve mechanism. These cysts originate along the margin of the meniscal tear ( Figure 18-10 ), although they may dissect some distance from the tear.
Meniscal cysts have been described in association with both medial and lateral meniscal tears. MRI is a highly sensitive means for the detection of meniscal cysts ; in fact, these cysts have been detected in asymptomatic knees using MRI.
Multiple bursae lie along the medial aspect of the knee. Bursal distension may result in medial joint line pain, simulating a meniscal tear and/or medial femorotibial DJD. The pes anserine bursa is located adjacent to the confluence of the semitendinosus, gracilis, and sartorius tendons as they course along the anterior aspect of the medial proximal tibia.
The pes anserinus (goose’s foot) bursa lies along the anteromedial tibia, deep to the sartorius, gracilis, and semitendinosus tendons and superficial to the medial collateral ligament.
This bursa is centered distal to the medial joint line. The clinical presentation of painful pes anserine bursitis may mimic that associated with a meniscal tear.
Pes anserinus bursitis is a cause of pain over the anteromedial tibia and is usually seen in overweight women, older patients with osteoarthritis, and young athletes (especially runners). Tenderness is present 2 to 5 cm below the anteromedial joint margin. Pain occurs especially with stair climbing.
The semimembranosus bursa is more posterior in location and slightly more proximal. Bursal distension results in a C-shaped collection draped over the semimembranosus tendon. Fluid may be observed in the semimembranosus bursa along with distension of the semimembranosus-gastrocnemius bursa (popliteal cyst). The medial collateral ligament (MCL) bursa is a potential space deep to the superficial MCL fibers. An MCL bursal collection will typically have a crescent shape, conforming to the undersurface of the MCL and spanning the joint line.
Anterior to the knee and adjacent to the extensor mechanism are the prepatellar and superficial infrapatellar bursae. Prepatellar bursitis is generally the result of friction on the prepatellar soft tissues, such as may be seen in the setting of repetitive kneeling. Fluid collects superficial to the patella and may result in considerable swelling. Hematomas may collect in the prepatellar bursa following direct trauma. Small amounts of fluid are frequently identified incidentally on MRI or sonography within the deep infrapatellar bursa, which lies just deep to the distal patellar tendon.
An important site of cyst formation about the knee is along the inner margin of the tibiofibular joint. These collections, referred to as tibiofibular joint ganglia , are variable in size. Due to the close proximity of tibiofibular joint ganglia to the peroneal nerve, some cysts may result in nerve irritation.
Intraarticular ganglia at the knee have been described. These often lie adjacent to the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) and are variable in size. Ganglia have been reported to occur in Hoffa’s fat pad. These lesions have also been reported in association with mucoid degeneration of the ACL. Some intraarticular ganglia may be associated with pain.
Shoulder
Cystic lesions at the shoulder may lie in proximity to the glenohumeral joint or to the acromioclavicular (AC) joint. Synovial cysts are commonly encountered in rheumatoid arthritis. Fluid may collect in the subacromial-subdeltoid bursa in the setting of direct trauma, impingement, or with a full-thickness rotator cuff tear when joint and bursa are in direct communication. The paralabral cyst forms secondary to glenoid labral tear, dissecting along the scapular neck. As detailed below, some paralabral cysts may become symptomatic.
Although the osseous and articular manifestations of rheumatoid arthritis at the shoulder are beyond the scope of this chapter, there are a number of soft tissue abnormalities to consider. Synovial cysts may extend from the glenohumeral joint capsule. These may become quite large, presenting as a soft tissue mass. On MRI, these cysts will show typical high signal intensity of fluid. In addition, the cyst contents will appear complex, with multiple, small filling defects representing hyperplastic synovial fronds, or “rice bodies.” Glenohumeral joint arthrography will show communication between joint and synovial cysts; it is therefore not surprising that the appearance of the cyst contents would parallel those of joint fluid. In addition to synovial cysts, distension of the subacromial/subdeltoid bursa may be seen with rheumatoid arthritis; this can be massive.
Massive distension of the subacromial bursa may be present in rheumatoid arthritis, tuberculosis, and amyloidosis, the latter described as the “shoulder pad sign.”
Of all cystic lesions at the shoulder, imaging plays the greatest role in the diagnosis and management of paralabral cysts. These cystic lesions form in response to glenoid labral tears, commonly posterior and/or superior in location. The mechanism of formation is probably analogous to meniscal cysts at the knee: a ball-valve mechanism forcing synovial fluid from the joint through the labral rent. Although cysts may be encountered anteriorly, they are far more common posterior and superior in location. Tiny paralabral cysts are of no clinical significance; when large, these cysts may exert mass effect resulting in nerve compression.
MRI has revolutionized the diagnosis of paralabral cysts and offers not only precise anatomic localization but also the ability to define the sequelae of nerve compression. By virtue of their location, these cysts may not be optimally visualized at glenohumeral arthroscopy, and paralabral cysts remain an important consideration in the differential diagnosis of shoulder pain, particularly in the setting of a normal rotator cuff. Prior to the widespread use of shoulder MRI, these cysts often went undiagnosed.
Most paralabral cysts dissect into the spinoglenoid notch ( Figure 18-11 ); some extend to the supraspinous region. Less commonly, the cysts may be entirely supraspinous in location. Within the spinoglenoid notch, paralabral cysts may compromise the suprascapular nerve motor branch to the infraspinatus. More superiorly, in the supraspinous region, the motor branch to the supraspinatus may be compromised as well. Although the suprascapular nerve is primarily a motor nerve, it must be emphasized that, due to sensory branches, some patients experience deep pain with paralabral cysts.
