One of the more common imaging findings when evaluating the knee, whether by ultrasound or MRI, is the “cystic lesion.” These “cystic lesions” may be related to bursae, cysts, or other cyst-like lesions and could be soft tissue or intraosseous in origin. They are mostly benign lesions; however, not all hypoechoic or T2 hyperintense lesions are cysts, and neoplastic lesions may be missed if close attention is not paid to the typical imaging findings. Therefore, knowledge of the normal bursae, common cysts, and cyst-like lesions which can occur about the knee is essential for the appropriate diagnosis and management in these cases.
In general, a bursa functions to reduce friction between two structures, at least one of which is moving relative to the other. Bursae can be divided into two types, anatomic and adventitial. Anatomic bursae are true synovial lined sacs which may be fluid-filled and located in expected positions near the joint. In contrast, adventitial bursae are not synovial lined and may occur away from the joint.
There are various bursae which are encountered during routine imaging of the knee. The bursae about the knee can be divided by location into anterior, medial and lateral (Table 1). These bursae can become distended from direct or repetitive trauma or secondary to local or systemic inflammatory changes. Bursitis can be further complicated by hemorrhage, rupture, synovial proliferation, chondromatosis, or infection.
The prepatellar bursa is located within the anterior subcutaneous soft tissues superficial to the patella (Fig. 1).1,2 Prepatellar bursitis, sometimes referred to as carpet-layer or housemaids knee, results from the bursa undergoing repetitive compressive and sheer forces between the skin and the patella owing to the positioning the individual’s weight on to the anterior knee over the patella.2 The superficial infrapatellar bursa, or pretibial bursa, resides within the subcutaneous fat superficial to the tibial tuberosity at the level of the distal patellar tendon (Figs. 2 and 3).1,2 The deep infrapatellar bursa, or retropatellar bursa, is a small bursa located directly posterior to the distal third of the patellar tendon, just proximal to its insertion on the tibial tubercle (Fig. 3 and 4).1,2,3 Trace fluid is normal in the deep infrapatellar bursa; bursitis is referred to when the fluid outpouches the recess inferior to the Hoffa’s fat pad. The suprapatellar bursa is a superior recess of the knee joint deep to the quadriceps femoris tendon anterior to the intracondylar fossa (Fig. 5). During embryologic development, the septum separating the suprapatellar bursa from the remainder of the joint will perforate and involute, leaving only a small transverse residuum of the septum.4
The medial gastrocnemius (MG) bursa is located between the medial head of the gastrocnemius and the joint capsule (Fig. 6).5 The medial collateral ligament (MCL) bursa is intimately related to the MCL (Fig. 7).4,6
The semimembranosus-tibial collateral ligament (SMTCL) bursa is located between the semimembranosus tendon and the deep posteroinferior portion of the medial collateral ligament complex (Fig. 8).1 The pes anserine consists of the conjoined tendons of the gracilis, sartorius, and semitendinosus tendons. The pes anserine bursa resides at, and distal to, the level of the joint, situated between the medial collateral ligament and the pes anserinus (Fig. 9).4 Anserine bursitis results from overuse, commonly in runners.
The iliotibial bursa is located deep to the distal iliotibial band proximal to its tibial attachment (Fig. 10) and may become distended in iliotibial band friction syndrome.2 Iliotibial band bursitis or iliotibial band syndrome is a common overuse injury in runners. The fibular bursa is located between the fibular collateral ligament and the distal biceps femoris tendon (Fig. 11).7 The fibulopopliteal bursa is contained between the fibular collateral ligament and the popliteus tendon. The popliteal bursa communicates with the joint and therefore may be referred to as the popliteal or subpopliteal recess (Fig. 12).8 This bursa extends between the popliteus tendon and lateral femoral condyle and in some individuals communicates with the fibulopopliteal bursa.9 The lateral gastrocnemius (LG) bursa is present between the lateral head of the gastrocnemius and joint capsule (Fig. 13). The gastrocnemius bursae and fibulopopliteal bursae will commonly become distended in chronic internal derangement of knee.
In contrast to the anatomic bursae described which reside over normal anatomic areas of friction, the body may also produce bursae as a result of friction between a normal anatomic structure and a pathologic or iatrogenic friction point. Some examples include adventitial bursae produced adjacent to an osteochondroma (Fig. 14) or orthopedic hardware.
Synovial cysts are juxta-articular fluid collections which are distinguished from other juxta-articular fluid lesions in that they are synovial lined, a feature which can normally be identified on post contrast images as peripheral enhancement. Because of their synovial lining, these cysts are subject to synovial processes, including synovial hyperplasia (e.g. inflammatory arthropathy), pigmented villonodular synovitis (PVNS), and synovial osteochondromatosis. Specific types of synovial cysts at the knee include the popliteal and tibiofibular cysts.1,10,11
A cyst which communicates with the joint through a defect in the joint capsule but does not insinuate between the semimembranosus and medial head of the gastrocnemius is given the general term of synovial cyst (Fig. 15). These synovial cysts are seen more commonly and with greater frequency in patients with increased internal knee pressure due to chronic derangement or from prior arthroscopy when excessive amounts of fluid are pushed into the joint as part of the procedure.10 The common mechanism includes capsular rupture, leading to the formation of a pseudodiverticulum, followed by closure of the neck, ultimately resulting in the formation of a simple synovial or multiloculated chronic ganglion cyst. Uncomplicated cysts are typically managed conservatively. If there is no intra-articular communication, these cysts can be decompressed in symptomatic patients by percutaneous aspiration and corticosteroid injection.12
Popliteal cysts are specific types of synovial cysts which communicate with the joint through a rent in the posteromedial capsule; they insinuate between the medial head of the gastrocnemius and the tendon of the distal semimembranosus (Fig. 16). These synovial cysts are commonly associated with joint effusions and are present in up to 40% of patients with posterior knee masses. Popliteal cysts are associated with internal derangement, even in the absence of a joint effusion.10 Patients may present acutely secondary to rupture of the cyst. Popliteal cysts may be complicated by hemorrhage, inflammation, or loculation from chronic or partial decompression. Also, due to their communication with the underlying joint, popliteal cysts may at times contain loose bodies (Fig. 17).
These synovial cysts arise from chronic arthritic changes of the proximal tibiofibular joint (Fig. 18). The size and location of the cyst will determine the clinical presentation. Tibiofibular cysts may compress the common peroneal nerve or extend intra-epineurally and retrograde along the intra-articular branches, causing neuropathy and foot drop.13-15
Parameniscal cysts are thought to be produced by the extension of joint fluid through a meniscal tear and into the adjacent parameniscal soft tissues (Fig. 19).16 These cysts may extend long distances from the joint, mimicking sarcomas; however, the neck can often be traced to the joint line and the meniscal level. Pericruciate meniscal cysts are seen adjacent to the PCL and arise from tears in the posterior horn of the medial meniscus.17 This particular location of a parameniscal cyst may be confused with a PCL ganglion. It is critical to differentiate the parameniscal cysts from PCL ganglia as the treatment approaches will often differ. Additionally, while parameniscal cysts reflect an underlying meniscal tear, cruciate ligament ganglia are not associated with tears of the ACL or PCL, but rather reflect chronic mucoid degeneration from micro-injuries.
Similar to parameniscal cysts, intrameniscal cysts may form from trauma or degeneration within the meniscus. When the fluid remains contained within the torn or degenerated meniscus, it is referred to as an intrameniscal cyst. An intrameniscal cyst may be seen as a small cystic focus of fluid signal to a larger cystic lesion with focal enlargement of the meniscus (Fig. 20). An adjacent horizontal tear may be demonstrated in some cases.16,18
In general, most ganglion cysts are found in locations which are under constant stress. Although somewhat controversial, one common explanation for the formation of ganglion cysts is that they are the result of mucoid cystic degeneration in collagenous structures.4,18 This degeneration produces a cyst with a well-defined fibrous capsule and highly viscous fluid internally, which will typically be demonstrated by intermediate or intermediate low T1 and high T2 signal.2,19 Intra-articular ganglia may be extrasynovial but intracapsular, commonly arising from the alar folds of either the infrapatellar fat pad or cruciate ligaments. Since patients with intra-articular ganglia often present with pain or locking and the lesions may mimic sarcoma, identification of these ganglia is of clinical importance. Contrast examination aids in differentiation of these lesions (peripheral enahancement) from malignancy (central, nodular peripheral, or solid enhancement) when the cyst is in an atypical location or the diagnosis is uncertain.
Most intra-articular ganglia are located within the intercondylar notch and are associated with the cruciate ligaments (Fig. 21).19-23 These ganglia may reside within or adjacent to the cruciate ligaments. Ganglia of the anterior cruciate ligament (ACL) are typically fusiform, often within and splaying the ACL fibers.1,19,20,23 In the case of posterior cruciate ligament (PCL) ganglia, the ganglia will appear as a well -defined multilocular cyst immediately adjacent to the PCL.1,19,20,23 There is no association with cruciate ligament tears in most cases. It is not uncommon for there to be an associated intraosseous ganglion at the cruciate ligament insertions (see intraosseous ganglia). Symptoms may include restricted range of motion (locking in terminal extension) and localized pain, exacerbated by squatting or full flexion.23
The infrapatellar fat pad is one of the most common locations for a ganglion cyst to occur at the knee, accounting for approximately 13% of all intra-articular ganglia (Fig. 22).24-25 Ganglion within the infrapatellar fat pad are most often located adjacent to the anterior horn of the lateral meniscus or transverse intermeniscal ligament. It is important to identify this relationship in order to avoid confusion of this ganglion with adventitial bursa formation due to Hoffa's fat pad impingement.24 Small ganglia are typically asymptomatic. However, larger ganglia within the infrapatellar fat pad may produce anterior knee pain at full extension and produce tenderness of the patellar tendon.21
Similar to intra-articular ganglia, extra-articular ganglia are well-defined, homogenous, round, or lobular cystic structures demonstrating intermediate or intermediate low T1 and high T2 signal. Fine internal septations may be present giving the ganglia its multiloculated or “bunch of grapes” appearance.2 Ganglia are common along tendon sheaths and adjacent to the joint capsule, but rarely have communication with the underlying joint.1,2,22 Soft tissue ganglia, however, may occur anywhere within the soft tissue with a predilection for peri-articular locations (Fig. 23).2
A subchondral cyst (Fig. 24) is an intraosseous cyst which occurs beneath an articular surface of a bone. These cysts are produced in areas of damaged articular cartilage, subjacent to the underlying subarticular cortical plate. The exact pathogenesis of these degenerative cysts is not certain.26,27 Subchondral cysts are most often seen in association with osteoarthritis, but may occur as the result of degeneration or injury of the overlying articular cartilage by other causes. Subchondral cysts are of variable size from a few millimeters to over a centimeter. A large subchondral cyst may be referred to as a geode. These cysts will appear as round, homogenous, intermediate-low T1, and high T2 signal foci within the articular bone marrow. Overlying full-thickness cartilage fissures or defects are often present.4,27
These intraosseous cystic lesions are typically located at the attachment of the cruciate ligaments or meniscotibial attachments (Fig. 25).2 Like soft tissue ganglia, insertional cysts and intraosseous ganglia are believed to be the result of chronic degeneration. Insertional cysts, also known as avulsive cystic changes, are formed by chronic tensile stress at the attachment of ligaments or menisci, producing focal necrosis and cyst formation.28 In the case of intraosseous ganglia, abnormal stress within the bone undergoes mucoid degeneration.28 Less commonly, these ganglia may result from extension of an adjacent soft tissue ganglia.29 The appearances of these degenerative intraosseous cysts are similar to the subchondral cyst, differing only on imaging by a location which is not subchondral.
There are a variety of cyst-like lesions which may mimic cysts depending on the modality utilized. Etiologies of these lesions include vascular, traumatic, infectious, and neoplastic.
Vascular lesions and normal and abnormal vascular structures may mimic cystic masses. The popliteal artery aneurysm or pseudoaneurysm is distinguished by its location and flow void. Slow flow lesions include popliteal vein varices, superficial varicosities (Fig. 26), lymphatic malformations, mixed venolymphatic malformations, and hemangiomas. Hemangiomas are rare in adults and should have an enhancing soft tissue mass. Most commonly, it is a slow flow venous malformation (Fig. 27) that is mistakenly referred to as a hemangioma. Normal vessels in and around the knee may occasionally be mistaken for cyst-like lesions (Fig. 28), depending upon the flow and signal intensity within the vascular structure. An understanding of the vascular anatomy and following the course of the vessel will help avoid this potential pitfall.
Fluid collections from trauma or infection may occur around the knee. The clinical findings are helpful in arriving at the definitive diagnosis. However, imaging clues include T1 shortening from blood products in hematoma versus diffuse fascial edema and enhancement noted with abscesses. These lesions should be followed to resolution clinically and/or by imaging. Expanding lesions may need further assessment with contrast to exclude underlying malignancy.
Of note, a degloving lesion may persist in the soft tissues for prolonged periods in the fascial or subcutaneous layers of the knee (Fig. 29). First identified by French surgeon Morel-Lavallée in 1863, the post-traumatic soft tissue degloving injury was originally described as occurring in the soft tissues of the proximal thigh over the greater trochanter.30 Closed degloving lesions most commonly occur adjacent to bony protuberances. They form as a result of direct or tangential shearing forces which separate the skin and subcutaneous tissues from the underlying fascia forming a potential space which can fill with blood, lymph and/or fat.31-33 Degloving lesions can be identified by their well defined oval or fusiform shape and tapering margins which merge with the subjacent fascia. These lesions may show fluid-fluid levels, septations, and variable internal signal intensities dependent on the concentration of hemolymphatic fluid and age of the lesion.
Synovial sarcoma is a relatively common primary soft tissue sarcoma. These lesions are most prevalent between the ages of 15 and 30 years of age and often present as a painful enlarging mass.34 Synovial sarcomas are seen on MR as nonspecific inhomogeneous mass with T1 signal isointense to skeletal muscle and T2 signal hyperintense relative to subcutaneous fat. These lesions may be multilocular and may have varying degrees of a cystic component with or without fluid-fluid levels (Fig. 30).34 Synovial sarcoma will often have a soft tissue component that heterogeneously enhances.34
To conclude, a variety of bursae, cysts, and cyst-like lesions exist about the knee joint, many of which have fairly characteristic locations and imaging features. Knowledge of these variants and lesions is essential for accurate diagnosis and to avoid mistaking them for aggressive soft tissue lesions, including sarcomas.
Chhabra A, Cerniglia CA. Bursae, Cysts and Cyst-like Lesions About the Knee . J Am Osteopath Coll Radiol. 2013;2(4):2-13.