Clinical History: A 73 year-old male complains of a gradually enlarging “knot” on the anterior knee for 8 years. He previously worked laying tile. Sagittal T1-weighted (1A), sagittal fat-suppressed proton density-weighted (1B), and axial fat-suppressed proton density-weighted (1C) images are shown. What are the pertinent imaging findings and diagnosis?
Chronic calcific superficial infrapatellar bursitis (“housemaid’s knee” or “clergyman’s knee”). This was treated successfully with resection.
Bursal distension and other cystic lesions about the knee are frequently encountered findings on MR imaging of the knee. It is important for those interpreting MRI of the knee to be familiar with the normally occurring anatomic bursa of the knee, potential clinical presentations, usual and variable imaging appearances, and differential diagnostic considerations. The aim of this web clinic is to review the anatomy of bursa of the knee and to discuss commonly occurring soft tissue cystic lesions around the knee.
Bursae of the Knee: Function and Anatomy
Definition and Function
Anatomic or primary bursae are synovial lined periarticular structures, which occur normally developmentally, contain variable amounts of fluid, and serve to reduce motion or pressure induced friction between adjacent anatomic periarticular structures, often between bone and the overlying soft tissues. An adventitial bursa describes an acquired fluid filled soft tissue abnormality that develops at a pathologic or iatrogenic site of increased friction between two structures, often between bone or hardware and the overlying soft tissues. The latter may not be synovial lined.
The Suprapatellar bursa or recess is an anterior midline structure located deep to the distal quadriceps and superficial to the pre-femoral fat pad and distal femur. This communicates widely with the knee joint in most adults functioning as a recess due to the involution of an embryonic septum that exists between the bursa and the remaining joint, which occurs at about the third fetal month.1,2 A suprapatellar plica is often seen, representing a shelf-like membranous remnant of part of that septum. In approximately 16% of adults, an isolated suprapatellar bursa may be present due to failure of the embryonic septum to involute leaving a complete septum. Distension of this bursa may mimic a mass in the pre-femoral region.3 A persistent partial septum or suprapatellar plica may trap chondral or osteochondral bodies in the suprapatellar recess.4 See Figure 15.
Prepatellar, Superficial and Deep Infrapatellar bursae: The prepatellar bursa lies superficial to the quadriceps continuation and is centered in the midline though it may extend slightly medially or laterally relative to the patella. Surgical and cadaveric studies have shown the prepatellar bursa to be trilaminar or bilaminar in nature,5 though on routine clinical MRI, that compartmentalization is often not apparent. The superficial infrapatellar bursa resides superficial to the anterior tibial tuberosity and patellar tendon insertion, and the deep infrapatellar bursa lies just deep to the distal patellar tendon. The latter frequently contains a small amount of fluid in asymptomatic knees.6 Normally none of these bursae communicate with the knee joint.1
Medial and Posteromedial Bursa
The pes anserine bursa is located anteromedially, about 3-4 cm caudal to the joint line and does not typically communicate with the joint. It is interposed between the distal tibial collateral ligament insertion and the distal sartorius, gracilis and semitendinosus tendons. The combined insertions of these tendons have a webbed configuration, which has been likened to that of a goose’s foot, from which is derived their Latin name, pes anserinus tendons.7 The Tibial collateral or MCL bursa has been found to be present in 90% of adults and in cadaveric specimens, separate femoral and tibial portions of the bursa were frequently seen.8 Distension of the MCL bursa typically appears as a vertically oriented well defined fluid collection extending along the medial aspect of the femoral and tibial cortices, being centered deep to the superficial MCL (tibial collateral ligament) between it and the deep MCL (the meniscofemoral and meniscotibial ligaments), typically occurring in association with medial compartment and or medial capsuloligamentous pathology.8,9 Distinction between fluid in the MCL bursa and a medial parameniscal cyst may be difficult, though usually medial parameniscal cysts are centered more posteriorly, and usually communication of a parameniscal cyst with a meniscal tear or focus of intra-meniscal degeneration is present.8 Additionally, parameniscal cysts are often more loculated and sometimes contain more inspissated contents.
The Semimembranosus-tibial collateral bursa is located along the posteromedial joint line, cranial to the pes anserine bursa, and was found to be consistently present in a cadaveric analysis of 50 knees.10,11 It has an inverted U or J configuration with a superficial distal arm and a deep proximal arm that drapes around the distal semimembranosus tendon.
The Semimembranosus-gastrocnemius bursa or recess (Sm-Gb) arises between the semimembranosus tendon and the medial head of the gastrocnemius, within the posteromedial popliteal fossa, cranial to the joint line at the level of the upper medial femoral condyle. They most often extend caudally, though they may extend in any direction.12 The bursa communicates with the knee joint in most adults and thus is sometimes referred to by many as a recess, being comprised of a distal gastrocnemius bursa and a proximal semimembranosus bursa, which sometimes are partially separated by a septum.13 The reported prevalence of this communication has varied dependent upon the population being studied, and the mode of investigation. It has been reported as occurring in 30-50% of cadaveric dissections, 50% in arthrograms of normal knees, 55% in open surgical excision, and 37% in diagnostic arthroscopies.14,15,16,17 Lindgren et al. studied 80 cadaver knee joints and concluded that communication of the Sm-Gb with the knee joint is acquired, due to degeneration of the capsule, occurring with greater frequency in older individuals, in more than 50% of those over the age of 50 years and in no patient under 10 years of age. The communication occurs via a slit-like defect in the posteromedial capsule most often measuring 15-20 mm, where the medial gastrocnemius tendon exits the joint.17,18 Various studies have shown unidirectional flow of fluid from the knee joint into the Sm-Gb, though the precise nature of that valvular mechanism is controversial with some proposing a Bunsen or ball valve mechanism and others theorizing that the dynamic actions of the semimembranosus, semitendinosus and gastrocnemius myotendinous units in flexion and extension contribute to a functional unidirectional communication.17,19,20
Lateral and Posterolateral Bursa
The Iliotibial Bursa is believed to be an adventitial bursa that may develop between the iliotibial tract and the lateral femoral condyle, possibly with extension further distally in the compartment like space between the iliotibial tract, and the lateral meniscocapsular ligaments in the setting of chronic inflammation and iliotibial band syndrome.21,22,23
The Popliteus Recess or Popliteus Hiatus occurs at the posterolateral corner of the knee, posterior to the posterior horn lateral meniscus around the popliteus tendon. It routinely communicates with the knee, may extend caudally deep to the popliteus myotendinous junction, and may communicate with the proximal tibiofibular joint in 10% of adults.24,25
The Fibular collateral- biceps femoris bursa has an inverted J shape in the axial plane with a curved short arm extending around the anteromedial portion of the distal fibular collateral ligament near the insertion, and a longer lateral limb lying along the superficial lateral aspect of the fibular collateral ligament. Terry et al. found this to be consistently present in cadaveric dissection of 30 knees, though other investigators discussing the anatomy of the posterolateral knee did not note the bursa to be uniformly present.26
Bursae of the Knee: Clinical Presentations and MRI Appearance
Bursae about the knee may or may not communicate with the joint, and with a few exceptions normally contain minimal or no fluid, such that they may not be detectable on MRI. Small amounts of fluid may be seen within some bursae and recesses in the absence of symptoms, particularly the semimembranosus gastrocnemius bursa, the deep infrapatellar bursa, the suprapatellar recess, the popliteal recess (hiatus) and occasionally the pes bursa. Though the significance of the finding of fluid within bursae of the knee on MRI is primarily determined by clinical factors, the MRI observation of prominent bursal distension, mild circumferential bursal wall thickening, adjacent soft tissue edema and/or reactive marrow edema, neurovascular impingement and indicators of bursal rupture (further discussed below) are imaging features that suggest symptomatic bursitis. Bursae that contain fluid may appear simple, demonstrating homogeneous fluid equivalent signal on T1 and T2-weighted images, or they may be complex and multiloculated with thin internal septations. Following administration of intravenous gadolinium, thin peripheral and/or septal enhancement is typical of bursae. Because synovial fluid may be absorbed by the synovial cell bursal lining, but fibrin and other debris typically is not, cyst contents may become inspissated or mineralized. Bursae containing proteinaceous or hemorrhagic material may demonstrate hyperintensity on T1-weighted images, while those containing calcification or hemosiderin will exhibit intrinsic low signal on T1 and T2-weighted images (see test case figure 1). Bursa may contain chondroid or osteochondral bodies, which may represent extruded joint bodies, or these may develop de novo due to chondro-metaplasia.27 Chondral or osteochondral bodies within bursa can enlarge over time as they may be nourished by synovium.
A distended semimembranosus-gastrocnemius bursa is the most commonly encountered fluid collection about the knee1,28 The term “Baker’s cyst” is now often used to describe distension of this anatomic bursa, after clinical observations in 8 patients with popliteal fossa swelling were described by William M. Baker in 1877.29 In adults these bursae frequently communicate with the knee joint and are most likely to occur in knees with an effusion due to traumatic or degenerative internal derangement, osteoarthritis, inflammatory or crystal deposition related arthropathy or other diseases of the synovium including PVNS and primary synovial osteochondromatosis.26 The reported prevalence of distension of this bursa varies, dependent upon the age and nature of the study population, defining criteria and imaging modality utilized, ranging from 5-32% of adults being evaluated for suspected internal derangement by MRI and arthrography, respectively. Higher detection rate using arthrography has been postulated to be due to iatrogenic distension of previously collapsed bursae.30 In an MRI evaluation of 102 asymptomatic knees, Tschirch et al. found prevalence of Sm-Gb of 19%, with the majority having a maximal size of 30 mm or less. In contradistinction, the majority of cases of semimembranosus-gastrocnemius bursal distension in pediatric patients, excluding those with juvenile rheumatoid arthritis (JRA), are not associated with effusion and intra-articular pathology and are thought to occur spontaneously due to local trauma or irritation involving the bursa.31,32 Pediatric patients with JRA and an effusion commonly develop semimembranosus- gastrocnemius bursal cysts.33
Baker’s cysts most commonly present in adults as a painless posteromedial swelling, but if large they may lead to limited range of motion. They may be associated with knee pain, though the pain is often due to the intraarticular pathology, rather than the cyst itself. Large cysts may lead to claudication due to mass effect upon the popliteal artery. Sudden onset of pain and swelling may occur mimicking thrombophlebitis, most often after cyst rupture due to overdistension and or trauma. Uncommonly, cyst rupture may be associated with compartment syndrome or with concomitant thrombophlebitis.34,35,36 Though bursal rupture is most often seen in the setting of a large Baker’s cyst, fluid may also extravasate from a distended posterolateral popliteus recess (hiatus) in which case the clinical presentation may mimic a lateral gastrocnemius muscle strain. In the setting of bursal rupture, MRI will show edema tracking in the fascial planes along the caudal, cranial medial or lateral aspects of the cyst, possibly with loss of smooth peripheral contour in the region of rupture and overlying subcutaneous edema. Though Sm-Gb cysts most often extend inferiorly and medially, they may extend along the fascial planes in any direction from the bursal origin, and may also dissect into adjacent musculature of the distal thigh and proximal calf.37 Particularly when large or complex the imaging appearance of a large semimembranosus gastrocnemius bursal may be confusing, but identification of a neck between the semimembranosus and gastrocnemius tendons confirms the diagnosis.
Semimembranosus-tibial collateral bursitis usually presents with nonspecific posteromedial pain, aggravated with activity. MR images show distension of the inverted U or J-shaped bursa partially draped over the semimembranosus, at the posteromedial joint line and abutting the posterior horn medial meniscus. Distension of this bursa may coexist with semimembranosus, pes anserine and or semimembranosus gastrocnemius bursal distension, though they do not typically communicate.8,11,38,39
Tibial collateral bursitis may present with pain along the medial joint line mimicking a medial meniscal tear or chondral abnormality. The bursa is visible only if distended with fluid, which is usually indicative of pathology. Tibial collateral bursitis does not often occur in isolation and may be associated with traumatic, degenerative, or inflammatory medial compartment pathology, severe genu valgus, and flat foot deformity. It has also been described in professional motorcycling and equestrian athletes.8 MRI will show a circumscribed fluid collection centered deep to the tibial collateral ligament interposed between it the meniscofemoral and meniscotibial ligaments, often with a bilobular appearance on axial images.1 At times only the tibial or femoral component of the bursa will be distended with fluid.
Pes anserine bursitis classically causes pain and or swelling medial and caudal to the joint line, which is often aggravated with crossing one leg over another, rising from a seated position or climbing stairs. It may be associated with vague medial pain mimicking medial meniscal tear(7). It often occurs in runners, basketball and racquetball players due to repetitive overuse, though it may also be seen in the setting of direct trauma, increased valgus or rotatory stresses upon the knee 40. There is also an increased incidence in middle age obese women, those with diabetes, osteoarthritis, medial meniscal or chondral pathology. Small amounts of fluid within the bursa may be asymptomatic. More prominent fluid collections, with or without a thickened synovial lining, and semi-solid appearance with complex contents have been described in symptomatic and chronic pes bursitis.38,40,41
Similarly, iliotibial band syndrome often occurs due to sport related overuse in cyclists and runners, particularly those training on sloped surfaces, with the more inferiorly positioned lower extremity being affected. It has also recognized in football, soccer, basketball players, and skiers. Other factors associated with greater incidence of IT band syndrome include genu varus, overpronation of the foot, hip adductor weakness, and a prominent lateral epicondyle.21–23,42 Though historically this syndrome was attributed to friction related to the movement of the ITB anterior and posterior to the lateral femoral condyle, some investigators have challenged this theory, suggesting that a compression or impingement like syndrome affecting the vascularized fatty tissue deep to the iliotibial tract may better reflect the etiology21,22,43 Despite some debate regarding the pathogenesis, the syndrome relatively consistently presents with lateral or anterolateral pain that may sometimes mimic lateral compartment pathology. MRI most often shows ill-defined edema within a compartment like space bounded laterally by the iliotibial tract laterally, and medially by the lateral femoral condyle, the periphery of the lateral meniscus and meniscocapsular ligaments. Thickening of the iliotibial band may or may not be seen. A minority of patients with iliotibial band syndrome develop a discrete fluid collection within the same space deep to the distal iliotibial band, thought to represent adventitial bursal development.
Pain and or swelling posterior and laterally, caudal to the joint line may be seen due to distension of the popliteus bursa, or due to a synovial cyst or ganglion arising from the proximal tibiofibular joint. The latter may be associated with clinical and or imaging findings of compression neuropathy due to direct mass effect upon or epineural extension involving branches of the peroneal or tibial nerves.1,44,45,46
Fluid normally occurs in the suprapatellar recess, though a symptomatic suprapatellar bursitis may occur usually in the setting of a complete suprapatellar plica. Patients commonly present with suprapatellar pain and swelling, often with concern for a mass involving the distal thigh.
A small amount of fluid may be seen in the infrapatellar bursa as a normal finding. Symptomatic deep infrapatellar bursitis most often shows more extensive fluid in the bursa with or without inflammation in the adjacent soft tissues, usually seen in jumper’s and runners, sometimes in association with patellar tendinosis or Osgood- Schlatter’s disease.42,47 Prepatellar and superficial infrapatellar bursitis may be seen in the setting of acute direct trauma and are also commonly associated with chronic repetitive microtrauma, most often due to occupational kneeling as has been called clergyman’s knee, housemaid’s knee, or carpet layer’s knee. In these cases, bursal imaging may show features of intrinsic hemorrhage or mineralization. Mineralization may also occur in the setting of tophaceous gout (see figure-18) which in the knee, has a predilection for the extensor tendons, anterior bursa and fat pad, as well as the intercondylar notch and popliteus tendon insertion.48,49 Infectious prepatellar or infrapatellar bursitis, most often due to staphylococcus aureus, may complicate chronic occupational related bursitis or penetrating anterior trauma. In addition to an increased prevalence of prepatellar and superficial bursitis, an increased incidence of multiloculated cysts of the popliteus recess or bursa has been reported in occupational kneelers.50
Synovial cyst refers to any synovial lined periarticular or peritendinous fluid collection. The term has been used in the literature synonymously with bursa, though most use it to describe synovial lined fluid collections that are acquired rather than being uniformly present developmentally.
Ganglia describes benign periarticular, intra-articular, subchondral or periosteal cystic structures which contain viscous fluid rich in hyaluronic acids and other mucopolysaccharides.51 They may arise from the joint capsule, ligaments, bursae, subchondral or subperiosteal bone adjacent to many joints, and have variable communication with the joint.1 Their precise etiology remains unclear with theorized origins including mesenchymal cell metaplasia and proliferation, versus mucoid connective tissue degeneration related to repetitive microtrauma, versus synovial herniation through a capsular defect. Although histologically they may often be differentiated from synovial cysts because they are lined by a dense fibroconnective tissue capsule with flat spindle cells rather than synovial cells, this distinction is less than perfect as ganglia may become synovialized1,52 Synovial cysts and ganglia are often not distinguishable on MRI, though this distinction is likely primarily of academic interest, as it usually does not directly alter management. More importantly, MRI does allow delineation of many other features of periarticular cysts that will often directly impact treatment, such as lesion size, location, effect upon adjacent structures, findings of rupture, as well as the nature and extent of associated intraarticular pathology.
Intraarticular ganglia have been reported with a prevalence of up to 1% of knee MR imaging53,54 and may be seen within or adjacent to the cruciate ligaments, most commonly occurring near the tibial insertion of the ACL and the adjacent anterior root lateral meniscus. Cruciate ligament ganglia may be associated with mucoid degeneration of the cruciate ligaments, likely representing different manifestations of the same pathogenesis. These ganglia may be asymptomatic, or dependent upon their size and location may be associated with pain mimicking a meniscal tear, clicking, or limited range of motion. Instability is not usually an associated finding.1,51,53,55
Juxta-articular ganglia (or synovial cysts) may be observed posterior to the distal femoral metaphysis, adjacent to the capsule, near the medial or lateral head gastrocnemius origins, and are often multiloculated. Unlike in cases of semimembranosus-gastrocnemius bursal distension, no extension between the semimembranosus gastrocnemius tendons is seen in medial lesions. In a small series of 10 cases only one patient presented with suspected popliteal fossa mass, none showed obvious MRI communication with the joint and none required surgical excision.57
Parameniscal and intrameniscal cysts have been theorized to develop when synovial fluid is pushed through a meniscal tear accumulating within or at the periphery of a torn or degenerated meniscus, or due to a myxoid degenerative process involving the meniscus. They have been found to have a prevalence of about 4% of knee MRI’s in 2 studies, one in symptomatic and another in asymptomatic knees.1,6,58 Parameniscal cysts typically appear as circumscribed T2 hyperintense structures with an epicenter at the joint line near the peripheral meniscal margin. Several studies have shown direct communication of parameniscal cysts with a meniscal tear in up to 98%, though the frequency of that association depends upon the definition used for meniscal tear. In cases in which communication with an articular surface tear is not identified, one may see communication with intra-meniscal degeneration which does not meet strict criteria for tear.51,58,59 They occur both medially and laterally, and most often are associated with tears having a horizontal component,9,58 commonly presenting with joint line pain with or without swelling. When large they may migrate away from the meniscal margin, in a distribution dependent upon the regional capsuloligamentous planes of the knee. Differentiation of these cysts from other periarticular ganglia is important as recurrence is common if only parameniscal cyst aspiration or excision is performed. Best treatment outcomes for symptomatic parameniscal cysts have been shown to include cyst decompression as well as the appropriate arthroscopic treatment of associated meniscal pathology.58,60,61 Medial parameniscal cysts should be differentiated from fluid within the MCL bursa, semimembranosus-TCL, and pes anserinus bursa. Communication with a meniscal tear (or horizontal intrasubstance degeneration) and location different from characteristic confines of the anatomic bursae, will allow differentiation. Edema around the MCL may occur in the setting of recent trauma and has also been described in the absence of recent trauma, having significant association with medial and lateral meniscal tears and chondromalacia.62 Such peri-ligamentous and meniscocapsular edema is ill defined, and readily distinguished from parameniscal cyst or medial bursal distension.
Synovial cysts or ganglia associated with the proximal tibiofibular joint appear as fusiform or multiloculated cystic collections along the distal aspect of the fibular head, with a pedicle of communication with the tibiofibular joint. The knee joint communicates with the proximal tibiofibular joint in 10% of individuals, and cysts of the proximal tibiofibular joint have a greater incidence in those with chronic knee effusions.9,42 Cysts may cause direct compression of the common peroneal nerve near the fibular head or may dissect along the epineurium of the common peroneal nerve and its branches or the articular branch of the tibial nerve with associated findings of denervation edema at MRI, and clinical findings of neuropathy.1,9,42 Ganglia of the proximal tibiofibular joint may occasionally cause pressure erosion of the adjacent bone,9 which can raise concern regarding an osseous lesion, though the sclerotic margination and adjacent ganglion communicating with the joint space allow diagnosis.
Morel-Lavallee lesions (also called Morel-Lavallee effusions) are a consideration in the differential diagnosis of anterior bursal collections. Though classically occurring lateral to the greater trochanter, these may involve the extensor aspect of the knee in contact athletes particularly wrestlers and football players, who have suffered a traumatic injury with shear forces tangential to the fascial planes. This results in a closed degloving injury which separates the subcutaneous fat from the underlying fascia and causes injury to small perforating capillaries and lymphatics. History and physical exam may sufficiently diagnosis the condition, but when utilized MRI will usually show a large lenticular fluid collection centered in the perifascial space deep to the subcutaneous fat with variable intrinsic signal characteristics consistent with contained fluid, hemorrhagic material and often globules of fat or lymphatic material.63,64,65 The peripheral margins of the collections may demonstrate acute margins related to the delamination of the subcutaneous fat from underlying fascia, and signal hypo-intensity on all sequences due to fibrous or hemosiderin laden pseudo-capsule. Eccentric medial, lateral and or proximal extension into the mid-thigh, beyond the normal confines of the prepatellar bursa is often seen. Due to their large size and extent, propensity to re-accumulate and lack of synovial lining, management of these collections differs from that of anterior bursa, often involving aspiration and compression banding, sometimes with sclerodesis or open evacuation of refractory lesions.
Post-traumatic or coagulopathy related hematomas may occur about the knee with the clinical history and MRI signal characteristics of hemorrhage usually leading to proper diagnosis.
Cystic adventitial disease of the popliteal artery is a very rare condition in which ganglia develop within the popliteal artery adventitia compromising blood flow, and potentially leading to claudication.45,66,67 The etiology is uncertain though synovial origin is favored with synovial fluid theorized to dissect through a capsular rent and along an articular vascular branch. MRI is a component of preoperative workup of patients with cystic adventitial disease, to identify any joint connections which should be ligated operatively.45
Certain solid tumors, especially myxomatous lesions, nerve sheath tumors, necrotic sarcomas, and vascular lesions may mimic cystic structures. These should be considered when any solid component other than a thin peripheral wall or septa is present, when the location is not in keeping with a recognized bursa or common ganglion location, when invasion of surrounding structures is present, or when contents are heterogeneous and differ from fluid or complex fluid. In uncertain cases, injection of intravenous contrast agent may be useful to distinguish cystic from solid or vascular lesions.
Many periarticular and intraarticular fluid collections, particularly those that are small, are asymptomatic requiring no treatment. Symptomatic bursae occurring in the setting of athletic overuse may be treated with rest, non-steroidal medication, corrective training measures, shoe wear or orthotics. Identification and treatment of treatable associated intraarticular pathology is important in the management of symptomatic semimembranosus gastrocnemius bursal cysts, and other synovial cysts that communicate with the knee joint. This may be supplemented with aspiration, often with steroid injection. Surgical resection is often reserved for recalcitrant symptomatic bursae, synovial cysts, and ganglia or those impinging upon neurovascular structures. The identification of pedicles of cyst extension and joint communication on preoperative MRI is important in preoperative planning to guide the ligation of any joint communication and complete cyst resection, such that the risk of cyst recurrence is diminished.
Cystic lesions and bursal distension about the knee joint are commonly occurring findings. While some lesions are asymptomatic, others may be associated with swelling, limited range of motion, pain due to impingement upon adjacent structures, or symptoms related to associated intraarticular pathology. MRI aids in characterization and treatment of symptomatic periarticular cystic or bursal collections by localizing them, delineating their relationship to adjacent structures, defining associated intra-articular pathology, and differentiating them from solid lesions or alternate causes of periarticular swelling.
A special thanks to Mike Stadnick, MD for illustrations in Figure 3, to David Rubin, MD for providing multiple cases used in this web clinic and to Liz Carpenter, MD for providing the case used in Figure 14.
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