Rheumatol Int DOI 10.1007/s00296-014-3120-1
Rheumatology INTERNATIONAL
REVIEW ARTICLE - IMAGING
Ganglion cyst versus synovial cyst? Ultrasound characteristics through a review of the literature Marie‑Claude Giard · Carlos Pineda
Received: 29 March 2014 / Accepted: 22 August 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Ultrasonographic characteristics of two common musculoskeletal lesions, ganglion cysts (GCs) and synovial cysts (SCs) are presented through a review of the literature. Although similar in many ways, these two lesions display different morphostructural characteristics justifying, in our view, their descriptions as separate entities. Mainly different from an anatomopathologic point of view, they also differ in their potential therapeutic implications. A symptomatic GC, refractory to conservative therapy, may require surgical excision of the cyst itself. For SC, therapy should primarily be oriented toward identifying and correcting the often coexisting intra-articular disease instead of only targeting merely its consequence, the SC. Keywords Synovial cysts · Ganglion cysts · Ultrasound imaging · Diagnostic
Para‑articular cysts definitions Ganglion cysts (GCs) and synovial cysts (SCs) are soft tissue masses that develop around diverse joints and that can be asymptomatic or cause symptoms such as pain, M.-C. Giard (*) Rheumatology Department, Verdun Hospital, 4000, Boulevard La Salle, Montreal, QC H4G 2A3, Canada e-mail:
[email protected] M.-C. Giard Rheumatology Department, Royal‑Victoria Hospital, McGill University Health Center, Montreal, Canada C. Pineda Rheumatology Department, Instituto Nacional de Rehabilitacion, Mexico City, Mexico
weakness, swelling or joint impairment with joint mobilization or compression of nearby structures. A GC is defined as a cystic, tumor-like lesion of unknown origin which is delimited by dense connective tissue and filled with gelatinous fluid rich in hyaluronic acid and other mucopolysaccharides [1]. Indeed, evaluation via electron microscopy demonstrated that the wall of GC is composed of randomly oriented sheets of collagen arranged in loose layers, one on top of another [2]. Most GCs display a focal cellular lining of its cavity. These cells are immunohistochemically different from synoviocytes and show characteristics of myofibroblasts or mesenchymal cells undergoing early muscle differentiation [2]. A fluid-filled stalk may, or may not, be seen connecting the GC to the adjacent joint or to diverse nearby soft tissue structures [1]. On the other end, a SC is a juxta-articular fluid-filled collection that is lined by synovial cells which histologically distinguishes them from other juxta-articular fluid collections, most importantly from GC. It represents a focal extension of joint fluid that again may, or may not, communicate with the joint [1]. Also, this fluid collection may extend in any anatomic direction, in opposition to a synovial effusion with its well-circumscribed anatomic boundary, the joint capsule, which guides its extension in a more predictable direction. The literature on the subject is still somehow confusing with both terms used interchangeably [3, 43]. For example, different sources will use GC to qualify what, with appropriate review of the pathologic description, would truly correspond to a SC according to the previous definitions. Even more frequently, the term “synovial cyst” will be used to mention what typically corresponds to a GC, a soft tissue mass lacking a synovial lining with a mucinous, gelatinous content.
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Etiology: a few theories Some aspects of these pathologic lesions are still to be clarified, for example, their exact pathogenesis remains unclear. The concept that a GC is a simple herniation of the joint capsule is difficult to support in light of the lack of synovial lining within the cyst itself. Some other theories exist for GCs, one of them, the metaplasia theory, implies that these GCs come from metaplasia of embryonic or post-traumatic peri-articular remnants. Another theory, the synovial theory, proposes that synovial fluid migrate from the joint cavity to form a collection into the vicinity of the joint keeping a connection to it through a small channel often referred to as “stalk” [4, 44]. With the demonstration of the neurovascular bundle linking the tibial as well as peroneal intraneural GC with the intra-articular cavity, this last theory gained support [5]. At the wrist joint level, a different theory suggests that stretching of the joint capsule or ligaments would stimulate mucin production, this mucin eventually being encapsulated to form GC [6]. In other words, the mucoid degeneration of ligaments and joint capsule would have as an end result the GC formation [7]. For SCs, the proposed theory is that they would originate from an effusion secondary to an internal joint derangement; this effusion would increase intra-articular pressure with the subsequent escape of joint fluid from its regular location through a one-way valve mechanism into the area of least resistance. The prototypical example of a SC, the Baker cyst, would therefore have its origins in an intraarticular process and, with time, would or would not keep its communication with the joint cavity [8]. Knee flexion would open this communication by increasing pressure in the intra-articular cavity, while knee extension would cause the tense surrounding muscles to close this “valve” [9]. A clinical sign proposed to distinguish Baker cysts from other structures at the popliteal fossa, the clinical Foucher’s sign, can be explained by a similar mechanism. Baker cysts are described as becoming firm with full extension of the knee while being soft when the joint is flexed. With knee extension, the gastrocnemius and the semimembranosus muscles approximate each other to the joint capsule compressing the cyst against the deep fascia and increasing the intracystic pressure explaining this clinical finding. Opposite effects in flexion allow the cyst to relax [11]. Some SCs are thought to arise from the bursa itself with no communication with the joint. In those cases, it can be questioned whether these entities should be classified as “synovial cysts” or whether a better use of terminology would rather be the terms “bursae” and “bursal enlargement” in those cases. Interestingly, a recent article about cysts and bursae about the knee indeed uses the terms “synovial cysts” and “bursae” interchangeably [12]. For our part, we still believe that the term SC should be used
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for those entities where a joint–bursa communication with a one-way valve-like mechanism is involved. Intra-articular derangement being often involved in such SC, in our view, it makes these entities worth being classified in a different category. Indeed, joint pathologies are often found coexisting with SCs. In a younger population, up to 70 % of popliteal SCs have been shown to coexist with medial meniscus tear while in older populations, SCs are most frequently related to degenerative conditions such as osteoarthritis [9]. In the pediatric population, unlike in the adult population, SCs of the popliteal area are not linked to an intra-articular process and should probably be called bursitis instead of SCs [10]. Indeed, the term “bursitis” should be reserved to synovium-lined structures not linked to the joint.
Pathology: main difference between these two entities An aspect that is mostly agreed upon in the literature is the different pathologic characteristics of these cysts. Most relate that the principal difference between a SC and a GC comes from their different content and histologic features. SCs have, by definition, a synovial cell lining, while a GC is delineated by dense fibrous connective tissue. Furthermore, when it comes to the content of each cyst, a SC is known to be filled up with synovial fluid ranging from normal to inflammatory synovial fluid, depending on the underlying joint disease. For GC, all report a more viscous content of hyaluronic acid and mucopolysaccharides representing the same constituents as the synovial fluid but at a higher concentration. Its content is also described as being clear or slightly xanthochromic with a gel-like consistency analogous to “apple jelly” [4].
Sonopalpation: US compressibility as an important indirect assessment of their pathologic content The sonographer has the ability to combine sonopalpation (i.e., direct pressure over structures of interest by the probe) and simultaneous visualization of a soft tissue mass with real-time feedback from the patient. This ability to palpate while directly visualizing a structure is a unique feature of ultrasonography that is very useful diagnostically. Knowing their different pathologic characteristics, it can be expected that each cyst will present different degrees of compressibility. Sonocompressibility is a criterion that can be very helpful to differentiate a SC, normally more easily compressible, from a GC, much harder to compress due to its thick, viscous content and its dense fibrous capsule [13, 43] (Table 1). Sonocompressibility provides an important clue about the cyst’s content, which, as mentioned previously, is the main characteristic distinguishing SCs from GCs. This
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criterion represents an important advantage of US imaging of these lesions over other imaging modalities. Of course, to be applicable, the cyst must be accessible, i.e., with good coverage of the ultrasound probe over soft tissues overlying the lesion in order to indirectly access its compressibility. Therefore, some locations prevent the use of such criteria, the paralabral cysts of the deep coxofemoral joint being one example. While SCs are usually associated with greater compressibility, SC related to active disease might show hypertrophied synovium altering its compressibility. A Baker cyst filled with hypertrophied synovial tissue related to rheumatoid arthritis is one example. These particular cases represent a small minority of SCs cases according to our experience and therefore should not affect the validity of this criterion. On the other end, GCs are known to present a thick and highly viscous content and to present a low compressibility. To the best of our knowledge, there is no exception to that rule.
Joint communication: an inconstant finding with standard imaging techniques for both GCs and SCs The main objectives of imaging techniques in the evaluation of cysts around joints are as follows: to confirm the cystic nature of the lesion, to determine whether there is a communication between the cyst, the joint cavity and the surrounding structures and finally, to evaluate the nearby joint for associated disorders. In this sense, US imaging is an appropriate and interesting tool to screen for suspected cystic lesions since it can demonstrate their content, their exact location and extent, and, in some cases, it can also demonstrate a communication with the underlying joint narrowing greatly the diagnostic possibilities. The main area of controversy in the imaging literature is regarding the potential communication of these cystic masses with the adjacent joint and how much importance this characteristic has for defining a GC or a SC. Some use this criterion as part of their definition of a GC, a GC having a communication with the joint space [3]. Some other sources allege that this communication may or may not be present both with SCs as well as with GCs at the time of assessment [1, 29]. For GCs, most data relate that a communication with the joint either sought by imaging or at time of surgery is, overall, an inconstant finding. It has been proposed that, in some cases, the communication is probably fairly thin and therefore easily missed at time of surgery [4]. From the imaging side, MRI, one of the preferred imaging modalities for evaluation of these cystic lesions, is said to demonstrate a stalk connecting the cyst to the joint cavity only after a review of multiple images in multiple planes in order to identify its frequently serpentine course
[14]. It can therefore be a laborious and time-consuming task sometimes unsuccessful. To improve sensitivity of imaging to the intra-articular communication, it has been suggested to use delayed plain radiography or CT perform ideally 1–2 h after arthrography. This allows contrast to diffuse into the thick and viscous content of the GC [15]. This technique of delayed arthrography would enhance the overall sensitivity of imaging to detect the very thin stalk [4, 15, 16]. Malghem et al. [15] reported 12 cases where this technique revealed the joint communication when standard MRI had revealed an evident stalk in only three of those cystic lesions of the knee. This is particularly useful in atypical GC where differential diagnosis with malignant tumors is of utmost importance. Indeed, demonstration of the stalk can allow exclusion of some malignant hypotheses such as neural tumor and liposarcoma. MR arthrography with delayed imaging is also used and was reported useful in depicting joint connections not detected on previous MRI in peroneal intraneural GC [17]. On the other hand, the need of a particular technique to improve imaging sensitivity for joint communication implies that it can be easily missed with standard MRI and US imaging techniques. Demonstration of a visible stalk by US was seen in only 25 % of cases in the largest series of GC, 60 GC of the hand and wrist [17]. Another series of 20 GCs depicted a joint extension in only 35 % (seven cases) [19]. Those percentages might be somehow underestimated since most series are of retrospective nature with only GC that required surgery included since relying on cases corroborated by pathology. GC with obvious communication by imaging could be underrepresented in those series. An obvious diagnosis could lessen the need for surgery in some cases while not depicting the cyst–joint communication could lead to more ambiguous cases and be the indication for surgery of the some GCs in these series. Nonetheless, with the previous data shown, it can be concluded that, overall, joint connections are not easily depicted in practice. To rely only on such a diagnostic criterion, presence of a communication with the joint, by standard MRI or US imaging could lead to misclassification. A proper diagnostic criterion for GS by US imaging would therefore be that they may or may not communicate with the joint cavity. For SCs, most mention that they may or may not be communication with the joint cavity [1, 12, 29]. From a pathophysiologic point of view, it is agreed that a normal communication with the joint, at some point, is responsible of enlargement of the bursa in most SC cases. A case report of an abnormal communication arising from a post-traumatic fistula between a pre-patellar bursa and the joint cavity is also described [20]. Only few SCs would arise from local or systemic inflammatory process involving the bursa itself without joint–bursa communication [9]. Although most SCs are thought to have displayed such communication
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Table 1 Ultrasonographic characteristics of ganglion cysts (GCs) versus synovial cysts (SCs) Ultrasound characteristics
Ganglion cyst
Synovial cyst
No Wrist, hand, ankle, foot
Yes Knee (gastrocnemiussemimembranosus bursa), hip (iliopsoas bursa)
Most discriminative findings : Compressibility Locations (most frequent a )
Similar findings Blood flow by Doppler evaluation Echogenicity Configuration Borders Joint communication Posterior acoustic shadowing
Absence b Absence or possible b Anechoic to hypoechoic Anechoic to hypoechoic Uni or multilocular Uni or multilocular ± septations ± septations Thin and well-defined c Thin and well-defined c ± ± ± ±
a
See Tables 2 and 3 for an overview of possible locations
b
Only four cases described with GC and Doppler versus more synovial pathologies with possible blow flow at Doppler evaluation with SC
c
Ill-defined walls described (rare finding)
leading to their formation, this joint–cyst communication has been depicted in a much smaller proportion of SCs, from 30 to 55 % found either by dissection, surgery or by arthrography [9]. Unlike GCs where delayed arthrography has been proposed to enhance visualization of a cyst–joint communication, in SCs, such imaging technique has been alleged to create arthrographic distension of communication with normal bursa possibly leading to false-positive “synovial cysts” [9, 21]. Lastly, this communication would increase with age. For Baker cysts, such communication rises from 26 %, in patients having age from 31 to 50 years to 53 % in patients having age 51–90 years while being almost absent in the pediatric population [9]. Not surprisingly, the incidence of SCs joint communications also increases with age. Overall, it can be concluded that less than 50 % of both GCs and SCs show a cyst–joint communication by standard US or MRI.
Location of a cyst: an important diagnostic tool Another criterion helpful in differentiating among the various cystic lesions is certainly their exact location. For example, it is proposed that clinical diagnosis of GCs of the wrist is obvious enough that the majority of them do not require imaging for their confirmation [4]. Indeed, 50–70 % of soft tissue masses of the wrist are consistent with GCs, and to the best of our knowledge, there are no
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reports of SC of the wrist [16, 22]. For many other pathologic lesions, location can strongly suggest a diagnosis over another. A good example is Baker cysts, mostly expected in the popliteal region of the knee. Such diagnosis is therefore most plausible when a lesion is encountered in that anatomic region because of its well-known characteristic location with many previous pathologic validations described in the literature [23]. Therefore, data already available on pathologic findings of diverse cysts at different locations should definitely be used to make location a strong criteria part of the diagnostic algorithm for GCs as well as for SCs. GCs can be observed in different anatomic structures and in the vicinity of various diarthrodial joints: in the surrounding fat tissue, in the juxta-articular soft tissues, diffusing in adjacent muscles, peritendinous or even in an intratendinous, intraosseous, periosteal or intraneural locations. No rigid classification for GCs exists, Beaman and Peterson [1] provided one of the first classification composed of three general categories: juxta-articular, intra-articular and periosteal. A variety of locations has been described and can serve to elaborate a general topographic map for possible anatomic sites most compatible with GC diagnosis. Different GC locations, presented to dress an overview of different locations found in the literature, can be found in Tables 2 and 3. The wrist is the most frequent overall location, particularly described in relation to the scapholunate ligament of the dorsal wrist, followed secondly by the foot. To
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mention a few of them, in the upper limb, there are reports of GC from the temporomandibular joint, the acromioclavicular joint down to the small joints of the hand, at the level of the metacarpophalangeal and interphalangeal joint. In the lower limb, there are descriptions of GCs at the tibioperoneal joint as well as various other locations of the knee joint, the anterior cruciate ligament being among the most frequently reported [24]. Diverse locations around the ankle and foot, even at the small metatarsophalangeal joints, have also been reported [25]. Some of these locations rely on isolated case reports and do not constitute frequent findings. Another class of GC described is intraneural GCs. Some nerves are principally affected; the most cited being the common peroneal nerve. Its close proximity to the tibiofibular joint could explain why it is the most affected nerve. The actual theory for common peroneal nerve GC pathogenesis is its connection to the joint by the articular nerve ramus, in concordance with the synovial theory for GCs etiology. Lastly, intraosseous lesions can also be encountered, mostly at the epiphysis of long bones; the humeral head, the olecranon and the malleolus represent some examples. They can also be localized at the level of the carpal or tarsal bones [26]. When it comes to SCs, two most important locations, among those accessible by US, can be very useful landmarks to orient proper diagnosis. Not only location but also typical direction of the cyst extension is of value. The most frequent location is around the knee joint. The gastrocnemius-semimembranosus bursa is by far the most common SC, present in about 5 % of MRI perform for diverse reasons or when an internal joint derangement is suspected [27]. Its typical extension pattern is inferior and medial. In rare instances, it can extend laterally or proximally and even be intramuscular [1, 21, 25]. Although the cyst origin can be a little more laborious to determine in voluminous SC, the epicenter can normally be localized in the center of two tendons, the medial head of gastrocnemius and the semimembranosus tendons. This finding represents an important clue to the proper diagnosis [27]. A tibiofibular SC with communication with the knee joint is also an infrequent finding described but its differentiation from intraneural GC of the peroneal nerve is questioned by some [1, 21]. Although not accessed by US, after the gastrocnemius-semimembranosus bursa, another important site for SCs to develop is the facet joints, particularly at the level of the lumbar spine, which may occasionally cause neurogenic claudication [8, 28]. For other SC accessible by US imaging, after the knee, it is mostly at the hip level that the term SC is used to describe finding of a cystic lesion consistent with a SC, the “iliopectineal bursitis” or, mostly referred to as the “iliopsoas bursitis” [29]. The iliopsoas bursa was present in 98 % of cadavers in one study and in 15 % of the normal population a communication
Table 2 An overview of ganglion cysts (GCs) locations: upper limb Upper limb Hand and wrist (dorsal-sided) Radioscaphocapitate ligament Radiolunotriquetral ligament Scaphotrapeziotrapezoid joint All carpal or carpometacarpal joints Annular pulleys (mostly A1 and A2) Metacarpophalangeal joint Proximal interphalangeal joint Distal interphalangeal jointa Extensor tendons Hand and wrist (volar-sided) Scaphotriquetral ligament Pisotriquetral joint Guyon’s canal Intraneural Ulnar nerve Post interosseous nerve
Elbow Arcade of Frohse (“supinator arch”) Radiocapitellar joint (anterior) Radioulnar joint (proximal) Radioulnar interspace (proximal) Cubital tunnel Shoulder Acromioclavicular joint b Paralabral cystb Intramuscular cyst
a
“mucoid cyst”
b
Content similar to GC or controversy as if typical GC or not
Table 3 An overview of ganglion cysts (GCs) locations: lower limb Lower limb Knee Tibiofibulara Iliotibial band Medial patellar retinaculum Hoffa’s fat pad Alar folds (“plicae synovialis infrapatellaris”) Posterior joint capsule Anterior cruciate ligament (ACL) Posterior cruciate ligament (PCL) Epiphyses of long bones near ACL/PCL insertion (intraosseous) Periosteum at the level of the pes anserinus (periosteal) End of long tubular bones (periosteal) Intraneural Common peroneal nerve Tibial nerve Obturator nerve Sciatic nerve Lumbosacral plexus elements Tibial posterior nerve Sural nerve
Hip Greater trochanter (posterior) Hip joint capsule (anterior) Hip joint capsule (posterior) Ankle and foot Tarsal joints Tibiotalar joint Subtalar Metatarsophalangeal joint Tibial posterior tendon
a
Controversy regarding if different from common peroneal intraneural GC
between this bursa and the hip joint could be found [30]. It is believed that this percentage increases with hip derangement [30]. It has been said that this bursa could be an equivalent of the Baker cyst but at the hip level [31]. Giant SCs are also described, mostly related to rheumatoid arthritis [32]. They can involve large joints such as the knee, the
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shoulder and the elbow. One recent case report mentioned up to 24 cm, of maximal length, at the posterolateral knee joint [33]. Apart from these three proper SC descriptions (the gastrocnemius-semimembranosus bursa, the iliopsoas bursa and the facet joint SC), there is very few other sites where SC have been reported. Another site reported is at the level of temporomandibular joint, where both SC and GC have been described [34]. Overall, when evaluating if a SC case reported corresponds truly to a SC based on pathologic findings, it can be realized that most cases described at other anatomic sites than the knee or the hip joints are, according to our definition, GC with the terminology used interchangeably. At the level of the elbow joint, a few cases have been reported in relation to this joint. From Japan, where a rare disease of multiple extra-articular SC formation is also described, three cases of SC of the elbow have been described [35, 36]. Those three cases were located at the proximal ulna with no communication with the elbow joint, adherent to the median nerve and as an extension of the anterior recess anterolaterally affecting the posterior interosseous nerve. In the pediatric population, at the level of the shoulder joint, SCs leading to a distension of the biceps tendon sheath are also described [38]. Overall, in the adult population, two main peripheral locations should be recognized as characteristic of proper SC: between the tendons of the semimembranosus and the medial head of the gastrocnemius muscles for the Baker cyst and between the iliopsoas tendon and the hip joint capsule for SC of the hip for the so-called “iliopsoas bursitis” [29, 37].
Cysts arising from fibrocartilaginous tears: part of a different class of cysts? Following such enumeration of different sites where cysts can be found, it may be observed that shoulder paralabral cysts, hip paralabral cysts and meniscal cysts have not been mentioned. Indeed, these cysts are mostly described in a category apart from GCs or SCs. If not, they are mostly described as being part of GC with an intracystic content similar to the gelatinous mucoid content typical of GCs. These three cysts are all related to tears of different fibrocartilaginous structures: the glenoid labrum, acetabular labrum and knee meniscus [22, 39]. Such characteristic could justify classifying them in a different category since this strong correlation is of the utmost clinical importance [22]. In paralabral cysts of the hip, for example, the high association of paralabral cysts and labral tear is said to allow inference of an underlying tear even when not specifically visualized by imaging [40]. For meniscal cyst, to be in contiguity with an horizontal tear is proposed as a distinguishing feature [41]. It is even proposed to use the absence of meniscal tear to exclude parameniscal cyst diagnosis in
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Fig. 1 Ganglion cyst of the wrist: absence of blood flow at Doppler evaluation
Fig. 2 Ganglion cyst (GC) of the wrist: unilocular, anechoic with posterior acoustic enhancement
parameniscal cystic lesions evaluated by MRI [12]. Finally, for shoulder paralabral cysts, in one study, up to 89 % of the 63 patients with paralabral cysts had associated superior labral pathology [42].
Ganglion cysts: most commonly avascular on Doppler evaluation Another ultrasonographic characteristic, the presence or absence of blood flow on Doppler evaluation, is of great importance. It used to be sought inconstantly but its importance is now well recognized. Indeed, the great majority of GCs do not show blood flow at Doppler evaluation (see Fig. 1) [6, 19]. Pseudocystic venous malformations could have a similar appearance on gray-scale imaging and should be excluded with Doppler evaluation [43]. Some report that finding of internal blood flow in GC would even raise the suspicion for a malignant neoplasm [25]. Others have found that atypical thick walls cysts can present vascularity on power Doppler imaging [6, 43]. Teefey et al. reported four cases, out of 60 GC, presenting color Doppler flow. Those cases, to our knowledge, were the first
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report of GC showing blood flow. Of these four cases, three were solid-appearing GCs. It can be deduced that only one case out of the 57 “non-solid-appearing” GC of this study showed vascularity. This corroborates that GCs are generally without internal vascularity and do not show color Doppler signals. One possible explanation for the few cases presenting blood flow is chronic inflammation. Indeed, from the data in Teefey et al., it can be understood that three out of the four-color Doppler-positive lesions showed pathologic description of inflammation. Blow flow might also be expected in collapsed, ruptured GC since the three solid-appearing lesions were collapsed and showed increased color Doppler flow according to their findings. When it comes to SCs, being lined by synovial cells, they could present internal blood flow when associated with an active inflammatory process, an inflammatory arthritis or pigmented villonodular synovitis for example.
Other characteristics by US imaging In order to complete the different criteria used in evaluating cysts with musculoskeletal US imaging, their echogenicity, their configuration, presence of loculations and/or septations, borders, presence of internal echoes and, lastly, posterior acoustic shadowing will be further discussed. Most data come from imaging of GCs of the wrist, the overall most frequent GC [2]. Data from the foot and ankle GCs, other frequent sites, corroborated that similar findings for GC are found at these locations [25]. In the past, GCs have mainly been described as purely anechoic lesions [18]. With the advent of higher frequency transducers and better resolution, it is now well recognized that they can also be hypoechoic or mixed anechoic and hypoechoic [19]. The term “simple cyst” is used to define lesions presenting the most common findings of a cyst. Simple cysts are, by definition, well-circumscribed and anechoic lesion with posterior acoustic enhancement [43] (Fig. 2). Cysts with atypical features or so-called “complex cysts” are then used when they do not comply with the previous simple cyst definition. Teefey et al. [6], in a study of 60 pathologically proven GCs, found that most GCs are complex (57 %) rather than simple (38 %) on sonography with a few ones even having a solid appearance (5 %). In term of configuration, they can be round or oval and unilocular or multilocular, the latter being more often encountered with large lesions (See Fig. 3). Sharply defined septations can be observed, again more often with large GCs. Borders are usually thin and well-defined, especially with cysts purely anechoic [19]. Ill-defined walls in pathologically proven GCs have been defined but do not represent a frequent finding [19]. Thick-walled cysts are also described, being part of atypical cysts, with mention to carefully assess those cysts since
Fig. 3 Ganglion cyst of the wrist: multilocular cyst
compression could give a false impression of solid mass [43]. Internal hyperechoic foci have been described, more commonly in small GCs. With regard to posterior acoustic enhancement, although a simple cyst is normally expected to present such ultrasonographic characteristic, Wang et al. [21], reported that small GCs, 10 mm or less at the largest dimension, tend to present this US characteristic inconstantly. Another study found posterior acoustic enhancement in only 47 % out of 60 GCs of the hand and wrist [6] (See Fig. 2). In comparison, SCs show an echogenicity that is also variable, from anechoic to hypoechoic. Although, most cases of the prototypical SCs, when simple and not complicated by diseases affecting the synovium, are described as being anechoic. Of course, since lined by synovium tissue, SCs are prone to the same pathologic process that occurs at the joint level. They can be complicated by hemorrhage, especially in patients taking anticoagulants, osseous loose bodies, synovial hypertrophy in relation to inflammatory arthritis, infections, and tumors such as pigmented villonodular synovitis (PVNS) and synovial sarcoma. They can present variable shapes but are usually round to oval and unilocular. Although more frequently unilocular, they can also be multilocular [21]. Multiples septations can also be present and large Baker cyst would more often present such septations. Finally, SC can also present posterior acoustic enhancement being also a fluid-filled structure.
Conclusion In conclusion, there is still confusion between these two entities, GCs and SCs, and no consensus on the proper terminology to use seems to have been reached in the literature. In our view, such consensus on clear terminology to use when it comes to the different para-articular cysts (SC, GC, and possibly “fibrocartilaginous tear-related cysts”) still has its importance. For SCs, the nature of the cystic mass, being a
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SC and not a GC, might give therapy a different direction. Indeed, since SCs are mostly associated with an effusion producing joint derangement, it has been suggested to first direct the treatment toward identifying and correcting the intra-articular disease instead of targeting its consequence, the SC [9, 44]. Baker cysts or iliopsoas bursitis deriving from an active inflammatory arthritis being a good example. On the other hand side, for most GCs, when therapy is needed, it is mostly directed toward the cyst itself (aspiration and intracystic injection of corticosteroid and excision when refractory to conservative treatment) [45]. Lastly, another possible category of cysts, the “fibrocartilaginous tear-related cysts”, are so strongly associated with fibrocartilaginous tears that therapy toward correcting these tears is recognized as the proper therapeutic intervention [44]. Once it is agreed upon that GCs and SCs should be classified separately, US imaging of these different cysts can be helpful in distinguishing between these different paraarticular soft tissue masses. Low compressibility seems a constant and characteristic US feature of GCs, compared to the more compressible SCs. Of course, this applies when such cysts allow “sonopalpation”. The other important feature would be their anatomic location, with the classic Baker cyst expected when a cystic lesion arise between two specific tendons while most cysts of the wrist being consistent with GCs at pathology. Other US characteristics, echogenicity and joint communication among others, are less discriminative features since they are possibly similar for both GCs and SCs (see Table 1). US imaging having some limitations further imaging might be necessary in some cases. Assessment of concomitant intra-articular joint derangement should be sought by MRI or arthroMRI, especially for clinically significant SCs, since US has limited value for deep intra-articular diseases like meniscal diseases [44]. MRI can also complete the cyst’s evaluation and assessment of its relation to the joint if doubts persist after US imaging and surgery is planned [43, 46]. Acknowledgments Dr. Cristina Hernández-Díaz and Dr. Lucio Ventura. Conflict of interest None.
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