Ganglion Cyst

Introduction

Knee joint intra-articular ganglion cysts may originate from the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), menisci or meniscal tears, popliteus tendon, alar folds, or areas of chondral fracture in descending order of frequency. Although the term ganglion cyst is widely accepted, it needs to be differentiated from synovial cysts and mucoid degeneration. Most ligament cysts (75.4%) in the knee joint are known to be located within the ACL. Ganglion cyst of the ACL is still a rare condition and a mostly incidental finding on magnetic resonance imaging (MRI) and arthroscopy. However, these ganglion cysts may produce knee pain or discomfort, swelling, extension block, and limitation of knee flexion without a clear cause or a preceding major traumatic episode. These clinical manifestations may also resemble those of internal derangement of the knee.

This chapter summarizes the background knowledge for this pathology, from the pathophysiology and histology up to the clinical, arthroscopic, and imaging findings, and ends with the optional treatment.

Epidemiology

ACL ganglion cyst is a rare condition that is seen infrequently in clinical setting, and was described for first time in 1924 by Caan in a cadaveric specimen of an elderly male. Reported incidence of ganglion cysts related to ACL varies from 0.29% to 1.3% on MRI studies and 0.54%–2% in arthroscopic studies. Although the widespread use of MRI and arthroscopy has resulted in an increased number of reports on ganglion cysts, these published case reports mainly involve intra-articular soft tissue masses; true ganglion cysts of the ACL are still rarely reported.

Even more rarely, the ACL ganglion cysts may present bilaterally. The incidence of intra-articular ganglion cysts of the knee joint has been reported to be more frequent in males in the fourth or fifth decade of life. The youngest patient reported to have an ACL ganglion cyst was a 2-year-old child, while cases of 4- and 7-year-old children have also been reported.

Epidemiology

ACL ganglion cyst is a rare condition that is seen infrequently in clinical setting, and was described for first time in 1924 by Caan in a cadaveric specimen of an elderly male. Reported incidence of ganglion cysts related to ACL varies from 0.29% to 1.3% on MRI studies and 0.54%–2% in arthroscopic studies. Although the widespread use of MRI and arthroscopy has resulted in an increased number of reports on ganglion cysts, these published case reports mainly involve intra-articular soft tissue masses; true ganglion cysts of the ACL are still rarely reported.

Even more rarely, the ACL ganglion cysts may present bilaterally. The incidence of intra-articular ganglion cysts of the knee joint has been reported to be more frequent in males in the fourth or fifth decade of life. The youngest patient reported to have an ACL ganglion cyst was a 2-year-old child, while cases of 4- and 7-year-old children have also been reported.

Pathophysiology

Ganglions usually arise from tendon sheaths, joint capsules, or muscles and can be solitary or multilobulated. Although well reported in the literature, intra-articular ganglion cysts of the knee joint are a rare clinical finding, often presenting incidentally. Knee joint intra-articular ganglion cysts may originate from the ACL, PCL, menisci or meniscal tears, popliteus tendon, chondral fracture or subchondral bone cysts, alar folds, and infrapatellar fat pad. Most of ligament ganglion cysts (75.4%) in the knee joint are known to be located in the ACL.

The pathogenesis of ACL ganglion cyst is not completely known. It is believed that one or more causative factors may contribute to the formation of an ACL ganglion cyst, with the most predominant being the synovial tissue herniation, mucoid degeneration of connective tissue, cyst formation after trauma, ectopia of synovial tissue, congenitally displaced synovial tissue, and proliferation of pluripotential mesenchymal stem cells .

Some of these theories relating to the pathogenesis of a ganglion cyst of ACL are more common than others; for example, mucoid degeneration has been frequently associated with ganglia formation. The relationship between ACL ganglion cysts and mucoid degeneration has been theorized, but its existence is still unproven. Bergin et al. conducted a retrospective study on 4221 consecutive knees referred for MRI, which were screened systematically for ACL mucoid lesions. They found 74 (1.8%) knees with mucoid lesions; 24% showed mucoid degeneration and 76% a mucoid cyst (MC), referring to an entity similar to ganglion cyst. One-third (35%) of knees had MRI criteria for both MC and mucoid degeneration suggesting common pathogenesis, as previous literature had done. The MC concerned only the proximal or distal insertion of ACL in 50% of cases, whereas mucoid degeneration concerned the entire length of ACL in 93% of cases. The authors suggested that ACL ganglia and mucoid degeneration commonly coexist, and that these two entities may share a similar pathogenesis.

Another theory suggests that herniation of synovial tissue through a defect in the tendon sheath causes ganglia formation, while a third theory describes displacement of synovial tissue during embryogenesis. However, the histological observation that these fluid-filled structures have no epithelial lining confirms that they are not true cysts and thus dispels the theories of synovial herniation favoring a degenerative cause.

Trauma has been advocated as playing a role in the pathogenesis of ACL ganglion cyst, although the exact relationship is unknown. One theory involves the cellular response to trauma that liberates a mucin substance, hyaluronic acid. This is interspersed with the fibers of the ligament, causing its fusiform dilation. With joint and tissue motion, the mucin substance dissects the ligament fibers and may be found at the ligament attachments or in the intercondylar notch of the knee. For this reason, repetitive minor knee trauma has been theorized as a key parameter to the initiation or the development of the process. In contrast, usually there is no report of a certain serious injury of the knee joint to the past medical history of the patients.

Ganglion cysts of the cruciate ligaments may expand not only outside along the fibers (anterior to the ACL and posterior to the PCL), but also between the two cruciates (intercruciate expansion or distension), sometimes interspersing within the fibers. The isolated location within the cruciate fibers (intraligamentous distension) has been theorized as extremely rare, and it may result from a mucoid degenerative process within the ligament.

Ganglion cysts of the cruciate ligaments originated more often from the ACL than from the PCL, and mainly at the tibial attachment. Intraligamentous ganglion cysts are detectable by intrafibrous probing, provoking an outflow of whitish or yellowish gelatinous material. They are normally fusiform or spindle-shaped. All other ganglion cysts show rounded, ovoid, and well-demarcated outlines, with a normal size of 5–30 mm, but they only rarely reach up to 40 mm in diameter. They appear uni- or multilocular, and mostly isolated in each knee.

Clinical Findings

ACL ganglion cysts may be either symptomatic or asymptomatic. Most often they are asymptomatic and are diagnosed incidentally during MRI examination or arthroscopy. In one of the largest case series to date, Krudwig et al. reported 11% symptomatic ACL ganglion cysts. Since the intercondylar notch is relatively spacious, it may take time for the mass to develop dimensions large enough to cause symptoms. Therefore the relatively slow progression of symptoms may considerably delay the patient’s decision to seek medical attention. When symptomatic, the complaints are nonspecific and are dependent on the exact location of the ganglion cyst within the knee joint. The symptoms may include pain or tenderness at the joint line, anterior knee pain, mechanical locking, restriction of extension or flexion motion, intermittent swelling, or clicking sensations. Most patients present with pain localized mainly around the joint line, accompanied with some restriction in flexion or extension because of the worsening pain. These symptoms can also be suggestive of other intra-articular lesions such as a meniscal tear, a cartilage lesion, or a free body, although the limitation in the range of knee motion is relatively greater with the ACL ganglion cysts. The incidence, severity, and duration of pain seem to vary depending on size and location of the cyst. Also, the limitation of knee motion seemed to be influenced by the location of the cyst. Ganglion cysts arising anterior to the ACL tend to limit knee extension, while those occurring posterior to the cruciate ligaments tend to limit knee flexion. One possible reason for this is that a cyst located anteriorly can impinge between the ACL and the intercondylar roof, resulting in an extension block. However, it has been shown that an ACL ganglion cyst anterior to the ligament can limit both knee flexion and extension. Additionally, cysts between the ACL and PCL can also limit both knee flexion and extension range of motion. Such cases show that the limitation cannot be completely explained by the location of the cyst only. Changes in the length and torsion of the cruciate ligaments with knee motion may provoke traction or compression on the cyst. These excessive compression or traction forces on the synovial membrane around the ganglion cyst may stimulate nerve endings, and the knee may be positioned in such a way that the synovial membrane is relaxed, resulting in a limitation in knee motion.

ACL ganglion cysts can produce symptoms that are disturbing to the patient, which makes diagnosis and specific treatment necessary. Given the fact that these symptoms and especially knee pain can be caused by a variety of conditions, diagnosis of ganglion cyst can rarely be made on clinical grounds alone, and frequently it is the MRI findings that lead to the diagnosis.

Imaging Findings

Prior to arthroscopy, MRI yields valuable information not only on the differential diagnosis of ACL ganglion cysts but also on the location and size of it, although it should be noted that MRI alone is not entirely diagnostic ( Fig. 106.1 ). Before the advent of MRI, these ACL ganglia were identified only at open surgery or arthroscopy. MRI usually depicts a well-demarcated and relatively round mass in the knee joint of a patient that usually presents without any specific history of trauma. The ganglion cysts usually demonstrate fluid characteristics with low or intermediate signal intensity on T1-weighted images and increased signal on T2-weighted images, and they are well-delineated structures that are either lobulated or multilobulated. Cruciate ganglion cysts can be intraligamentous and can be directly adjacent to the ligaments or some distance away from them. The cysts arising from the ACL and lying adjacent to it are often anterior to the ligament, are fusiform in shape because they tend to align along the ACL fibers, and are usually unilocular ( Fig. 106.2 ). They may, however, be more complex and multilocular, extending anteriorly into Hoffa’s fat pad ( Fig. 106.3 ). For intratendinous ganglia of the ACL, common MRI findings are high signal on T2-weighted images thickening the ACL with a “celery-stalk” appearance. ACL ganglion cysts are easily distinguishable from Baker cysts or menisci cysts on the T2-weighted images because they are usually located within or surrounding the cruciate ligament and do not extend to the medial and lateral head of the gastrocnemius, nor are they connected with the menisci. MRI is also used to exclude neoplastic lesions, and to detect additional intra-articular pathologies. Occasionally it may be difficult from MRI only to distinguish whether the ganglion cyst originates from the ACL or PCL. In cases that are unclear from the MRI, ultrasound, computed tomography (CT), and arthrography, other imaging techniques may assist.

Arthroscopic Findings

Arthroscopically it can be seen that the cysts of the cruciate ligaments can distend outside along the fibers (anterior to the ACL and posterior to the PCL), between the two cruciate ligaments (intercruciate distension), or interspersing within the fibers. Nearly two-thirds of all ganglion cysts in the knee originate from the ACL; they usually arise from its tibial insertion ( Fig. 106.4 ).Typically the ACL ganglion cyst may be clearly observed by a routine anteromedial or anterolateral arthroscopic approach. The cyst mainly presents as round- or ellipse-shaped, or it may be fusiform, spindle-shaped, or ovoid ( Fig. 106.5 ). It is associated with the ACL, and it shows well-demarcated outlines and a clear boundary from adjacent structures. Intraligamentous ganglion cysts are detectable by intrafibrous probing during surgery, which yields an outflow of whitish or yellowish gelatinous material.