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.

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.