Clinical History: A 41 year-old male presents with a history of lateral knee pain for 6 months. (1a) Axial proton density-weighted with fat saturation and (1b) Sagittal T1-weighted images of the left knee are provided. What are the findings? What is your diagnosis?
Infrapatellar (Hoffa’s fat pad) ganglion cyst arising from the anterior cruciate ligament.
Soft tissue ganglia are a very common finding with MR Imaging, particularly about the wrist, often presenting due to mass-like symptoms. However, they are also commonly found in other parts of the body and can be sometimes confused with other benign and malignant processes. Intraosseous ganglia are also very often identified on MR scans.
Soft tissue ganglia are fluid-filled sacs containing mucinous material which have a thin connective tissue capsule but no synovial lining. Though the etiology is not clear, they may represent a synovial herniation or coalescence of small degenerative cysts arising from the tendon sheath, joint capsule, or bursae.1 Although they are histologically distinct from synovial cysts, which have a true synovial lining, these entities are typically indistinguishable on imaging.2 Histologically, there are no major differences between intraosseous ganglia and extraosseous ganglia. They can be of primary etiology or secondary due to an inflammatory process in the joint.3 Intrasubstance anterior cruciate ligament ganglia are due to mucinous degeneration of connective tissue.4
Often both soft tissue and intraosseous ganglia are incidental findings with no clinical symptoms. However, when large or adjacent to neurovascular structures, symptoms due to mass effect may be present. Soft tissue ganglia often present as non-painful, round, firm, smooth “masses” about the joints (3a). They are variable in size at presentation, often changing depending on the level of activity. Ganglia are the most common soft-tissue mass in the foot and ankle.5 Though usually asymptomatic, ankle ganglia can present with pain, weakness, swelling, osseous erosion, and tarsal tunnel syndrome.2 When found in the knee, pain, clicking, stiffness, incomplete extension of the knee and pain at the extremes of motion are commonly encountered symptoms. Occasional findings include a palpable mass and bone erosion (4a). Cysts anterior to the ACL tend to limit extension and those posterior to the PCL to limit flexion.6
Soft tissue ganglia appear as simple, single lobule to multi-septated and/or multi-lobulated fluid signal foci, usually contiguous with a joint capsule or tendon sheath (5a-7a). Intraosseous ganglia present as fluid signal foci, with or without surrounding marrow edema, usually in close proximity to an articular surface. Ganglia are usually of low to intermediate signal on T1-weighted and of increased signal on T2-weighted sequences, similar to pure fluid. However, due to hemorrhage or high proteinaceous content, they can be of increased signal on T1-weighted sequences. Mild peripheral or septal enhancement may be seen on contrast enhanced imaging.
Especially when large, soft tissue ganglia may present with osseous erosion (8a,9a). Intraosseous ganglia may also extend outward through cortex and communicate with an extra-osseous component. Ganglia may also present with neurological findings due to mass effect upon nerves. (10a-14a) In a study on knee ganglia, discrete intraosseous ganglia were observed in 66% of studies with intrasubstance anterior cruciate ligament ganglia and 77% of patients with mucoid degeneration.7
The pathogenesis of ganglia is uncertain with theories being that they develop from an outpouching of a joint capsule or as the result of soft tissue irritation or chronic damage leading to connective tissue degeneration and subsequent mucinous fluid production rich in hyaluronic acid from lining cells.8
The term “complex ganglion” is one not often found in the literature. The author and colleagues have used this term in the setting of a ganglion that is not of simple fluid signal, with this often due to hemorrhage. However, by ultrasound, the term complex ganglion has been used to describe ganglia that are larger, likely due to less constraint of surrounding soft tissues, containing septations, thick walls, and locules.9 The loculations and septa may be due to episodes of rupture and subsequent recurrence.9 Since ganglia can also collapse following rupture, they may have the appearance of a solid mass, similar to a giant cell tumor of tendon sheath.
Usually ganglia do not require treatment. However, when necessary, due to pain, mass effect on surrounding structures, limitation of range of motion or for cosmetic reasons, they can be drained or surgically removed. The surgical recurrence rate has been found to be as low as 4% in one study10 and up to 40% in another study.11
The differential diagnosis of these cystic lesions includes synovial myxoma12, meniscal or parameniscal cyst, synovial cysts, pigmented villonodular synovitis, synovial hemangioma, aneurysm (15a,16a), and synovial or other sarcoma (17a-19a). In general, ganglion cysts are more well defined and in particular more homogeneous than the other diagnostic considerations. It is important to recognize that lesions that are not of typical fluid signal intensity on T1 or T2-weighted images often do not represent a ganglion.
MR is considered to be the gold-standard for ganglion imaging. The full extent of the lesion as well as its effect on surrounding soft tissues and osseous structures can be evaluated. Though ganglia are usually characteristic lesions on MR, other etiologies, including malignancies, can appear similar, and thus careful consideration of the morphology and signal characteristics of the mass is required in order to avoid a misdiagnosis.
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