ReviewMRI features of cystic lesions around the knee
Introduction
A variety of cystic lesions may be encountered around the knee joint during routine MRI, from benign cysts to complications of various disorders such as infection, inflammatory or degenerative arthritis, and malignant lesions. Most cystic lesions around the knee represent encapsulated fluid collections and exhibit low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. As MRI provides better soft tissue contrast and multi-planar imaging capability than other imaging methods, it is considered the technique of choice to confirm the cystic nature of the lesion, to evaluate the anatomical relationship to the joint and surrounding tissues, and to identify associated intra-articular disorders. MRI findings are important to guide specific therapy and to avoid unwarranted arthroscopy. This article covers the etiology, clinical features, MRI findings and differential diagnosis of cystic lesions around the knee.
Section snippets
Popliteal (Baker's) cysts
Popliteal cysts are not true cysts and represent the semimembranosus–medial gastrocnemius bursa. They are the most frequently encountered cystic masses around the knee. Their incidence on MRI reported in the literature ranges from 4.7% to 19% [1], [2], [3]. They are formed by a communication between the posterior joint capsule and gastrocnemius–semimembranosus bursa. A previous adult cadaveric study have shown this communication to be present in approximately half of the study population [4].
Other bursae around the knee
Bursae around the knee are numerous and act to reduce friction between adjacent moving structures, such as tendons, ligaments, and bone surfaces. Bursae are synovium-lined structures, usually collapsed but often containing a small amount of synovial fluid. They are not usually visible on MRI. However, inflammation from local and systemic processes such as overuse, trauma, infection, hemorrhage, internal joint derangement, inflammatory arthropathy, and collagen vascular disease, may cause
Meniscal cysts
A meniscal cyst is a focal collection of synovial fluid located within or adjacent to the meniscus. Several theories on the etiology of meniscal cysts have been proposed. Intrameniscal cysts are believed to be formed by an accumulation of joint fluid within a torn or degenerated meniscus. Parameniscal cysts are thought to form when there is fluid extravasation through a meniscal tear into the parameniscal soft tissue [8], [10], [14]. The tear has a horizontal component in most cases [8].
Ganglion cysts
A ganglion cyst is a benign cystic mass containing clear, highly viscous fluid that is rich in hyaluronic acid and other mucopolysaccharides, within a dense fibrous connective tissue wall without a synovial lining. The pathogenesis of ganglion cysts remains controversial. Although most authors believe that mucoid cystic degeneration in a collagenous structure near areas under continuous stress is the probable cause, many other theories have been put forth: synovial tissue herniation, ectopia of
Insertional cysts [35]
Insertional cysts most likely result from bone resorption due to chronic avulsive stress at the insertion of the cruciate ligaments, and eventually at the meniscotibial attachments. These stresses may also lead to focal necrosis of the bone, resulting in the formation of the cyst. These lesions have been reported to occur in 1% of patients with MRI examinations of the knee, and are usually asymptomatic with no clinical significance. On MRI, insertional cysts appear as well-defined homogeneous
Subchondral cysts (geode)
Subchondral cysts or geodes are often associated with osteoarthritis. Two possible mechanisms have been described regarding the pathogenesis of these lesions in osteoarthritis: 1) elevated intra-articular pressure causing intrusion of synovial fluid through the compromised cartilage, and 2) fracture and vascular insufficiency of the subchondral bone due to impaction of apposing bony surfaces, leading to cystic necrosis [36]. On MRI, subchondral cysts are often multiple, segmental in
Proximal tibiofibular synovial cysts
In 10% of adults, the proximal tibiofibular joint communicates with the knee joint. The probable pathogenesis of proximal tibiofibular joint lesions is increased pressure in the knee joint leading to an out pouching of the tibiofibular joint capsule which then herniates to form the synovial cyst [37]. They are therefore more prevalent in patients with chronic knee effusions. Synovial cysts in this location are rare, with a reported prevalence between 0.09% and 0.76% [10]. Small lesions are
Other lesions mimicking cysts
Normal synovial recesses may fill with synovial fluid and mimic cystic lesions around the knee, such as meniscal cysts and ganglion cysts. The absence of an associated meniscal tear and the homogeneous appearance of the fluid-filled recess lined with smooth and thin synovium are useful features to rule out meniscal cysts and ganglia, respectively. Solid tumors such as synovial sarcomas, synovial hemangiomas, schwannomas (Fig. 23), neurofibromas, and intra-osseous giant cell tumors may have a
Conclusion
Cystic lesions around the knee are frequently encountered on MRI examinations. Their origins vary, as do prognosis and therapy. Clinical presentation depends on cyst location, size, and relationship with surrounding tissues, and MRI is the best technique for visualizing these factors. Clinicians must know the characteristic appearance and location of cystic masses around the knee on MRI to make the correct diagnosis and determine the appropriate therapy.
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