Imaging Quiz Answer

A Medial Soft-Tissue Mass of the Knee

John Lin, MD; Christine Chang, MD

THE PHYSICIAN AND SPORTSMEDICINE - VOL 27 - NO. 10 - OCTOBER 1, 1999

Diagnosis

Return to case presentation.

The MRI (figure 2) demonstrates a large (5 cm in diameter), well-defined soft-tissue lesion at the medial joint line, adjacent to the medial meniscus, and abnormal increased signal within the substance of the medial meniscus that is consistent with a horizontal cleavage tear. The patient was diagnosed as having a meniscal cyst associated with a medial meniscal tear, findings confirmed at surgery.

Discussion
Meniscal cysts are well-defined cystic lesions, from millimeters to several centimeters in diameter, located adjacent to the peripheral margin of the meniscus. They may be multiloculate and may demonstrate thin internal septations. Meniscal cysts are almost always associated with a horizontal meniscal tear (1-4). However, horizontal meniscal tears can occur without a meniscal cyst and are actually more common than those with a meniscal cyst.

One study (1) reports meniscal cysts in up to 7% of patients undergoing meniscectomy, but the true prevalence is probably about 1% of patients who undergo meniscectomy (2,5). Lateral meniscal cysts are three to four times more common than medial cysts (3). Meniscal cysts are seen most often in young adults and more frequently in men than women.

Etiology. The etiology of meniscal cysts has been controversial. Various theories propose trauma, chronic infection, hemorrhage, and mucoid degeneration as the source of these entities (6). Since the fluid found in meniscal cysts is similar to synovial fluid, the prevailing view is that cysts form from joint fluid that is forced through a peripherally extended meniscal tear and accumulates outside the joint capsule (3,4,6).

Signs and symptoms. Meniscal cysts usually occur at the level of the joint line as a focal mass or swelling, frequently with localized tenderness. Pain is most likely related to associated meniscal tears, but discomfort may also be due to stretching of the knee capsule and other parameniscal soft tissues (7). The consistency of the mass can vary from soft and fluctuant to firm or bone-hard.

A characteristic clinical feature of a meniscal cyst is that its size can vary. If the size of a knee mass changes, particularly when the knee is flexed, a meniscal cyst should be suspected (6). However, the absence of this finding does not exclude the diagnosis of a meniscal cyst.

Imaging studies. MRI is an accurate, noninvasive method of assessing the soft tissues and internal structures of the knee. Although ultrasonography (2,8), computed tomography (9,10), and arthrography (11) have been used in evaluating meniscal cysts, MRI is preferred (2,4,7), because it shows structures such as the menisci, cartilage, and ligaments and is the most effective modality for evaluating soft-tissue masses.

Typically, meniscal cysts have uniformly low signal intensity on T1-weighted images and increased signal intensity on T2-weighted or short-T1 inversion-recovery images, consistent with fluid (3,4,7). Because cysts may also contain bloody or gelatinous fluid with an increased protein content, the signal intensity may vary.

Differential diagnosis. The differential diagnosis for soft-tissue masses about the knee should include synovial cyst formation, bursal fluid collections, ganglion cysts, severe degenerative changes with osteophytic spurring, and soft-tissue masses such as pigmented villonodular synovitis, lipoma, hemangioma, and sarcomas (6,7). (For more information, see "Cysts and Other Masses About the Knee," July 1999, page 59.)

MRI is useful in the differential because the images it provides are distinctly different for soft-tissue masses and cysts. The close association of a cyst with the peripheral border of a meniscus and the presence of a meniscal tear that communicates with the cyst also help to distinguish a meniscal cyst from other fluid-filled lesions.

Treatment. Meniscal cysts tend to recur after aspiration or simple resection (4). Therefore, open or arthroscopic intra-articular surgery to treat the underlying meniscal tear is necessary for successful therapy (4-7). Distinguishing meniscal cysts from other cystic lesions is important, since meniscal cysts more often require surgery.

Case Treatment and Outcome
The planned therapy for our patient was surgical resection of the meniscal cyst and treatment of the medial meniscal tear. However, the cyst ruptured before surgery, rendering resection unnecessary. Arthroscopic partial meniscectomy and debridement of the medial meniscal tear was performed to prevent the cyst from recurring and to alleviate pain related to the meniscal injury. The patient was placed in a knee immobilizer for 2 weeks and then started physical therapy. Three months after surgery, he was asymptomatic and resumed his preinjury level of activity.

References

1. Barrie HJ: The pathogenesis and significance of meniscal cysts. J Bone Joint Surg (Br) 1979;61(2):184-189
2. Coral A, van Holsbeeck M, Adler RS: Imaging of meniscal cyst of the knee in three cases. Skeletal Radiol 1989;18(6):451-455
3. Janzen DL, Peterfy CG, Forbes JR, et al: Cystic lesions around the knee joint: MR imaging findings. Am J Roentgenol 1994;163(1):155-161
4. Burk DL, Dalinka MK, Kanal E, et al: Meniscal and ganglion cysts of the knee: MR evaluation. Am J Roentgenol 1988;150(2):331-336
5. Mills CA, Henderson IJ: Cysts of the medial meniscus: arthroscopic diagnosis and management. J Bone Joint Surg (Br) 1993;75(2):293-298
6. Ryu RK, Ting AJ: Arthroscopic treatment of meniscal cysts. Arthroscopy 1993;9(5):591-595
7. Tyson LL, Daughters TC Jr, Ryu RK, et al: MRI appearance of meniscal cysts. Skeletal Radiol 1995;24(6):421-424
8. Seymour R, Lloyd DC: Sonographic appearances of meniscal cysts. J Clin Ultrasound 1998;26(1):15-20
9. Chen WC, Wu JJ, Chang CY, et al: Computed tomography of a meniscal cyst. Orthopedics 1987;10(11):1569-1572
10. Passariello R, Trecco F, de Paulis F, et al: Meniscal lesions of the knee joint: CT diagnosis. Radiology 1985;157(1):29-34
11. Schuldt DR, Wolfe RD: Clinical and arthrographic findings in meniscal cysts. Radiology 1980;134(1):49-52

Dr Lin is a clinical assistant professor in the department of radiology at the University of Michigan Medical Center in Ann Arbor and a member of the Society of Skeletal Radiologists. Dr Chang is a staff physician at the University of Michigan Medical Center and St Joseph Mercy Hospital in Ypsilanti, Michigan; she is a fellow of the American Academy of Pediatrics. Address correspondence to John Lin, MD, The University of Michigan Medical Center, 1500 E Medical Center Dr, TC 2910, Ann Arbor, MI 48109-0326; address e-mail to johnlin@umich.edu.