This article presents the case of a 63-year-old man who noted painless swelling within the suprapatellar pouch, which he attributed to an effusion. Small-magnet, lower-extremity magnetic resonance imaging (MRI) and subsequent arthroscopy missed the lesion. On MRI with gadolinium contrast, the lesion was defined but misdiagnosed as a suprapatellar pouch ganglion cyst. Following resection of the 6 x 5 x 1.8-cm lesion, histology confirmed a lobular benign tumor with cystic elements recognized in the pathology and radiology literature as juxta-articular myxoma. Such lesions are characteristically multilobulated and contained within a capsular rim that enhances with gadolinium venous contrast. Otherwise, they appear hyperintense to fat on T2-weighted images and hypointense to muscle on T1-weighted images. This is an uncommonly encountered but known cystic lesion around the knee that is most often confused with ganglion cyst, synovial lipoma, lipoma arborescens, or pigmented or non-pigmented villonodular synovitis. Given its more cellular nature and thicker encapsulation, it should be able to be differentiated from ganglion cyst on MRI with a high index of suspicion, and should be recognized because of its high rate of recurrence. Unusual in this case was its location within the suprapatellar pouch, demonstrating the ease with which such lesions can be missed arthroscopically despite significant mass.
Many soft tissue tumors and nodules can arise about the knee, the nomenclature of which can be confusing because of similar presentations and shared histological characteristics. Despite the frequency with which nodular lesions are seen about the femorotibial joint line, lesions involving the suprapatellar pouch are rare.
This article presents a case of cystic swelling within the suprapatellar pouch due to juxta-articular myxoma, an infrequent subtype of myxoma. The lesion was mistaken for an effusion on initial physical examination and small-magnet lower extremity magnetic resonance imaging (MRI). The lesion was also overlooked at initial arthroscopic surgery. Subsequently, on second MRI study performed with a large, closed magnet and intravascular gadolinium contrast, the lesion was misinterpreted as an intra-articular ganglion cyst.
A 63-year-old male recreational runner averaging 10 miles per week and engaging in occasional competitive runs presented with suprapatellar symptoms 15 months following 2 consecutive fun-runs. Lower-extremity MRI on a 0.2-Tesla magnet identified meniscal pathology of both the posterior horn medial meniscus and anterior horn lateral meniscus, and a large volume of fluid in the knee joint consistent with large knee effusion. A radial tear of the posterior horn medial meniscus and grade III chondromalacia involving the trochlea of the patella were identified; however, suprapatellar pathology was not identified.
The patient exhibited a palpable, resilient nodule involving the suprapatellar pouch of the right knee, distinctly nonfluctuant and asymmetric to the contralateral side. Tenderness, crepitus, and ligament instability were absent. Motion of the involved knee was full and symmetric to the contralateral side.
Gadolinium-contrast MRI revealed a multilobulated, 5.5 x 6.2 x 2.4-cm mass within the suprapatellar pouch, with low signal on T1-weighted images and high signal on T2-weighted images. Postcontrast images demonstrated smooth rim enhancement suggestive of a ganglion cyst (Figures 1, 2). Aspiration was attempted but yielded only scant clear, xanthochromic, gelatinous material.
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Figure 1: Axial fat saturation T1-weighted MRI post-gadolinium contrast. Figure 2: Sagittal fat saturation T1-weighted MRI post-gadolinium contrast.
The patient underwent repeat arthroscopy at 9 months with debridement of a complex tear at the anterior horn of the lateral meniscus and chondral surface of the medial femoral condyle (Figure 3). After arthroscopic mobilization of the intrabursal lesion, the mass was resected through a separate incision centered over the suprapatellar pouch.
Gross examination revealed a 6 x 5 x 1.8-cm glistening, white-grey mass that demonstrated a gelatinous cut surface with focal cystic features when bisected. Whole-mount, low-power magnification revealed the lesion to have a well-defined fibrous capsule and abundant amphophilic, hypocellular myxoid stroma. High-power microscopic examination revealed a bland pattern of round and spindle-shaped myxomatous cells suspended in a myxoid matrix, supported by a network of delicate fibrous connective tissue (Figure 4). No cellular atypia, mitotic activity, or zones of geographic necrosis were observed in the multiple sections examined.
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Figure 3: Arthroscopic image of lesion within the suprapatellar pouch. Figure 4: Representative high-power field photomicrograph demonstrating a bland myxoid neoplasm without mesenchymal atypia or hypercellularity (hematoxylin-eosin, 400×).
A rarely encountered soft tissue lesion, juxta-articular myxoma is predominantly seen about the knee.1-4 Because of its resemblance to other entities, it can create a diagnostic dilemma. The largest series, from the Armed Forces Institute of Pathology in 1992, described 65 cases from military and civilian hospital consultations spanning 28 years and including patients aged 16 to 83 years.1 Another case report documents an intra-articular myxoma in a 9-year-old girl.2
Juxta-articular myxoma is a variant of myxoma, which includes other entities such as intramuscular myxoma, superficial angiomyxoma, aggressive angiomyxoma, and neurothekeoma.5 Several other soft tissue lesions include myxoid elements (ie, poorly organized fibrous stroma with cystic foci), although they are not technically myxomata.
Juxta-articular myxoma, although discussed in the radiology and pathology literature, is relatively new to the orthopedic literature. In the Armed Forces Institute of Pathology series, 72% of the 65 cases involved men and 88% were in the vicinity of the knee.1 Other sites of occurrence included the elbow, shoulder, hip, and ankle. In addition, the recurrence rate was high: 34% among the patients available for follow-up.
Juxta-articular myxoma most frequently involves the subcutaneous adipose tissue of the knee, but it may arise from any of the tissues about the knee, including adjacent dermis, tendons/tendon sheaths, the joint capsule, and the menisci. Most occurrences of juxta-articular myxoma are extracapsular; however, in the Armed Forces Institute of Pathology review, of the 4 cases that were intra-articular, 3 were reported in the suprapatellar bursa and 1 was reported in the infrapatellar bursa.1 Minkoff et al3 reported an intra-articular lesion that encircled the medial femoral condyle, restricting knee motion.
There has been some debate regarding the etiology of these lesions, as well as whether they are neoplastic or degenerative and whether formation of the cystic spaces is inherent to the tumor or is the result of trauma (ie, friction to the expanding mass).1 Multiple authors have noted the concomitance of degenerative processes within the knee, such as arthritis or meniscal pathology, which may be typical for the representative male population, median age 43.1
The MRI appearance of these lesions is typically hypointense to muscle on T1-weighted images and hyperintense to fat on T2-weighted images.4 On fat-saturated short-tau inversion-recovery sequences, the lesion is hyperintense, dissimilar from an effusion by the presence of a thickened capsule or presence of heterogeneous signal within the lesion, depending on the extent to which there is an internal cellular framework. With intravenous gadolinium contrast, there is frequently higher signal intensity on T1-weighted images, corresponding to the thin rim of vascularized capsular tissue.6
In our case, the juxta-articular myxoma was located within the suprapatellar pouch. Other common benign soft tissue lesions such as meniscal and parameniscal cysts could be eliminated from the differential diagnosis. Also eliminated were Bakers cyst, popliteal bursal cyst, tibiofibular synovial cyst, and desmoid tumor, all of which occur in distinctly extra-articular locations.7
The gadolinium MRI report suggested a ganglion cyst within the suprapatellar pouch, which in itself would have been a rare occurrence; however, with intravenous contrast there was smooth rim enhancement, which suggested a more cellular, encapsulated lesion. In contrast, a distinguishing feature of ganglion cyst is the absence of an enhancing capsule and heterogeneous myxoid tissue.
Other differential diagnoses considered included pigmented and nonpigmented villonodular synovitis. These entities may occur in a focal nodular area, but more frequently occur about the infrapatellar fat pad. Other fatty tumors in the differential included synovial lipoma and lipoma arborescens. Lipoma arborescens may occur at any age but most frequently involves men and women in their fifth and sixth decades.8 Although there is a predilection for the suprapatellar pouch, the diagnosis is generally evident on MRI by a characteristic, frond-like appearance and by fat consistency. Finally, consideration was also given to a chronic, posttraumatic loculated hematoma.
Although preoperatively diagnosed as an intra-articular ganglion cyst, there was enough MRI evidence in this case to identify the nodule as a juxta-articular myxoma. Juxta-articular myxomas recur in upwards of 30% of cases within 5 years of excision. Although most occurrences have been treated by direct excision, it is unclear what the recurrence rate may be after arthroscopic resection.
The literature is inconsistent regarding the growth rate of these lesions, but presentation typically includes knee swelling with little pain and, as in our case, the lesion can be present for a considerable time before the patient seeks treatment. Skeletal erosion is not reported and expansion seems to cease once moderate size is reached.
- Meis JM, Enzinger FM. Juxta-articular myxoma: a clinical and pathologic study of 65 cases. Hum Pathol. 1992; 23(6):639-646.
- Daluiski A, Seeger LL, Doberneck SA, Finerman GA, Eckardt JJ. A case of juxta-articular myxoma of the knee. Skeletal Radiol. 1995; 24(5):389-391.
- Minkoff J, Strecker S, Irizarry J, Whiteman M, Woodhouse S. Juxta-articular myxoma: a rare cause of painful restricted motion of the knee. Arthroscopy. 2003; 19(10):E6-13.
- King DG, Saifuddin A, Preston HV, Hardy GJ, Reeves BF. Magnetic resonance imaging of juxta-articular myxoma. Skeletal Radiol. 1995; 24(2):145-147.
- Allen PW. Myxoma is not a single entity: a review of the concept of myxoma. Ann Diagn Pathol. 2000; 4(2):99-123.
- Murphey MD, McRae GA, Fanburg-Smith JC, Temple HT, Levine AM, Aboulafia AJ. Imaging of soft-tissue myxoma with emphasis on CT and MR and comparison of radiologic and pathologic findings. Radiology. 2002; 225(1):215-224.
- Damron TA, Sim FH. Soft-tissue tumors about the knee. J Am Acad Orthop Surg. 1997; 5(3):141-152.
- Narváez J, Narváez JA, Ortega R, Juan-Mas A, Roig-Escofet D. Lipoma arborescens of the knee. Rev Rhum Engl Ed. 1999; 66(6):351-353.
Dr Kosty is from the Department of Orthopedic Surgery and Rehabilitation, The University of Texas Medical Branch, Galveston, and Dr Moore is from the Department of Pathology, Christus St John Hospital, Nassau Bay, Texas.
Drs Kosty and Moore have no relevant financial relationships to disclose.
Correspondence should be addressed to: John W. Kosty, MD, Department of Orthopedic Surgery and Rehabilitation, The University of Texas Medical Branch, 301 University Blvd, Galveston, TX 77555-0165.