AJR 2002; 178:595-599
© American Roentgen Ray Society
Neoplastic and Tumorlike Lesions Detected on MR Imaging of the Knee in Patients with Suspected Internal Derangement
Part 2, Articular and Juxtaarticular Entities
Gregory S. Stacy1,
Robert K. Heck2,3,
Terrance D. Peabody2 and
Larry B. Dixon1
1
Department of Radiology, The University of Chicago Hospitals, 5841 S. Maryland
Ave., MC 2026, Chicago, IL 60637.
2
Department of Orthopedic Surgery, The University of Chicago Hospitals, MC
3079, Chicago, IL 60637.
3
Present address: Campbell Clinic, University of Tennessee at Memphis, 1400 S.
Germantown Rd., Germantown, TN 38138.
Received July 19, 2001;
accepted after revision September 14, 2001.
Presented at the annual meeting of the American Roentgen Ray Society,
Seattle, April-May 2001.
Address correspondence to G. S. Stacy.
Introduction
We illustrate the MR imaging findings of several articular and
juxtaarticular lesions in patients who were referred to the orthopedic
oncology clinic at our hospital because their lesions were originally thought
to represent malignancies. In many instances, the MR imaging features indicate
a specific, benign diagnosis.
Lesions of Fluid Signal Intensity
Parameniscal cysts are associated with meniscal tears. The diagnosis is
straightforward if a cystic structure of homogeneous low fluid signal
intensity is noted adjacent to a torn meniscus
(Fig. 1). The cysts may be
lobulated and septate. Cysts that originate posteromedial to the medial
meniscus may become quite large and even penetrate the joint capsule
[1]. Recurrence of the cyst
after aspiration is typical if the meniscal tear itself is not treated.
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Fig. 1. —43-year-old woman with parameniscal cyst. Fat-suppressed
T2-weighted coronal MR image of knee shows horizontal tear (arrow)
through posterior horn of medial meniscus associated with large cyst
(arrowheads).
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Ganglion cysts, on the other hand, are not associated with meniscal tears,
although their cause is uncertain. Intraarticular ganglia are usually seen
adjacent to one of the cruciate ligaments (Figs.
2 and
3) and may cause pain or
locking of the knee [2,
3]. These lesions may also
arise from the joint capsule and extraarticular tendon sheaths. Those lesions
arising in the vicinity of the proximal tibiofibular joint may cause a
neuropathy of the peroneal nerve as a result of compression. Like parameniscal
cysts, ganglion cysts are typically of homogeneous low fluid signal intensity
on T1-weighted images, are of homogeneous high fluid signal intensity on
T2-weighted images, and may be lobulated and septate. Arthroscopic excision of
ganglion cysts may be considered in symptomatic patients, but percutaneous
aspiration is often successful and curative.
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Fig. 2. —27-year-old woman with intraarticular ganglion. T2-weighted
sagittal MR image of knee shows bilobed mass of high signal intensity at
tibial attachment of anterior cruciate ligament extending into Hoffa's fat
pad.
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Several bursae are located about the knee joint, some of which, when
inflamed and filled with fluid, may mimic tumor masses both clinically and on
MR imaging. Fluid collections in bursae can be diagnosed with relative
certainty on the basis of anatomy. Joint fluid may extend between the tendons
of the semimembranosus and medial head of the gastrocnemius muscles into the
gastrocnemius-semimembranosus bursa. Distention results in a Baker's cyst
(Fig. 4), which, if of
sufficient size, may be palpable. Pes anserine bursitis can present as a mass
lesion along the medial border of the proximal tibia (Fig.
5A,5B).
Diagnosis rests on its characteristic location deep relative to the tendons of
the sartorius, gracilis, and semitendinosus muscles. Prepatellar bursitis is
also known as housemaid's knee or carpet-layer's knee because of its
association with recurrent kneeling
[4]. Prepatellar bursitis may
present as a mass located in the subcutaneous tissues between the patella and
the skin (Fig. 6).
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Fig. 4. —39-year-old woman with Baker's cyst. Fat-suppressed proton
density—weighted axial MR image of knee shows fluid extending between
medial head of gastrocnemius muscle (short arrow) and tendon of
semimembranosus muscle (long arrow) into distended bursa.
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Lesions Not of Fluid Signal Intensity
Hematomas have varied signal patterns on MR imaging. On T1-weighted images,
subacute hematomas may appear slightly hyperintense relative to skeletal
muscle because of the presence of methemoglobin, and this signal will remain
bright on fat-suppressed sequences (Fig.
7A). The signal of hematomas on T2-weighted images also varies
with the age of the blood products; a dark hemosiderin rim may be seen in
hematomas from several days to weeks after the injury
(Fig. 7B). Diagnosis is often
suggested by a history of trauma, but follow-up studies may be indicated to
document resolution and the lack of an underlying neoplasm. Popliteal artery
pseudoaneurysms may also show an internal high signal on T1-weighted images
when methemoglobin is present, and direct continuity with the artery is often
seen (Fig.
8A,8B).
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Fig. 7A. —35-year-old woman with suprapatellar hematoma. Noninfused
fat-suppressed T1-weighted sagittal MR image of knee shows
high-signal-intensity mass above patella, typical of methemoglobin-containing
hematoma.
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Fig. 8A. —58-year-old man with popliteal artery pseudoaneurysm.
Noninfused T1-weighted axial MR image of knee shows popliteal fossa mass of
heterogeneous signal (arrow), including high-signal-intensity
components suggesting subacute blood products.
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Other tumorlike lesions may show characteristic signal patterns as a result
of old blood products. Pigmented villonodular synovitis is a synovial
proliferative disorder of uncertain etiology. MR imaging may reveal multiple
intraarticular masses of low signal intensity (or even signal void) on
T1-weighted images that remain low signal intensity on T2-weighted images
because of hemosiderin deposition (Fig.
9). Apparent enlargement of these hypointense foci on
gradient-echo imaging is due to blooming from local susceptibility artifact.
Bony erosions may be present and large. A large joint effusion is often
present. Many patients require synovectomy for treatment. No calcific or
ossific matrix will be seen on conventional radiography, a fact that can help
distinguish pigmented villonodular synovitis from idiopathic synovial
osteochondromatosis (which will usually show multiple calcified or ossified
intraarticular bodies) or even an ossified parosteal osteosarcoma, which is
classically located posterior to the distal femur
(Fig. 10).
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Fig. 9. —43-year-old woman with pigmented villonodular synovitis.
T2-weighted sagittal MR image of knee shows intraarticular masses
(arrows) of low signal intensity representing hemosiderin-laden
villonodular synovitis.
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Fig. 10. —20-year-old woman with parosteal osteosarcoma. T2-weighted
sagittal MR image of knee shows mass (arrow) posterior to distal
femur. Its low signal intensity reflects ossific matrix, and location is
classic for parosteal osteosarcoma.
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Intracapsular (osteo) chondromas are rare and are usually located inferior
to the patella [3]. MR imaging
reveals a mass of isointensity or low signal intensity on T1-weighted images
relative to skeletal muscle and a variable and heterogeneous signal on
T2-weighted images. Foci of signal void represent calcification cation or
ossification (which can be confirmed on radiographs), whereas high signal on
T2-weighted images reflects chondroid elements (Fig.
11A,11B).
Differentiation of an intracapsular chondroma from a mineralized synovial
sarcoma, an uncommon malignancy that occurs most frequently at extraarticular
sites about the knee joint and commonly calcifies, may be difficult or
impossible (Fig. 12).
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Fig. 11A. —47-year-old man with tenosynovial osteochondroma. Proton
density-weighted sagittal MR image of knee shows heterogeneous but
predominantly high-signal mass occupying Hoffa's fat pad and eroding anterior
tibia (arrow). Foci of low signal correspond to ossification seen on
radiograph (B).
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Malignant soft-tissue tumors are usually of isointensity or low signal
intensity on T1-weighted images relative to skeletal muscle and high signal
intensity on T2-weighted images. They are often heterogeneous in signal and
large. Unlike cystic lesions, malignant soft-tissue tumors may enhance
centrally after IV gadolinium administration. A specific histologic diagnosis
(e.g., malignant fibrous histiocytoma vs liposarcoma) usually cannot be
rendered. A fusiform mass oriented longitudinally along the distribution of a
known nerve suggests a nerve sheath tumor, however, particularly if adjacent
and distal muscle atrophy is noted
[5].
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Introduction
Lesions of Fluid Signal...
