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A popliteal cyst, also known as a Baker’s cyst, is the most
common tumorous mass found around the popliteal fossa
of the knee joint, and is usually treated conservatively with
the identification of underlying conditions.
1)
However,
other tumorous masses such as lipoma, synovial sarcoma,
meniscal cysts, muscular herniation, bursae of biceps and
semitendinosus tendons mimick the appearance of pop-
liteal cysts.
2)
Furthermore, some papers in the literature
also report a vascular problem, namely popliteal artery
and vein aneurysm, that also present as a popliteal mass.
3)
Therefore, careful differential diagnosis of popliteal masses
is needed because occasionally popliteal masses require
surgical intervention.
The case presented here describes a patient with
a soft tissue tumor of the popliteal fossa, which was di-
agnosed as osteochondrolipoma. To our knowledge, a
popliteal mass of this pathology has never been reported,
suggesting that it is exceptional.
CASE REPORT
A 63-year-old housewife presented with sustained knee
pain and a palpable mass on her popliteal fossa that had
lasted more than a year. After failed control of the pain
with medication, a primary physician referred this patient
to our clinic after taking an ultrasonography with the im-
pression of an ‘unusual soft tissue tumor in the popliteal
fossa.’ Physical examination revealed a soft, non-tender
mass in the posteromedial area of the left knee. The mass
was non-movable, without evidence of palpable pulsation,
erythema or venous congestion. Overall alignment of the
lower extremity was varus by a finger breadth, and the left
knee joint had a mild effusion without any instability or
tenderness. The range of motion of the knee was slightly
decreased, and the patient complained of some discomfort
with forced flexion.
Lateral plain radiography of the left knee revealed a
round, soft tissue density lesion containing bony fragments
on the popliteal fossa (Fig. 1). Ultrasonography showed a
solid mass of the dimensions 4 × 5 × 3 cm, well-encapsulated
with heterogeneous echogenicity in the popliteal fossa (Fig. 2).
A magnetic resonance imaging (MRI) investigation showed
Osteochondrolipoma Presenting
as a Popliteal Cyst
Young-Joon Choi, MD, Jeong-Ho Kang, MD, Gil-Hyun Kang, MD
*
, Soo-Jung Choi, MD
†
Departments of Orthopaedic Surgery,
*
Pathology, and
†
Radiology, Gangneung Asan Hospital,
University of Ulsan College of Medicine, Gangneung, Korea
Here, we describe a popliteal mass that was initially misdiagnosed as a simple popliteal cyst, which finally turned out to be osteo-
chondrolipoma. A 63-year-old housewife presented with sustained knee pain in association with a palpable mass on the popliteal
fossa. The mass was in the posteromedial area and soft, non-tender, non-movable in the posteromedial area. Using plain radiog-
raphy, the mass appeared as a round, soft tissue density lesion containing bony fragments. We performed an ultrasound-guided
needle biopsy in conjunction with magnetic resonance imaging, followed by an open excisional biopsy. Microscopically, histologi-
cal sections showed a lipoma with cartilaginous and osseous differentiation, finally diagnosed as osteochondrolipoma. In conclu-
sion, popliteal masses are not always simple cysts, and the evaluation of masses in the popliteal fossa is always necessary.
Keywords: Popliteal cyst, Neoplasm, Biopsy
Copyright ©
2015 by The Korean Orthopaedic Association
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0)
which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Clinics in Orthopedic Surgery
•
pISSN 2005-291X eISSN 2005-4408
Received March 16, 2012; Accepted June 1, 2012
Correspondence to: Jeong-Ho Kang, MD
Department of Orthopaedic Surgery, Gangneung Asan Hospital, University of
Ulsan College of Medicine, 38 Bangdong-gil, Sacheon-myeon, Gangneung
210-711, Korea
Tel: +82-33-610-3243, Fax: +82-33-641-8050
E-mail: theknee@naver.com
Case Report
Clinics in Orthopedic Surgery 2015;7:264-268 • http://dx.doi.org/10.4055/cios.2015.7.2.264
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a well-defined round mass located just behind the posteri-
or capsule, in close proximity to the popliteal neurovascu-
lar bundle (Fig. 3). The mass was mixed with hypointense
and hyperintense lesions on both T1-weighted and T2-
weighted images, and heterogeneous enhancement of the
lesion was observed.
Due to the fact that the mass was located relatively
far from the tibial nerve, but near to the joint capsule,
and contained both fat and bone components, a tumor of
neural origin or a vascular mass, can be excluded. With a
tentative diagnosis of malignant mesenchymoma, synovial
sarcoma, or teratoma, an ultrasound-guided needle bi-
opsy was performed, and five pieces of whitish gray linear
specimen obtained. A cytological examination revealed
fragments of fibro-collagenous tissue with myxoid stromal
change of a benign nature.
An open excisional biopsy was performed using a
posterior approach. The mass was located near the medial
head of the gastrocnemius, under a superficial fascia. The
joint capsule was adhered to the mass in certain areas,
otherwise separation from the surrounding soft tissue was
relatively easy. Finally, a moderately solid and yellowish
mass was excised and sent to a pathologist.
The mass was well-circumscribed and the cut
surface was yellow and fatty with traversing whitish fibro-
chondroid bands and nodules (Fig. 4). Microscopically, the
Fig. 1. The plain radiograph of the left knee shows a mass-like lesion
con taining fat (long arrow) and internal ossifications (short arrows) in the
posterior area of the left knee.
A B C D
Fig. 3. Sagittal plane magnetic resonance imaging scans: (A) T1-weighted image, (B) T2-weighted image, (C) fat-suppressed gadolinium-enhanced
image, and (D) axial T1-weighted image. The contents of the mass are presumed to be fat (arrows), enhancing fibrous tissue (arrow heads), and
ossifications. The tibial nerve (curved arrow) is shown separately.
Fig. 2. The ultrasound (US) scan of US-guided biopsy reveals a well-
defined soft tissue mass and prominent acoustic shadowing caused by
internal ossifications.
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specimen showed chondroid matrix and chondrocytes aris-
ing in the fibromyxoid band, as well as lamellar bone with
osteoblasts and osteoclasts within the chondroid nodule
(Fig. 5).
There were no abnormal neurological or vascular
symptoms postoperatively. Two months postoperative, the
patient had showed improvement of the chronic discom-
fort, and no calcified density remained on a plain radio-
graph.
DISCUSSION
The common evaluation steps of a popliteal mass are
physical examination, plain radiography, and aspiration of
cystic fluid. The most common popliteal mass is a Baker’s
cyst, and the prevalence rate is between 5% and 19% in
a large series of knee MRI studies.
4,5)
The diagnosis of a
Baker’s cyst is easily made using these procedures. How-
ever, many papers in the literature have reported unpre-
dictable disease of the popliteal area, therefore differential
diagnosis and evaluation of a popliteal mass should be
performed in some cases. Physical examination can give
important clues. In the case of a lipoma in the popliteal
area, it is generally less renitent on palpation compared
with the tightness of a popliteal cyst. When the popliteal
mass is accompanied by erythema with leg edema, deep
vein thrombosis or popliteal vein aneurysm can be consid-
ered as a diagnosis. In addition, a pulsatile popliteal mass
may be caused by pseudoaneurysm of the popliteal artery.
Tseng et al.
6)
reported a nerve sheath ganglion of the tibial
nerve in the popliteal area. Due to the fact that their case
showed a positive Tinel’s sign and a moderate degree of
paresthesia, they decided to perform an MRI, and found
an unexpected ganglion. If the character of the mass ap-
pears unusual upon physical examination, the next step
should include ultrasonography or MRI.
Ultrasonography is a very useful imaging method to
identify the nature of a cystic or solid mass, and arterial or
venous origin, however sometimes it is not sufficient. Fiori
et al.
3)
reported that a popliteal venous aneurysm in the
A B C
Fig. 5. Histological sections of the tumor specimen show lipoma with cartilaginous and osseous differentiation. (A) Ossification in the chondroid nodule.
Lamellar bone with osteoblasts and osteoclasts is being formed in the chondroid nodule (H&E, ×40). (B) Fibroblast proliferation and fibrous band is
intimately apposed to the fat necrosis area of lipoma (H&E, ×100). (C) A chondroid nodule in the lipoma. Chondroid matrix and chondrocytes are arising
in the fibromyxoid band (H&E, ×40).
Fig. 4. Photograph of the popliteal mass. The mass is well-circumscribed
and the cut surface is yellow and fatty with traversing whitish fibro-
chondroid bands and nodules.
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popliteal fossa demonstrated only a mixed echogenicity
mass, with no evidence of an arterial or venous nature, us-
ing color-Doppler ultrasonography. Tatari et al.
7)
reported
a well-circumscribed mass, resembling a Baker’s cyst, us-
ing ultrasonography; however, it was finally confirmed to
be pigmented villonodular synovitis. Additional investiga-
tion such as needle biopsy in conjunction with ultraso-
nography can prevent misdiagnosis. Needle biopsy of the
mass described in our study was performed with a view
to uncovering important information about the nature of
mass.
MRI is widely considered the best imaging tech-
nique for the examination of popliteal masses. Shin et al.
8)
reported a case of synovial sarcoma that was located at the
popliteal fossa, adjacent to the proximal tibia, and which
failed to show any abnormalities on a plain radiograph.
The soft tissue mass accompanied by bony lesions in close
proximity to the neurovascular structure was uncovered
during an MRI scan. MRI has the additional advantage of
being able to show the anatomical orientation, important
for excision of the mass.
In this case, histological findings were a well-encap-
sulated mass containing a mixture of adipose, fibrous, and
chondroid tissue, with bone formation. The lipoma area is
composed of mature fat, and there is no atypical adipocyte
or lipoblast to be concerned with liposarcoma, including
any dedifferentiated cells. On the basis of fat necrosis ad-
jacent to the chondroid and osseous differentiation, this
osteochondroid feature is thought to be metaplasia, related
to fat necrosis in a longstanding lipoma. Certain authors
regard this tumor as a benign mesenchymoma, due to the
presence of various cell types. However, the predominant
components encapsulated within the lipoma are fat tis-
sue, cartilage and bone, thus it was finally diagnosed as
osteochondrolipoma. Differentiation into a diverse set of
mesenchymal elements, such as blood vessels, fibrous tis-
sue or muscle is relatively common. Nevertheless, mature
cartilage and bone arising within a lipoma is extremely
rare. Only a few cases of osteochondrolipoma have been
reported, particularly in the thigh and chest wall, but never
in the popliteal fossa.
9,10)
It is apparent from this case that popliteal masses are
not always popliteal cysts, and careful evaluation of any
mass in the popliteal fossa is essential. The following are
a few suggestions to keep in mind when being presented
with a popliteal mass. Careful physical examination is es-
sential to check the nature of the mass including tender-
ness, pulsation and a tingling sensation that do not usually
exist with a simple cyst. If there are any abnormal findings
upon the physical examination or plain radiography, do
not hesitate to perform further imaging investigations
such as ultrasonography or MRI. Needle biopsy should be
chosen primarily over open excisional biopsy if a malig-
nant lesion is suspected from the imaging investigation.
As described earlier, the popliteal mass that was
initially misdiagnosed as a simple popliteal cyst, finally
turned out to be osteochondrolipoma. Therefore, when-
ever we encounter a popliteal mass, being open to all
diagnostic possibilities and evaluation procedures is para-
mount.
CONFLICT OF INTEREST
No potential conflict of interest relevant to this article was
reported.
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