A seventy-three-year-old woman who had been previously healthy
was admitted to our hospital because of a soft-tissue mass involving
the right knee joint. She had first noted the mass ten years earlier,
when it appeared to be much smaller. Because she had no pain or
limitation of activities, she had not sought medical attention.
Upon discovering the mass, the patient's local physician referred
her to us in September 1999. The medical history and the family
history were unremarkable, and the patient remembered no trauma
to the knee.
Physical examination revealed a soft-tissue mass measuring 6
by 9 cm on the anteromedial aspect of the right knee joint (Fig. 1Fig. 1). On palpation,
the mass was found to be adherent to the joint, soft, nontender,
and similar in temperature to the surrounding tissue. The mass became more
prominent and firm with contraction of the quadriceps muscle. No
joint effusion was present. Full extension and 140 degrees of flexion
of the knee were possible, without pain or locking. No muscle atrophy
was evident. The results of blood tests, including erythrocyte sedimentation
rate, rheumatoid factor, and C-reactive protein level, were essentially
normal.
Plain radiographs of the right knee joint showed no abnormalities
except for soft-tissue swelling (Fig. 2Fig. 2). No osteoarthritic change
or bone erosion was present. T1 and T2-weighted axial magnetic resonance
images showed a soft-tissue tumor with high signal intensity containing
linear structures of low signal intensity (Figs. 3-AFigs. 3-A and 3-B3-B). The lateral
part of the mass was located between the femur and the patella.
A small amount of joint fluid also was demonstrated. The capsule
of the mass was enhanced strongly on T1-weighted, fat-suppressed
images after injection of gadolinium-diethylenetriamine penta-acetic
acid (Fig. 3-CFig.
3-C). The linear structures within the tumor were not enhanced.
A gallium scan showed no uptake in the lesion. The preoperative
differential diagnosis included intra-articular lipoma, extra-articular lipoma
protruding into the joint, lipoma arborescens, and liposarcoma .
An excision was performed. The mass was not directly apparent
following the skin incision, but arthrotomy by means of a medial
parapatellar incision revealed an ovoid soft-tissue mass within
a white fibrous capsule (Fig. 4Fig. 4). The mass was not directly
adherent to the joint capsule but was attached by a fibrous stalk
to the posterior part of the joint capsule in the area of the suprapatellar
bursa. The lateral portion of the lesion was located within the
patellofemoral joint. No villous synovial proliferation was observed
either over the surface of the tumor or elsewhere in the joint. The
lesion was easily and completely removed after the stalk was cut.
Slight erythema of the synovial tissue overlying the anterior surface
of the femur was observed after the excision. Neither degenerative
changes of the joint nor meniscal tears were seen. The cut surface
of the mass showed lipomatous tissue. Histological examination revealed
a proliferation of benign lipocytes (Fig. 5Fig. 5). Between the areas of tumor
cells, collagen bundles were seen; these bundles were relatively
thick compared with those seen in most lipomas. Mitotic activity
was absent, and careful searching failed to reveal lipoblasts or
malignant cells. The capsule of the tumor was composed of thick
collagenous tissue. Synovial lining cells were absent at the surface of
the capsule. Many vessels were present beneath the capsule of the
tumor, accounting for capsular enhancement of the tumor on magnetic
resonance imaging. Neither villous proliferation of the synovial
tissue nor fatty replacement of the underlying connective tissue
was demonstrated. The stalk of the tumor was fibrous and contained
feeding vessels. Mild, nonspecific chronic synovitis was noted to
involve the anterior surface of the femur.
The postoperative course was uneventful. Four months after the
surgery, the patient was doing well with no symptoms and had regained
the preoperative range of motion.