Keywords
knee joint - soft tissue neoplasms - lipoma - synovectomy
Introduction
Lipomatous tumors are common soft tissue neoplasms, although their intra-articular
occurrence is unusual. Most of them are benign and the knee is the most affected joint.[1]
[2]
[3]
[4]
[5]
[6] Regarding these, although few cases of lipoma arborescens (LA) have been described
in the literature, true intra-articular lipomas are exceedingly rare.[3]
[4]
[6] Reporting two distinct cases of rare intra-articular lipomatous tumors, the authors
intend to discuss their etiology, clinical and imaging aspects and treatment approach.
Case Report
Case 1: A 34-year-old female (recent diagnosis of psoriasis), evaluated for an atraumatic
bilateral knee pain for the last 5 years, sometimes having edema and morning stiffness;
the right knee had increased volume and was slightly painful at extreme flexion. Bloodwork
and X-rays were normal; magnetic resonance imaging (MRI) of the right knee (without
contrast - patient's hypersensitivity) showed joint effusion and diffuse thickening
of the synovial of the subquadricipital pouch (with isointense signal to subcutaneous
fat on T1-weighted and fat-saturated sequences, and without hemosiderin artifacts)
with multiple frond-like villi projecting from the synovium, measuring in total 45 × 19 × 18 mm
([Fig. 1]). Complete surgical excision and partial synovectomy was performed ([Fig. 2]). Histopathological examination showed mature adipose cells projecting in a villiform
fashion and covered by hyperplastic inflamed synovium – the diagnosis of LA was confirmed.
Fig. 1 Magnetic resonance imaging of the right knee (case 1): upper images are T1-weighted
axial (A), coronal (C) and sagittal (E) incidences showing the mass (black arrows) with intensity similar
to subcutaneous fat and multiple villous projections of the synovium; lower imagens
are proton density fat-saturated (PD-FS) axial (B), coronal (D) and sagittal (F) incidences, showing high intensity (*) compatible with a lipomatous tumor.
Fig. 2 Surgical approach (supra-patellar medial midvastus knee arthrotomy) of case 1: (A) lipomatous proliferation of the synovium with hyperplastic villous polypoid projections
(black arrow); (B) Excised tumor – 22,8 g of a yellowish adipose villous mass and some small synovial
chondromatosis foci (*).
Case 2: A 61-year-old female referred for an atraumatic superolateral mass on her
right knee with intermittent pain for the past 4 years and progressive joint enlargement.
Besides a nontender palpable mass, knee examination was unremarkable. X-rays showed
mild degenerative aspects. Magnetic resonance imaging showed a complete articular
multilobular mass in the superolateral subquadricipital pouch, extending to the lateral
recess and posterior to the femur, measuring 116 × 60 mm, compatible with a lipomatous
tumor in all sequences and irregularly nonenhanced by contrast ([Fig. 3]). Complete surgical excision and partial synovectomy was performed ([Fig. 4]); no villous structures were observed within the joint. Histopathological examination
revealed a low grade lipomatous tumor (lipoma-like) with mature adipocytes and no
atypical nuclei, as well as many vascularized septa beneath the capsule.
Fig. 3 Magnetic resonance imaging of the right knee (case 2): T1-weighted axial with fat-saturation
(A) and sagittal proton density fat-saturated (B) views after gadolinium contrast administration, showing a globally homogenous lobulated
mass with a very irregular peripheric enhancement (but no uptake in the lesion), compatible
with a lipomatous tumor.
Fig. 4 Surgical approach of case 2: (A) preoperative aspect of the knee; (B) lateral arthrotomy; (C and D) excision of the mass (note the lipomatous and capsulated nature of the tumor in
communication with the prefemoral fat pad); (E) excised lobulated and capsulated lipomatous mass (232,6 g).
Both our patients agreed on surgical consent for the use of their data for scientific/educational
purposes.
Discussion
Although initially described as different spectra of the same lesion (benign, chronic
and slow-growing),[5]
[6]
[7] true intra-articular lipomas and the less rare LA are different in presentation
and pathogenesis.[3]
[6] Lipomas arborescens are well established lipomatous lesions, characterized by the
replacement of the subsynovial layer by mature fat cells in a villiform proliferated
synovium, whose etiology is unknown.[1]
[2]
[3]
[5]
[7] The majority of LA is thought to rise from a nonspecific chronic synovial irritation
and is usually associated with trauma, degenerative joint disease and inflammatory
conditions (even the sparsely described psoriatic arthritis, like in case 1), yet
a true causal relationship is difficult to establish.[1]
[5]
[6]
[7] Lipomas arborescens seem to occur equally in both genders;[5]
[7] nonetheless, there are two types of LA: typical (secondary) are more common, larger,
occur in older patients (bilateral occurrence in the knee is estimated in 20% of cases[1]
[2]
[7]) and are often associated with other intra-articular pathology; on the other hand,
atypical (primary) LAs are rarer, smaller, occur in younger patients, usually with
monoarticular involvement or in other locations besides the knee, and present without
other joint alterations[1]
[3]
[5]
[7]
[8]
As for intra-articular lipomas, they are rare de novo entities, usually solitary small
fatty ovoid masses (unlike our case 2, which had an extremely rare morphology) that
are involved by a fibrous capsule of synovial tissue; they can emerge from the synovial
membrane or overgrow from the intra-articular subsynovial fat and are most commonly
found in the knee over the femur in connection with the fat pads (suprapatellar, prefemoral
– like in our case – or infrapatellar), although they can occur more rarely in other
locations (e.g., hip, spine, elbow, shoulders and wrists).[3]
[4]
[6]
These tumors are usually asymptomatic until mechanical effects arise (e.g., local
space-occupying effect), which could be an explanation for the symptoms of the patient
in case 2, although we also consider the concomitant mild degenerative disease found
in diagnostic imaging, which is not frequent.[3]
[4]
[6] However, in the cases of LA, one should account for the classical presentation of
a chronic painless swelling (over years) that intermittently has cyclical exacerbations;
this is thought to be related with trapping of the villi inside the joint, causing
effusion (which is almost always present), or from pre-existing joint disease in secondary
LA (like in our case 1–psoriatic arthritis – which could explain the inflammatory
nature of the complaints).[1]
[2]
[3]
[5]
[7]
Bloodwork is usually unremarkable and X-rays can show a soft tissue density or degenerative
alterations, particularly in LA.[1]
[5] Magnetic resonance imaging is the gold standard for evaluation of these lesions:
they show hyperintense signal in T1 and T2-weighted sequences, which can be suppressed
in fat-saturated or Short Tau Inversion Recovery (STIR) sequences, giving more detail;
the hypertrophied subsynovial tissue (in LA) or lipomatous tissue classically do not
enhance with gadolinium contrast (although this is highly variable especially in the
presence of inflammatory cells or vascularized capsular septa, which can produce irregular
enhancement images like in our case 2).[1]
[2]
[5]
[6]
[7] Besides morphological detail, MRI can help in differential diagnosis and show other
intra-articular pathology particularly in LA patients; in these, synovial chondromatosis
(in 13% of cases[2]) is believed to occur as a result of osteochondral differentiation of synovial tissue
as a response to the nonspecific synovial irritation that can occur simultaneously
with adipocyte differentiation as shown in case 1. The differential diagnosis of intra-articular
masses should contemplate the less infrequent noninfectious synovial proliferative
processes (LA, synovial lipoma, synovial chondromatosis, pigmented villonodular synovitis,
inflammatory arthritis) and deposition diseases (e.g., gout), and the more unusual
infectious granulomatous diseases (e.g., tuberculous arthritis), malignancies (e.g.,
metastases, chondrosarcoma, liposarcoma), vascular malformations and/or tumors (e.g.,
synovial hemangioma).[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8] For most, specific MRI patterns can outline the diagnosis; occasionally, laboratorial
analyses of blood, joint fluid and even biopsy samples are needed.
In LA and synovial lipomas, surgical treatment is indicated in symptomatic patients.
Mass excision and synovectomy are the standard care regardless of surgical approach;
although arthroscopic resection is deemed as equally effective and more advantageous,
some lesions (especially larger ones) benefit from joint arthrotomy for a complete
resection. Recurrence of these lesions after effective surgical treatment is rare.[1]
[4]
[6]
[7]
