La tunisie Medicale - 2019 ; Vol 97 ( n°02 ) : 393-395
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Université Tunis El Manar.
Lipomas are the most common soft tissue tumors and may include in addition to mature fat cells various other mesenchymal elements. The World Health Organization describes 14 types of benign tumors including mature adipose tissue such as myxolipoma, fibrolipoma, angiolipoma... . Osteolipoma or ossifying lipoma is a rare subtype [1]. The coexistence within a fat mass of bone and chondral components is even more exceptional. This entity has several names: osteochondrolipoma, lipoma with bone and chondral metaplasia reflecting its uncertain origin.
Histologically, it is composed of mature adipocytes associated with mature bone trabeculae, mature hyaline cartilage and osteoid substance [3]
This tumor has been most often described in the region of the head and neck and around the oral cavity [3]. However, some isolated cases located to the limbs have been reported [2,3].
We report a case of osteochondrolipoma of the knee specifying the clinical, radiological and therapeutic aspects of this rare tumor.
A 59-year-old woman with no medical history presented for a painless swelling of the anterolateral side of the right knee, evolving for 3 years, gradually increasing in volume. He reports a direct knee trauma that occurred 4 years ago. This tumefaction evolved without a deterioration of the general status.
Clinical examination showed a superficial oval mass of the anteromedial surface of the right knee, well limited, 8 cm long axis, soft, painless to palpation, not adherent to superficial and deep planes and without local inflammatory signs. The range of motion of the knee was normal.
X-Rays revealed rounded and arcuate calcifications of the juxta-articular soft tissues on the antero-medial side (Fig.1). The CT scan confirmed the presence of a heterogeneous fat mass containing peripheral calcification areas (Fig.2).
MRI revealed a well-defined anteromedial juxta-articular mass of the right knee, at the contact of the medial collateral ligament and medially to the hamstring tendons, measuring 8x3 cm. This mass was of fatty signal hyper T1 signal fading on the fat saturation sequences, hyper T2 signal and enhancing shortly after Gadolinium injection.
In peripheral, it contains millimeter-sized nodular formations in iso signal intensity to the bone in T1 and T2 weighted images suggesting bone metaplasia. There was no abnormal morphology or signal of the bony structures of the knee
Total resection of the tumor was performed. Macroscopic examination revealed a well-encapsulated firm oval tumor that was vascularized yellowish in color and not communicating with the joint (fig.4).
Histopathological examination confirmed the diagnosis of lipoma with osteochondral metaplasia by showing mainly mature adipose tissue associated with micronodules of mature hyaline cartilage and lamellar bone areas containing osteocytes (Fig.5)
With a follow-up of 8 years postoperatively, the evolution was favorable and the patient did not present a local recurrence.
Lipoma with osteochondral metaplasia is an extremely rare benign tumor that often affects the area of the head and neck. Knee localization is uncommon. The nomenclature and physiopathology of this lesion remain controversial. Diagnostic confirmation is anatomopathological only. The treatment is surgical and consists of complete excision.

Figure 1: Standard knee radiographs showing rounded and arciform calcifications of the soft tissue of the antero-internal aspect of the knee.

Figure 2: CT scan of the knee: juxta-articular heterogeneous mass of fatty density containing peripheral calcification ranges.

Figure 3: MRI: antero-medial well-defined mass of the right knee located in contact with the medial collateral ligament, fatty signal in T1 and T2 hyper signal, fading on FatSat sequences and enhancing shortly after injection of Gadolinium. In the periphery there are millimeter-sized nodular formations in T1 and iso signal on T2 relative to the bone evoking bone metaplasia.

Figure 4: firm oval tumor well encapsulated finely vascularized of yellowish color not communicating with the joint

Figure 5: (A) Histological sections of the tumor specimen show lipoma with cartilaginous and osseous differentiation (HEx20). (B) Enchondral ossification within lipoma (HEx100).

  1. Huynh T-PV, Cipriano CA, Hagemann IS, Friedman MV. Osteolipoma of the knee. Radiol Case Rep. 2017 Mar;12(1):124–9.
  2. Nishio J, Ideta S, Iwasaki H, Naito M. Scapular osteochondrolipoma: Imaging features with pathological correlation. Oncol Lett. sept 2013;6(3):817‑20.
  3. Choi Y-J, Kang J-H, Kang G-H, Choi S-J. Osteochondrolipoma presenting as a popliteal cyst. Clin Orthop Surg. 2015;7(2):264.
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