A 27-year-old male patient with intermittent pain in the right knee that started 6 months earlier, after falling from his bicycle. Knee X-rays were assessed in the Emergency Department 3 weeks after the initial trauma, with no remarkable findings. MRI was performed to rule out other causes of pain.
Right knee X-ray (Fig. 1) revealed an eccentric medullary osteolytic lesion in the posterolateral aspect of the tibial metaphysis, with type IA geographic pattern and well-defined sclerotic margins (arrow). No cortical involvement or adjacent periosteal reaction was observed.
MRI (Fig. 2) shows a well-defined lesion with a peripheral area of low signal intensity in PD FSE fat-sat sequences and increased signal intensity in T1 FSE sequences, similar to the bone marrow, suggesting lipid component (arrows). The central area of the lesion shows high signal intensity in PD FSE fat-sat sequences and low-intermediate intensity in T1 FSE sequences (asterisk), in relation to cystic changes.
These findings are highly suggestive of intraosseous lipoma (IL) with cystic degeneration.
IL is an adipose-derived benign tumour of very low incidence (0.1% of all primary bone tumours). It may undergo varying degrees of degenerative changes and manifest with areas of fat necrosis, cystic changes, and calcification [1, 2].
No differences in age nor gender distribution have been clearly established, although some authors describe a slightly higher incidence in males [2-4] and a mean age peak at diagnosis in the fourth decade of life .
Although IL may appear anywhere in the skeleton , it is usually located in the lower limbs [1, 2], predominantly in the calcaneus (30%)  and in the metaphysis of long bones [1, 3, 5]. Its clinical presentation remains controversial. Some authors indicate that up to 70% of ILs cause pain [2, 4], while others consider that it is an asymptomatic incidental finding in most cases [4, 5]. Imaging techniques are required to exclude other causes of pain since IL usually mimics ligamentous or soft tissue injuries .
According to Milgram’s classification , three types of IL can be distinguished according to the histological features: mature adipocytes without necrosis (stage I); mature adipocytes with partial fat necrosis and/or focal calcifications (stage II); and regressive changes with fat necrosis, cystic degeneration, calcifications and reactive bone formation (stage III). In our case, cystic degeneration occurred in most of the lesion, so it could be classified as stage III.
Radiographically, IL usually appears as an osteolytic, well-circumscribed lesion with a sclerotic rim . Sometimes, findings may be subtle, as in the case presented. A cystic lesion with a central nidus of calcification (“cockade” image) is considered pathognomonic for IL of the calcaneus.
In CT, IL appears as a low-attenuating lesion (-110 to -40 HU) due to the presence of adipose tissue, with or without calcifications. Areas of fluid-like density can be observed in case of cystic degeneration.
In MRI, IL usually appears as a hyperintense lesion in T1 and T2-weighted images, with signal suppression in fat-saturation sequences. Calcifications can be identified as hypointense foci, whilst cystic degeneration areas normally show high signal intensity in T2-weighted images and low-intermediate signal intensity in T1-weighted sequences. No contrast enhancement is detected within the lesion .
IL is widely regarded as a non-malignant entity and usually does not increase in size.
Recently with the capability of CT scans or MR imaging in confirming the diagnosis of intramedullary lipoma, the necessity of surgical biopsy for the definite diagnosis has become controversial. Some authors believe that the discretion of fat in the lesion in CT and/or MR images can confirm the benign nature of the lesion and terminate the diagnostic process, because the presence of fat in malignant lesions is extremely rare. Additionally, with a high degree of assurance, distinction of intraosseous lipoma from liposarcoma can be made with the aid of CT and MRI. Absence of usual negative Hounsfield unit and loss of homogeneity can differentiate the liposarcoma from the lipoma. Also, with the application of a short repetition time on MR Imaging, liposarcoma (long T1) showed lower intensity than lipoma (short T1). 
According to the typical findings of the X-ray and MR images and the stability of the lesion during years of follow-up, the intraosseous lipoma was diagnosed and the need for a histopathological examination was eliminated.
Therefore, radiological follow-up with conservative treatment is recommended, except for rare cases with risk of pathological fracture, pain or malignant transformation . In the latter case, the treatment consists of curettage of the lesion with bone grafting [2, 4, 6]. Milgram described four cases of malignant transformation of intraosseous lipomas. As the literature lacks other reports of malignant transformations of intraosseous lipomas, some authors believe that the report by Milgram should be considered only as a proof of difficulties associated with differentiation between bone infarct and an intraosseous lipoma, rather than of the malignant potential of this tumour. A malignant transformation of bone infarct is a common complication. .
Written informed patient consent for publication has been obtained.
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