Teaching Case

A 51-year-old female came to Orthopedic Out Patient Department with complaints of pain and swelling in her left forearm for one month.

Plain radiograph

Well-defined lytic lesion in the proximal 1/3^rd of the radius with a rim of sclerosis, focal cortical defect in the dorsal aspect, and associated with adjacent soft tissue swelling.

Computed tomography

Hypodense lesion with fatty attenuation (Average attenuation of -120HU) in the proximal shaft of the radius with a cortical defect measuring 7mm in the posterior aspect and extension of the lesion into intermuscular planes.

MRI

Well-defined intramedullary lesion in the proximal 1/3^rd of the radius, which is hyperintense on T1 & T2 images. Loss of signal in fat suppression sequence (STIR). Cortical defect in the postero-lateral aspect through which the lesion is seen extending into the intermuscular planes pushing adjacent muscles laterally and neurovascular bundle medially. The intermuscular component of the lesion is measuring 5.3x 4.6 x 3.7 cm (Craniocaudal x Transverse x Anteroposterior) with thin internal septations (thickness 3mm) and no evidence of a solid component. Constellation of above-mentioned findings indicates, it is a stage 1 lipoma.

Background

 Intraosseous lipomas are one of the rarest benign soft tissue tumours, accounting for only 0.1% of all primary osseous tumours [4] with histologically identical tissue to adipose fat. Lipomas in general occur at a rate of up to 2.1 per 100 people [1], whilst the incidence of intraosseous lipomas is significantly lower than this. They can be subcutaneous or in the deeper planes like intermuscular, intramuscular, intraosseous, or intraglandular locations. They typically grow expansively between different fascial planes without infiltrating neighbouring structures, allowing them to be easily excised or aspirated with low recurrence rates. They have various stages of presentation due to their stages of evolution and can be mistaken for various osteolytic lesions [non-ossifying fibroma, aneurismal bone cyst, simple bone cyst, bone infarct, or chondroid tumours] on a plain radiograph [5].

Stages of evolution include:

1. Solid tumours of viable lipocytes
2. Transitional cases with partial fat necrosis and focal calcification but also regions of viable lipocytes and
3. Late cases in which the fat cells have died with variable degrees of cyst formation, calcification, and reactive new bone formation of a characteristic morphology.

Ischemia and infarction within the lesion cause the progression from stage I to stage III, which could be related to the rigid honeycomb structure of bone, fat cell expansion, and multiplication. These lesions may also cause bone resorption and expansion, whereas bone infarcts do not cause bone expansion. [6]

Clinical perspective

The majority of lipomas can be treated conservatively. The most common reasons for surgical intervention are suspicion or evidence of malignancy or the risk of pathological fracture. Cosmetic deformity or pain are two other indications. Curettage and packing with bone chips are common surgical treatments, similar to the one presented in this case. [7]

Imaging perspective

On CT, they appear as homogeneous encapsulated masses with low attenuation. All MRI sequences including the fat suppression sequence, show characteristic complete signal loss of the mass, revealing that the signal intensity of lipomas was consistent with that of subcutaneous fat [2]. They can cause mass effects on the adjacent neurovascular bundles if any. MRI is the imaging modality of choice if the patient is symptomatic [3].

Teaching points

As lipomas are the most common benign soft tissue tumours, it is important to evaluate their localization, extent, involvement of neurovascular bundle, and malignant transformation [Solid component and thick septae]. MRI is the best imaging modality for the characterization of the lesion. Evaluation of lipomas is necessary for a symptomatic patient to plan the treatment accordingly.

Written informed patient consent for publication has been obtained.