Teaching Case

A 31-year-old male presented with a history of chronic intermittent pain in his right shoulder after a trivial trauma 5 months back. No signs of local inflammation were noted in the examination. The pain was increasing in severity and without significant reliance on analgesic medications.

The anteroposterior (Figure 1) and anteroposterior with internal rotation (Figure 2) view of the right shoulder showed the relatively well-defined osteolytic lesion with a sclerotic border in the inferomedial aspect of lesser tuberosity of the right humerus at the level of the surgical neck (black arrow).

For better characterization of the lesion, MRI right shoulder was done which showed the well-defined eccentric lobulated T1 low, T2 fat sat high signal intramedullary lesion in the medial aspect of the neck of the humerus. Peripheral thick rim of T1 high signal which showed suppression in fat saturated images, suggesting fat content is noted. Normal overlying cortex and adjacent marrow signal is noted.

It is one of the rarest bone tumors with an incidence of about 1 in 1000 population with male predominance. However, it is the most common lipogenic tumor of bone. It is most frequently observed in the 3^rd to 4^th decade of life [1]. It mainly affects the metaphysis of the long bones with a higher incidence of the lower limb. It rarely affects the upper limb with an incidence of 7% [2]. Based on the histopathological appearances, it is classified into three types by Milgram. Stage I contains only the adipose tissue without any necrotic areas. Stage II contains adipose tissue with interspersed necrotic area and dystrophic calcification. Stage III contains intraosseous lipoma with extensive necrotic changes, cyst formation, and dystrophic calcification [3]. Although the exact etiology is unknown multiple theories have been kept forward but none of these carries strong evidence [4].

Most of the lesions are symptomatic with a dull pain at the lesion sites. Soft tissue swelling may also occur. There may be complete resolution or recurrence of pain after surgery. About 30-40% are asymptomatic and are diagnosed incidentally [5].

Milgram correlated radiographic appearance with histopathological staging. Stage I shows a well-defined lytic lesion with a narrow zone of sclerosis. In stage II there are hyperdense foci of calcification and expansion of the lytic area due to necrosis. In stage III the sclerotic zone becomes thickened with or without calcification. Our patient belongs to Milgram stage III [3,6].

CT is another radiological modality that helps in the diagnosis based on the fat attenuating (-40 to -110 HU) areas as well as areas of dystrophic calcification, thereby avoiding unnecessary biopsy [7]. MRI helps to accurately diagnose and characterize the stage of intraosseous lipoma. Stage I shows T1 high, T2 fat sat low signal lesion with T1/T2 low signal sclerotic rim. Stage II shows similar findings with internal T1/T2 low signal foci of dystrophic calcification. Stage III shows T1/T2 low signal intensity thick sclerotic rim. Necrotic and cystic areas show T1 low, and T2 high signal intensity content [6].

Symptomatic or with risk of imminent fracture is managed surgically by curettage and bone grafting. However, those with normal bone stability are managed conservatively [8]. Very few cases of sporadic malignant transformation were mentioned by Milgram [3]. Radiological follow up is needed as it may undergo spontaneous involution.

A simple bone cyst is a very important differential diagnosis that may undergo lipomatous degeneration from the periphery with advancing age showing similar radiological appearances [9].

Written informed consent was taken from the patient for publication.