Encapsulated fat necrosis: a form of traumatic panniculitis

Editor We present three cases of encapsulated fat necrosis, which characteristically appears in the form of solitary or multiple well-circumscribed subcutaneous nodules. One male and two female patients presented with solitary (case 1) or multiple (cases 2 and 3) nodular lesions that had evolved over time on the lower extremities. No previous trauma was reported. The two female patients were skiers and the male a footballer. The lesions consisted of subcutaneous, firm, freely movable and non-painful nodules (fig. 1). No clinical signs of stasis dermatitis of the lower extremities were present. Multiple (five in case 2 and 16 in case 3) or solitary (case 1), well-encapsulated nodules of variable size (3–6 mm in diameter in cases 2 and 3, and 4 mm in case 1) were observed. An excision was made in case 1 and a biopsy of one of the nodules was performed in cases 2 and 3. The nodules were composed of adipose tissue showing variable amounts of fat necrosis. These were completely encapsulated by a thin-to-thick, fibrous capsule and sharply separated from the surrounding structures (fig. 2). The degree of necrotic fat cells was variable in the different lesions and from patient to patient. Some nodules were almost acellular (cases 2 and 3), but the adipose tissue conserved the normal fat architecture. Central calcification was observed in cases 1 and 2. ‘Well circumscribed fat necrosis’ first appeared in the German medical literature in 1975 as multiple calcified nodules in the breast of a 52-year-old woman. Since this publication, several reports have appeared using other nomenclatures such as ‘nodular-cystic fat necrosis’, ‘mobile encapsulated lipoma’, ‘nodular fat necrosis and posttraumatic fat degeneration and herniation’ and ‘encapsulated necrosis’. In 1988, Ezoe et al. proposed the term ‘encapsulated fat necrosis’, and later Ohtake et al. and Kiryu et al. stated that this term is the most representative for this condition. The clinical features are well-demarcated, movable nodules that appear most frequently on the lower limbs, and later on the elbows or hips. Most of the lesions are movable and 75% of patients present multiple nodules. Only 40% of patients refer to a previous trauma. In our cases none of the patients remembered any traumatic event. All sports practised probably result in small traumas in the pretibial area where the lesions appeared. The male to female ratio in the literature is 1 : 2. Histopathologically, these lesions are totally or nearly totally encapsulated by thin to thick, fibrous tissue that contains well-preserved adipocytes with occasional inflammation. At the periphery of the lesions, necrotic adipocytes in the form of anucleated fat cells are usually seen. The absence of nuclei in well-preserved but necrotic adipocytes at the periphery of the lesion permits differential diagnosis of a lipoma. Membranocystic or lipomembranous changes have also been described. Calcification is occasionally present. We think this is a frequent pathology that is underreported and that has different clinical and histopathological characteristics. We agree with other authors that it can be a form of traumatic panniculitis.