Lipoma of the calcaneus (stage 2)
Intra-osseous lipomas are rare lesions (0.1% of primary osseous tumors). Up to 15% are within the calcaneus. They probably rather correspond to a hyperplasia of the fatty tissue of the calcaneus than to a real tumor.
Calcaneus lipomas may be observed in men and women of all ages with equal frequency, although most are identified between the fourth and the sixth decade of life.
50% of those lipomas present calcifications or central ossifications secondary to fat necrosis.
According to Milgran, 3 evolutive stages can be found.
In stage 1, lesions contain a viable non necrotic fat with a trabecular resorption.
In stage 2, lesions contain viable fat, central fat necrosis and dystrophic calcifications.
In stage 3, lesions present a diffuse eosinophilic fat necrosis with extension of calcifications and ossifications. Thus, lipomas and bone infarcts are same lesions at different stages of their evolution.
CHARACTERISTIC CLINICAL FEATURES AND RADIOLOGIC FINDINGS
Lipomas of the calcaneus are usually entirely asymptomatic and are incidental discoveries. In case of heel pain, the rule is to look for an associated lesion. Nevertheless, in some cases of acute fat necrosis, pain may be present.
Radiological appearance depends on the stage due to evolutionary changes of the tumor.
Plain films show a central calcaneal area of geographic osteolysis with sharp edges underlined by a thin osteoblastic rim. On later phases, calcifications appear within the areas of fat necrosis, they may be central or peripheral.
CT allows identification of the fatty content of the lesion as well as absence of residual trabecular bone, sclerotic margins and degree of expansion and thinning of cortices.
MR imaging demonstrates the fatty nature of the lesion with a high signal intensity on SE T1 and T2 weighted sequences and a low signal intensity on fat suppressed sequences.
Treatment: Biopsy is not necessary when the lesion is asymptomatic and well characterized by MRI. Therapeutic abstention is the rule.
Differential diagnoses below can be considered:
- Pseudocyst of the calcaneus (or “fatty pseudo cyst”), generally considered as a variant of the normal trabecular pattern.
- Simple bone cyst of the calcaneus.
As a matter of fact, the relationship between these different entities is often discussed in the literature.
Actually, differential diagnosis between these different lesions and especially between a pseudocyst and a small lipoma limited to the central area of the calcaneus is very subtle and may be impossible. This is not critical because such a distinction has no clinical nor therapeutical implication.
- The main differential diagnosis is subchondral calcaneus cyst secondary to a subtalar joint arthropathy, usually observed in older patients (usually fifth or sixth decade).
MR imaging easily differentiates simple bone cysts from pseudo cysts and lipomas (which have a fat content), aneurysmal bone cysts, and other tissular tumors or pseudo tumors such as fibrous dysplasia which may present the same pattern on plain films.
Diard F., Hauger O., Moinard M., Bruno S., Marcet B. Pseudo-cysts, lipomas, infarcts and simple cysts of the calcaneus: Are there different or related lesions? JBR-BTR (90.5), 2007: 315-324.
Milgram J.W. Intraosseous lipomas: radiologic and pathologic manifestations. Radiology, 1988, 167: 155-160.