A hibernoma is a benign neoplasm of vestigial brown fat. The term was originally used by the French anatomist Louis Gery in 1914. Signs and symptoms
Patients present with a slow-growing, painless, solitary mass, usually of the subcutaneous tissues. It is much less frequently noted in the intramuscular tissue. It is not uncommon for symptoms to be present for years. Benign neoplasm with brown fat is noted. Diagnosis
Imaging findings
In general, imaging studies show a well-defined, heterogeneous mass, usually showing a mass which is hypointense to subcutaneous fat on magnetic resonance T1-weight images. Serpentine, thin, low signal bands (septations or vessels) are often seen throughout the tumor. Pathology findings
From a macroscopic perspective, there is a well-defined, encapsulated or circumscribed mass, showing a soft, yellow tan to deep brown mass. The size ranges from 1 to 27 cm, although the mean is about 10 cm. The tumors histologically resemble brown fat. There are four histologic types recognized, but one is the most frequently seen (typical). There is a background of rich vascularity. Lobular type: Variable degrees of differentiation of uniform, round to oval cells with granular eosinophilic cells with prominent borders, alternating with coarsely multivacuolated fat cells (pale cells). There are usually small centrally placed nuclei without pleomorphism. The cells have large cytoplasmic lipid droplets interspersed throughout.
It is then considered as being a mechanical function problem of the forefoot. Cause
The exact cause is unclear. It may be due to a combination of internal and external causes. Proposed factors include wearing overly tight shoes, high-heeled shoes, family history  and rheumatoid arthritis. The American College of Foot and Ankle Surgeons states that footwear only worsens a problem caused by genetics.Excessive pronation of the foot causes increased pressure on the inside of the big toe that can result in a deformation of the medial capsular structures of the joint, subsequently increasing the risk of developing bunions. Pathophysiology
The bump itself is partly due to the swollen bursal sac or an osseous (bony) anomaly on the metatarsophalangeal joint. The larger part of the bump is a normal part of the head of the first metatarsal bone that has tilted sideways to stick out at its distal (far) end (metatarsus primus varus). Bunions are commonly associated with a deviated position of the big toe toward the second toe, and the deviation in the angle between the first and second metatarsal bones of the foot. The small sesamoid bones found beneath the first metatarsal (which help the flexor tendon bend the big toe downwards) may also become deviated over time as the first metatarsal bone drifts away from its normal position. Osteoarthritis of the first metatarsophalangeal joint, diminished and/or altered range of motion, and discomfort with pressure applied to the bump or with motion of the joint, may all accompany bunion development.