Os sustentaculi

The os sustentaculi or os sustentaculum is a rare accessory ossicle of the ankle and a normal anatomical variant.

Epidemiology

The estimated prevalence is approximately 0.3-0.4% ¹.

Associations

It has been found in up 24% of talocalcaneal coalitions ².

Gross anatomy

The os sustentaculi is usually found at the posterior aspect of the sustentaculum tali and is connected to the latter by a fibrous or fibrocartilaginous synchondrosis ^3,4.

Variant anatomy

A bony bridge between the posterior aspect of the sustentaculum and the talus has been suggested as a variant of the os sustentaculi.

Radiographic features

Plain radiograph

The os sustentaculi can be best visualised on an AP view of the ankle with a slight amount of external rotation ⁴.

CT

The Osos sustentaculi can be nicely demonstratedvisualised on CT and might demonstrate degenerative changes such as irregular or hypertrophic cortical contours, subchondral cysts or vacuum phenomenon ³.

MRI

On MRI of the ankle, the os sustentaculi can be visualised as a separated bony structure from the sustentaculum.

Degenerative changes of the synchondrosis with contour irregularities, sclerosis, and cyst formation might occur. Bone marrow oedema might be present if the patient experiences symptoms and pain ^1-3.

History and etymology

The os sustentaculi was first reported by the German anatomist Wilhelm Pfitzner in 1896 ^2-6.

Clinical importance

An os sustentaculi can cause pain, e.g. as a result of overuse, and might be mistaken for a sustentaculum fracture, an unusual exostosis or an os calcanei secundarium ^1-5.

Related pathology

The os sustentaculi has been related to an extraarticularextra-articular talocalcaneal coalition (see Case 1) and can cause pain in the setting of overuse ^1,2.