The os sustentaculi or os sustentaculum is a rare accessory ossicle of the ankle and a normal anatomical variant.
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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 visualized on an AP view of the ankle with a slight amount of external rotation 4.
CT
The os sustentaculi can be nicely visualized 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 visualized as a separated bony structure from the sustentaculum.
Degenerative changes of the synchondrosis with contour irregularities, sclerosis, and cyst formation might occur. Bone marrow edema 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 os calcanei secundarium 1-5.
Related pathology
The os sustentaculi has been related to an extra-articular talocalcaneal coalition (see Case 1) and can cause pain in the setting of overuse 1,2.