Accessory Bones

Accessory Bones

These are “extra” bones that can be commonly seen. They arise from secondary centres of ossification that do not join with the main centres of ossification or are hereditary, such as sesamoid bones enclosed in tendons.

Os Trigonum:
Lateral tubercle of talus that is separate from body of talus at posterior aspect – variable size and shape
seen on lateral view at posterior margin of talus
need to differentiate from a fracture
occurs in 14-25% of population – often bilateral

Os tibiale externum/secondary navicular/accessory navicular:
Forms from a secondary ossification centre on the medial side of the navicular
prevalence of 4-14%
Commonly associated with a pronated/flat foot
3 types:
Type 1 – independent of navicular and is considered a sesamoid in posterior tibial tendon – well-defined oval shape. 2-3mm in diameter and about 3mm posteromedial to navicular – not usually symptomatic
Type 2 – united to navicular by cartilaginous bar – secondary ossification centre – usually 9-12mm and situated 1-2mm from posteromedial aspect of navicular tuberosity
Type 3 – enlarged posteromedial tuberosity – possibly due to fusion of type 2

Os supra naviculare:
Seen over the dorsal aspect of the talonavicular joint on lateral view

Os interphalangeum of the hallux:
Seen below the IP joint of the hallux on a dorsoplantar view
Sesamoid bone in tendon of flexor hallucis longus

Os peroneum:
In tendon of peroneus longus, articulating with cuboid on lateral or lateral oblique projection – seen lateral and plantar to cuboid
in 15% of population – often bilateral

Os vesalianum:
At base of fifth metatarsal
Needs to be differentiated from a fracture

Os subtibiale:
Seen on mortise view of ankle, just below the tip of the medial malleolus
Needs to be differentiated from a fracture

Os sufibulare:
Seen on mortise view of ankle, just below the tip of the lateral malleolus
Needs to be differentiated from a fracture

Os supratalare:
Seen on lateral projection over dorsum of the neck of the talus

Os sustentaculi:
Seen on axial view of calcaneus beneath the sustentaculum tali

Os intermetatarseum
Seen as an ossicle between the bases of the first and second metatarsals

Accessory sesamoids:
small sesamoids plantar to lateral four metatarsal heads or interphalangeal joint of the hallux – most common the fifth metatarsal head.

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