Part 3: Anatomical Variation and Bone Density

ANATOMICAL VARIATION

Accessory ossicles

Unfused secondary ossification centres; common in the foot.

Usually asymptomatic but a small proportion may be symptomatic, particularly following (minor) trauma or sporting activity; needs to be differentiated from a fracture.

Accessory ossicle:

Well defined sclerotic margin with normal adjacent bones.

Recent fracture:

Tender, with at least one edge where the sclerotic margin is absent and irregular margin of an adjacent bone indicating the origin of the fracture fragment.

Common accessory ossicles:

1. os trigonum
2. os supratalare
3. os supranaviculare
4. os calcaneus secundaris
5. os peroneum
6. os vesalianum
7. os naviculare

Os trigonum

Located posterior to the talus; ~8% of the population.

May cause posterior ankle pain if compressed between the posterior tibia and posterior calcaneus during forced plantar flexion, as occurs in footballers and ballet dancers; referred to as os trigonum syndrome or posterior ankle impingement.

Os peroneum

Located in the peroneal groove of the cuboid, embedded in the peroneus longus tendon. Associated with peroneus longus tenosynovitis and tear.

Os vesalianum

Located at the fifth metatarsal base. Must be differentiated from an avulsion fracture;

a fracture has a longitudinal orientation and an irregular outline, compared to the transverse orientation and smooth contour of the ossicle.

Os naviculare

Located proximal to the navicular tuberosity (medial side); often embedded in distal tibialis posterior tendon.

Synchondrosis can be ‘tweaked’ by minor trauma, leading to painful micro-movement and subsequent degenerative changes between the accessory ossicle and navicular; development of bursitis and localised swelling / prominence. May be associated with a higher incidence of tibialis posterior tendinopathy and tears.

Hallux Sesamoids

Symptomatic sesamoids may be due to fractures, osteonecrosis, inflammation (sesamoiditis), infection and involvement in degenerative or inflammatory joint disease.

Sesamoiditis

Sclerotic, irregular, fragmented (MRI more sensitive in early phase)

Sesamoid fracture

Commonly mistaken for a bipartite sesamoid.

Bipartite sesamoid:

Smooth, well-corticated transverse lucency with rounded margins; bipartite usually larger than single sesamoid thus the sum of the two parts is greater than the adjacent sesamoid.

Sesamoid fracture:

Painful, non-corticated fracture line separating irregular opposing sesamoid fragments with sharp edges.

Bipartite sesamoid                                          Sesamoid fracture

Tarsal Coalitions

Occur in about 6% of the population.

Thought to represent failure of proper segmentation of the tarsal bones.

May be fibrous (syndesmosis), cartilaginous (synchondrosis) or osseous (synostosis).

May be bilateral and result in restricted subtalar joint motion.

Calcaneo-navicular coalition

Best seen on oblique views.

‘Anterior nose’ sign or ‘anteater’ sign = anterior extension of enlarged anterolateral calcaneal process.

Osseous coalition: connecting osseous bar (bony bridge) between anterior calcaneal process and navicular.

Fibrous coalition: Sclerosis and irregularity at coalition interface (non-osseous coalition).

Possible hypoplastic talar head.

Osseous calcaneo-navicular coalition (arrows)

Talo-calcaneal coalition

Occurs on the medial side of the talus.

Usually at the middle subtalar facet.

Prominent talar beak arising from dorsal talar head or neck (not obvious in this image).

C sign – Formed by prominence of inferior border of sustentaculum and talar dome on lateral projection (NB this is not diagnostic of coalition as it can often be seen with a valgus heel e.g. flat foot)

Talocalcaneal coalition with characteristic C shape (arrows) on lateral view