Usually,
accessory and sesamoid bones are incidental findings on radiographic examinations.
Superior modalities as magnetic resonance imaging,
computed tomography,
ultrasound and even scintigraphy can be helpful in difficult diagnostic cases or when associate pathology is suspected.
Magnetic resonance imaging is of a great importance in early depiction of bone and joint abnormalities and soft-tissues changes.
Os trigonum is formed by failure of fusion of a secondary ossification center at the posterolateral aspect of the talus with which articulates via a fibrocartilaginous synchondrosis (Fig.
1,
2).
Posterior impingement syndrome is also known as os trigonum syndrome or posterior tibiotalar compression syndrome.
It arises from the compression of posterior soft tissues between the posterior process of the calcaneus and the posterior tibia and is associated with repeated plantar flexion.
The presence of os trigonum may contribute and/or precipitate the presentation of posterior impingement syndrome.
Clinically manifestations include posterior chronic ankle pain and swelling.
Imaging findings may depict soft tissue swelling,
local synovitis,
flexor hallucis longus tendinitis,
joint effusion and degenerative changes at the synchondrosis with cystic and sclerotic changes (Fig.
3,
4).
The accessory navicular bone is also known as os tibiale,
os naviculare secundarium or os tibiale externum.
Three types of accessory navicular bones are described in the literature,
of which type II is the most frequently encountered (Fig.
5).
Type II accessory navicular bone results from a persistent accessory ossification center,
being located adjacent to the posteromedial tuberosity of the navicular bone.
It is connected to the navicular bone by a 1- 2 mm wide synchondrosis and it is the insertion point for the posterior tibial tendon.
A symptomatic accessory navicular bone is known as os naviculare syndrome.
It is caused by repetitive shear stress and tension at the synchondrosis by pulling the posterior tibial tendon.
Clinical symptoms are associated with pain and tenderness in the medial foot related to disruption of the synchondrosis,
posterior tibial tenosynovitis and osteonecrosis.
MR imaging may demonstrate bone marrow oedema of the accessory navicular and of the articulating navicular,
as well as thickening or even tears of the posterior tibial tendon (Fig.
6,
7).
Hallux sesamoid bones present with a medial and lateral component embedded within the medial and lateral slips of the flexor hallucis brevis tendon.
Their role is to reduce friction during halux flexion.
The medial component,
more common than the lateral component,
can be bipartite or even tripartite (Fig.
8,
9).
Chronic pain caused by chronic stress to the hallux sesamoid bones is defined clinically as sesamoiditis,
a spectrum of entities that include chondromalacia,
osteochondritis,
osteonecrosis and stress fractures.
MR-imaging is valuable in assessing abnormalities of clinical sesamoiditis,
depicting bone marrow edema and inflammation of surrounding soft tissues (tendinitis,
bursitis or synovitis) (Fig.
10,
11).