Musculoskeletal ImagingTalus
1. Anatomy
the most proximal of the tarsal bones, which articulates with the tibia and fibula to form the ankle joint. Also, see talus.
2. Pathology
Beaking of the talus (see beaking, talar (III:1), Fig. 1) occurs when an excrescence forms at the dorsal surface of the talar head adjacent to the talonavicular space. This condition is seen in patients with talocalcaneal coalition, diffuse idiopathic skeletal hyperostosis DISH , acromegaly and osteoarthritis of the ankle.
Congenital vertical talus
, or the existence of
congenital flatfoot with talonavicular dislocation, frequently occurs in association with
arthrogryposis multiplex congenita or
meningomyelocoele, although it can occur as an isolated condition or as part of other generalized malformation syndromes. Anteroposterior radiographs reveal severe heel valgus and forefoot abduction, resulting in an increased talocalcaneal angle such that the talar axis lies medial to the first metatarsal bone. Lateral radiographs show an equinus heel with plantar flexion of both the calcaneus and the talus. Dorsiflexion of the forefoot at the midtarsal level is observed, resulting in the
rocker bottom foot deformity. The navicular bone is dislocated dorsally, locking the talus into its plantar flexed position.
Osteochondritis dissecans
may involve the talar dome as a result of
trauma (
Fig.1,
Fig.2). The osteochondral fragment may remain in place or be dislodged. On radiographs the osseous defects may be quite subtle.
Arthrography, computed arthrotomography and MR imaging provide better data regarding the condition of the overlying chondral coat and the presence of intra-articular osseous and cartilaginous bodies.
Traumatic lesions.
The talus is one of three structures forming a ring whose integrity is essential for stability of the ankle joint. A single break in the ring does not allow subluxation of the talus in the mortise, whereas two or more breaks in the ring, whether fractures or a
fracture in combination with a ruptured
ligament, will allow abnormal talar motion.
In ankle injuries the talus can be displaced in five possible ways: external rotation, in which the talus is displaced externally or laterally; internal rotation, in which the talus is displaced internally or medially; abduction, in which the talus is displaced laterally without significant rotation; adduction, in which the talus is displaced medially without significant rotation; and dorsiflexion, in which the talus is dorsiflexed on the tibia. Various combinations of injuries can be recognized, such as supination external rotation fracture or supination adduction fracture.
The talus is second only to the calcaneus as a site of fracture in the tarsal bones; avulsion fractures predominate, especially in the superior surface of the talar neck and the lateral, medial, and posterior aspects of the body. Fractures of the head of the talus are rare and probably are related to a longitudinal compression force combined with plantar flexion of the foot. Fractures of the talar neck may be complicated by delayed union or nonunion, infection, osteoarthritis of adjacent joints, and ischaemic necrosis of the proximal portion of the bone. A radiolucent area (Hawkins' sign) in the talar dome on radiographs obtained 1 to 3 months after a talar neck fracture represents hyperaemia and should not be misinterpreted as a crescent sign of osteonecrosis. Fractures of the body of the talus are infrequent; complications include ischaemic necrosis, osteoarthritis, and delayed union.
Subluxations and dislocations of the talus generally occur in association with fractures of the bone and usually are classified as subtalar (peritalar) dislocations and total talar dislocations.
Osteonecrosis of the talus is a recognized and disabling complication of various fractures and injuries. Usually radiographic diagnosis is not made until osteoporosis of the surrounding viable bone becomes obvious as a ragion of relatively increased density in the talar body.
DR/RB
DR/RB