Introduction:
No muscle or tendon attachment on talus! 60% covered by articular cartilage. Small nutrient arteries, but no collaterals. All these make it prone for AVN.
Etiology:
Trauma - increased risk with fracture dislocation of talar NECK
Non-traumatic - steroids, alcohol, SLE, renal transplants, sickle cell anemia, hyperlipidemia, radiotherapy, thrombophilias.
Talar neck fracture (Hawkin's classification) and AVN:
Risk depends on the magnitude of displacement and dislocation
Type I: nondisplaced talar neck fracture, risk of AVN 0%–15%
Type II: displaced fractures with dislocation or subluxation of the subtalar joint, risk of AVN 20%–50%
Type III: displaced fractures with dislocation or subluxation of ankle and subtalar joints, risk of AVN 100%.
Type IV: displaced fractures with dislocation or subluxation of the subtalar, tibiotalar, and talonavicular joints, risk of AVN 100%
If the inferior fracture line involves posterior facet of talus, it is body fracture and fractures involving talar body have higher prevalence of AVN and a poorer prognosis (??)
Hawkins sign:
Evident in 6–8 weeks after injury
Disuse osteopenia due to resorption of subchondral bone
Seen as thin subchondral lucent line along all or part of the talar dome
Best seen on AP film, can be missed on lateral view
This sign indicates adequate blood supply
Partial Hawkins sign may indicate partial AVN
Incomplete AVN:
Common in lateral talus due to rich medial blood supply
Radiograph and CT:
Progression:
1. Necrotic bone and viable bone are equal in opacity
2. Hyperemia leads healthy bone to be resorbed, becomes osteopenic
3. Necrotic bone does not undergo resorption due to lack of vascular supply, hence becomes sclerotic
4. Reossification over necrotic trabeculae causing further sclerosis. Sclerosis occurs in talar dome and may extend into talar body
5. Revascularization and resorption around necrotic bone causes lucent rim
6. Collapse of articular surface. Fragmentation of talar dome and body
References:
Pearce DH et al.Avascular Necrosis of the Talus: A Pictorial Essay. Radiographics 2005;25:399-410
