The blood supply of the talus is very limited and unique. This creates the risk of osteonecrosis with fractures of the talar neck. Osteonecrosis is death of a segment of bone that interrupts the blood supply. The artery of the tarsal canal is the dominant blood supply. The deltoid branch of the posterior tibial artery is the only remaining blood supply with displaced fractures. Talar neck fractures are classified into four types. Type I fractures are nondisplaced with 10% occurrence of avascular necrosis. Type II fractures include a fracture with subtalar dislocation or subluxation and this type has a 50% chance of AVN. Type III fractures consist of a fracture with subtalar and tibiotalar dislocation or subluxation; these fractures have up to 90% avascular necrosis. Type IV fractures occur with subtalar and tibiotalar dislocation in addition to a talonavicular subluxation. Type IV fractures have up to 100% AVN.
The fracture is usually reduced and fixed with the patient following up clinically and radiologically for healing of the fracture and the development of avascular necrosis. The status of talar vascularity can be check by the Hawkin’s sign. The Hawkin’s sign helps in the diagnosis of AVN.
What is the Hawkin's Sign?
The Hawkin’s Sign is a subchondral osteopenia (lucency) seen at six to eight weeks on the mortise view x-ray of the ankle on the dome of the talus. The physician should look for the radiolucent line below the subchondral bone. This line is more commonly seen on the medial side of the mortise view. The Hawkin’s sign is a good indication of intact vascularity with resorption of the subchondral bone following a fracture of the talar neck. It is 100% sensitive and 58% is specific. This indicates that the talus is alive! This is a good prognosis. Its absence does not rule out an intact vascularity.
Once the fracture heals, begin weight bearing. Restricting weight bearing beyond that which is needed for healing of the fracture does not decrease the risk of osteonecrosis. At 3-6 months post-operatively, AVN can be seen on the plain x-ray as sclerosis. An MRI is sensitive for detecting AVN as it shows a decreased signal on T1, but it does not guide the treatment. In MRI studies, titanium implants have better visualization than stainless steel. Osteonecrosis does not usually involve the entire talar body. Treatment is usually done using conservative methods. A surgical procedure in the form of tibiotalar fusion (ankle fusion) may be done in some cases. In cases of excessive osteonecrosis, tibiocalcaneal fusion or Blair fusion may be useful.
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