Subtalar Dislocation
On this page
Direct answer
A subtalar dislocation is a displacement of the calcaneus and foot from the talus, with the talus remaining in the ankle mortise, and it is usually described as medial or lateral. Closed reduction is attempted first, but interposed tendons or fragments often block it, and irreducible cases need open reduction. Post-reduction CT is then used to look for osteochondral fragments.
What you must remember
- Definition: the talus stays in the ankle mortise while the calcaneus and navicular displace from beneath it, a dislocation through the subtalar joint.
- Variants: medial and lateral dislocation differ in direction and in how readily they reduce, and many texts describe the medial form as the commoner one.
- Mechanism: a high-energy fall or twisting injury with the foot forced into inversion or eversion.
- Interposed soft tissue: the posterior tibial tendon is a classic block in lateral dislocations, while extensor structures can block medial dislocations.
- Closed reduction: plantar flexion, traction and inversion or eversion of the heel under adequate sedation or anaesthesia.
- Open reduction: indicated for irreducible cases, through a sinus tarsi or medial approach depending on the variant.
- Associated injuries: osteochondral fractures of the talus or calcaneus and neurovascular injury, which post-reduction CT helps to exclude.
A stepwise pathway from recognition to definitive care
A 30-year-old man falls from a ladder and has a deformed foot with the heel displaced, while the ankle joint looks intact on a quick view. The steps below show the sequence from recognition to definitive care, which is the order an examiner expects when the injury is described.
- Recognise the pattern. Look for a foot displaced relative to a talus that is still seated in the mortise, with the heel shifted medially or laterally.
- Check neurovascular status. Assess the distal pulses and sensation before manipulation, because the dislocation can compromise the posterior tibial vessels and nerves.
- Attempt closed reduction. Apply traction with plantar flexion, then invert or evert the heel to restore alignment, under adequate sedation or anaesthesia.
- Check the reduction. Take AP, lateral and oblique films and confirm that the subtalar joint is congruent with the talus in the mortise.
- Move to open reduction if blocked. If closed reduction fails, expose the joint through the sinus tarsi or a medial approach, and remove the interposed tendon or fragment.
- Image for osteochondral injury. Use CT after reduction to detect talar or calcaneal osteochondral fractures, and plan fixation or immobilisation accordingly.
Where the exam tries to catch you
Subtalar questions test whether the candidate notices the displacement below the talus and anticipates a failed closed reduction.
- Trap: missing a subtalar dislocation because the ankle joint looks reduced. Correct: the talus stays in the mortise while the foot is displaced, so the subtalar joint must be inspected on the lateral and oblique views.
- Trap: assuming closed reduction always succeeds. Correct: interposed tendons, especially the posterior tibial tendon in lateral dislocations, commonly block reduction and need open surgery.
- Trap: stopping after reduction. Correct: CT looks for osteochondral fragments, which must be treated because they affect long-term function.
Frequently asked questions
What is a subtalar dislocation?
A subtalar dislocation is a displacement of the calcaneus and foot from the talus, with the talus remaining in the ankle mortise. It is usually medial or lateral, and it results from a high-energy injury.
Why does closed reduction fail in subtalar dislocation?
Closed reduction in subtalar dislocation fails most often because soft tissue becomes trapped between the bones. The posterior tibial tendon is a classic block in lateral dislocations, and extensor structures can block medial dislocations.
Which imaging is used after reduction of a subtalar dislocation?
CT is used after reduction of a subtalar dislocation to look for osteochondral fractures and intra-articular fragments. Plain radiographs confirm alignment, but they can miss the fragments that CT demonstrates.
Why is neurovascular status checked in a subtalar dislocation?
Neurovascular status is checked because the dislocation can stretch or compress the posterior tibial artery and nerves, and the foot may become ischaemic. Pulses and sensation are documented before and after reduction.