The posterior process has a smaller medial and a larger lateral tubercle, separated by the groove of FHL. The process has a separate ossification centre and may remain a separate bone, the os trigonum. A fibrous union between the talar body and an ostrigonum may be injured. The posterior process is an attachment for the posterior talofibular and posterior tibiotalar ligaments.
Fractures of the posterior process may be produced by forced plantarflexion with a “nutcracker effect” between the posterior tibial plafond and the calcaneum, or by a twisting injury that subluxes the subtalar joint or avulses the ligamentous attachments. The lateral process is more commonly injured and is known as Shepard’s fracture from the original description.
Like lateral process fractures, posterior process fractures may range in size, from small avulsions up to large fragments which carry a large amount of the posterior subtalar and ankle joint surface and may lead to arthritis of both joints.
They tend to present acutely simply as a severely injured ankle joint; in our experience localised posterior pain is not usually detectable in the acute stage. They may be picked up incidentally at this stage on ankle radiographs. More often, the fracture presents with persistent posterior, posteromedial or posterolateral ankle pain, posterior impingement or mechanical symptoms. Pain on dorsiflexion or resisted plantarflexion of the hallux suggests FHL injury, compression or tendonopathy. Two of our patients had a tibial nerve injury at presentation.
Plain films often miss the fracture, but a lateral view in 30deg of external rotation showed all fractures in Ebraheim's cadaver series (2007). CT is useful to assess the fragment size but the FHL tendon is best assessed with ultrasound or MR. Arthroscopy allows assessment of the ankle and subtalar joints, and has been reported as useful in fracture reduction.
Acutely, small fragments can be managed non-operatively with as much early mobilisation as the stability of the ankle-hindfoot complex will stand. Larger fragments are reduced through a posterior approach and fixed, protecting the FHL and neurovascular bundle. Small fragments presenting late may be symptomatic enough to warrant excision. Large fragments presenting late often have significant subtalar joint surface damage requiring subtalar or even tibiotalocalcaneal fusion, although a few can be reduced and fixed.