Traumatic medial subtalar dislocation

A 45-year-old female patient presented to the emergency room following a fall from a height of +/- 2m landing on the left foot.

She complains of sharp pain, deviation of the axis of the foot with swelling and severe limitation of movement.

She does not present sensory deficit and her foot pulse is present, the skin recolouration time is normal (of the order of 2 sec).

Based on clinical presentation, she was suspected of having a fracture or dislocation.

She was referred to us in radiology for exploration.

An x-ray followed by a CT scan was performed for lesion characterization.

An x-ray of the left foot (figures 1 and 2) showed a fracture of the external side of the navicular bone and internal dislocation of the talonavicular joint, probably first degree (talo-navicular dislocation with posterior talo-calcaneal subluxation) with opening of the tarsal sinus.

Given the presence of an intra-articular fracture and a dislocation on the x-ray, a CT scan was performed for a better appreciation of the anatomical relationships, as well as to search for a possible occult fracture.

CT scan confirmed the internal subtalar dislocation, a multifragmentary fracture of the lateral side of the navicular bone (blue arrow), a small sunken fracture of the medial part of the head of the talus (which is in contact with the navicular bone) (red arrow) and an undisplaced fracture of the calcaneal rostrum (orange star) (Figures 3, 4 and 5) with associated swelling and densification of the surrounding soft tissues.

Subtalar dislocation is a dislocation of the hindfoot resulting from high energy trauma.
Its diagnosis is made clinically and confirmed by x-rays of the foot.

Epidemiologically speaking, this traumatic entity is extremely rare, it represents 1% of all dislocations and its incidence is of the order of <1 per 100,000 per year. [1,2]

The rarity of this condition is due to the presence of a very strong interosseous ligament connecting the talus and the calcaneus, the strong biomechanical properties of the ankle and a tight joint capsule. [3,4]

This dislocation is classified by Broca into 3 entities: medial, lateral and posterior. Malaigne Burger described a 4^th variety: anterior.

In order of frequency we find:

Medial or internal (65-85%) also known under the name of “acquired clubfoot” results from a forced inversion of the forefoot which exerts a stress on the lateral collateral ligament when the foot is in plantar flexion.

The lateral or external variant (15-35%) also known under the name of “acquired flat foot”.

Finally, the posterior and anterior variants represent nearly 1%. [5]

Internal subtalar dislocations can be divided into 2 degrees: [6,7]

• 1st degree dislocation: rotation of the talo-navicular joint, anterior talo-calcaneal dislocation (since this is an integral part of the chopart line), and posterior talo-calcaneal subluxation.

The talus still remains attached to the calcaneus by the postero-internal part of the talo-calcaneal ligament; and there is integrity of the fibulocalcaneal ligament at this stage.
This degree corresponds to what Baumgartner and Huguir call "torsional dislocation" or "oblique dislocation" of Malgaine and Quenu. This stage is reflected radiologically by the opening of the tarsal sinus.

• 2nd degree dislocation: total dislocation of the anterior and posterior subtalar joints, and is accompanied by significant ligament damage. An internal talonavicular dislocation occurs with rupture of the dorsal talonavicular ligament, the head of the talus then tears the frondiform ligament (splitting of the anterior annular ligament of the tarsus). If pressure continues, the calcaneofibular bundle of the external lateral ligament ends up rupturing.

Treatment is based on clinical reduction by the boot-pulling manoeuvre under general anaesthesia, but given the difficulty of this, due to a complex anatomy and the presence of several obstacles, surgery is very common for these patients. [2] [5]

Complications [5]

Osteoarthritis

Avascular necrosis of the talus

Instability of the subtalar joint