Subtalar Dislocation: Mechanisms, Types, and Management

What is the Mechanism of Subtalar Dislocation?

Subtalar dislocation is a rare, high-energy injury characterized by the complete displacement of the talocalcaneal joint, where the talus remains in the ankle mortise while the rest of the foot (calcaneus, navicular, cuboid, and forefoot) dislocates relative to it, typically preserving the tibiotalar articulation.

Understanding the Subtalar Joint

To comprehend the mechanism of subtalar dislocation, a foundational understanding of the subtalar joint's anatomy is essential. The subtalar joint, also known as the talocalcaneal joint, is formed by the articulation between the inferior surface of the talus and the superior surface of the calcaneus. This joint is crucial for the complex movements of the foot, primarily inversion (sole of the foot turns inward) and eversion (sole of the foot turns outward), which are vital for adapting the foot to uneven terrain.

Key anatomical features contributing to its stability and vulnerability include:

• Articular Surfaces: Three distinct facets on the talus articulate with corresponding facets on the calcaneus: anterior, middle, and posterior.
• Ligamentous Support:
  • Interosseous Talocalcaneal Ligament: Located within the tarsal sinus, this is the strongest ligament stabilizing the subtalar joint. Its rupture is a prerequisite for complete dislocation.
  • Cervical Ligament: Also within the tarsal sinus, anterior to the interosseous ligament.
  • Medial Talocalcaneal Ligament, Lateral Talocalcaneal Ligament, Posterior Talocalcaneal Ligament: Provide additional capsular reinforcement.
  • Calcaneofibular Ligament and Deltoid Ligament: While primarily ankle joint ligaments, their integrity and tension can influence the direction and stability of subtalar dislocations.

Classification of Subtalar Dislocation

Subtalar dislocations are classified based on the direction of the calcaneus and the rest of the foot relative to the talus, which remains fixed within the ankle mortise. This classification directly relates to the specific mechanism of injury:

• Medial Dislocation (Most Common): The calcaneus and foot displace medially and often superiorly relative to the talus. Also known as "acquired clubfoot."
• Lateral Dislocation: The calcaneus and foot displace laterally relative to the talus. Also known as "acquired flatfoot."
• Posterior Dislocation: The calcaneus and foot displace posteriorly relative to the talus.
• Anterior Dislocation: The calcaneus and foot displace anteriorly relative to the talus.
• Rotational Dislocation: A combination of the above, often with an axial rotation component.

Common Mechanisms of Injury

The mechanism of subtalar dislocation invariably involves high-energy trauma and a specific combination of axial loading and rotational forces applied to the foot in a particular position. The talus, being tightly secured within the ankle mortise by the tibia and fibula, acts as a fulcrum, while the rest of the foot displaces around it.

Medial Subtalar Dislocation Mechanism

This is the most prevalent type, accounting for approximately 80% of cases. The primary mechanism involves:

• Foot Position: The foot is typically in a position of strong plantarflexion and inversion.
• Force Application: An axial load, often from a fall from a height or a direct impact (e.g., motor vehicle accident), is applied to the foot.
• Resultant Displacement: The combined forces cause the calcaneus to rotate and displace medially and superiorly relative to the talus. The talar head and neck are often "buttonholed" through the extensor retinaculum or joint capsule.
• Ligamentous Disruption: This mechanism almost always results in the rupture of the interosseous talocalcaneal ligament and the cervical ligament, crucial for subtalar stability. The lateral ankle ligaments (anterior talofibular, calcaneofibular) may also be injured or avulsed. The strong medial deltoid ligament usually remains intact, contributing to the medial displacement.

Lateral Subtalar Dislocation Mechanism

Less common than medial dislocations, the mechanism for lateral dislocation involves:

• Foot Position: The foot is typically in a position of dorsiflexion and eversion.
• Force Application: A high-energy axial load or an abduction force is applied to the foot.
• Resultant Displacement: The calcaneus and foot displace laterally and often inferiorly relative to the talus.
• Ligamentous Disruption: This mechanism often involves the rupture of the medial deltoid ligament and the interosseous talocalcaneal ligament. The lateral ankle ligaments may remain relatively intact, acting as a hinge.

Posterior and Anterior Subtalar Dislocation Mechanisms

These are exceedingly rare and typically occur with extreme forces:

• Posterior Dislocation: Results from a severe plantarflexion force, driving the calcaneus posteriorly. Often associated with significant fractures.
• Anterior Dislocation: Results from a severe dorsiflexion force, driving the calcaneus anteriorly. Also frequently associated with fractures.

Contributing Factors

Several factors can influence the likelihood and type of subtalar dislocation:

• High-Energy Trauma: Almost all subtalar dislocations are the result of significant force, such as falls from height, motor vehicle collisions, or sports injuries.
• Foot Position at Impact: The exact position of the foot (plantarflexion/dorsiflexion, inversion/eversion) at the moment of impact dictates the direction of dislocation.
• Ligamentous Integrity: Pre-existing ligamentous laxity, though rare, could theoretically predispose to dislocation with less force. However, in most cases, healthy ligaments are forcefully ruptured.
• Associated Fractures: While not a mechanism, associated fractures of the talus, calcaneus, malleoli, or navicular are common and can complicate the injury, sometimes indicating a different or combined injury pattern.

Clinical Presentation and Diagnosis

Clinically, a subtalar dislocation presents as a severely deformed foot, often with significant swelling and pain. The ankle joint itself may appear normal, but the midfoot and hindfoot are obviously displaced. Diagnosis is primarily made through plain radiographs (X-rays) of the foot and ankle in multiple views (AP, lateral, oblique), which clearly show the talus remaining in the ankle mortise while the calcaneus is displaced. CT scans are often used to identify associated fractures and better define the extent of the injury.

Treatment Principles

Immediate closed reduction (manual repositioning) under sedation or anesthesia is the primary treatment, followed by immobilization in a cast. Surgical intervention may be required for irreducible dislocations, open dislocations, or in cases with significant associated fractures or instability after reduction.

Prevention and Prognosis

Given the high-energy nature of these injuries, direct prevention is difficult. However, appropriate safety measures in high-risk activities (e.g., proper footwear, safe landing techniques in sports, seatbelt use) can reduce the overall risk of such traumatic events. Prognosis varies, but complications such as subtalar arthritis, chronic pain, stiffness, and avascular necrosis of the talus are not uncommon due to the severe soft tissue and cartilaginous damage.

Conclusion

Subtalar dislocation is a severe and relatively uncommon orthopedic injury resulting from specific high-energy trauma mechanisms. Understanding the intricate anatomy of the subtalar joint and the precise forces involved – predominantly axial loading combined with extreme inversion/plantarflexion for medial dislocations, or eversion/dorsiflexion for lateral dislocations – is critical for accurate diagnosis and effective management. While rare, its potential for long-term functional impairment underscores the importance of prompt recognition and appropriate intervention.