Subtalar Dislocation: Causes, Injury Mechanisms, and Associated Risks

What Causes Subtalar Dislocation?

Subtalar dislocation is a rare but severe injury primarily caused by high-energy trauma, typically involving a sudden, forceful twisting motion of the foot that disrupts the strong ligamentous connections between the talus and calcaneus bones.

Understanding the Subtalar Joint

To grasp the causes of subtalar dislocation, it's essential to first understand the anatomy and biomechanics of the subtalar joint. This critical joint, also known as the talocalcaneal joint, is located beneath the ankle joint and is formed by the articulation of two primary bones:

• Talus: The uppermost bone, which connects the leg to the foot.
• Calcaneus: The heel bone, which sits directly beneath the talus.

These two bones are intricately connected and stabilized by a robust network of ligaments, including the interosseous talocalcaneal ligament, cervical ligament, and various subtalar ligaments. The subtalar joint is primarily responsible for the foot's inversion (turning the sole inward) and eversion (turning the sole outward) movements, which are crucial for adapting to uneven terrain and absorbing impact during gait. Its stability is paramount for proper foot function.

Primary Mechanisms of Injury

Subtalar dislocation is almost exclusively the result of a significant traumatic event, rather than a gradual process or minor sprain. The forces involved typically exceed the tensile strength of the stabilizing ligaments, leading to the displacement of the talus relative to the calcaneus.

• High-Energy Trauma: The most common cause, involving substantial force applied to the foot and ankle.

  • Motor Vehicle Accidents (MVAs): Often involve direct impact or severe twisting forces when the foot is braced against a pedal or floorboard.
  • Falls from Height: Landing awkwardly on a plantarflexed and inverted foot can generate immense compressive and rotational forces.
  • Sports Injuries: Activities involving jumping, rapid changes in direction, or direct contact, such as football, basketball, gymnastics, and skiing, can produce the necessary trauma. A common scenario is landing from a jump or roll with the foot aggressively pointed and turned inward.

• Specific Foot Positions and Applied Forces: The precise mechanism often dictates the type of dislocation.

  • Plantarflexion and Inversion (Most Common): This is the predominant mechanism for medial subtalar dislocations. When the foot is pointed downwards (plantarflexed) and turned inward (inverted), the talus is relatively locked within the ankle mortise, and the calcaneus is forcefully rotated medially or laterally beneath it. The heel bone is then driven medially or laterally relative to the talus, tearing the strong interosseous ligament and other stabilizing structures.
  • Dorsiflexion and Eversion (Less Common): This mechanism is typically associated with lateral subtalar dislocations. Here, the foot is pointed upwards (dorsiflexed) and turned outward (everted). While less frequent, similar high-energy forces can lead to disruption.
  • Axial Loading: Direct compression along the axis of the leg combined with rotational forces can also contribute.

Classifications of Subtalar Dislocation

Subtalar dislocations are classified based on the direction of the calcaneus and foot relative to the talus, which remains relatively fixed within the ankle mortise. This classification often reflects the specific causative mechanism:

• Medial Dislocation (Most Common, "Inward"): The calcaneus and the rest of the foot are displaced medially (inward) relative to the talus. This typically occurs with severe plantarflexion and inversion forces. The talonavicular joint is often disrupted, and the talar head may be palpable laterally.
• Lateral Dislocation (Less Common, "Outward"): The calcaneus and foot are displaced laterally (outward) relative to the talus. This usually results from significant dorsiflexion and eversion forces. The talar head may be palpable medially.
• Posterior Dislocation (Rare): The calcaneus and foot are displaced posteriorly relative to the talus.
• Anterior Dislocation (Extremely Rare): The calcaneus and foot are displaced anteriorly relative to the talus.

Contributing Factors and Risk Modifiers

While acute, high-energy trauma is the direct cause, certain factors can potentially increase an individual's susceptibility or influence the severity of the injury:

• Ligamentous Laxity: Individuals with naturally looser ligaments or a history of ankle sprains may have slightly reduced static stability, though the forces required for dislocation are typically so extreme that even robust ligaments can fail.
• Previous Injuries: Prior fractures around the ankle or subtalar joint, or chronic instability, could theoretically predispose the joint to dislocation under stress, though evidence directly linking these to subtalar dislocation is limited compared to acute trauma.
• Foot Morphology: While not a direct cause, extreme foot types (e.g., severe pes cavus or pes planus) might alter biomechanical stress distribution, potentially influencing injury patterns in the event of trauma. However, this is largely speculative in the context of subtalar dislocation.
• Inadequate Footwear/Bracing: While good footwear and support can mitigate less severe ankle injuries, the forces involved in subtalar dislocation are often so great that even well-supported ankles can succumb.

Associated Injuries

Due to the high-energy nature of subtalar dislocations, they are frequently accompanied by other significant injuries, which can complicate diagnosis and treatment:

• Fractures: Common associated fractures include those of the malleoli (ankle bones), talus (especially the talar neck or body), calcaneus, or navicular bone.
• Ligamentous Ruptures: Other major ankle and foot ligaments, such as the anterior talofibular ligament (ATFL), calcaneofibular ligament (CFL), posterior talofibular ligament (PTFL), or components of the deltoid ligament, may also be torn.
• Tendon Injuries: Damage to surrounding tendons, particularly the peroneal tendons with lateral dislocations or the tibialis posterior tendon with medial dislocations, can occur.
• Neurovascular Compromise: In severe cases, the displaced bones can compress or stretch nearby nerves and blood vessels, leading to neurological deficits or compromised blood supply to the foot, which is a medical emergency.

Why Prompt Medical Attention is Crucial

Subtalar dislocation is a severe orthopedic emergency. Prompt diagnosis and reduction (repositioning of the bones) are critical to minimize complications. Delayed treatment significantly increases the risk of:

• Avascular Necrosis (AVN) of the Talus: Disruption of the blood supply to the talus, potentially leading to bone death.
• Post-Traumatic Arthritis: Long-term joint degeneration due to cartilage damage and altered joint mechanics.
• Chronic Instability: Persistent weakness and recurrent sprains or dislocations.
• Nerve Damage: Permanent neurological deficits.

Understanding the causes and mechanisms of subtalar dislocation underscores the importance of proper protective measures in high-risk activities and immediate medical evaluation following significant foot and ankle trauma.