Elbow Dislocation: Mechanisms, Anatomy, and Associated Injuries

What is the mechanism of injury in the elbow dislocation?

Elbow dislocation typically results from a high-energy trauma, most commonly a fall on an outstretched hand (FOOSH), leading to the forceful displacement of the ulna and radius from their articulation with the humerus, often in a posterolateral direction, disrupting the joint capsule and stabilizing ligaments.

Understanding the Elbow Joint Anatomy

To fully grasp the mechanism of elbow dislocation, it's essential to first understand the intricate anatomy that provides its stability and range of motion. The elbow is a complex hinge joint formed by the articulation of three bones: the humerus (upper arm bone), and the ulna and radius (forearm bones).

• Bony Articulations:
  • Humeroulnar Joint: The trochlea of the humerus articulates with the trochlear notch of the ulna, forming the primary hinge for flexion and extension.
  • Humeroradial Joint: The capitellum of the humerus articulates with the radial head, contributing to flexion/extension and forearm rotation.
  • Proximal Radioulnar Joint: The radial head pivots within the annular ligament and the radial notch of the ulna, facilitating pronation and supination of the forearm.
• Ligamentous Stability: The primary static stabilizers of the elbow are its collateral ligaments and joint capsule.
  • Medial Collateral Ligament (MCL) Complex / Ulnar Collateral Ligament (UCL): Located on the inner side of the elbow, this complex provides critical valgus stability (resisting outward bending).
  • Lateral Collateral Ligament (LCL) Complex: On the outer side, this complex includes the Radial Collateral Ligament, Annular Ligament, and the Lateral Ulnar Collateral Ligament (LUCL). The LUCL is paramount in resisting posterolateral rotatory instability.
• Joint Capsule: A fibrous capsule encloses the joint, providing additional stability.
• Dynamic Stabilizers: Muscles crossing the joint, such as the biceps, triceps, and brachialis, provide dynamic stability, especially during movement.

The Primary Mechanism: Fall On Outstretched Hand (FOOSH)

The vast majority of elbow dislocations occur due to a Fall On Outstretched Hand (FOOSH). This high-energy mechanism involves a specific sequence of forces and movements that overwhelm the elbow's static and dynamic stabilizers:

• Axial Compression: Upon impact, a compressive force is transmitted up the forearm through the wrist and elbow.
• Elbow Position: The elbow is typically in slight flexion (often 20-45 degrees), which is its position of least bony congruence and greatest instability.
• Forearm Supination/Pronation: The position of the forearm significantly influences the direction of dislocation.
• Valgus or Varus Stress:
  • Valgus Stress (most common): An outward bending force is applied to the elbow, pushing the forearm away from the body's midline. This force often combines with axial compression and external rotation.
  • Varus Stress (less common): An inward bending force, pushing the forearm towards the body's midline.
• External Rotation of the Forearm: As the body continues to fall and rotate over the fixed hand, an external rotation force is applied to the forearm relative to the humerus.

This combination of axial compression, valgus stress, and external rotation is the classic pathway for posterolateral rotatory instability (PLRI), which is the most common type of elbow dislocation. The sequence of tissue failure in PLRI typically starts with the Lateral Ulnar Collateral Ligament (LUCL). Once the LUCL is torn, the elbow joint rotates externally, allowing the ulna and radius to pivot posterolaterally around the humerus, leading to complete dislocation.

Other Mechanisms of Injury

While FOOSH is predominant, other mechanisms can also lead to elbow dislocation:

• Direct Trauma: A direct blow to the elbow joint, such as from a sports impact or motor vehicle accident, can force the bones out of alignment.
• Hyperextension: Although less common as a sole mechanism, forceful hyperextension (straightening beyond the normal range) can cause the olecranon (the bony tip of the ulna) to lever against the humerus, leading to dislocation, often combined with other rotational or valgus forces.
• Traction/Distraction: A strong pulling force on the forearm, such as in "nursemaid's elbow" (radial head subluxation in children), can, in severe cases or with additional forces, lead to complete dislocation. This is less common in adults for complete dislocation.
• High-Energy Sports Injuries: Activities involving falls from height, wrestling, gymnastics, or contact sports can generate sufficient force to dislocate the elbow through various combinations of the above mechanisms.

Common Associated Injuries

Due to the significant forces involved in an elbow dislocation, it's common for other structures to be damaged concurrently:

• Ligamentous Tears:
  • Lateral Ulnar Collateral Ligament (LUCL): Almost always torn in posterolateral dislocations.
  • Medial Collateral Ligament (MCL): Often involved in more severe dislocations or those with a significant valgus component.
• Fractures:
  • Radial Head Fractures: Common, especially with compressive forces during FOOSH.
  • Coronoid Process Fractures: Occur when the coronoid process of the ulna impacts the humerus during dislocation, indicating a more severe injury.
  • Medial Epicondyle Fractures: Can occur, particularly in children, due to avulsion by the MCL.
  • Olecranon Fractures: Less common but can occur with hyperextension or direct impact.
• Neurovascular Injury: While rare, damage to the brachial artery or median, ulnar, or radial nerves can occur and represents a medical emergency.

Factors Influencing Dislocation Risk

Several factors can influence an individual's susceptibility to elbow dislocation:

• Joint Laxity: Individuals with generalized ligamentous laxity or hypermobility may be more prone to dislocation.
• Prior Dislocation: A history of elbow dislocation significantly increases the risk of recurrence due to residual ligamentous laxity and capsular damage.
• Muscle Weakness/Imbalance: Compromised dynamic stability from surrounding musculature can predispose the joint to injury.
• Sport-Specific Risks: Participation in sports with a high risk of falls, direct impact, or repetitive stress (e.g., gymnastics, wrestling, football, rock climbing).

Conclusion

Elbow dislocation is a significant, high-energy injury primarily driven by a complex interplay of axial compression, valgus/varus stress, and rotational forces, most often from a fall on an outstretched hand. The characteristic posterolateral dislocation involves a progressive failure of the lateral ligamentous complex, particularly the Lateral Ulnar Collateral Ligament (LUCL). Understanding these precise mechanisms is crucial for proper diagnosis, effective treatment, and targeted rehabilitation to restore joint stability and function, minimizing the risk of recurrence and long-term complications.