Ulnar Dislocation: Causes, Mechanisms, and Prevention

What causes ulnar dislocation?

Ulnar dislocations primarily result from traumatic forces that disrupt the stability of the joints involving the ulna, most commonly the distal radioulnar joint (DRUJ) at the wrist or, less frequently, the proximal radioulnar joint (PRUJ) or the ulnohumeral joint at the elbow.

Understanding Ulnar Dislocation

An ulnar dislocation refers to the abnormal displacement of the ulna bone from its normal articulation with an adjacent bone. The ulna is one of the two long bones of the forearm, extending from the elbow to the wrist, running parallel to the radius. While a complete dislocation of the entire ulna from both its proximal and distal articulations is exceedingly rare, the term typically refers to a dislocation at one of its primary joints: the distal radioulnar joint (DRUJ) at the wrist, the proximal radioulnar joint (PRUJ) near the elbow, or its articulation with the humerus in an elbow dislocation. Understanding the specific joint involved is crucial as the causes and mechanisms differ.

Anatomy of the Forearm and Wrist Pertinent to Dislocation

To grasp the causes of ulnar dislocation, it's essential to understand the key anatomical structures that provide stability:

• Distal Radioulnar Joint (DRUJ): Located at the wrist, this pivot joint allows pronation and supination of the forearm. Its stability is primarily provided by the triangular fibrocartilage complex (TFCC), a complex of ligaments and cartilage that binds the distal ulna and radius, and the surrounding joint capsule and muscles.
• Proximal Radioulnar Joint (PRUJ): Located at the elbow, this joint allows the radial head to rotate against the ulna. It is stabilized by the annular ligament, which encircles the radial head and binds it to the ulna, along with the joint capsule.
• Ulnohumeral Joint: This is the primary articulation of the elbow, where the trochlear notch of the ulna articulates with the trochlea of the humerus. Stability is provided by the congruent bony surfaces, the ulnar collateral ligament (UCL), and the joint capsule.

Disruption of these stabilizing structures, whether bony or ligamentous, is the direct cause of dislocation.

Primary Mechanisms of Ulnar Dislocation

The vast majority of ulnar dislocations are traumatic in nature, resulting from significant external forces.

• High-Energy Trauma:

  • Falls onto an Outstretched Hand (FOOSH): This is the most common mechanism, particularly for DRUJ dislocations. The impact force travels up the arm, and depending on the position of the forearm (pronation or supination) at impact, can drive the distal ulna dorsally or volarly relative to the radius.
  • Direct Blows: A direct impact to the forearm or wrist can force the ulna out of alignment.
  • Sports Injuries: Activities involving falls, collisions, or high-velocity movements (e.g., gymnastics, football, wrestling, skiing) frequently lead to the forces necessary for dislocation.
  • Motor Vehicle Accidents (MVAs): High-impact collisions can generate extreme forces capable of causing complex forearm and elbow dislocations, often accompanied by fractures.

• Ligamentous Laxity or Instability: While less common as a sole cause, pre-existing laxity in the supporting ligaments (e.g., TFCC for the DRUJ, or the annular ligament for the PRUJ) can predispose an individual to dislocation with less force, or lead to recurrent instability. This can be congenital or acquired.

• Anatomical Anomalies: In rare cases, congenital deformities or developmental abnormalities of the bones or joints can lead to inherent instability and an increased risk of dislocation.

• Degenerative Conditions: While true dislocations are rare, severe degenerative arthritis or chronic inflammatory conditions (e.g., rheumatoid arthritis) can weaken joint structures and lead to chronic subluxation (partial dislocation) or instability, particularly at the DRUJ.

Specific Types and Causes of Ulnar Dislocation

The specific joint involved dictates the precise mechanisms:

• Distal Radioulnar Joint (DRUJ) Dislocation:

  • Dorsal Dislocation (more common): Typically occurs from a FOOSH injury with the forearm in pronation. The force drives the distal ulna posteriorly (dorsally) relative to the radius.
  • Volar Dislocation: Usually results from a FOOSH injury with the forearm in supination, driving the distal ulna anteriorly (volarly) relative to the radius.
  • Often associated with Essex-Lopresti fracture-dislocation (radial head fracture, DRUJ dislocation, and interosseous membrane disruption) or Galeazzi fracture-dislocation (distal radial shaft fracture with DRUJ dislocation).

• Proximal Radioulnar Joint (PRUJ) Dislocation:

  • Monteggia Fracture-Dislocation: This is the most common cause, involving a fracture of the ulna shaft with concomitant dislocation of the radial head from the PRUJ. It typically results from a direct blow to the forearm or a FOOSH injury.
  • "Nursemaid's Elbow" (Radial Head Subluxation): While technically a subluxation of the radial head from the annular ligament, it's a common pediatric injury where the ulna's articulation is indirectly affected. It occurs from a sudden pull on the child's extended arm (e.g., lifting a child by one hand).

• Elbow Dislocation (Ulnohumeral Dislocation):

  • This involves the displacement of the ulna (and radius) from its articulation with the humerus.
  • Most commonly results from a FOOSH injury with the elbow hyperextended, or a direct blow to the elbow.
  • Often involves significant ligamentous damage (UCL, lateral collateral ligament complex).

Risk Factors for Ulnar Dislocation

Certain factors can increase an individual's susceptibility to ulnar dislocations:

• Participation in High-Impact Sports: Activities like gymnastics, wrestling, football, skiing, and skateboarding carry a higher risk due to the nature of falls and collisions.
• Occupational Hazards: Jobs involving manual labor, working at heights, or repetitive forceful movements can increase risk.
• Pre-existing Joint Laxity: Individuals with generalized ligamentous laxity or conditions like Ehlers-Danlos syndrome may be more prone.
• Previous Injuries: A history of sprains, fractures, or dislocations in the same joint can weaken supporting structures, making recurrent dislocations more likely.
• Age: Children are susceptible to "nursemaid's elbow," while older adults may be at higher risk for falls leading to wrist or elbow dislocations due to decreased balance and bone density.

Clinical Presentation and Diagnosis

An ulnar dislocation typically presents with acute pain, swelling, deformity, and significant loss of function in the affected joint. There may be an inability to pronate or supinate the forearm, or to flex/extend the elbow fully. Diagnosis is confirmed with physical examination and imaging studies, primarily X-rays, which clearly show the displacement of the ulna relative to its articulating bones. In some cases, CT or MRI may be used to assess associated soft tissue injuries (e.g., TFCC tears, ligament ruptures).

Prevention Strategies

While not all dislocations are preventable, several strategies can reduce the risk:

• Proper Technique in Sports and Activities: Learning and employing correct body mechanics during sports can minimize awkward falls and impacts.
• Strength and Conditioning: Developing strong muscles around the joints (forearm, wrist, elbow) enhances dynamic stability.
• Balance Training: Especially for older adults, improving balance can reduce the risk of falls.
• Protective Gear: Using appropriate wrist guards or elbow pads in high-risk sports can absorb impact forces.
• Awareness of Surroundings: Avoiding slippery surfaces and hazards that could lead to falls.
• Addressing Pre-existing Instability: For individuals with known ligamentous laxity or previous injuries, specific rehabilitation exercises or bracing may be recommended.

Conclusion

Ulnar dislocations are significant injuries almost universally caused by traumatic events that overwhelm the stability of the forearm and elbow joints. Understanding the specific joint involved (distal radioulnar, proximal radioulnar, or ulnohumeral) is key, as each has distinct mechanisms and common associated injuries. While prevention focuses on minimizing traumatic forces and enhancing joint stability, any suspected ulnar dislocation requires immediate medical attention for proper diagnosis and management to prevent long-term complications and restore optimal function.