Radial Head Dislocation: Mechanisms, Types, and Risk Factors

How does radial head dislocation occur?

Radial head dislocation occurs when the radial head, the top part of the radius bone, displaces from its normal articulation with the capitellum of the humerus and the ulna, most commonly due to traumatic forces like a fall on an outstretched hand, direct impact, or a strong pulling force on the forearm, particularly in children.

Understanding the Elbow Joint Anatomy

To comprehend how a radial head dislocation occurs, it's essential to first understand the intricate anatomy of the elbow joint. The elbow is a complex hinge joint designed for flexion, extension, pronation, and supination, formed by the articulation of three bones:

• Humerus: The upper arm bone.
• Ulna: The larger forearm bone on the pinky finger side.
• Radius: The smaller forearm bone on the thumb side.

Within the elbow, three distinct articulations work in concert:

• Humeroulnar Joint: The primary hinge joint between the trochlea of the humerus and the trochlear notch of the ulna, responsible for flexion and extension.
• Humeroradial Joint: The articulation between the capitellum of the humerus and the concave radial head, facilitating elbow flexion/extension and forearm rotation.
• Proximal Radioulnar Joint: The articulation between the radial head and the radial notch of the ulna, crucial for forearm pronation and supination.

The stability of the radial head within the proximal radioulnar joint is primarily maintained by the annular ligament. This strong, fibrous band encircles the radial head, holding it firmly against the ulna. Additional stability is provided by the medial (ulnar) and lateral (radial) collateral ligaments, which prevent excessive side-to-side motion of the elbow.

Mechanisms of Radial Head Dislocation

Radial head dislocations are typically the result of significant force, leading to the disruption of the ligaments and joint capsule. The mechanisms vary depending on whether the dislocation is isolated or part of a more complex injury.

Isolated Radial Head Dislocation

While less common than complex elbow dislocations, isolated radial head dislocations primarily occur through two main mechanisms:

• Fall On an Outstretched Hand (FOOSH) with Valgus Stress: This is the most frequent mechanism in adults. When an individual falls and attempts to break their fall with an outstretched arm, the impact transmits an axial load up the forearm. If the force also includes a valgus (outward) stress on the elbow, it can drive the radial head anteriorly, posteriorly, or laterally, tearing the annular ligament and potentially the collateral ligaments.
• Direct Trauma: A direct impact to the radial head area, though less common, can forcefully displace the bone.

Traction/Pulling Injuries (Nursemaid's Elbow)

This is a very common type of radial head subluxation (partial dislocation) seen almost exclusively in young children (typically under 5 years old) due to their developing ligaments and bone structure.

• Mechanism: It occurs when a sudden, strong pulling force is applied to the child's forearm, such as pulling a child up by the hand, swinging them by the arms, or catching them by the wrist to prevent a fall. This traction causes the radial head to slip out from under the annular ligament. The ligament can then become entrapped between the radial head and the capitellum, preventing spontaneous reduction.

Radial Head Dislocation as Part of a Complex Elbow Injury

More frequently, radial head dislocation is not an isolated event but occurs in conjunction with other injuries to the elbow, indicating a higher energy trauma.

• Monteggia Fracture-Dislocation: This classic injury involves a fracture of the ulna (usually in the proximal or middle third) combined with a dislocation of the radial head. The ulna fracture destabilizes the forearm, allowing the radial head to dislocate. The mechanism is often a direct blow to the ulna or a FOOSH with a pronated forearm.
• Complex Elbow Dislocation: In a full elbow dislocation, the humeroulnar and humeroradial joints are disrupted. The radial head often dislocates along with the ulna, typically posteriorly or posterolaterally, due to a FOOSH injury with an axial load and supination/valgus forces.
• "Terrible Triad" Injury: This severe injury involves an elbow dislocation, a fracture of the radial head, and a fracture of the coronoid process of the ulna. It results from high-energy trauma, usually a FOOSH, leading to significant instability and often requiring surgical intervention.

Risk Factors for Radial Head Dislocation

Several factors can increase the likelihood of experiencing a radial head dislocation:

• Age: Young children are highly susceptible to Nursemaid's elbow due to the laxity of their annular ligament. Adults involved in high-impact sports or activities with a risk of falls are at higher risk for traumatic dislocations.
• Sports and Activities: Gymnastics, wrestling, contact sports, and activities with a high fall risk (e.g., cycling, skiing) increase exposure to the traumatic mechanisms.
• Pre-existing Ligamentous Laxity: While rare as a primary cause, individuals with generalized joint hypermobility or specific ligamentous laxity around the elbow may be more prone to dislocations.
• Congenital Anomalies: Certain genetic conditions or developmental abnormalities can lead to a congenitally dislocated radial head, though this is distinct from traumatic dislocation.

Symptoms and Diagnosis

Symptoms of a radial head dislocation typically include immediate, severe pain, swelling around the elbow, an inability or significant difficulty to move the elbow or forearm, and often a visible deformity. In children with Nursemaid's elbow, the child will typically hold their arm still, often pronated, and refuse to use it.

Diagnosis is confirmed through physical examination and imaging studies. X-rays are crucial to identify the dislocation and rule out associated fractures. In complex cases, a CT scan or MRI may be used to assess soft tissue damage and subtle bone injuries.

Importance of Prompt Medical Attention

Any suspected radial head dislocation requires immediate medical attention. Prompt reduction (repositioning of the bone) is critical to minimize pain, prevent long-term complications such as stiffness, nerve damage, or chronic instability, and facilitate a full recovery. Self-reduction should never be attempted.