Wrist Dislocation: Mechanism of Injury, Anatomy, and Cascade

What is the mechanism of injury for a wrist dislocation?

A wrist dislocation occurs when the bones of the wrist are forced out of their normal alignment, most commonly due to high-energy trauma involving a fall onto an outstretched hand (FOOSH), leading to a sequential failure of the strong wrist ligaments.

Understanding Wrist Anatomy and Stability

To comprehend the mechanism of a wrist dislocation, it's essential to first understand the complex anatomy of the wrist joint. The wrist is not a single joint but a highly intricate complex of eight small carpal bones arranged in two rows (proximal and distal) that articulate with the forearm bones (radius and ulna) and the metacarpal bones of the hand.

• Radiocarpal Joint: The primary articulation between the distal radius and the proximal carpal row (scaphoid, lunate, triquetrum). The ulna is primarily involved in forearm rotation and articulates with the carpals via the triangular fibrocartilage complex (TFCC).
• Intercarpal Joints: Numerous small joints between the carpal bones themselves.
• Ligamentous Support: The stability of the wrist relies heavily on a complex network of strong intrinsic (connecting carpal bones) and extrinsic (connecting forearm to carpals) ligaments. Key ligaments include:
  • Scapholunate Ligament: Connects the scaphoid and lunate, crucial for proximal carpal row stability.
  • Lunotriquetral Ligament: Connects the lunate and triquetrum.
  • Radiocarpal Ligaments: Volar (palmar) and dorsal ligaments that tether the carpal bones to the radius. These are critical in limiting wrist motion and absorbing forces.

General Principles of Dislocation

A dislocation is defined as a complete separation of the articular surfaces of a joint. Unlike a subluxation, which is a partial displacement, a dislocation implies a full loss of congruity between the bones. In the wrist, this typically involves the carpal bones being forced out of their normal relationship with each other or with the radius.

Common Mechanisms of Injury

The vast majority of wrist dislocations result from high-energy trauma, with specific force vectors dictating the type and severity of the injury.

Fall On Outstretched Hand (FOOSH)

The Fall On Outstretched Hand (FOOSH) is by far the most common mechanism leading to wrist dislocations. This scenario typically involves:

• Axial Load: The primary force is an axial compression transmitted up the arm through the hand and wrist.
• Hyperextension (Dorsiflexion): The wrist is forced into extreme dorsiflexion, often combined with ulnar deviation. This position places significant stress on the volar (palmar) ligaments.
• Forearm Pronation: The forearm is often pronated at the moment of impact, adding a torsional component to the forces.

The Perilunate Dislocation Cascade

The FOOSH mechanism frequently leads to a perilunate dislocation, which is the most common type of carpal dislocation. This injury often follows a predictable pattern of ligamentous disruption, known as the "perilunate dislocation cascade" or "stage-wise carpal instability":

1. Stage I (Scapholunate Disassociation): Initial force causes tearing of the crucial scapholunate ligament. This disrupts the stable linkage between the scaphoid and lunate, leading to rotational instability of the scaphoid.
2. Stage II (Capitate Dislocation): With continued force, the injury propagates dorsally and radially. The capitate bone, the central keystone of the distal carpal row, is forced to dislocate dorsally relative to the lunate. At this stage, the lunate maintains its articulation with the radius, but the rest of the carpal bones (and the hand) are dislocated dorsally around it. This is the classic perilunate dislocation.
3. Stage III (Triquetral Disruption): Further force can lead to tearing of the lunotriquetral ligament and other intercarpal ligaments on the ulnar side, adding to overall carpal instability.
4. Stage IV (Lunate Dislocation): In the most severe cases, often with persistent or recurrent force, the lunate itself can then dislocate volarly (anteriorly) out of its articulation with the radius, rotating out of the carpal tunnel. This is a lunate dislocation, which is often a progression from an unreduced perilunate dislocation.

Other Mechanisms

While less common for pure dislocations, other mechanisms can contribute:

• Hyperflexion: Extreme palmar flexion can cause dorsal dislocations, though these are rarer and often associated with fractures.
• Direct Impact: A direct blow to the wrist, especially from a high-energy source, can cause a dislocation, often combined with fractures.
• Rotational Forces: Severe twisting injuries can disrupt ligamentous integrity and lead to complex carpal dislocations or fracture-dislocations.
• Chronic Instability: Less common for acute dislocations, but pre-existing ligamentous laxity or chronic instability (e.g., from repetitive stress or prior untreated injuries) can lower the threshold for dislocation with less significant trauma.

Forces Involved

The magnitude and direction of forces are critical to the mechanism of a wrist dislocation:

• Compressive Forces: Axial loading through the long axis of the forearm and hand.
• Shear Forces: Forces acting parallel to the articular surfaces, causing bones to slide past each other.
• Torsional Forces: Twisting forces, especially in combination with hyperextension or hyperflexion.
• Distraction Forces: While not the primary cause of pure dislocations, distraction can contribute to ligamentous tearing.

It's the combination of these forces, particularly high-energy compressive and shear forces applied to an hyperextended wrist, that overwhelms the strength of the intrinsic and extrinsic wrist ligaments, leading to their sequential failure and subsequent displacement of the carpal bones.

Clinical Implications

Understanding the mechanism of injury is paramount for healthcare professionals:

• Diagnosis: Knowing the typical mechanisms helps clinicians anticipate potential injuries, even when initial X-rays might appear subtle. Oblique views and advanced imaging (CT, MRI) are often necessary to fully appreciate the extent of ligamentous damage and bone displacement.
• Treatment: The type of dislocation and the extent of ligamentous injury dictate the treatment approach, which can range from closed reduction and casting to complex open surgical repair and reconstruction.
• Prevention: For athletes and individuals in high-risk occupations, understanding these mechanisms can inform preventative strategies, such as proper falling techniques, use of protective gear (e.g., wrist guards in snowboarding), and strengthening exercises to improve wrist stability and proprioception.

In conclusion, a wrist dislocation is a severe injury primarily caused by high-energy trauma, most notably a fall onto an outstretched, hyperextended hand. This mechanism initiates a progressive cascade of ligamentous failures, culminating in the displacement of carpal bones and significant wrist instability.