Carpal Dislocations: Understanding Perilunate, Lunate, and Other Types

What are the Different Types of Carpal Dislocations?

Carpal dislocations represent severe injuries where one or more of the eight carpal bones in the wrist are displaced from their normal anatomical alignment, often resulting from high-energy trauma and leading to significant instability and potential long-term complications.

Understanding the Carpal Bones: A Foundation

The wrist is a complex joint, not just a simple hinge. It comprises eight small, irregularly shaped carpal bones arranged in two rows:

• Proximal Row (from radial to ulnar): Scaphoid, Lunate, Triquetrum, Pisiform (anterior to triquetrum). These bones articulate with the radius and ulna.
• Distal Row (from radial to ulnar): Trapezium, Trapezoid, Capitate, Hamate. These bones articulate with the metacarpals of the hand.

These bones are intricately connected by numerous strong ligaments, forming a kinematic chain that allows for a wide range of motion while maintaining stability. A disruption of these ligamentous connections or the bony articulations constitutes a dislocation.

What is a Carpal Dislocation?

A carpal dislocation occurs when the normal articulation between two or more carpal bones, or between the carpal bones and the radius/ulna or metacarpals, is completely lost. This differs from a subluxation, where the articulation is merely partially disrupted. Carpal dislocations are often high-energy injuries, typically resulting from falls or direct trauma, and are critical to diagnose promptly due to their potential for long-term pain, instability, reduced range of motion, and early-onset arthritis if left untreated.

Primary Types of Carpal Dislocations

While any carpal bone can theoretically dislocate, certain patterns are far more common and clinically significant due to the biomechanics of wrist injury.

Perilunate Dislocation

This is the most common and archetypal carpal dislocation pattern, often considered the "great carpal disruptor."

• Description: In a perilunate dislocation, the capitate (the largest carpal bone) displaces dorsally (towards the back of the hand) relative to the lunate, which remains in its normal articulation with the radius. Essentially, the entire hand and distal carpal row dislocate dorsally around the lunate.
• Mechanism: Typically results from a Fall On an Outstretched Hand (FOOSH) with the wrist in hyperextension, ulnar deviation, and supination. The force drives the capitate dorsally, tearing the strong ligaments that normally tether the carpal bones.
• Clinical Significance: Often associated with significant ligamentous disruption, particularly the scapholunate ligament. It is part of a spectrum of carpal instability.

Lunate Dislocation

A lunate dislocation often represents a progression of an untreated or more severe perilunate injury.

• Description: If the force continues after a perilunate dislocation, the lunate itself can then dislocate. It typically rotates and displaces volarly (towards the palm) into the carpal tunnel, losing its articulation with both the radius and the capitate. The capitate then re-aligns with the radius, but the lunate is now free-floating and displaced.
• Mechanism: Same as perilunate, but with greater force or continued loading.
• Clinical Significance: Due to its volar displacement, the lunate commonly compresses the median nerve within the carpal tunnel, leading to acute carpal tunnel syndrome symptoms (numbness, tingling, weakness in the thumb, index, middle, and radial half of the ring finger). This is a medical emergency requiring urgent reduction.

Trans-scaphoid Perilunate Dislocation

This is a very common variant of a perilunate dislocation.

• Description: Instead of the ligaments between the scaphoid and capitate tearing, the force causes a fracture through the waist of the scaphoid bone. The distal fragment of the scaphoid dislocates dorsally with the capitate and distal carpal row, while the proximal scaphoid fragment remains articulated with the lunate and radius.
• Mechanism: Similar high-energy FOOSH injury.
• Clinical Significance: Combines the challenges of a carpal dislocation with a scaphoid fracture, which is notorious for slow healing and risk of non-union or avascular necrosis due to its precarious blood supply.

Scapholunate Dissociation (and SLAC Wrist)

While technically a ligamentous injury leading to instability rather than a true dislocation of a bone from its primary articulation, scapholunate dissociation is a crucial form of carpal instability frequently discussed alongside dislocations due to its severe implications.

• Description: This involves a tear of the scapholunate ligament, the primary stabilizer between the scaphoid and lunate. This disruption allows the scaphoid to flex excessively and the lunate to extend, creating a gap between them (often visible on X-ray as the "Terry Thomas sign").
• Mechanism: Often a less severe FOOSH than a full dislocation, but still significant.
• Clinical Significance: Left untreated, scapholunate dissociation can lead to Scapholunate Advanced Collapse (SLAC) wrist, a progressive pattern of degenerative arthritis that is one of the most common forms of wrist arthritis, severely impacting function.

Isolated Carpal Bone Dislocations

These are much rarer than the perilunate spectrum injuries but can occur.

• Description: Dislocation of a single carpal bone without involvement of the entire carpal row.
  • Pisiform Dislocation: Most common isolated carpal dislocation, often due to direct trauma or forceful wrist flexion/ulnar deviation.
  • Trapezium/Trapezoid/Hamate Dislocations: Extremely rare, usually requiring very specific and high-energy direct trauma.
• Mechanism: Typically direct impact or very specific forceful movements.
• Clinical Significance: While less devastating than perilunate injuries, they still require accurate diagnosis and treatment to restore wrist mechanics and prevent chronic pain.

Mechanisms of Injury

The overwhelming majority of carpal dislocations result from a Fall On an Outstretched Hand (FOOSH). The specific position of the wrist at impact (degree of hyperextension, ulnar or radial deviation, forearm rotation) and the magnitude of the force dictate the precise pattern of ligamentous and bony injury. High-energy trauma, such as motor vehicle accidents or significant athletic impacts, are also common culprits.

Clinical Presentation and Importance of Diagnosis

Symptoms of a carpal dislocation include:

• Severe Pain: Often immediate and intense.
• Swelling: Rapid accumulation of fluid around the injured area.
• Deformity: Visible alteration of the wrist contour, though sometimes subtle.
• Limited Range of Motion: Difficulty or inability to move the wrist.
• Neurological Symptoms: Tingling, numbness, or weakness, particularly if the median nerve is compressed (e.g., in lunate dislocation).

Accurate and prompt diagnosis is paramount. Initial assessment involves physical examination and X-rays (including true lateral views which are crucial for identifying dislocations). Often, CT scans are needed to fully delineate the extent of bony injury and displacement, and MRI may be used to assess associated ligamentous damage. Delay in diagnosis or inadequate treatment significantly increases the risk of chronic pain, instability, stiffness, and early-onset osteoarthritis.

Conclusion

Carpal dislocations are complex and serious injuries that demand immediate medical attention. Understanding the different types – primarily the perilunate spectrum (perilunate, lunate, trans-scaphoid perilunate dislocations) and the critical implications of scapholunate dissociation – is vital for anyone involved in fitness, sports, or healthcare. While fitness professionals cannot diagnose, recognizing the potential signs and symptoms of these high-energy wrist injuries is crucial for advising clients to seek urgent medical evaluation, ensuring optimal outcomes and preventing long-term disability.