The dislocation of carpal bones about the lunate, with or without fracture, usually results from dramatic wrist hyperextension. High velocity trauma or falls from substantial heights may produce forceful palmar tension and dorsal compression that exceeds the limits of ligament and/or bony carpal constraints. The progressive perilunar instability model for such injury predicts sequential disruption of ligamentous attachments, forces transmitted from the radial to the ulna aspect of the carpus.

In stage I, there is disruption of the scapho-lunate ligament and subsequent scapho-lunate dissociation.

In stage II, the capitate and scaphoid separate from the lunate and triquetrum .

In stage III, there is lunotriquetral dissociation allowing the entire carpus to separate from the lunate almost always with dorsal displacement, hence dorsal perilunate dislocation.

In stage IV, the dorsal extrinsic ligaments fail and the lunate may dislocate volarly. This represents the most severe form and highest degree of instability. Such transmission of force purely about the lunate follows the so-called lesser arc pathway. Perilunar forces may also be transmitted through adjacent bone, so-called greater arc injuries.

The scaphoid, trapezium, capitate, hamate and triquetrum may fracture. As one might expect, there may be concomitant fracture and ligamentous injury resulting in the perilunar dislocation stages described. The lesser and greater arcs together comprise the vulnerable zone for all such injuries.

Scapholunate dissociation represents a stage I lesser arc injury and therefore is not considered severe. It is, however, a component of scapholunate instability, the most frequent form of carpal instability. In its most general sense, instability refers to a clinical perception by the patient that the joint can not withstand normal loads. Scapholunate dissociation refers to the anatomic, ligamentous disruption between scaphoid and lunate. This may only be detectable with stress radiographs or fluoroscopy and hence is referred to as dynamic instability. This may be readily apparent with routine radiographs, hence static instability. The scapholunate angle may be normal as seen on lateral radiographs. Only when the scaphoid assumes a flexed position however, is there rotatory subluxation. C. Incorrect. An intercalated segment is a middle segment of a three segment system under compression. This middle segment is inherently unstable unless stabilized by its connection to the other two segments, hence the proximal carpal row. DISI (dorsal intercalated segment instability) refers to the dorsiflexed posture or extension of all or part of the proximal carpal row, relative to the radius and capitate. This may be associated with numerous injuries, with and without dissociation of intrinsic or interosseous ligaments. It is less severe than lunate dislocation. D. Incorrect. Peri-lunate dislocation is a stage III perilunar injury and, therefore, less severe than stage IV lunate dislocation.

Scaphoid fracture

Around 80% of the blood supply is derived from the dorsal carpal branch (branch of the radial artery), in a retrograde manner. Interruption of the blood supply risks avascular necrosis of the scaphoid, with this most commonly complicating proximal injuries.

Most non-displaced fractures of the scaphoid and distal pole fractures can be managed conservatively with a cast for 6 weeks. Displaced scaphoid fractures generally require surgical fixation, and all proximal pole fractures require surgical fixation due to the risk of the development of avascular necrosis.

Patients typically present with:

• Pain along the radial aspect of the wrist, at the base of the thumb
• Loss of grip / pinch strength

Signs:

• 1. Point of maximal tenderness over the anatomical snuffbox
  • This is a highly sensitive (around 90-95%), but poorly specific test (<40%)in isolation
• 2. Wrist joint effusion
  • Hyperacute injuries (<4hrs old), and delayed presentations (>4days old) may not present with joint effusions.
• 3. Pain elicited by telescoping of the thumb (pain on longitudinal compression)
• 4. Tenderness of the scaphoid tubercle (on the volar aspect of the wrist)
• 5. Pain on ulnar deviation of the wrist

Clinical examination has a high diagnostic probability (sensitivity 100%; specificity 74%) when [1], [3], and [4] are positive on examination.