• Anatomy of the Carpus
• The wrist consists of eight carpal (wrist) bones divided into two rows of four bones each
• The proximal carpal row is composed of (from radial to ulnar) the scaphoid lunate, triquetrum, and pisiform
• The distal carpal row is composed of (from radial to ulnar) the trapezium, trapezoid, capitate, and hamate.
• The carpus is supported by the intrinsic (originate and insert within the carpus) and extrinsic (originate or insert outside the carpus) ligaments.
• Ligaments comprising the proximal row interosseous ligaments are the scapholunate and lunotriquetral interosseous ligaments. 
• Extrinsic Ligaments
• Along the volar side of the wrist from radial to ulnar, the ligamentous elements include radioscaphocapitate ligament , long radiolunate ligament, the ligament of testut or the radioscapholuante ligament, the short radiolunate ligaments, followed by the ulno lunate and ulnotriquetral ligaments
• The dorsal extrinsic ligaments include the (dorsal radiocarpal) ligament, which originates at the radius between the scaphoid and lunate fossa and extends obliquely across to attach to the lunate and triquetrum, and the dorsal intercarpal ligament, which extends obliquely in a radial direction to insert along the distal scaphoid and the trapezium

• These ligamentous elements form dorsal and palmar V-shaped ligaments that support the carpus and are directly perpendicular to forces that favor carpal dissociation from the radius. Injuries to these ligaments can cause carpal instability patterns.
• Carpal Kinematics: The wrist and distal radioulnar joint (DRUJ) combine to form essentially a universal joint, normally capable of stable motion with six degrees of freedom in the cardinal planes (flexion, extension, radial deviation, ulnar deviation, pronation, and supination). 
• Overall, the bones of the distal carpal row are tightly bound to each other, creating an essentially a single functional unit.
• The bones of the distal row move with the hand through all six degrees of freedom.
• The proximal carpal row behaves differently as there is motion between the adjacent bones as the entire row moves in essentially the same direction.
• The proximal carpal row bones move with the distal row bones during flexion and extension of the wrist (adjunct rotation) but continue to experience flexion and extension during radial and ulnar deviation of the wrist, respectively (conjunct motion). 
• The scaphoid wants to rotate into flexion, but when coupled to the lunate, through the scapholunate ligament, flexion is limited.
• The lunate wants to extend, because of the longitudinal force of the capitate (the capitate head articulates slightly dorsal to the central aspect of the lunate.
• The balance of the scaphoid flexion and lunate extension maintains the neutral alignment of the proximal carpal row.
• Wrist radial deviation is accomplished with flexion of the scaphoid to get out of the way of the trapezium and trapezoid. This pulls the lunate into slight flexion, but less than the scaphoid.
• Wrist ulnar deviation comprises primarily extension of the scaphoid, along with the lunate and triquetrum as the trapezium and trapezoid move away from the radius, allowing room for the scaphoid to extend.
• In the dart throwers axis, there is minimal motion within the proximal row (this was an inservice question several years ago). With radial deviation and extension, the scaphoid has little movement as the extension wants to extend the scaphoid and the radial deviation wants to flex the scaphoid. The corollary is true with ulnar deviation and flexion. Although the scaphoid wants to extend from the ulnar deviation, the wrist flexion counteracts this, again resulting in little movement. This is the reason this motion is used in rehabilitation following scapholunate ligament injuries.

• Scapholunate Ligament: C shaped and attaches along dorsal proximal and volar margins. Contains dorsal, volar, and membranous portions.
• Dorsal is the thickest, strongest (true ligament) with transversely oriented fibers
• Important secondary stabilizers include RSC, LRL, SRL and STT as well as dorsal structures DRC and DIC (V arrangement confers important secondary stabilizers against repetitive motion)
• Scapholunate Dissociation (SLD):  most common form of carpal instability
• Mechanism: axial loading a wrist that is 1) extended 2) ulnarly deviated 3) and in intracarpal supination
• Presenting Symptoms: pain over SL (1cm distal to listers); can have positive Watsons (or scaphoid shift test), painful clunking or clicking

i. Scaphoid (Watson) shift maneuver (for scapholunate ligament tear):  Place examiner’s thumb on the palmar surface of the patient’s distal pole of the scaphoid–>Place the examiner’s index finger over the dorsal surface of the scapholunate joint–> Passively move the patient’s wrist from ulnar deviation to radial deviation while applying a dorsally directed force on the distal pole of the scaphoid–> A positive response is dorsal pain with detection of dorsal subluxation of the proximal pole of the scaphoid, out of the scaphoid fossa of the radius (the clunk) 
 

• Specific x-ray findings:
• Perfect scapholunate view is 10 degree hyperpronation
• Best test for dynamic SL is clenched pencil view
• Remember that the scapholunate interval may appear normal on X-raysà however, this will eventually progress to static changes
• Scaphoid will flex as injury progresses leading to >SL and radioscaphoid angle (cortical ring sign), collapsed carpal height due to capitate subsidence, and extended lunate.
• Criteria for radiographic diagnosis of SL injury include: Increased scapholunate angle (30-60 nl) >70 SL gap >3mm 

• Ancillary imaging: includes MRI and MRI arthrogram
• Arthroscopy is gold standard for diagnosis (anatomic and functional evaluation) and includes a grading system based on the appearance of the SL ligament and the ability to pass an arthroscope through the scapholunate interval. (Geissler Grade)

• The progression of scapholunate instability occur over time or acutely with trauma. Progression typically follows as occult injuries, dynamic injuries, scapholunate dissociation (static deformity), DISI deformity (in which the lunate is extended and the scaphoid is flexed) and finally the development of SLAC wrist or scapholunate advanced collapse

• The stages of SLAC wrist are:

a. Stage 1: arthritis at the radial styloid
b. Stage 2: arthritis of the entire scaphoid fossa of the distal radius
c. Stage 3: arthritis of the capitolunate articulation.

• Treatment of these injuries is very complex and depends on the stage of scapholunate injury as well as surgeon preference (dynamic or static injury vs SLAC)
• In general: 
• Acute: Dorsal ligament repair through bone tunnels or suture anchors with supplemental K wire fixation of SL and SC joints. Protective motion 2-3 months
• Partial tears: debridement (arthroscopic) pinning and immobilization 
• Chronic injuries (those that are still reducible) : salvage procedures- dorsal capsulodesis (dynamic instability), tendoesis (Brunelli recosntruction with FCR), bone ligament bone reconstruction, or arthrodesis (scaphotrapezial/scaphocapitiate) 
• Arthritis: radial styloidectomy (Stage 1 SLAC), proximal row, scaphoid excision and 4 corner fusion, complete arthrodesis or arthroplasty