Scaphoid & Lunate - Orthopaedics

SCAPHOID

Anatomy & Blood Supply

• In ~10% of individuals, the scaphoid has a sole blood supply from the radial artery entering at the waist, making those patients especially vulnerable.
• It is located in the anatomical snuffbox and forms part of the lateral border of the carpal tunnel.

Fracture Classification (by location)

Clinical Features

• Mechanism: fall on an outstretched hand (FOOSH) - hyperextension of wrist
• Pain in the anatomical snuffbox (radial-sided, distal to radial styloid)
• Decreased wrist and thumb ROM
• Clinical tests: anatomical snuffbox tenderness, scaphoid tubercle tenderness, Watson scaphoid shift test, axial compression of 1st metacarpal, resisted supination, "clamp sign"
• Key rule: Snuffbox tenderness = treat as scaphoid fracture until proven otherwise

Imaging

• Plain X-ray: ulnar-deviated PA view (scaphoid view) helps, but fracture may be occult initially
• Pronator quadratus fat pad obliteration on lateral view = clue to wrist injury
• MRI: highest sensitivity; also detects soft tissue injury - preferred for occult fractures
• CT: good for cortical disruption, fracture geometry
• Bone scan: highest sensitivity but low specificity (false positives)
• If X-ray negative but clinically suspected: thumb spica + repeat X-ray at 10-14 days, or early MRI

Management

• No consensus on whether thumb should be included - most surgeons terminate at the IP joint
• Long arm spica (prevents pronation/supination) may be used for the first few weeks

Complications

• AVN (especially proximal pole fractures) - due to retrograde blood supply
• Nonunion - most clinically problematic complication
• Malunion
• Carpal instability (DISI deformity from scapholunate dissociation)

LUNATE

Anatomy

Lunate Fracture

• Relatively uncommon; tends to occur with congenitally short ulna (negative ulnar variance)
• Mechanism: FOOSH causing extreme dorsiflexion - force transmitted from capitate to lunate
• Signs: dorsal wrist pain, tenderness just distal to Lister's tubercle, pain on axial loading of long finger metacarpal
• Imaging: often missed on plain X-ray (overlap of radius/ulna); CT or MRI needed; arthroscopy is gold standard
• Management:
  • Nondisplaced: short arm cast (double sugar-tong or thumb spica); ortho follow-up 5-7 days
  • Displaced (>1-2 mm) / avulsion / comminuted: ORIF (K-wires, cannulated screws, suture anchors)

KIENBÖCK'S DISEASE (AVN of the Lunate)

Overview

• Progressive osteonecrosis of the lunate leading to fragmentation and collapse
• Demographics: men aged 20-40 years
• Multifactorial etiology: negative ulnar variance, single arterial supply, increased intraosseous pressure from venous stasis, decreased radial inclination
• Negative ulnar variance increases shear stress on the marginally perfused lunate - a key risk factor

Diagnosis

• Dorsal wrist pain, mild swelling, limited motion, weakness
• Unexplained persistent non-activity-related dorsal wrist pain in a young adult with negative ulnar variance = do MRI
• X-ray: initially normal or linear fracture → later: lunate sclerosis → lunate collapse
• MRI (early diagnosis): diffuse low signal intensity throughout lunate on T1-weighted images; increased T2 signal = revascularization

Lichtman Classification

Treatment (based on Lichtman stage + ulnar variance)

• Goal in stages I-IIIA: save the lunate
• Stage I: trial of cast immobilization (limited long-term success)
• Stage II+: surgical treatment per MRI findings
• Joint-leveling procedure is first-line surgery:
  • Negative ulnar variance: radial shortening osteotomy preferred (goal: neutral or +1mm)
  • Positive ulnar variance: capitate shortening + capitohamate fusion
• Core decompression of radius and ulna: option in early stages (stimulates local vascular healing)
• Vascularized bone grafting (stages I-IIIA): 4th-5th extracompartmental artery (4-5 ECA) is preferred pedicle; may combine with scaphocapitate pinning
• Stage IIIB: must address carpal instability - options: STT fusion, scaphocapitate fusion, proximal row carpectomy (PRC)
• Stage IV: PRC or wrist arthrodesis
• Stages IIIB-IV: salvage procedures only (lunate cannot be revascularized)

SCAPHOLUNATE DISSOCIATION (SLD)

• Most common form of carpal ligament injury
• Mechanism: wrist hyperextension + ulnar deviation + intracarpal supination
• 40% of distal radius fractures may have concomitant SL injury

Diagnosis

• Wrist pain, reduced grip strength
• X-ray: Terry Thomas sign (UK) / David Letterman sign (USA) = gap between scaphoid and lunate >3 mm on PA view, indicating SLD
• Lateral X-ray: DISI deformity - scapholunate angle >60° (normal: 30-60°), capitolunate angle >30° (normal: 0-30°)

PERILUNATE DISLOCATION (Mayfield Classification)

• Lesser arc injuries: purely ligamentous
• Greater arc injuries: ligamentous + fractures (scaphoid, radial styloid, capitate, triquetrum, etc.)

Mayfield Stages

• Perilunate dislocation = lunate stays in radiocarpal fossa; rest of carpus dislocates around it (Stages I-III)
• Lunate dislocation = lunate itself dislocates palmarly (Stage IV) - can cause median nerve compression / acute carpal tunnel syndrome

Management

• Closed reduction in ED to decompress median nerve
• Definitive: surgical reconstruction of intrinsic ligaments + K-wire fixation
• K-wires for 8 weeks; cast/splint for 3 months
• These injuries are easily missed - always check lateral X-ray carpal alignment carefully: the capitate should sit in the "cup" of the lunate

Key High-Yield Summary