Scaphoid
Os Scaphoideum
location_on Proximal row of carpal bones, on the radial (thumb) side of the wrist
The scaphoid is the largest bone of the proximal carpal row and the most commonly fractured carpal bone. It spans both the proximal and distal carpal rows, acting as a bridge, which makes it especially vulnerable to injury during falls on an outstretched hand. Its retrograde blood supply enters primarily at the distal pole, making proximal pole fractures prone to avascular necrosis.
Key Anatomical Features
- Tubercle on the palmar surface is palpable and provides attachment for the flexor retinaculum
- Waist is the narrow middle portion and the most common fracture site
- Retrograde blood supply enters distally, making the proximal pole vulnerable to avascular necrosis
- Articulates with five bones: radius, lunate, capitate, trapezium, and trapezoid
- Located in the anatomical snuffbox, bounded by the tendons of extensor pollicis longus and brevis
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Abductor pollicis brevis | Scaphoid tubercle (partial origin) | Abducts the thumb |
| Flexor retinaculum | Scaphoid tubercle (lateral attachment) | Maintains the carpal tunnel and prevents bowstringing of tendons |
| Flexor pollicis brevis (superficial head) | Scaphoid tubercle via flexor retinaculum | Flexes the proximal phalanx of the thumb |
| Opponens pollicis | Scaphoid tubercle via flexor retinaculum | Opposes the thumb |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Radiocarpal joint | Synovial ellipsoid | Distal radius |
| Scaphotrapeziotrapezoid (STT) joint | Synovial plane | Trapezium and trapezoid |
| Scaphocapitate joint | Synovial plane | Capitate |
Common Pathologies
Scaphoid fracture
The most common carpal fracture, typically from a fall on an outstretched hand. May not be visible on initial X-rays; clinical suspicion (snuffbox tenderness) warrants immobilization and repeat imaging or MRI.
Scaphoid nonunion
Failure of fracture healing, more common with proximal pole fractures. Leads to progressive scaphoid nonunion advanced collapse (SNAC) wrist arthritis.
Avascular necrosis of scaphoid
Death of the proximal pole bone due to disrupted blood supply following fracture. The retrograde blood supply pattern means proximal pole fractures have the highest risk.
Scapholunate dissociation
Tear of the scapholunate ligament causing abnormal carpal kinematics. Widening of the scapholunate interval (Terry Thomas sign) is seen on AP radiographs.
Clinical Relevance
Tenderness in the anatomical snuffbox (between the extensor pollicis longus and extensor pollicis brevis tendons) is the classic sign of scaphoid fracture. Initial radiographs are negative in up to 20% of cases, so clinical suspicion warrants thumb spica immobilization with repeat radiographs at 10-14 days or immediate MRI. The Herbert screw is the standard fixation device for displaced scaphoid fractures.
Development and Ossification
The scaphoid ossifies from a single center that appears between ages 5 and 6. It is entirely cartilaginous at birth. The bone is the last of the radial column carpals to ossify. Bipartite scaphoid (failure to fuse from two centers) is a rare variant that can mimic fracture.
Did You Know?
- Scaphoid means boat-shaped in Greek (skaphe = boat), referring to its elongated concave shape
- The scaphoid is the most commonly fractured carpal bone, accounting for about 70% of all carpal fractures
- The blood supply to the scaphoid was not well understood until the 1960s, explaining historically poor treatment outcomes
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