Scaphoid bone

At a glance

English

Scaphoid bone

Latin (TA2)

Os scaphoideum

System

Bone

Region

Hand

Side

Paired, left and right

Series

Carpal bones

About the scaphoid bone

The scaphoid is a carpal bone at the thumb edge of the wrist. It is the largest member of the proximal carpal row, about the size and curve of a cashew nut, and its long axis slants down, outwards and forwards. Unlike its neighbours it reaches across both carpal rows, so it acts as a link between them. Its name comes from the Greek for a boat; older texts called it the navicular of the hand, a name now kept for the foot.

Most of its surface, over four-fifths, is joint cartilage. Its convex upper surface meets the lower end of the radius at the radiocarpal joint. On its inner side a flat, half-moon facet meets the lunate and a hollow lower facet takes the head of the capitate. Its smooth lower surface meets the trapezium and trapezoid. Only narrow strips are left for ligaments: a rough groove along the back, a narrow outer surface where the radial collateral ligament attaches, and a tubercle on the front for the flexor retinaculum. The scapholunate ligament ties it to the lunate.

Its blood supply is the key to its injuries. Branches of the radial artery enter mainly through the dorsal ridge and the distal part, and blood reaches the proximal pole by flowing back through the bone. Because the bone spans both rows, a fall on the outstretched hand often snaps it, usually across its narrow waist. A break can cut off the proximal pole, which then heals slowly, fails to join, or loses its blood supply and collapses. Early fractures may not show on plain X-ray, so a tender wrist is often splinted and imaged again, or scanned by MRI or CT. Displaced or slow-healing breaks may be fixed with a compression screw; untreated non-union leads to wrist arthritis.

The scaphoid can be felt in the floor of the anatomical snuffbox, the hollow between the thumb tendons when the thumb is extended, and its tubercle where the thenar eminence begins at the wrist. Tenderness at either point after a fall is treated as a scaphoid fracture until shown otherwise. Tearing of the scapholunate ligament widens the gap between scaphoid and lunate, causing instability.

Function and supply

Function

Bridges the proximal and distal carpal rows and, with the lunate, carries load from the hand to the radius; it tilts between upright and flat as the wrist bends and deviates.

Nerve supply

Not innervated as a muscle is; its joints receive branches from the anterior and posterior interosseous nerves, and the superficial branch of the radial nerve crosses the snuffbox above it.

Blood supply

Palmar and dorsal branches of the radial artery enter its middle and distal parts; the proximal pole depends on blood flowing back through the bone from these.