Wrist Bone Anatomy and Radiographs

Overview

This lecture covers the anatomy of the wrist bones, their arrangement on radiographs, and key features for assessing wrist alignment in different radiographic projections.

Wrist Bone Anatomy

• The wrist contains 8 carpal bones, arranged in proximal and distal rows between the metacarpals and the radius and ulna.
• Proximal row (lateral to medial): scaphoid (boat-shaped), lunate (moon-shaped), triquetrum (three-cornered), pisiform (pea-shaped sesamoid).
• Distal row (medial to lateral): hamate (has a hook), capitate (largest, head-shaped), trapezoid, trapezium (under the thumb).
• The triangular fibrocartilage complex (TFCC) fills space above the ulna, absorbing force away from the ulna.

Radiographic Projections of the Wrist

• AP View: Shows both rows of carpal bones; key for tracing bone margins and identifying normal spacing.
• Oblique View: Highlights the anterior tilt of the scaphoid and crescent shape of the lunate.
• Lateral View: Emphasizes alignment; lunate, scaphoid, and capitate should stack properly.

Carpal Bone Relationships and Shapes

• The scaphoid tilts from proximal/posterior to distal/anterior, which can cause superimposition on AP view.
• The lunate has a crescent shape and cups the capitate above it.
• The trapezium and trapezoid are lateral, with the trapezium under the thumb.

Alignment & Assessment on X-rays

• Even spacing between carpal bones and smooth carpal arcs indicate normal alignment.
• The carpal tunnel is formed by anteriorly projecting carpal bones (scaphoid, trapezium, pisiform, hamate hook) and the flexor retinaculum.
• Radial inclination on AP: normal is 20–25°; less may suggest distal radius fracture.
• Volar inclination (on lateral): normal is 10–20°.
• Ulnar variance: difference between radius and ulna articular surfaces should be <2.5 mm.
• Scapholunate angle (lateral): normal is 30–60°.

Scaphoid Fractures and Blood Supply

• The scaphoid has a distal and proximal pole separated by a waist—most common adult fracture site.
• Fractures in the waist or proximal pole risk avascular necrosis due to lack of dual blood supply.

Key Terms & Definitions

• Carpal bones — eight small bones forming the wrist.
• Triangular fibrocartilage complex (TFCC) — structure stabilizing ulna side of wrist.
• Radial inclination — angle of the radius’ articular surface relative to its shaft on AP view.
• Volar inclination — anterior tilt of the distal articular surface of the radius on lateral view.
• Ulnar variance — relative length of the ulna compared to the radius at the wrist.
• Carpal tunnel — passageway in the wrist formed by carpal bones and the flexor retinaculum.
• Scapholunate angle — angle between the long axes of the scaphoid and lunate bones.

Action Items / Next Steps

• Practice identifying wrist bones by shape and relation on radiographs.
• Review the alignment criteria and normal measurement ranges for wrist X-rays.
• Learn the names and shapes of each carpal bone for exam preparation.