Carpal Bones
Eight carpal bones arranged in two rows form the skeleton of the wrist. Their precise order — proximal then distal, lateral to medial — determines vulnerability to fracture, avascular necrosis, and clinical presentation of wrist injuries.
✦ The Mnemonic
"Straight Lines Travel Purposefully; Trained Trappers Capture Hares"
Proximal row (lateral→medial) then distal row (lateral→medial)
Clinical Breakdown
The carpal bones sit in two rows within the carpal tunnel. The proximal row (scaphoid, lunate, triquetrum, pisiform) articulates with the radius and ulna to form the radiocarpal joint. The distal row (trapezium, trapezoid, capitate, hamate) articulates with the five metacarpals.
The scaphoid spans both rows and receives its blood supply from distal to proximal — a fracture through the waist cuts off supply to the proximal pole, causing avascular necrosis in up to 30% of cases. Anatomical snuffbox tenderness after a fall demands X-ray; if negative, a CT or MRI should follow given the high-stakes diagnosis.
The lunate is the most commonly dislocated carpal bone. In a perilunate dislocation, the remaining carpals dislocate around a still-articulating lunate. In a lunate dislocation, the lunate itself tilts anteriorly — compressing the median nerve and producing an acute carpal tunnel presentation.
⭐ Clinical Pearl
Anatomical snuffbox tenderness = scaphoid fracture until proven otherwise. The snuffbox borders are: EPL (medial), EPB + APL (lateral), with the floor being the scaphoid and trapezium. In a fresh fracture, initial X-rays are negative in up to 20% of cases — do not discharge without follow-up imaging or immobilisation.
⚠ Exam Trap
The pisiform is a sesamoid bone — it articulates with the triquetrum and lies within the FCU tendon. On a lateral X-ray, it projects anterior to the other proximal carpals. Pisiform fractures are associated with direct trauma to the hypothenar eminence.