Carpal tunnel

The carpal tunnel is an osteofibrous passage with rigid boundaries. Because it cannot expand easily, all of its contents share a confined space, making the median nerve particularly vulnerable to any increase in pressure within the tunnel.

Anatomical structure

The floor and walls are formed by the concavity of the carpal bones, while the roof is formed by the flexor retinaculum.

Boundary	Anatomical landmarks
Lateral	Scaphoid and trapezium
Medial	Pisiform and hook of hamate
Roof	Flexor retinaculum
Floor	Concavity of the carpal bones

Contents

The carpal tunnel contains ten structures.

Structure	Number
Median nerve	1
Flexor digitorum superficialis tendons	4
Flexor digitorum profundus tendons	4
Flexor pollicis longus tendon	1

Total: 9 tendons and 1 nerve.

A common examination mistake is including the flexor carpi radialis tendon as a tunnel content. It passes through a separate compartment within the flexor retinaculum, not through the carpal tunnel itself.

Relationship to the median nerve

The median nerve enters the hand through the tunnel and divides into its terminal branches after emerging. Because the space is confined, it is the structure most affected by any increase in the volume of the surrounding tendons or synovial sheaths.

Important anatomical relationships

Structure	Passes through the tunnel?
Median nerve	Yes
FDS tendons	Yes
FDP tendons	Yes
FPL tendon	Yes
Flexor carpi radialis tendon	No
Ulnar nerve	No
Ulnar artery	No

The ulnar nerve and artery pass superficial to the flexor retinaculum through Guyon's canal.

Anatomical significance

The carpal tunnel shows how fixed anatomical boundaries influence nerve function. Since the carpal bones and retinaculum allow little expansion, any increase in tunnel contents raises compartment pressure, and the median nerve is affected first.

How to review this topic

Draw a transverse section of the wrist with the four bony landmarks and the retinaculum as the roof. Memorise the contents in order: median nerve, four FDS, four FDP, one FPL. Then review the structures that do not pass through — they are the common exam points.