Anatomy Atlas

Carpal tunnel

Canalis carpi
Space / regionHigh-yielddraftMSKNeuro

Osseofibrous tunnel on the palmar wrist, formed by the carpal bones and the flexor retinaculum, that carries the median nerve and nine long flexor tendons.

Core anatomy

Location

Palmar aspect of the wrist, deep to the flexor retinaculum.

Boundaries
roof
Flexor retinaculum (transverse carpal ligament). Medial attachments: pisiform and hook of hamate. Lateral attachments: scaphoid tubercle and trapezium tubercle.
floor and walls
The concave palmar arch formed by the carpal bones
Contents
  • Median nerve (the most superficial structure in the tunnel)
  • Four tendons of flexor digitorum superficialis
  • Four tendons of flexor digitorum profundus
  • One tendon of flexor pollicis longus
  • Synovial sheaths: the common flexor sheath (ulnar bursa) for FDS/FDP and a separate sheath (radial bursa) for FPL
Communications

Leads from the anterior compartment of the forearm to the central compartment of the palm.

Notes

Not in the tunnel: the ulnar nerve and artery (superficial to the retinaculum, in Guyon's canal); the palmar cutaneous branch of the median nerve and the palmaris longus tendon (superficial to the retinaculum). The flexor carpi radialis tendon runs in its own split within the retinaculum (a separate compartment). Mnemonic: 9 tendons + 1 nerve.

Mentioned in

Generated automatically from links in other entries.

Carpal bones (right hand, palmar view)Palmar view of the right wrist with the thumb on the left. The proximal row, from lateral to medial, is the scaphoid, lunate, triquetrum and pisiform; the distal row is the trapezium, trapezoid, capitate and hamate. The radius articulates with the scaphoid and lunate; the ulna is separated from the carpus by the triangular fibrocartilage complex. The flexor retinaculum spans from the scaphoid tubercle and trapezium to the pisiform and hook of hamate, roofing the carpal tunnel, through which the median nerve passes. The ulnar nerve passes through Guyon canal, between the pisiform and the hook of hamate, superficial to the retinaculum.SLTPTTCHScaphoidmost fractured carpal; snuffboxtenderness; proximal pole AVN riskTrapeziumTrapezoidMedian nerve (in the tunnel)Flexor retinaculum(roof of the carpal tunnel)Radiusradial styloid is the more distalLunatemost dislocated carpal (palmar);AVN = Kienböck diseaseTriquetrumPisiform (sesamoid in FCU)CapitateHamateHook of hamate: fractures ingolf, baseball; Guyon canal edgeTFCC (triangular fibrocartilage)UlnaUlnar nerve (Guyon canal,between pisiform and hook)Right hand, palmar view · thumb ←Not to scaleLateral → medial: proximal row S L T P,distal row T T C H ("Some Lovers TryPositions That They Can't Handle")

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Carpal bones (right hand, palmar view). Thumb on the left. The flexor retinaculum (dashed) is anchored to the scaphoid tubercle and trapezium laterally and to the pisiform and hook of hamate medially, forming the roof of the carpal tunnel. Highlighted bones are the high-yield ones: scaphoid (fracture), lunate (dislocation) and hamate (hook fracture).
Original schematic, created for this atlas.
Palmar view of the hand showing flexor tendon sheaths passing deep to the transverse carpal ligament
Flexor tendon sheaths passing through the carpal tunnel deep to the transverse carpal ligament (flexor retinaculum).
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 1233. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Carpal tunnel syndrome PANCE

Mechanism / lesion

Anything that reduces the space or enlarges the contents compresses the median nerve: pregnancy (fluid), hypothyroidism, diabetes, rheumatoid arthritis (flexor tenosynovitis), obesity, acromegaly, dialysis-related amyloidosis, distal radius fracture, lunate dislocation, and repetitive wrist use.

Classic presentation

Nocturnal paresthesias of the thumb, index, middle and radial half of the ring finger. Hand-shaking relief. Clumsiness. Late thenar atrophy and weakness of thumb abduction. Thenar skin sensation is spared.

Carpal tunnel release

Mechanism / lesion

Surgical division of the flexor retinaculum (open or endoscopic) for severe or refractory symptoms.

Classic presentation

The recurrent (thenar motor) branch of the median nerve is at risk, particularly with anatomic variants where it passes through the retinaculum. Injury leaves the patient unable to oppose the thumb.

High-yield PANCE pearls

  • Carpal tunnel contents: median nerve + 9 flexor tendons (4 FDS, 4 FDP, 1 FPL).
  • The ulnar nerve and artery are not in the carpal tunnel (Guyon's canal).
  • CTS associations: pregnancy, hypothyroidism, diabetes, RA, acromegaly, amyloidosis.
  • Diagnosis is clinical (Tinel, Phalen, Durkan); nerve conduction studies/EMG confirm and grade severity.

Imaging anatomy

X-ray

The carpal tunnel view (axial projection with the wrist dorsiflexed) shows the pisiform, the hook of hamate, and the tubercles of the scaphoid and trapezium, the attachments of the retinaculum. It is useful for hook of hamate fractures and bony encroachment.

CT

Hook of hamate fractures and bony masses.

MRI

Swelling of the median nerve proximally, flattening distally, increased T2 signal, bowing of the flexor retinaculum, and tenosynovitis or masses.

Ultrasound

Shows the median nerve's cross-sectional area at the tunnel inlet, flexor tenosynovitis, anatomic variants (bifid median nerve, persistent median artery), and ganglia. Guides corticosteroid injection.

Key views & landmarks
  • Carpal tunnel (axial) radiograph
  • Ultrasound transverse at the pisiform and hook of hamate levels

Physical exam

Surface landmarks

The distal wrist crease roughly marks the proximal edge of the flexor retinaculum, which extends distally into the palm. Palpable attachments: pisiform (base of the hypothenar eminence), hook of hamate (deep, distal and radial to the pisiform), scaphoid tubercle and trapezium tubercle (base of the thenar eminence).

Palpation

Palpate the four retinacular attachments. Look for thenar atrophy (flattening of the lateral eminence).

Strength / function testing

Thumb abduction strength (APB). Two-point discrimination on the index pad. Thenar sensation (spared in CTS).

Tinel sign

Technique

Tap over the carpal tunnel.

Positive test

Tingling in the median-innervated digits.

Significance

Carpal tunnel syndrome.

Phalen test

Technique

Full wrist flexion for 60 seconds.

Positive test

Median-distribution paresthesias.

Significance

Carpal tunnel syndrome.

Carpal compression (Durkan) test

Technique

Direct pressure over the carpal tunnel for up to 30 seconds.

Positive test

Median-distribution paresthesias.

Significance

Carpal tunnel syndrome.

Self-quiz

Q1.

Which structure passes through the carpal tunnel?

Show answer

C

The carpal tunnel contains the median nerve and nine tendons: 4 FDS, 4 FDP and FPL. The ulnar nerve and artery pass through Guyon's canal. The palmar cutaneous branch and palmaris longus pass superficial to the retinaculum.

Q2.

A 48-year-old woman with fatigue, cold intolerance and weight gain reports bilateral nocturnal hand numbness in the thumb, index and middle fingers. Which test is most appropriate to evaluate an underlying cause?

Show answer

B

Her symptoms suggest hypothyroidism, a recognized cause of carpal tunnel syndrome (myxedematous tissue deposition). Check TSH. Other associations include pregnancy, diabetes, RA and acromegaly.

Q3.

The medial attachments of the flexor retinaculum are which structures?

Show answer

B

The flexor retinaculum attaches medially to the pisiform and hook of hamate and laterally to the scaphoid tubercle and trapezium.

Q4.

During carpal tunnel release, injury to which branch would cause loss of thumb opposition?

Show answer

B

The recurrent branch supplies abductor pollicis brevis, opponens pollicis and the superficial head of FPB. Injuring it causes loss of opposition. The palmar cutaneous branch is purely sensory.

Q5.

In carpal tunnel syndrome, sensation over the thenar eminence is usually normal. Why?

Show answer

D

The palmar cutaneous branch arises proximal to the flexor retinaculum and passes superficial to it, so it isn't compressed in the tunnel. Reduced thenar sensation points to a more proximal lesion (e.g. pronator syndrome). The recurrent branch is motor to the thenar muscles.

Flashcards

See also