Carpal Tunnel : Anatomy, Boundaries, Contents, Imaging

Topic

Carpal tunnel region

The carpal tunnel is a narrow osteofibrous passage located on the palmar aspect of the wrist, through which important tendons and the median nerve pass into the hand. It serves as a confined space formed by the carpal bones posteriorly and the flexor retinaculum (transverse carpal ligament) anteriorly.

This region plays a critical role in hand flexion and finger movement, housing the tendons of the flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus, along with the median nerve, which provides motor and sensory innervation to the hand. The carpal tunnel’s rigid boundaries make it a common site of nerve compression — known as carpal tunnel syndrome (CTS).

Synonyms

Carpal canal
Transverse carpal tunnel
Flexor retinacular tunnel

Location and Boundaries

Location: Anterior wrist region, between distal forearm and proximal palm.
Roof (anterior boundary): Flexor retinaculum (transverse carpal ligament).
Floor (posterior boundary): Concave arch of the carpal bones (scaphoid, lunate, triquetrum, pisiform proximally; trapezium, trapezoid, capitate, and hamate distally).
Medial wall: Pisiform and hook of hamate.
Lateral wall: Scaphoid tubercle and ridge of trapezium.

Median nerve: Principal neural structure traversing the tunnel, lying superficial and slightly radial to the tendons.
Tendons:
- Four tendons of flexor digitorum superficialis (FDS).
- Four tendons of flexor digitorum profundus (FDP).
- One tendon of flexor pollicis longus (FPL).
All tendons are enclosed in synovial sheaths (ulnar bursa for FDS/FDP, radial bursa for FPL).

Relations

Superficial (anterior): Palmar fascia, palmaris longus tendon, palmar cutaneous branch of median nerve.
Deep (posterior): Carpal bones and intercarpal ligaments.
Medial: Guyon’s canal (housing the ulnar nerve and artery, separated by pisohamate ligament).
Lateral: Thenar muscles (abductor pollicis brevis, flexor pollicis brevis).

Supporting Structures

Flexor retinaculum: Thick fibrous band maintaining tendon alignment, preventing bowstringing during flexion.
Synovial sheaths: Facilitate tendon gliding; inflammation leads to swelling and compression of the median nerve.
Carpal arch: Bony concavity forming the tunnel’s floor, crucial for shape and stability.

Function

Mechanical protection: Encloses and stabilizes flexor tendons and median nerve during wrist motion.
Tendon guidance: Maintains alignment for efficient finger and thumb flexion.
Neural passage: Allows median nerve transmission to thenar muscles and palmar digits.
Force distribution: Balances compressive and tensile forces during gripping.

Clinical Significance

Carpal tunnel syndrome (CTS):
- Caused by median nerve compression due to swelling, tenosynovitis, or fibrosis.
- Symptoms: Numbness, tingling, pain in thumb, index, middle, and radial half of ring finger.
- Chronic compression leads to thenar muscle weakness or atrophy.
Inflammatory conditions: Rheumatoid arthritis, gout, and synovitis increase tunnel pressure.
Trauma: Fracture or dislocation of carpal bones can alter tunnel dimensions.
Mass lesions: Ganglion cysts, lipomas, or tenosynovial tumors may occupy tunnel space.
Post-surgical assessment: MRI or CT used to evaluate decompression success or recurrent fibrosis.

MRI Appearance

T1-weighted images:
- Flexor retinaculum: Low-signal (dark) linear band across anterior wrist.
- Tendons: Low-signal tubular structures surrounded by intermediate synovial tissue.
- Median nerve: Oval or round structure with intermediate signal, superficial and radial within the tunnel.
- Fat planes: Bright signal outlining tunnel margins and separating contents.
- Pathology: Flattened median nerve or obliterated fat plane indicates compression.
T2-weighted images:
- Normal tendons: Low signal.
- Median nerve: Slightly brighter than tendons but darker than fat.
- Inflammation or edema: Appears as bright hyperintense signal in nerve or synovium.
- Tenosynovitis: High T2 signal surrounding flexor tendons with thickened synovial sheath.
- CTS: Flattened nerve with proximal enlargement and increased intraneural T2 signal.
STIR:
- Normal structures: Tendons and retinaculum appear dark; nerve intermediate-to-dark.
- Pathology: Bright hyperintensity of the median nerve or peritendinous region in edema, inflammation, or entrapment.
- Highlights perineural edema and flexor tendon sheath thickening.
Proton Density Fat-Saturated (PD FS):
- Normal: Tendons and retinaculum are dark; nerve intermediate.
- Abnormal: Bright hyperintense signal in nerve or surrounding synovium indicates neuritis or synovitis.
- Useful for detecting subtle perineural or peritendinous inflammation.
T1 Fat-Sat Post-Contrast:
- Normal: Minimal or no enhancement of nerve or synovium.
- Inflamed synovium or nerve: Moderate-to-strong enhancement in tenosynovitis or neuritis.
- Postoperative fibrosis: Shows irregular enhancement distinguishing scar tissue from recurrent compression.

CT Appearance

Non-Contrast CT:

Flexor retinaculum: Dense fibrous band bridging carpal arch.
Tendons: Soft-tissue density cylindrical structures within tunnel.
Median nerve: Oval, slightly hypodense compared to tendons.
Bony boundaries: Carpal arch clearly defined by scaphoid, lunate, and hamate contours.
Pathology: Detects carpal bone fractures, tunnel narrowing, or calcific tenosynovitis.

Post-Contrast CT (standard):

Synovial inflammation: Enhancing soft-tissue around tendons.
Mass lesions: Contrast uptake in ganglia, tumors, or post-surgical fibrosis.
Median nerve: May enhance subtly in neuritis or entrapment neuropathy.
Excellent for osseous abnormalities, space-occupying lesions, and postoperative evaluation.

MRI images

CT images

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