M01.03.009 The Carpal Tunnel

The carpal tunnel is a narrow passage on the anterior side of the wrist, providing a gateway to the palm for several tendons and the median nerve. This structure plays a critical role in wrist and hand function. This guide explores its anatomy, contents, and clinical importance, especially for medical students.

Anatomy of the Carpal Tunnel

Borders

The carpal tunnel is composed of two key layers:

1. Carpal Arch (base and sides)
2. Flexor Retinaculum (roof)

Carpal Arch

• Shape: Concave on the palmar side.
• Lateral Borders:
  • Scaphoid tubercle
  • Trapezium tubercle
• Medial Borders:
  • Hook of hamate
  • Pisiform

Flexor Retinaculum

• Structure: A thick connective tissue forming the roof of the tunnel.
• Function: Converts the carpal arch into the tunnel by bridging the medial and lateral borders.
• Attachments:
  • Medially: Hook of hamate, pisiform
  • Laterally: Scaphoid, trapezium

Pro Tip: To locate the carpal tunnel’s entry point on yourself, find your distal wrist crease.

Contents of the Carpal Tunnel

Tendons (9 Total)

1. Flexor Pollicis Longus
   • Surrounded by its synovial sheath.
2. Flexor Digitorum Profundus (4 tendons)
3. Flexor Digitorum Superficialis (4 tendons)
   • These 8 tendons share a single synovial sheath, facilitating free movement.

Note: The Flexor Carpi Radialis tendon is located in the flexor retinaculum, not within the carpal tunnel.

Median Nerve

• Divides into two branches upon exiting the carpal tunnel:
  1. Recurrent Branch: Supplies the thenar muscles.
  2. Palmar Digital Nerves:
     • Sensory: Lateral 3½ digits (palmar skin, dorsal nail beds).
     • Motor: Lateral two lumbricals.

Clinical Relevance: Carpal Tunnel Syndrome (CTS)

Overview

CTS is caused by compression of the median nerve in the carpal tunnel. It is the most common mononeuropathy and can result from thickened ligaments, tendon sheaths, or idiopathic causes.

Symptoms

• Sensory:
  • Numbness and tingling in the median nerve distribution.
  • Pain radiating to the forearm.
  • Worsens at night and upon waking.
• Motor:
  • Weakness or atrophy of the thenar muscles (e.g., thumb).

Physical Examination Tests

1. Tinel’s Sign: Tapping over the carpal tunnel elicits pain.
2. Phalen’s Test: Holding the wrist in flexion for 60 seconds induces numbness or pain.

Management

1. Conservative Treatment:
   • Splinting the wrist in dorsiflexion overnight.
   • Corticosteroid injections if symptoms persist.
2. Surgical Intervention:
   • Decompression surgery for severe cases to relieve pressure on the median nerve.

Key Points to Remember

• The carpal tunnel is formed by the carpal arch and flexor retinaculum.
• It contains 9 tendons and the median nerve, which divides into sensory and motor branches.
• Carpal Tunnel Syndrome (CTS) is a common condition caused by median nerve compression.
• Diagnosis includes Tinel’s Sign and Phalen’s Test.
• Treatments range from splinting to surgical decompression in severe cases.