Hand Anatomy: Carpal Tunnel and Flexor Retinaculum

Hand Anatomy: Flexor Retinaculum and Carpal Tunnel

• The flexor retinaculum is a strong fibrous band covering structures passing through the carpal tunnel.
• The carpal tunnel is a narrow passageway in the wrist containing flexor tendons and nerves.
• Medial attachment of the flexor retinaculum: pisiform and hook of hamate
• Lateral attachment of the flexor retinaculum: scaphoid and trapezium
• Eight tendons of flexor superficialis and profundus share a common flexor sheath in two rows that open laterally.
• Flexor pollicis longus and flexor carpi radialis have separate sheaths.
• The ulnar artery and nerve pass above the flexor retinaculum in Guyon's canal.

Synovial Flexor Sheath

• Only the thumb and little finger have complete synovial sheaths.
• Other fingers have separate synovial sheaths lining the fibrous flexor sheath.
• Bare parts of the tendons are windows for lumbricals.

Digital Attachment of Long Tendons

• Branches of tendons insert into the dorsal digital expansion of the hand.
• Common digital artery

Extensor Expansion

• The extensor tendon's expansion across the metacarpophalangeal joint is called the extensor hood.
• This area provides insertion points for lumbricals, interossei, extensor indicis, and extensor digiti minimi muscles.

Thenar Muscles

• These muscles are located at the base of the thumb.
• Include abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, and adductor pollicis
• The origin is the flexor retinaculum with scaphoid and trapezium.
• The insertion of these muscles is lateral to the base of the proximal phalanx.
• These muscles are innervated by the recurrent branch of the median nerve.
• Thenar muscles are used to abduct and flex the thumb.

Hypothenar Muscles

• These muscles are located at the base of the little finger.
• Include abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi
• Origin of these muscles are the flexor retinaculum and hook of hamate
• Insertion of these muscles is to the ulnar side of the 5th metacarpal
• These muscles are innervated by the deep branch of the ulnar nerve.
• Hypothenar muscles act on little finger–abduction, flexion, and opposition.

Palmaris Brevis Muscle

• Origin is the palmar aponeurosis.
• Insertion is on the skin of the ulnar border over the hypothenar base.
• Its action is to improve grip by steadying the skin on the ulnar side.
• Innervation by the superficial palmar branch of the ulnar nerve.

Interossei Muscles

• Palmar interossei muscles adduct the fingers towards the middle finger.
• Dorsal interossei abduct the fingers away from the middle finger.
• Both muscle groups originate from the metacarpals and insert into the extensor expansion and proximal phalanx.
• Both are innervated by the deep branch of the ulnar nerve

Lumbricals

• The origin of the lumbrical muscles is from the radial side of four profundus tendons.
• Their insertion is into the extensor expansion on the lateral side of the 2nd to 5th fingers.
• Innervation of these muscles is by median nerve and the deep branch of ulnar nerve.
• Lumbricals flex the metacarpophalangeal joints and extend the interphalangeal joints.