Structure: the carpal bones
The eight carpal bones sit in two rows of four, forming an arch whose concavity faces the palm and is roofed by the flexor retinaculum to make the carpal tunnel.
- Proximal row, lateral to medial: scaphoid, lunate, triquetrum and pisiform. The scaphoid, lunate and triquetrum articulate with the radius and the articular disc; the pisiform is a sesamoid bone in the tendon of flexor carpi ulnaris, sitting on the front of the triquetrum.
- Distal row, lateral to medial: trapezium, trapezoid, capitate and hamate. The trapezium carries the saddle joint for the thumb metacarpal; the capitate is the largest carpal bone; the hamate has a hook that projects into the palm.
The scaphoid is boat-shaped and bridges both rows. It has a proximal pole articulating with the radius, a narrow waist, and a distal pole carrying the tubercle, which can be felt at the base of the thenar eminence. Its blood supply enters mainly at the distal end and dorsal ridge and flows backwards to the proximal pole.
Attachments
The intrinsic muscles fall into five groups, each with a distinct job.
| Muscle | Origin | Insertion | Nerve |
|---|---|---|---|
| Abductor pollicis brevis | Flexor retinaculum, scaphoid and trapezium tubercles | Radial side of base of proximal phalanx of thumb | Median (recurrent) |
| Flexor pollicis brevis | Flexor retinaculum and trapezium (superficial head); trapezoid and capitate (deep head) | Radial side of base of proximal phalanx of thumb | Median (superficial head); ulnar (deep head) |
| Opponens pollicis | Flexor retinaculum and trapezium | Radial border of the first metacarpal | Median (recurrent) |
| Adductor pollicis | Capitate and bases of second and third metacarpals (oblique head); shaft of third metacarpal (transverse head) | Ulnar side of base of proximal phalanx of thumb | Ulnar (deep) |
| Abductor digiti minimi | Pisiform | Ulnar side of base of proximal phalanx of little finger | Ulnar (deep) |
| Flexor digiti minimi brevis | Hook of hamate, flexor retinaculum | Base of proximal phalanx of little finger | Ulnar (deep) |
| Opponens digiti minimi | Hook of hamate, flexor retinaculum | Ulnar border of the fifth metacarpal | Ulnar (deep) |
| Lumbricals (four) | FDP tendons | Radial side of the extensor expansions | First and second median; third and fourth ulnar |
| Palmar interossei (three) | Metacarpals of index, ring and little fingers | Proximal phalanx and expansion, towards the middle finger | Ulnar (deep) |
| Dorsal interossei (four) | Adjacent sides of all five metacarpals, bipennate | Proximal phalanx and expansion, away from the middle finger axis | Ulnar (deep) |
The dorsal interossei abduct and the palmar interossei adduct the fingers relative to the middle finger. The middle finger has two dorsal interossei, the second on its radial side and the third on its ulnar side, and no palmar interosseous. Lumbricals and interossei together flex the metacarpophalangeal joints and extend the interphalangeal joints.
Blood supply and innervation
The hand is supplied by two palmar arches fed from both forearm arteries, and its intrinsic muscles are almost all T1 muscles.
The superficial palmar arch is the termination of the ulnar artery, usually completed by the superficial palmar branch of the radial artery, and lies just deep to the palmar aponeurosis. The deep palmar arch is the termination of the radial artery, which enters the palm between the two heads of the first dorsal interosseous and then between the two heads of adductor pollicis; the deep branch of the ulnar artery completes it. The deep branch of the ulnar nerve runs in the concavity of the deep arch.
The ulnar nerve supplies every intrinsic muscle except abductor pollicis brevis, the superficial head of flexor pollicis brevis, opponens pollicis and the first two lumbricals, which are median.
Clinical relevance
Scaphoid fractures and patterns of muscle wasting are the two clinical themes of the hand station.
- Scaphoid fracture: the commonest carpal fracture, usually at the waist, after a fall on the outstretched hand. Tenderness in the anatomical snuffbox or over the tubercle is the clue; the first radiographs may be normal, so suspected fractures are immobilised and imaged again or scanned. Because the blood supply flows from distal to proximal, a waist fracture can leave the proximal pole avascular, leading to non-union and avascular necrosis.
- Hook of hamate fracture: from a club or racquet handle, with pain in the hypothenar eminence and risk to the ulnar nerve.
- Thenar wasting: median nerve lesion, usually carpal tunnel syndrome.
- Hypothenar and first dorsal interosseous wasting: ulnar nerve lesion, with clawing of the ring and little fingers.
- Wasting of all the small muscles: a T1 root or lower trunk lesion, such as a cervical rib or apical lung tumour.
On the specimen
The hand stations show the carpal bones, the thenar and hypothenar muscles from the palm, and the extensor tendons from the dorsum.
- Carpal bones: start with the pisiform (small, pea-shaped, on the triquetrum) and the hook of the hamate, then name the rest in rows. The scaphoid's proximal pole, waist and distal pole are pinned separately on the same bone.
- Thenar eminence: abductor pollicis brevis is the superficial lateral muscle; flexor pollicis brevis lies medial to it; opponens pollicis lies deep to abductor. Adductor pollicis lies deeper, in the first web space, with oblique and transverse heads separated by the radial artery.
- Hypothenar eminence: abductor digiti minimi is the most medial, flexor digiti minimi lateral to it, and opponens digiti minimi deep to both. Flexor carpi ulnaris is traced into the pisiform.
- Dorsum: the third dorsal interosseous lies between the third and fourth metacarpals and inserts on the ulnar side of the middle finger. Extensor carpi radialis longus reaches the second metacarpal base, brevis the third. Extensor digitorum has four tendons linked by juncturae tendinum; extensor digiti minimi runs to the little finger. Abductor pollicis longus and extensor pollicis brevis run together to the thumb, and extensor pollicis longus separately.