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TSM77: ANATOMY OF THE WRIST
14/11/08
LEARNING OUTCOMES
Describe the structure of the wrist joint and the normal range of movement
ANATOMY OF THE WRIST JOINT
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The wrist joint (or carpus) is the point of articulation between the distal radius and carpal bones
o The ulna does not articulate directly with any of the carpal bones
o The carpal bones articulate distally with the five metacarpal bones at the base of the digits
o The phalanges are the bones of the digits themselves – three in each finger, two in the thumb
The ulnar and radial arteries both pass superficially into the wrist to supply structures of the hand
o The ulnar artery passes medially from the flexor compartment just lateral to the ulnar nerve
o The radial artery passes laterally from the extensor compartment
There are eight carpal bones arranged roughly in two rows:
o Proximal row (lateral to medial)
 Scaphoid – articulates with distal radius
 Lunate – articulates with distal radius
 Triquetrum – slightly anterior displacement in relation to the above
 Pisiform – point of insertion flexor carpi ulnaris (see below)
o Distal row (lateral to medial)
 Trapezium – articulates with the first metacarpal
 Trapezoid – articulates with the second metacarpal
 Capitate – articulates with the third metacarpal
 Hamate – articulates with the fourth and fifth metacarpals
o Can be remembered with the mnemonic some lovers try positions that they cannot handle
MUSCLES ACTING AT THE WRIST
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Muscles acting on the wrist itself are contained in the two compartments of the forearm
o Tendons entering the hand are covered by the two retinacula – strong fibrous bands
o The flexor retinaculum forms the roof of the carpal tunnel (see below)
The flexor compartment muscles all have common flexor origin – medial epicondyle of humerus
o Superficial layer – all wrist flexors
 Flexor carpi radialis – inserts onto 2nd metacarpal – also radial abductor
 Palmaris longus – inserts onto flexor retinaculum
 Flexor carpi ulnaris – inserts onto pisiform – also ulnar abductor
o The median nerve supplies the former two whilst the ulnar nerve supplies the latter
The extensor compartment muscles all have common extensor origin – lateral epicondyle of humerus
o Superficial layer – both wrist extensors
 Extensor carpi radialis – inserts onto 2nd/3rd metacarpals – also radial abductor
 Extensor carpi ulnaris – inserts onto 5th metacarpal – also ulnar abductor
o Both of the above are supplied by the radial nerve
Explain the causes and consequences of nerve compression syndromes
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Nerve compression distorts the structural elements of the nerve (fascicles) and impedes blood supply
o This is most likely to occur at joints or where nerves pass through narrow tunnels
o Ulnar nerve – second most common; often at the elbow (see case 32)
o Radial nerve – at the elbow; generalised extensor paralysis of hand / wrist
o Median nerve – most common; often at the carpal tunnel
The carpal tunnel is formed by the carpals inferiorly and laterally and the flexor retinaculum superiorly
o The median nerve passes laterally just deep to the flexor retinaculum
o The tendon of flexor pollicis longus passes lateral to the median nerve
o All eight tendons of flexor digitorum superficialis and profundus pass centrally
o No blood vessels pass through the carpal tunnel
Carpal tunnel syndrome involves elevated pressure in the tunnel – up to 15-fold
o Often uncertain aetiology – affects females more than males (3:1)
o May be associated with repetitive tasks over a long period of time
o Can result from rheumatoid arthritis (swelling of synovial sheathes) or oedema
o Symptoms arise due to compression of the median nerve:
 Pain and paraesthesia over its sensory distribution (lateral palm etc.)
 Weakness of thenar muscles – impaired thumb opposition
o Percussion of the nerve at the wrist exacerbates symptoms – Tinel’s sign
o Treatment involves anti-inflammatories and surgery in severe cases