Axillary nerve injuryoften results from shoulder joint dislocation or
fractures at the surgical neck of the humerus. The injury causes deltoid
muscle paralysis and skin anesthesia over the lateral deltoid region.
Shoulder contour may be lost with time as the deltoid atrophies. Arm
abduction is lost when the arm is abducted beyond the first 15 degrees.
Midhumeral fracturemay involve the deep brachial artery and the
radial nerve as they wind about the posterior aspect of the humerus.
Arterial injury produces ischemic contracture; nerve injury paralyzes the
wrist extensors and extrinsic extensors of the hand (“wrist-drop”).
Except on the ulnar side (flexor carpi ulnaris and 4–5 flexor digitorum
profundus), the forearm flexor compartmentis innervated by the
median nerve.
Scaphoid fractureis the most common hand bone break because it
transmits forces from the abducted hand directly to the radius. Because
the blood supply enters distally, the proximal portionof the scaphoid is
especiallyprone to avascular necrosis.
Lunate dislocationis most common in falls on the out-stretched hand,
compressing the median nerve within the carpal tunnel and producing
carpal tunnel syndrome.
Extensionof the medial four digits at the metacarpophalangeal jointsand
interphalangeal jointsis accomplished by the extensor digitorum in the
forearm, innervated by the radial nerve. In addition, extension of the inter-
phalangeal joints of the medial four digits is carried out by the lumbricals,
which are innervated by both the median nerve (lumbricals 1–2 on the lat-
eral side) and ulnar nerve (lumbricals 3–4 on the medial side). Lumbricals
also flex the metacarpophalangeal joints of the medial four digits.
Proximal phalangeal flexionat the metacarpophalangeal joint is by:
(a) the interossei (ulnar nerve) and lumbricals muscles; (b) the flexor dig-
itorum superficialis (median nerve) muscle; (c) the flexor digitorum pro-
fundus (medial and ulnar nerves) muscles. Middle phalangeal flexion at
the proximal interphalangeal joint is by (b) and (c). Distal phalangeal
flexionat the distal interphalangeal joint is by (c).
