The extensor pollicis longus takes origin from the middle third of the posterior surface of the ulna and adjacent interosseous membrane [Fig. 8.6]. Its tendon passes deep to the extensor retinaculum and bends laterally around the dorsal tubercle of the radius. It crosses the tendons of the extensor carpi radialis longus and brevis and runs along the dorsum of the first metacarpal. At the first metacar- pophalangeal joint, it is joined by the first palmar interosseous muscle and an extension from the ab- ductor pollicis brevis to form a limited extensor expansion.
The extensor pollicis brevis and abductor pollicis longus arise together—the abductor from the posterior surfaces of the ulna, radius, and
Fig. 8.28 Schematic transverse section through the hand to show the motor distribution of the median and ulnar nerves. FDS = flexor digitorum superficialis; FDP = flexor digitorum profundus; M1–5 = metacarpals 1–5; L1–4 = lumbricals 1–4; 1–4 = palmar interossei 1–4;
D1–4 = dorsal interossei 1–4; Pb = palmaris brevis; Fm = flexor minimi brevis; Am = abductor digiti minimi; Om = opponens digiti minimi;
Ap = abductor pollicis brevis; Fp = flexor pollicis brevis; Op = opponens pollicis.
Reproduced with kind permission of CMC Vellore and Mrs Harsha.
Superficial branch ULNAR NERVE
AmOm Fm Pb
FDS FDS FDS FDS
FDP FDP FDP FDP L4
Deep branch 4
3
D3 D2
2
M2 D1
M1 1
Op Fp Ap Med. Lat.
MEDIAN NERVE
M4 M3
L3 L2 L1
M5 D4
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The forearm and hand
carpometacarpal and metacarpophalangeal joints.
The abductor pollicis brevis is used principally when the thumb is opposed to the little finger; the abductor longus is continuously active during op- position and the reverse movement. In this reverse movement of straightening of the opposed thumb, extension is first produced by the extensor pollicis brevis, which helps to maintain abduction at the carpometacarpal joint. The extensor pollicis longus comes more and more into play, as the movement progresses, helping to produce lateral rotation of the thumb. The extensor pollicis longus may also act as an adductor of the fully abducted thumb, thus assisting the adductor pollicis or mimicking its activity when paralysed.
Confirm these movements in your own hand, and check the contraction of the muscles by not- ing, as far as possible, their tendons or the harden- ing of the muscles themselves.
Clinical Applications 8.1, 8.2, and 8.3 explore how the anatomy of the forearm and hand applies to clinical practice.
the palmar surface of the thumb to oppose that of one of the fingers.
Movements at the metacarpophalangeal joint of the thumb is mainly flexion and extension.
A small amount of abduction (produced by the ab- ductor pollicis brevis) and adduction by the adductor pollicis and the first palmar interossei are possible.
Only flexion and extension occur at the inter- phalangeal joint—brought about mainly by the flexor pollicis longus and extensor pollicis longus.
Opposition is produced by the combined ac- tion of the abductor pollicis longus and brevis, followed by the action of the opponens pollicis (medial rotation) synchronously with the flexor pollicis brevis. Opposition of the thumb is usually (but not necessarily) accompanied by flexion at the metacarpophalangeal and interphalangeal joints of the thumb.
The flexor pollicis longus is used principally when the tip of the thumb is opposed to the tip of a finger or when power is required. The flexor pol- licis brevis is used when the main flexion is at the
CLINICAL APPLICATION 8.1 Carpal tunnel syndrome A 52-year-old domestic worker developed tingling and
burning pain over the palmar aspect of the thumb, index, and middle finger. On inspection, the doctor noticed that there was flattening of the thenar eminence.
Study question 1: what forms the thenar eminence? (An- swer: muscles of the thenar eminence, the abductor pol- licis brevis, the flexor pollicis brevis, and the opponens pol- licis.) On examination, it was found that she was not able to abduct or oppose her thumb effectively. She also had decreased sensation over the palmar aspect of the thumb, index finger, middle finger, and lateral part of the ring finger.
Study question 2: if this were due to a nerve lesion, which nerve is affected? (Answer: the median nerve.)
Study question 3: how does the affected nerve enter the hand? Name other structures that lie in the same space at the wrist. (Answer: the median nerve passes deep to the flexor retinaculum (in the carpal tunnel) to enter the hand. Other structures in the carpal tunnel are the tendons of the flexor pollicis longus and flexor digi- torum superficialis and profundus.) Death of the tendons due to interference with their blood supply can occur in extreme cases.
CLINICAL APPLICATION 8.2 Tenosynovitis A 12-year-old schoolgirl pricked her thumb with a nee-
dle, just proximal to the interphalangeal joint on the pal- mar aspect. Over the next few days, she developed pain and swelling over the site of injury which soon spread to the lateral side of the hand and palm up to the wrist. Her parents treated her with painkillers and made her rest.
The swelling continued to spread and extended to the lower part of the wrist. Movements of the thumb and
wrist became excruciatingly painful, and she developed a fever on the tenth day. She was taken to a hospital. On examination, her entire thumb, lateral side of the hand, and lower part of the forearm were swollen and tender.
Movements of the thumb were restricted due to the swelling and painful. There was no lymphadenopathy.
Study question 1: from your knowledge of anatomy of this region, which structure, when infected, is most likely
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Extensor compartment of the forearm and hand
to present with these signs and symptoms? (Answer: syn- ovial sheath of the thumb.)
Study question 2: name the tendon which lies in this sheath. What is the proximal and distal extent of this sheath? (Answer: the tendon of the flexor pollicis longus.
The sheath starts from a few centimetres proximal to the flexor retinaculum and ends on the distal phalanx.)
Study question 3: do you think it is possible for the infection to spread outside the sheath? (Answer: Yes.) The radial synovial sheath may communicate with the ulnar (common) synovial sheath for the long flexors, and infec- tion can spread through this communication. The sheath could rupture because of distension, and infection could spread to the surrounding tissue.
CLINICAL APPLICATION 8.3 Dupuytren’s contracture Progressive shortening of the palmar aponeurosis re-
sults in a condition known as Dupuytren’s contracture.
The patient’s fingers are flexed, because the aponeurosis is attached to the proximal phalanges through the deep
transverse metacarpal ligament. The shortening usually affects the medial part of the aponeurosis, and hence the little and ring fingers. Surgical division of the aponeurosis is required to straighten the fingers.