Illustrated Encyclopedia of Human Anatomic Variation: Opus I: Muscular System: Alphabetical Listing of Muscles: A
Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed
Variations in the muscles of the thumb have been reported to occur in one of every six subjects dissected. The most common variations occur in the long abductor and consist of an extensive cleavage of its tendon, or even the whole muscle, into separate parts. The belly of the muscle may be doubled. The long abductor may be reduced to a small slip arising only from the radius. The abductor pollicis longus may also arise from both the ulna and the mediolateral aspect of the radius. The slips into which its tendon is divided may find attachment to the flexor retinaculum, scaphoid, trapezium, abductor pollicis brevis, opponens pollicis, first phalanx, or the tendon may be inserted onto the base of the first metacarpal bone. An additional tendon may give origin to opponens pollicis. In one study of 134 wrists, 132 (95%) abductor pollicis longus muscles had accessory tendons. They inserted into abductor pollicis brevis in 25%; the trapezium, joint capsule, fascia in 34%; and the trapezium, joint capsule, and the abductor pollicis brevis muscle in 39%. The insertion of the abductor pollicis longus tendon varied in almost every instance from anatomy textbook descriptions. These involved, in addition to the base of the first carpometacarpal joint, the trapezium, navicular (scaphoid), surrounding fascial structures or the abductor pollicis brevis muscle. In another study of 38 forearms, only seven abductor pollicis longus muscles had a single tendon, the usual condition, 19 had an additional tendon, 20 had two, and six had three anomalous tendons. In the 19 cases with a second tendon, the aberrant tendon inserted into the tendon of abductor pollicis brevis in 12 cases, inserted onto the trapezium in six, and joined the volar carpal ligament in one. In the 20 cases of doubled aberrant tendon, 10 inserted into the abductor pollicis brevis, eight inserted onto the trapezium, and two inserted into the opponens pollicis. In the six cases of triple aberrant tendon, two tendons inserted into the abductor pollicis brevis, two inserted onto the trapezium, and two inserted into the tendon of opponens pollicis. In another study of 127 dissected forearms, 10.2% of the abductor pollicis longus muscles had a single tendon that inserted upon the radial side of the first metacarpal only. In the remaining 89.8%, the abductor had a dual insertion: into the abductor pollicis brevis, 56%; onto the trapezium, 18%; into the opponens pollicis, 15.7%; onto the first phalanx of the thumb, 7.1%; and into the carpal-metacarpal joint, 2.3%.
Macalister reported the following variations of extensor ossis metacarpi pollicis (abductor pollicis longus):
Abductor pollicis tertius (extensor atque abductor pollicis accessorius), a rare muscle, arises from the dorsal aspect of the radius with abductor pollicis longus and inserts, after fusion with abductor pollicis brevis, onto the first metacarpal. Syn.: m. abductor pollicis bicornis, m. extensor ossis metacarpi pollicis (Sharpey).
Variations of abductor pollicis longus tendons of
a, usual tendon of insertion at base of the first metacarpal;
b, tendinous slip to the greater multangular;
c, tendinous slip to abductor pollicis brevis; and
d, tendinous slip to opponens pollicis.
From Lacey, et al. (1951)
Accessory abductor pollicis longus.
An accessory muscular slip to abductor pollicis longus had its origin from the tendon of brachioradialis.
from Calori, 1867
Abu-Hijleh, M.F. (1993) Extensor pollicis tertius: an additional extensor muscle to the thumb. Plast. Reconstructive Surg. 92:340-343.
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