Fetal development and growth of extensor tendons and their sheaths in the dorsal side of the wrist and hand: a histological study.

Background: To understand considerable variations in number of adult extensor ten-dons at the dorsal side of the wrist and hand.

Materials and methods: We examined histological sections from 30 human fetuses of gestational age 7-39 weeks and crown-rump length 22-323 mm.

Results: At the carpal level, earlier or smaller fetuses showed a simpler configuration with fewer tendon slips, whereas later or larger fetuses had a greater number of tendon slips with considerable variations in number and topographical relationships. Tendon slips of the early extensor digitorum to the middle and ring fingers were always 1 or 2, but were seven or more at late term. A tendon of the extensor digitorum to the little fin-ger could not be distinguished from other tendons at the carpal bone level. At the meta-carpal bone level, however, it extended from the ring finger tendon toward two slips of the extensor digiti minimi tendon. At the distal carpal level at midterm and late term, in which the tendon sheath was lost, each of the extensor digitorum tendon slips further divided to provide a mediolateral linear cluster of thin bundles. This large number of tendon components joined and united together to provide a single tight tendon at the level of the metacarpophalangeal joints. The extensor pollicis longs tendon usually lost a membranous septation when it crossed the extensor carpi radialis brevis tendon.

Conclusions: Tendon slips in the fourth and fifth canals of the sheath were most likely to reduce in number after birth depending on the mechanical demand.