Anatomical studies on the extensor carpi radialis longus and brevis muscles in Japanese.

In order to determine whether the muscle bellies of the extensor carpi radialis longus and brevis (including additional slips) show laminar disposition, they were observed in 490 upper limbs of 245 Japanese cadavers. The author examined them from the point of view of lamination of the muscle slips (bellies) and classified them into 22 types in this study. The extensor carpi radialis longus muscle consists of five layers (A, B, C, D and E). Their proximal portions are disposed from radial deep to ulnar superficial as A, B, C, D and E, and the distal portions from ulnar deep to radial superficial as A, B, C, D and E. C is the main part of the extensor carpi radialis longus muscle, and A, B, D and E are additional (accessory) slips. The main part (C) is inserted into the second metacarpal bone A and B cross beneath the main part (C) to the ulnar side of the hand and are inserted into the third to second metacarpal bones. D and E pass across on the main part (C) to the radial side of the hand and are inserted into the second to first metacarpal bones. The extensor carpi radialis brevis consists of three layers (X, Y and Z). Their proximal portions are disposed from radial deep to ulnar superficial as X, Y and Z, and the distal portions from ulnar deep to radial superficial as X, Y and Z. Y is the main part of the extensor carpi radialis brevis muscle, and X and Z are additional (accessory) slips. The main part (Y) is inserted into the third metacarpal bone. X passes deep to Y to the ulnar side of the hand and is inserted into the fourth or third metacarpal bone. Z passes superficial to Y to the radial side of the hand and is inserted into the third or second metacarpal bone. The extensor carpi radialis longus muscle showed the normal form (C only present) in 326 of 490 cases, and had the main part (C) and one to three additional slips (A, B, D, E) in 164. The extensor carpi radialis brevis showed the normal form (Y only present) in 389, and had the main part (Y) and one or two accessory slips (X, Z) in 101 of 490 cases.