Ulnar dimelia is a very rare congenital abnormality with approximately 60 cases documented worldwide . Also known as ''mirror hand,'' the forearm and hand are essentially symmetrical about the mid-line with duplication of the ulna and ulna rays and an absence of the radius and radial rays. The condition is typically unilateral, although instances of bilateral ulnar dimelia have been documented [60,61].
No specific genetic abnormality has been linked to ulnar dimelia. Spontaneous mutation has been suggested as a possible cause . Regarding embryogenesis, Gorriz  explained the development of ulnar dimelia as a failure of complete anteroposterior differentiation of the limb bud rather than a true ulnar duplication. Chineg-wundoh and colleagues  support this theory, directing their attention toward the ability of the preaxial ulna to mature into a near-normal distal radius.
The preaxial ulna is often broader than a normal ulna, resembling a normal distal radius. Ulnar dimelia is characterized by the absence of the thumb, scaphoid, and trapezium. The hand consists of seven or eight digits, with the radial digits performing the function of opposition as a group. Duplication of the remaining carpal bones (ie, pisiform, triquetrum, hamate, capitate, and lunate) also occurs in a symmetrical fashion . The proximal row contains two triquetral bones, each articulating with its own pisiform. An abnormally wide lunate formed from two lunate bones fused side-by-side sits in the center of the proximal
row. The distal row usually consists of two hamate and two capitate bones articulating with each other at the midline. A single bone which may represent a trapezoid is sometimes present between the two capitate .
Several abnormalities are also present in the forearm. At the elbow, the distal humerus lacks a capitellum and is composed of two ill-defined trochleas. Elbow flexion and extension are limited by the degree of joint abnormality. Forearm rotation is impossible because of the absence of proximal and distal radioulnar joints . In addition, the wrist is often fixed in a flexed posture partly because of the relative abundance of flexor muscles and lack of extensors .
Surgical treatment of ulnar dimelia should address limitations of elbow flexion, wrist contracture, and digital excess. Pollicization of one of the preaxial digits, deletion of the remaining supernumerary preaxial digits, and reinforcement with muscle and tendon taken from the deleted digits help normalize the appearance of the hand .
The flexion contracture at the wrist reflects excessive flexor forces and can be addressed with several procedures, including proximal muscle release, distal tenotomy, volar wrist capsulotomy, and proximal row carpectomy . Tendon transfer from the volar to the dorsal aspect of the hand may be necessary to maintain a corrected wrist posture through increasing active wrist extension . Although resection of the proximal ulna has been suggested as a means of improving elbow motion and forearm rotation, long-term preservation of the motion achieved has been variable .
Because these forearms lack an interosseous membrane, resection of the proximal preaxial ulna may allow the remaining ulna to migrate proximally, which may then lead to further abnormalities in carpal posture and wrist motion.
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