Ulnar Dimelia

Ulnar dimelia is a rare congenital disorder characterized by duplication of the ulna, absence of the radius, and polydactyly. Patients can also have arterial anomalies, such as absence of the radial artery, duplication of the ulnar artery, and abnormal arterial arches in the hand. Nerve anomalies may also be present and include shortening of the radial nerve and duplication of ulnar nerve (with or without connections to the median nerve)

Radiopaedia has some great images of type I ulnar dimelia

Distinguishing features of the two types of ulnar dimelia
Feature	Type I	Type II
Index finger	1	2
Lunate	1	2
Trapezoid	1	2

References

• Afshar A. Ulnar dimelia without duplicated arterial anatomy. J Bone Joint Surg Br. 2010 Feb;92(2):293-6. doi: 10.1302/0301-620X.92B2.23057.
• Tomaszewski R, Bulandra A. Ulnar dimelia-diagnosis and management of a rare congenital anomaly of the upper limb. J Orthop. 2015 Feb 18;12(Suppl 1):S121-4.