Anatomic Study of Variations of Hypothenar Muscles and Arborization Patterns of the Ulnar Nerve in the Hand Keiichi Murata, MD, FRCS, Makoto Tamai, MD, Amit Gupta, MD, Louisville, KY Purpose: To show the variations in the hypothenar muscles and the arborization patterns of the ulnar nerve and to investigate the relationship between the hypothenar muscles and the ulnar nerve. Methods: We performed an anatomic study of 35 hands from embalmed cadavers. After dissecting the ulnar side of the hand we recorded the number of hypothenar muscles and their variations, the site of the hiatus for the deep branch of the ulnar nerve, and the branching patterns of the ulnar nerve in each hand. We then investigated the relationship between the variations in the hypothenar muscles and the arborization patterns of the ulnar nerve. Results: The abductor digiti minimi (ADM) had 1 belly in 6 hands, 2 bellies in 28 hands, and 3 bellies in 1 hand. The flexor digiti minimi brevis was absent in 8 hands, had 1 belly in 24 hands, and had 2 bellies in 3 hands. The opponens digiti minimi had 2 layers of origin and the deep branch of the ulnar nerve passed between these 2 layers in all hands. We classified the structural patterns of the hiatus into 3 types, the arborization patterns of the ulnar nerve into 5 types, and the branching patterns of the motor branch to the ADM into 4 types according to their morphologic characteristics. We found variations of the arborization pattern in which the deep branch origi- nated from the ulnar trunk of the ulnar nerve distal to bifurcation in 3 hands, a communicating branch was present between the 2 sensory branches to the fingers in 3 hands, and the ulnar sensory branch pierced the ADM in 2 hands. Conclusions: We confirmed that the anatomic relationship between the ulnar nerve and the hypothe- nar muscle is complex and that the formation of the hiatus varies. This knowledge can assist the surgeon in the diagnosis and treatment of conditions associated with the ulnar aspect of the hand. (J Hand Surg 2004;29A:500 –509. Copyright © 2004 by the American Society for Surgery of the Hand.) Key words: Aberrant muscle, Guyon’s canal, hypothenar muscle, ulnar nerve. The complex anatomic variations of the ulnar nerve and the hypothenar muscles in the hand have been described in the literature. 1–6 The purposes of this study are to (1) show the variations in the hypothenar muscles and the arborization patterns and course of the ulnar nerve and (2) to investigate the relationship between the hypothenar muscles and the ulnar nerve. This knowledge can assist the surgeon in the diag- nosis and treatment of conditions affecting the ulnar aspect of the hand such as tumors, trauma, nerve compression, abductor digiti quinti opposition trans- From the Christine M. Kleinert Institute for Hand and Microsurgery and University of Louisville, School of Medicine, Louisville, KY. Received for publication May 21, 2003; accepted in revised form January 8, 2004. No benefits in any form have been received or will be received by a commercial party related directly or indirectly to the subject of this article. Reprint requests: Amit Gupta, MD, FRCS, Christine M. Kleinert Institute for Hand and Microsurgery, 225 Abraham Flexner Way, Ste 850, Louisville, KY 40202. Copyright © 2004 by the American Society for Surgery of the Hand 0363-5023/04/29A03-0026$30.00/0 doi:10.1016/j.jhsa.2004.01.006 500 The Journal of Hand Surgery