Anatomic Study of the Motor Branch of the Ulnar Nerve Regarding Carpal Tunnel Surgery: A Cadaveric Study

Abstract

Background: Motor branch of the ulnar nerve (MUN) injury during carpal tunnel surgery is rare and it should never be injured during carpal tunnel release (CTR). However, an iatrogenic injury of the MUN can cause catastrophic physical and mental suffering. The aim of our study is to understand the anatomy of the MUN in relation to carpal tunnel in order to prevent iatrogenic injury during CTR. Methods: We dissected 34 fresh cadaver hands and located the MUN in relation to the anatomical axis used for carpal tunnel surgery. Possible mechanisms of injury and the vulnerable area of the MUN were determined along the dissection. Results: The MUN turned towards the thumb distal to hook of hamate. It then travelled on the floor of the carpal tunnel which was formed by intrinsic hand muscles under flexor tendons. The nerve located at 29.39 ± 7.41, 35.01 ± 3.14 and 38.79 ± 4.03 mm (Mean ± SD) in the central axis of ring finger, the vertical axis of the third web-space and the central axis of middle finger respectively. The nerve’s turning point, 10.9 ± 2.63 mm distal to the centre of hook of hamate where it lies just below the level of the transverse carpal ligament. Conclusions: Surgeons should be aware of the nerve’s location. Surgical dissection or passing of any surgical instruments around the hook of hamate should be done with care. Level of Evidence: Level IV (Therapeutic)

The Isolated Motor Branch of the Ulnar Nerve Injury During Open Carpal Tunnel Release

“…The MUN’s turning point is about 11 mm distal to the center of the palpable surface of the hook of the hamate and situates just under the level of the TCL, along the axis of the ring finger. There could be an anatomical variation where the MUN’s turning point resides at the distal border of the hook of the hamate [ 9 ]. Every MUN injury during OCTR is associated with an incision placed on the ulnar side of the skin.…”

“…Radial to the axis of the ring finger, the MUN then turns anatomical posteriorly to the muscular floor of the palm, which is formed by the intrinsic hand muscles. It then traverses on this floor, giving branches to supply intrinsic hand muscles, and enters the adductor pollicis muscle radial to the central axis of the middle finger, under the flexor tendons [ 9 , 10 ]. Along the axis of the third web space, the MUN is covered by flexor tendons.…”

“…Along the axis of the third web space, the MUN is covered by flexor tendons. The MUN is located about 35.1 and 9.2 mm from distal wrist crease and Kaplan’s cardinal line, respectively [ 9 , 11 ]. In our case and in all previously reported MUN injuries during OCTR, the nerve was found to be divided in the mid-palmar space.…”