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Carpal tunnel: Is the “Safe Zone” really avascular?

Introduction: Numerous publications have studied the regional anatomy of the carpal tunnel to define a “Safe Zone” in order to reduce the risk of perioperative neurovascular complications. This zone is located between the ulnar neurovascular bundle and the median nerve. It is considered to be safe essentially because of the absence of vascular structures. This study aims at assessing the presence of arterioles within this area using Superb Microvascular Imaging (“SMI”).

Materials and Methods: Twenty-seven patients underwent a bilateral routine wrist ultrasound, including SMI between January 28th and February 28th, 2019. The images from 54 wrists were retrospectively reviewed by two radiologists to evaluate presence and location of arterioles in the Safe Zone. In addition, 5 cadaveric wrists injected with intra-arterial red latex underwent a dissection of the carpal tunnel.

Results: In the Safe Zone, arterioles were seen above the flexor retinaculum in 36 wrists (36/54; 66.7%) and under the flexor retinaculum in 21 wrists (21/54; 38.9%). The arterioles located under the flexor retinaculum were more frequently located close to the median nerve (21/54; 38.9%) than to the ulnar artery (9/54; 16.7%). In cadaveric wrists, arterioles were seen above the flexor retinaculum in 3 wrists (3/5; 60%) and under the flexor retinaculum in 2 wrists (2/5; 40%).

Conclusion: Arterioles can be seen in the Safe Zone both above and under the flexor retinaculum. Under the retinaculum, these vessels are essentially observed in the proximal aspect of the carpal tunnel and more frequently close to the median nerve.