Anatomic delineation of the ulnar nerve and ulnar artery in relation to the carpal tunnel by axial magnetic resonance imaging scanning

In a number of publications the distal branches of both the ulnar artery and ulnar nerve have been identified as being positioned ulnar to the hook of the hamate. We undertook a magnetic resonance imaging project in patients who presented for carpal tunnel release to determine how far radially vital structures of Guyon's canal may be located and if they may overlap the carpal tunnel at the hook of the hamate. Cross-sectional magnetic resonance imaging scans of 20 patients were performed prior to carpal tunnel release with the wrist positioned in neutral, flexed, and extended positions. Linear measurements were made either radial or ulnar to the hook of the hamate taken as the reference point. In the neutral position, the ulnar nerve was found to be, on average, 3.6 mm ulnar to the hook of the hamate (range of 5.8 mm radial to 7.5 mm ulnar). The ulnar artery averaged 0.7 mm to the radial side of the hook of the hamate (range 7.8 mm radial to 2.8 mm ulnar). Guyon's canal extended 28% of the way across the carpal ligament (range 9 to 63%). With the wrist in flexion and extension, there was an ulnar displacement and a radial displacement, respectively, of these structures relative to the hook of the hamate.