Ultrasound of the Musculoskeletal System
Authors: Bianchi, Stefano, Martinoli, Carlo
The most useful bony landmarks to identify the proximal carpal tunnel are the pisiform at its ulnar side and the scaphoid at its radial side. At ultrasound examination, these bones appear as round hyperechoic structures with posterior acoustic shadowing. Once these landmarks are demonstrated in a single image, the orientation of the probe should be adjusted to optimize the depiction of the soft tissues contained within the tunnel (Fig. 26). Tilting the probe back and forth may be helpful to distinguish the hypoechoic median nerve by the adjacent anisotropic tendons. Relative to the flexor carpi radialis, the flexor pollicis longus tendon runs in a deeper location, slightly closer to the midline. Oblique longitudinal ultrasound images can depict these tendons in the same plane. The proximal carpal tunnel is larger in size compared with the distal tunnel. In a comparative ultrasound-cadaveric study, ultrasound has proved to be accurate in evaluating the different diameters, the outline and the cross-sectional area of the carpal tunnel and the median nerve (Kamolz et al. 2001). The transverse carpal ligament appears as a thin slightly convex band of 1−1.5 mm thickness (Fig. 26). Its attachments to the pisiform and the scaphoid are readily detected with US.
Fig. 26 a,b. Proximal carpal tunnel and Guyon tunnel. a Schematic drawing and b corresponding transverse 12−5 MHz US image show the proximal level of the carpal tunnel delimited by the scaphoid (Sca) and the pisiform (Pis). The transverse carpal ligament (arrowheads) forms the roof of the carpal tunnel and the floor of the Guyon tunnel. The palmar carpal ligament (light gray) forms the volar boundary of the Guyon tunnel. US image demonstrates the tendons of the flexor digitorum superficialis (s) and profundus (p), the tendons of the flexor pollicis longus (fpl) and flexor carpi radialis (fcr) and the median nerve (straight arrow) extending through the carpal tunnel, with the nerve lying palmar-radially. At the pisiform level, the ulnar nerve (curved arrow) courses medial to the ulnar artery (a) within the Guyon tunnel.