Carpal tunnel: Normal anatomy, anatomical variants and ultrasound technique

Abstract

The carpal tunnel is an osteofibrous canal situated in the volar wrist. The boundaries are the carpal bones and the flexor retinaculum. In addition to the medial nerve, the carpal tunnel contains nine tendons: the flexor pollicis longus, the four flexor digitorum superficialis and the four flexor digitorum profundus. Ultrasound (US) study of the carpal tunnel generally involves short-axis imaging of the tendons, and in the presence of disease, long-axis imaging and dynamic maneuvers are added. There are numerous reports of anatomical variants of the wrist involving vessels, nerves, tendons and muscles, and they can all be studied by US. Some are particularly relevant from a clinical point of view and will therefore be accurately described. The anatomy is complex, and the US operator should therefore be thoroughly familiar with the normal anatomy as well as the anatomical variants that may have a role in the pathogenesis of carpal tunnel syndrome or influence treatment.

Keywords: Carpal tunnel; Flexor tendons; Median nerve; Ultrasound.

Fig. 1

Proximal portion of the carpal tunnel. The carpal tunnel is an osteofibrous canal situated in the volar wrist. The boundaries are the carpal bones and the flexor retinaculum (A). The carpal tunnel contains nine tendons: the flexor pollicis longus, the four flexor digitorum superficialis and the four flexor digitorum profundus as well as the median nerve (B, C). The retinaculum (arrows) appears as a ribbon-like structure, hypoechoic (B) or hypointense (C), proximally it inserts to the tuberosity of the scaphoid and the pisiform.

Fig. 2

Distal carpal tunnel. Distally the retinaculum, which is about 3–4 cm wide, inserts to the trapezium and the hook of the hamate (arrows).

Fig. 3

Coronal three-dimensional reconstruction of the median nerve. The median nerve is located at the carpal tunnel immediately below the flexor retinaculum, superficial to the flexor pollicis longus and the superficial flexor tendons to the index and middle fingers.

Fig. 4

Persistent median artery. In adults, the hand receives blood from the ulnar and radial arteries. However, some people have a third artery, the median artery (A, B), which normally regresses during the second month of intrauterine life.

Fig. 5

Persistent median artery associated with bifid median nerve. The median artery (arrows) is situated between the two nerves (A); identification can be facilitated by color-Doppler examination (B).

Fig. 6

Veins travelling in the carpal tunnel. In addition to the structures which are normally situated in the carpal tunnel, the image shows a median artery and two veins (arrows) (A). Differentiation between artery and vein is possible by using the transducer to compress the wrist; unlike the artery the vein will collapse (B). A reversed palmaris longus muscle is evident on the volar side compared to the vascular-nervous structures.

Fig. 7

Bifid median nerve. Proximal bifurcation (A, B) is a quite common anatomic variant. US shows two nerves travelling adjacent to each other in the distal forearm (A).

Fig. 8

Median nerve divided into three branches. Proximal trifurcation is an extremely rare anatomical variant. US image shows three nerves travelling in the carpal tunnel just below the flexor retinaculum (arrows).

Fig. 9

Reversed palmaris longus. Normally, the muscle belly in the palm is in proximal position and the tendon in distal position, but in some people this position is reversed and the tendon is in proximal position and the muscle belly in distal position. In this variant, the muscle often enters into conflict with the carpal tunnel structures and may cause carpal tunnel syndrome.

Fig. 10

Flexor digitorum superficialis with the muscle belly in the carpal tunnel. Normally only the flexor digitorum tendons travel in the carpal tunnel whereas the muscle belly stops before reaching it; this variant can cause carpal tunnel syndrome.