CHAPTER 22 Wrist blocks
Figure 22.1 Axial MR image showing anatomy of relevant structures at the wrist. 1; Median nerve; 2; palmaris longus tendon; 3; ulnar artery; 4; ulnar nerve; 5; radial artery; 6; radial nerve; 7; radius.
The median nerve approaches the wrist between the palmaris longus (if present) and the flexor carpi radialis. It can also lie beneath the palmaris longus (Fig. 22.2). The median nerve provides sensation to the lateral half of the palm, flexor aspect of the thumb, index finger, middle finger, and radial side of the ring finger.
Figure 22.2 Cadaver structures illustrating anatomy pertinent to nerve blocks at the wrist. 1; Flexor carpi ulnaris; 2; ulnar nerve and artery; 3; flexor digitorum superficialis; 4; palmaris longus; 5; median nerve; 6; flexor carpi radialis; 7; superficial branches of the radial nerve; 8; radial artery.
The ulnar nerve, in the middle of the forearm between the flexor digitorum profundus and the flexor carpi ulnaris, gives off a dorsal and a ventral cutaneous branch. At the wrist, the ulnar nerve lies between the ulnar artery and the lateral border of the flexor carpi ulnaris (Fig. 22.2), which inserts on the pisiform bone. Near the pisiform bone, it passes superficial to the flexor retinaculum and ends by dividing into superficial and deep branches. The ulnar nerve provides sensation for the ulnar half of the back and front of the hand, little finger, and ulnar side of the ring finger.
The radial nerve at the wrist lies between the flexor carpi radialis and the radial artery (Fig. 22.3). The radial nerve provides sensation for the radial half of the dorsum of the hand, back of the thumb, and part of the dorsum of the index finger.
Bony landmarks include the ulnar styloid and the radial styloid carpal bones. Other landmarks include the wrist crease, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris tendons (Fig. 22.4). These tendons can be accentuated by having the patient flex the wrist while making a fist. The radial artery can be palpated lateral to the tendon of the flexor carpi radialis, and the ulnar artery lateral to the flexor carpi ulnaris tendon.
The examination begins with the patient supine, the arm abducted, the forearm and wrist in supination (Fig. 22.5). A systematic survey should be performed from superficial to deep and medial to lateral. A high frequency ultrasound transducer is used with a transverse orientation.
The median nerve passes distally in the volar aspect of the forearm between the flexor digitorum superficialis and the flexor digitorum profundus muscles (Fig. 22.6A). Approximately 5 cm proximal to the flexor retinaculum, the median nerve courses around the radial or lateral edge of the flexor digitorum superficialis, where its position becomes more superficial. Just proximal to the carpus, the nerve lies between the tendons of the flexor digitorum superficialis and the flexor carpi radialis, partially deep to the tendon of palmaris longus (if it is present; Fig. 22.6B). The nerve then passes deep to the flexor retinaculum into the carpal tunnel of the wrist.
When scanning in the axial plane, at the level of the carpal tunnel, it is often difficult to differentiate hyperechoic tendons from the normal median nerve. However, when the transducer is moved proximally in the axial plane, several findings on sonography allow identification of the median nerve. First, the median nerve courses around the radial aspect of the flexor digitorum superficialis to ultimately lie between the flexor digitorum superficialis and the flexor digitorum profundus. Second, the hyperechoic tendons of the flexor digitorum superficialis and the flexor digitorum profundus are contiguous with hypoechoic muscle at the musculotendinous junctions, whereas the median nerve remains relatively hyperechoic. The hyperechoic fat surrounding the median nerve accentuates the contrast between the hyperechoic nerve and hypoechoic muscle proximally. Once the median nerve is identified proximally, it can then be followed distally in the transverse-axial plane to the carpal tunnel. The echogenic surface of the pisiform at the ulnar aspect of the wrist defines the proximal carpal tunnel.
The ulnar nerve at the wrist lies within the Guyon canal, an oblique fibro-osseous tunnel, formed by the flexor retinaculum and palmar carpal ligaments, that lies within the proximal part of the hypothenar eminence. The canal contains the ulnar nerve, the ulnar artery with its venae comitantes, and loose fibrofatty tissue. On transverse sonograms, the ulnar nerve appears as a rounded structure with a location medial to the artery (Fig. 22.7).