Wrist and Hand Ultrasound

Median Nerve, Flexor Digitorum Tendons, and Volar Joint Recesses

The primary structures evaluated from the volar aspect at midline are the median nerve, the flexor tendons, and the volar aspects of the wrist joints. Examination begins short axis to the tendons and median nerve in the center of the wrist. For evaluation of the median nerve, the transducer is placed in the transverse plane at the level of the wrist crease, which is at the proximal aspect of the carpal tunnel ( Fig. 5.2 ). Normal peripheral nerves have a honeycomb appearance when they are imaged in short axis from hypoechoic nerve fascicles and surrounding hyperechoic connective tissue ( Fig. 5.2B ). Toggling the transducer to angle the sound beam along the long axis of the median nerve will help to show the characteristic appearance of the nerve when the sound beam is perpendicular ( Video 5.1 ). Because peripheral nerve trunks are composed of both hypoechoic and hyperechoic elements, the median nerve appears relatively hypoechoic when surrounded by hyperechoic tissue in the carpal tunnel and relatively hyperechoic when surrounded by hypoechoic muscle in the forearm. At the wrist crease, the median nerve is identified by its hypoechoic nerve fascicles, which are most conspicuous surrounded by the adjacent hyperechoic tendons. If differentiation between the median nerve and adjacent tendons is difficult, median nerve identification is easily accomplished with movement of the transducer proximally in the transverse plane. During this maneuver, the normal median nerve courses radial to the flexor tendons and then moves ulnar and deep between the flexor digitorum superficialis and profundus ( Fig. 5.3 ) ( Video 5.2 ). In addition, the median nerve now appears relatively hyperechoic as a result of the connective tissue and fat because it is surrounded by hypoechoic muscle. The characteristic course, location, and echogenicity assist in identification of the median nerve. An additional method to differentiate the median nerve from the adjacent flexor tendons at the wrist crease in the transverse plane is angulation of or toggling the transducer along the long axis of the tendons. This maneuver causes the hyperechoic tendons to become hypoechoic as a result of anisotropy, whereas the hypoechoic median nerve fascicles remain unchanged ( Video 5.1 ). Evaluation of the median nerve is then continued distally into the carpal tunnel, where the thin and hyperechoic flexor retinaculum can be visualized, which attaches to the pisiform and trapezium proximally ( Fig. 5.2C ) and the hamate and scaphoid distally ( Fig. 5.2D ). A small branch of the median nerve, the palmar cutaneous branch, originates proximal to the carpal tunnel and courses superficial to the flexor retinaculum and ulnar to the flexor carpi radialis tendon ( Fig. 5.2E ). The thenar motor branch can also be visualized at the carpal tunnel outlet originating from the palmar ulnar aspect of the radial division of the median nerve, coursing vertically in a palmar direction and extending proximal between the abductor pollicis brevis and flexor pollicis brevis ( Fig. 5.2F ). The transducer is turned 90 degrees to visualize the median nerve in long axis ( Fig. 5.4 ) ( Video 5.3 ). The variable appearance of peripheral nerve echogenicity relative to the surrounding tissue echogenicity is well demonstrated when imaging the median nerve in long axis in the distal forearm ( Fig. 5.4D ). Proximal to the wrist joint in the transverse plane, the pronator quadratus can be identified extending between the distal radius and ulna, a soft tissue landmark that is used for proximal measurement of the median nerve ( Fig. 5.3A ).

Attention is then turned back to the flexor tendons, with each tendon evaluated in both short axis and long axis. The flexor digitorum superficialis and profundus are identified about the median nerve as described earlier, proximally as hypoechoic muscle and distally as fibrillar and hyperechoic tendons (see Figs. 5.2 and 5.3 ). Just beyond the wrist crease, the thin hyperechoic flexor retinaculum is seen as it extends from the proximal scaphoid pole to the pisiform and from the trapezium to the hook of the hamate, which represents the roof the carpal tunnel (see Fig. 5.2D ). If the retinaculum is not imaged perpendicular to the ultrasound beam, it will appear hypoechoic as a result of anisotropy. The flexor digitorum tendons travel through the carpal tunnel to the digits, whereas the palmaris longus tendon, typically directly superficial to the median nerve, remains outside of the carpal tunnel. In the sagittal plane and long axis to the flexor tendons, the volar radiocarpal and midcarpal joint recesses are identified by the adjacent bone contours; the volar lip of the distal radius, the lunate bone, and the capitate bone have characteristic shapes (see Fig. 5.4B and C ). Between the distal radius and the lunate is the volar recess of the radiocarpal joint, and between the lunate and capitate bones is the volar recess of the midcarpal joint. The distal radioulnar joint is identified with placement of the transducer in the transverse plane between the distal radius and ulna.

Scaphoid, Flexor Carpi Radialis Tendon, Radial Artery, and Volar Ganglion Cysts

Evaluation of the radial aspect of the volar wrist begins in the transverse plane at the wrist crease. In this position, the various tendons of the volar wrist are identified. Just radial to the median nerve and somewhat similar in size is the flexor carpi radialis tendon, located outside the carpal tunnel in its own fibro-osseous canal (see Fig. 5.2B ). Ultrasound evaluation is completed in both long and short axis from proximal to the distal insertion of the flexor carpi radialis tendon on the second and third metacarpals, although some fibers insert onto the trapezium tuberosity. With placement of the transducer over the distal aspect of the flexor carpi radialis tendon in long axis ( Fig. 5.5 ), the characteristic bilobed or peanut-shaped bone contours of the scaphoid bone are identified deep to this tendon ( Fig. 5.5C ). The normal smooth and hyperechoic bone surface of the scaphoid bone is evaluated for cortical step-off fracture. Returning to the wrist crease in the transverse plane ( Fig. 5.6A ), the radial artery and veins are identified immediately radial to the flexor carpi radialis tendon ( Fig. 5.6B ). With the flexor carpi radialis tendon and radial artery in view, the transducer is moved both proximally and distally from the radiocarpal joint to evaluate for ganglion cysts. Placement of the transducer in the transverse plane between the scaphoid and lunate will show the normal hyperechoic and fibrillar volar component of the scapholunate ligament ( Fig. 5.6C ).

Ulnar Artery, Vein, and Nerve (Guyon Canal)

Evaluation of the ulnar aspect of the volar wrist begins in the transverse plane at the wrist crease. Moving the transducer ulnar to the carpal tunnel ( Fig. 5.7A ), the bone landmark of the pisiform is identified ( Fig. 5.7B ). Between the pisiform and the ulnar artery, the ulnar nerve is identified as hypoechoic nerve fascicles and surrounding hyperechoic connective tissue. The ulnar veins are usually not visible because they are easily compressed by pressure of the ultrasound transducer. As the transducer is moved distally, the hyperechoic and shadowing surface of the hook of the hamate is seen deep to the ulnar nerve and artery. The ulnar nerve branches, with a deep motor branch coursing along the ulnar side of the hamate hook and one to two predominantly sensory branches superficial to the hamate hook ( Fig. 5.7C ). This area should be evaluated as trauma can cause ulnar nerve and artery abnormalities as a result of compression by the hook of the hamate. Proximal to the pisiform, the flexor carpi ulnaris is identified as well as the dorsal cutaneous branch of the ulnar nerve coursing from volar to dorsal beneath the flexor carpi ulnaris ( Fig. 5.7D ). Evaluation of the ulnar artery and nerve is also completed in long axis ( Fig. 5.8 ).

Dorsal Wrist Tendons and Dorsal Joint Recesses

The primary structures of the dorsal wrist are the various extensor and abductor tendons of the six wrist compartments, and the dorsal radiocarpal, midcarpal, and distal radioulnar joint recesses. Evaluation of the dorsal tendons begins in the transverse plane over the Lister tubercle of the dorsal radius ( Fig. 5.9A and B ). This structure serves as an important starting point for dorsal wrist evaluation and assists in orientation and accurate identification of the wrist tendons. The Lister tubercle is seen as a pronounced bony prominence. Once the Lister tubercle is identified, the tendon immediately ulnar to it is the extensor pollicis longus of the third extensor compartment ( Fig. 5.9B ). Often there is an additional smaller dorsal radial protuberance at the ulnar aspect of the extensor pollicis longus as well. With movement of the transducer in the radial direction ( Fig. 5.9C ), the extensor carpi radialis brevis and then the extensor carpi radialis longus tendons are seen in the second extensor compartment ( Fig. 5.9D ). On further radial movement of the transducer, the extensor pollicis brevis and abductor pollicis longus tendons are seen in the first extensor compartment ( Fig. 5.9D ). It may be helpful to remember that the names of the tendons alternate from longus to brevis, beginning at the extensor pollicis longus and moving in a radial direction. The extensor pollicis longus tendon courses toward the thumb superficial to the extensor carpi radialis and ulnaris tendons in an oblique fashion proximally to distally. Therefore when short axis to the extensor carpi radialis brevis and longus tendons are moving distally, the extensor pollicis longus is seen moving in an ulnar to radial direction over the extensor carpi radialis brevis and longus tendons, a potential site of intersection syndrome ( Fig. 5.9E ) ( Video 5.4 ). In the region of the first extensor wrist compartment, the superficial branch of the radial nerve can be seen as it courses from the volar to the dorsal aspect of the distal forearm superficial to the first extensor wrist compartment tendons and extensor retinaculum, near branches of the cephalic vein ( Fig. 5.9F ).

Beginning again in the transverse plane at the Lister tubercle, transducer movement ulnar from the extensor pollicis longus tendon ( Fig. 5.10A ) shows the extensor indicis and multiple tendons of the extensor digitorum in the fourth wrist compartment, and the extensor digiti minimi in the fifth wrist compartment near the distal radioulnar joint ( Fig. 5.10B ). The posterior interosseous nerve is identified deep within the radial aspect of the fourth dorsal extensor compartment ( Fig. 5.9B ). Over the most ulnar aspect of the ulna, the extensor carpi ulnaris tendon is identified in a concave groove of the ulna in the sixth extensor compartment ( Fig. 5.10C ). The extensor carpi ulnaris tendon often has a normal thin hypoechoic longitudinal cleft that should not be interpreted as a tendon tear. The dorsal retinaculum and the deeper subsheath stabilize the extensor carpi ulnaris, with the latter attaching to the ulna. Up to 50% of the extensor carpi ulnar tendon can be located outside of the groove and still be considered normal.

Each of the extensor tendons is also imaged in long axis throughout the wrist ( Fig. 5.11 ). The extensor retinaculum courses transversely but slightly oblique over the extensor tendons and appears hyperechoic, measuring up to 1.7 mm thick and 23 mm wide in cross section ( Fig. 5.11B ). If imaged oblique to the ultrasound beam, the extensor retinaculum can appear artifactually hypoechoic as a result of anisotropy, which should not be misinterpreted as tenosynovitis. Similar to the volar wrist, the dorsal recesses of the radiocarpal joint (between the radius and proximal carpal row), the midcarpal joint, and the distal radioulnar joint are identified with recognition of the characteristic bone contours for orientation. The radiocarpal and midcarpal joint recesses are optimally evaluated in the sagittal plane, whereas the distal radioulnar joint is evaluated in the transverse plane ( Fig. 5.10B ).

Scapholunate Ligament (Dorsal Component) and Dorsal Ganglion Cysts

Similar to the dorsal tendons, evaluation of the scapholunate ligament begins in the transverse plane over the Lister tubercle ( Fig. 5.9A ). As the transducer is then moved distally, the bone contours of the radius are interrupted by the radiocarpal joint, and the next osseous structure visualized is the scaphoid. With movement of the transducer in the ulnar direction, the adjacent lunate bone is brought into view. Between the dorsal aspects of the scaphoid and the lunate is a triangular area where one sees the dorsal aspect of the scapholunate ligament, which has a compact hyperechoic fibrillar echotexture ( Fig. 5.12 ). Directly superficial to the dorsal aspect of the scapholunate ligament, the dorsal radiocarpal ligament (or dorsal radiotriquetral ligament) is identified. This area is a common site for dorsal wrist ganglion cysts.

Triangular Fibrocartilage Complex

The triangular fibrocartilage complex consists of the triangular fibrocartilage, the meniscus homologue, the extensor carpi ulnaris tendon sheath, and the volar and dorsal radiocarpal ligaments. For evaluation of the triangular fibrocartilage, the transducer is placed in the sagittal plane over the dorsal lateral wrist to identify the bone contours of the distal ulna and then moved toward the coronal plane with the wrist in slight radial deviation ( Fig. 5.13A ). A hyperechoic slab of tissue is identified as it extends from the ulnar styloid base to the radius, which represents the triangular fibrocartilage ( Fig. 5.13B ). The radial attachment of the triangular fibrocartilage must be identified to ensure complete evaluation as this may be a site of traumatic tears. Evaluation of the triangular fibrocartilage can be difficult given its orientation in the transverse plane extending away from the transducer, and often a lower frequency transducer is helpful. The meniscus homologue is seen as a hyperechoic triangular structure with its base adjacent to the extensor carpi ulnar tendon and in contact with the triquetrum, and this should not be mistaken for the triangular fibrocartilage, which is thinner, more proximal, and directly over the ulnar head.