A high frequency transducer – 12 MHz or higher – is used to evaluate the wrist, as most structures are superficial.
A standard wrist ultrasound starts with evaluating the dorsal aspect, followed by an assessment of the ventral side. Comparing findings with the contralateral wrist can be helpful in identifying asymmetries between normal and pathological wrists.
Normal tendons have variable echogenicity, depending on the relative orientation of the transducer and the structure being scanned. When the sound waves reflect off the tendon at 90 degrees, it will appear as echogenic structures characterized by multiple parallel fibrillar lines in the longitudinal view, and a dot-like pattern in the transverse view. This can be achieved by tilting or rocking the transducer until the tendon displays its normal echogenic appearance. When imaged at less than 90 degrees the strength of the reflections decreases, the tendon becomes hypoechoic, and the fibrillar pattern is lost. They are surrounded by anechoic synovial fluid and a thin echogenic sheath. Nerves, on the other hand, consist of multiple hypoechoic fascicles arranged in parallel and encased by an echogenic perineurium. In the transverse plane, nerves exhibit a distinctive "honeycomb-like" appearance.
Dorsal wrist
Depending on the specific clinical presentation, US images can be obtained in different positions of the wrist (flexion and extension, radial and ulnar deviation, pronation and supination).
Typically, the evaluation begins by identifying a specific tendon and systematically tracing it in short-axis views down to its distal insertion. Long-axis ultrasound images of the extensor tendons are generally less informative but can be useful for assessing tendon integrity and detailed dynamic movement.
Fig. 1 is a schematic display of the dorsal tendon compartments in relation to the radius, Lister's tubercle - a bony prominence on the distal radius - , and the ulna.
First compartment – Firstly position the patient’s wrist in a neutral position, midway between pronation and supination. Place the probe over the lateral side of the radial styloid to assess the first extensor tendon compartment, which includes the abductor pollicis longus (ventral) and extensor pollicis brevis (dorsal). The extensor retinaculum is depicted as a hypoechoic line over the tendons and should be checked for the presence of a vertical septum that may divide the compartment into two separate sections.
Second compartment – With the palm facing down on the examination table, move the probe medially in the transverse plane to visualize the second extensor compartment. On the radial side of the Lister’s tubercle, this compartment contains the extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB) tendons, which attach to the second and third proximal metacarpal bones, respectively.
Third compartment – The third extensor compartment, located on the ulnar side of Lister’s tubercle, contains the extensor pollicis longus (EPL) tendon. (Fig. 5)
Fourth compartment – The fourth extensor compartment contains the extensor digitorum communis (EDC) and extensor indicis proprius (EIP) tendons. This can be assessed by placing the transducer on the transverse plane over the mid dorsal wrist. Dynamic assessment during finger flexion and extension can help distinguish the individual tendons within the fourth compartment. (Fig. 7)
Fifth compartment – The fifth compartment lies over the distal radio-ulnar joint and contains the extensor digiti minimi tendon (EDM). (Fig. 8)
Sixth compartment – The sixth extensor compartment is located between the ulnar head and styloid process, housing the extensor carpi ulnaris (ECU) tendon, which sits within a shallow ulnar groove. For proper evaluation, position the wrist in slight radial deviation with the thumb facing downward. (Fig. 9)
Sonography allows, at least, partially visualization of some of the most important ligament structures in the wrist, with particular importance the scapholunate ligament.
Scapholunate ligament – With the Lister’s tubercle as the starting point, in a transverse plane, move the probe distally to visualize the dorsal portion of the scapholunate ligament. To enhance assessment of its integrity, position the wrist in ulnar deviation. (Fig. 11)
Radioulnar joint – Examine the distal radioulnar joint by placing the probe on the transverse plane at a more proximal level than the joint line, where the capsule has a greater compliance to distension. (Fig. 12)
Radiocarpal and midcarpal joints – Based on the hyperechoic profile of the carpal bones, identify the synovial recess of the radiocarpal and midcarpal joints in long-axis planes. Assess for the presence of effusion or synovial thickening. (Fig. 13)
Ventral wrist
Proximal carpal tunnel
When examining the volar aspect of the wrist, position the patient with the dorsal wrist resting on the examination table. Identify the bony landmarks of the proximal carpal tunnel—the scaphoid tubercle on the radial side, and the pisiform on the ulnar side— and place by placing the probe in the axial plane so that one edge rests over the scaphoid and the other over the pisiform. Tilting the probe back and forth helps with identification of the different structures within the canal.
Evaluate the flexor retinaculum and the nine long flexor tendons: four from the flexor digitorum superficialis, four from the flexor digitorum profundus, and the flexor pollicis longus on the radial side. Dynamic scanning during passive flexion and extension of the corresponding fingers can aid in tendon identification and assessing integrity. Additionally, examine the contents of the carpal tunnel for any abnormalities, such as anomalous muscles or flexor tenosynovitis. (Fig. 14)
Distal carpal tunnel
Move the probe to a more distal transverse plane to locate the two bony landmarks of the distal carpal tunnel: the trapezium tubercle on the radial side and the hamate hook on the ulnar side.
Slide the transducer up and down over the median nerve. Examine the median nerve in its short-axis, starting at the distal radius (cranial to the proximal edge of the retinaculum) and continuing through the palm (past the distal edge of the retinaculum). Look for anatomical variations, such as a bifid nerve or persistent median artery, and note any changes in the nerve’s cross-sectional area.
Guyon tunnel:
Guyon’s tunnel is a fibro-osseous canal space superficially on the ulnar side of the carpal tunnel formed by the flexor retinaculum and palmar carpal ligaments. It is bordered by the pisiform on the ulnar side and the hook of the hamate on the radial side. It contains the ulnar nerve (ulnar side), ulnar artery (radial side), as well as ulnar vein(s). It can be found by moving the probe medially from the carpal tunnel. (Fig. 15)
