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 because this allows proper orientation and accurate identification of the structures. 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 (see Fig. 5-2B).⁴ 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 (e.g., in the carpal tunnel) and relatively hyperechoic when surrounded by hypoechoic muscle (e.g., in the forearm). At the wrist crease, the round or oval median nerve is identified by its hypoechoic nerve fascicles, which are most conspicuous surrounded by the adjacent hyperechoic tendons (see Fig. 5-2C). 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).⁴ 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 (see 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 (see 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 flexor carpi radialis tendon (see Fig. 5-2E).⁵ 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 (see 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 (see Fig. 5-3A).

FIGURE 5-2  Carpal tunnel and volar wrist evaluation (transverse).

A, Transverse imaging over the volar wrist crease shows (B and C) the median nerve (arrowheads), flexor carpi radialis (F), palmaris longus (P), radial artery (r), flexor digitorum tendons (f), radius (R), and ulna (U). Transverse imaging distal to the volar wrist crease shows (D) the flexor retinaculum (arrows) and median nerve (arrowheads) within the carpal tunnel (H, hook of hamate; T, trapezium; U, ulnar artery). Transverse imaging proximal at wrist crease (E) shows the palmar cutaneous branch (arrow) of the median nerve (curved arrow) superficial to the flexor retinaculum (arrowheads).

FIGURE 5-3  Volar forearm evaluation (distal, transverse).

Sequential transverse ultrasound images (A and B) proximal to the volar wrist crease show that the median nerve (arrowheads) moves deep between the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS). PQ, pronator quadratus; R, radius, U, ulna.

FIGURE 5-4  Carpal tunnel and volar wrist evaluation (longitudinal).

A, Sagittal imaging over the volar wrist crease shows (B to D) the median nerve (arrowheads), flexor digitorum (F), palmaris longus (p), pronator quadratus (PQ), radius (R), lunate (L), and capitate (C). Note the median nerve in D, which appears relatively hyperechoic proximally and hypoechoic distally (left side of image is proximal).

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 around 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. This joint and the volar recesses are evaluated for anechoic fluid or variable-echogenicity synovial hypertrophy and other joint disorders.

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.⁶ 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 (see 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 (see 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 (see Fig. 5-6C).

FIGURE 5-5  Volar radial wrist evaluation (longitudinal).

A, Sagittal-oblique imaging over the thumb base shows (B and C) the flexor carpi radialis tendon (F) and scaphoid (S). R, radius.

FIGURE 5-6  Volar radial wrist evaluation (transverse).

A, Transverse imaging shows (B) the flexor carpi radialis tendon (F), radial artery (A), and veins (v). Transverse imaging between scaphoid (S) and lunate (L) shows (C) volar portion of the scapholunate ligament (arrowheads). R, radius.

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 (see Fig. 5-2A). Moving the transducer ulnar to the carpal tunnel (Fig. 5-7A), the bone landmark of the pisiform is identified (see 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 (see Fig. 5-7C). It is important to evaluate this area because ulnar nerve and artery abnormalities occur in this location as a result of compression by the adjacent hook of the hamate during trauma. Evaluation of the ulnar artery and nerve is also completed in long axis (Fig. 5-8). Another ulnar-sided tendon, the flexor carpi ulnaris, can also be identified by scanning proximal to the pisiform bone.

FIGURE 5-7  Guyon canal evaluation (transverse).

A, Transverse imaging over Guyon canal shows (B) the ulnar nerve (arrowheads) radial to the pisiform (P). Imaging distal to B over the hook of the hamate (H) shows (C) the superficial (arrows) and deep (open arrow) branches of the ulnar nerve. A, ulnar artery; M, median nerve.

FIGURE 5-8  Guyon canal evaluation (longitudinal).

A, Sagittal imaging shows (B) the ulnar nerve (arrowheads) and hook of the hamate (H) (left side of image is proximal).

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 accurate identification of the wrist tendons. The Lister tubercle is seen as a pronounced bony prominence. If one has difficulty finding this structure with ultrasound, it can easily be palpated at physical examination. Once the Lister tubercle is identified, the tendon immediately ulnar to it is the extensor pollicis longus of the third wrist compartment (see 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 (see Fig. 5-9C), the extensor carpi radialis brevis and then the extensor carpi radialis longus tendons are seen in the second wrist 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 wrist compartment (see 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 first digit 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 and moving distally, the extensor pollicis longus is seen moving in an ulnar to radial direction over the extensor carpi radialis brevis and longus tendons (see 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 (see Fig. 5-9F).

FIGURE 5-9  Dorsal wrist evaluation (extensor compartments 1 to 3).

A, Transverse imaging over the Lister tubercle of the radius shows (B) the extensor pollicis longus tendon (arrowheads) radial to the Lister tubercle (open arrow). C, Transverse imaging radial to A shows (D) the second and first extensor compartment. Distal imaging over the second wrist compartment shows (E) the extensor pollicis longus (arrowheads) moving superficial to the extensor carpi radialis tendons. F, The superficial branch of the radial nerve (arrowheads) can be identified superficial to the first extensor wrist compartment (T) and near a branch of the cephalic vein (v) (arrow, posterior interosseous nerve). A, radial artery; APL, abductor pollicis longus; ECRB/L, extensor carpi radialis brevis and longus; ED, extensor digitorum; EPB, extensor pollicis brevis; R, radius.

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 (see Fig. 5-10B). The posterior interosseous nerve is identified deep within the radial aspect of the fourth dorsal extensor compartment (see Fig. 5-9B).⁷ Finally, over the most ulnar aspect of the ulna, the extensor carpi ulnar tendon is identified in a concave groove of the ulna in the sixth wrist compartment (see Fig. 5-10C). The extensor carpi ulnaris tendon often has a thin hypoechoic longitudinal cleft that should not be interpreted as a tendon tear. In addition, up to 50% of the extensor carpi ulnar tendon can be located outside of the groove and still be considered within normal.⁸

FIGURE 5-10  Dorsal wrist evaluation (extensor compartments 4 to 6).

A, Transverse imaging ulnar to that shown in Figure 5-9A shows (B) the extensor digitorum and extensor indicis (ED), the extensor digiti minimi (arrow), and the distal radioulnar joint between the radius (R) and ulna (U). Transverse imaging over the lateral ulna shows (C) the extensor carpi ulnaris (ECU).

Each of the extensor tendons is also imaged in long axis throughout the wrist (Fig. 5-11). The extensor retinaculum courses transversely but slightly obliquely over the extensor tendons and appears hyperechoic, measuring up to 1.7 mm thick and 23 mm wide in cross section (see Fig. 5-11B).⁹ 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 (see Fig. 5-10B).

FIGURE 5-11  Dorsal wrist evaluation (longitudinal).

A, Sagittal imaging shows (B) the extensor retinaculum (arrowheads), extensor tendons (E), and dorsal recesses of radiocarpal (arrows) and midcarpal (curved arrow) joints. C, capitate; L, lunate; R, radius.

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 (see Fig. 5-9A). The transducer is then moved distally. The bone contours of the radius are interrupted by the radiocarpal joint, so the next osseous structure in view is the scaphoid bone. 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).¹⁰ Directly superficial to the dorsal aspect of the scapholunate ligament, the dorsal radiocarpal ligament (or dorsal radiotriquetral ligament) is identified.^11,12 This area is also a common site for dorsal wrist ganglion cysts.

FIGURE 5-12  Scapholunate ligament evaluation (dorsal component).

Transverse imaging over the proximal carpal row shows the dorsal aspect of the scapholunate ligament (arrowheads). L, lunate; S, scaphoid.

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 (see Fig. 5-13B).¹¹ It is important to ensure complete evaluation of the triangular fibrocartilage to the radial attachment because 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 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 and directly over the ulnar head.

FIGURE 5-13  Triangular fibrocartilage evaluation.

A, Coronal-oblique imaging dorsal to the ulnar styloid shows (B) the triangular fibrocartilage (arrowheads) and the meniscus homologue (M). ECU, extensor carpi ulnaris; L, lunate; R, radius; T, triquetrum; U, ulna.

(Courtesy of Tracy Boon, Ann Arbor, Michigan.)

Volar

At the volar aspect of the finger in long axis (Fig. 5-14A), both the hyperechoic and fibrillar flexor digitorum superficialis and profundus tendons can be seen at the level of the metacarpophalangeal joint with the overlying A1 pulley (see Fig. 5-14B). The pulleys often have a trilaminar appearance at ultrasound. At close inspection, a pulley itself is fibrous, fibrillar, and therefore hyperechoic when imaged perpendicular to the sound beam; however, a normal pulley often appears hypoechoic relative to the adjacent superficial hyperechoic fat and connective tissue and from anisotropy. The trilaminar appearance consists of the superficial reflective surface of the pulley, the relatively hypoechoic pulley, and the deeper hyper-reflective surface of the adjacent flexor tendon sheath. With regard to imaging the flexor tendons in long axis, the individual tendons can be distinguished from each other with isolated passive movement of the distal phalanx because this will cause movement of the flexor digitorum profundus. At the level of the proximal phalanx, the A2 pulley can be identified; slight obliquity of the transducer may make the pulley appear hypoechoic from anisotropy and can aid in its identification (see Fig. 5-14C). At the level of the proximal interphalangeal joint, the hyperechoic volar plate is identified (see Fig. 5-14D and E). The A3 and A4 pulleys are also identified superficial to the flexor tendons, at the level of the proximal interphalangeal joint and middle phalanx, respectively (see Fig. 5-14D).² Just distal to the proximal interphalangeal joint, the flexor digitorum superficialis inserts on the middle phalanx, whereas the flexor digitorum profundus extends distally over the volar plate of the distal interphalangeal joint to insert on the distal phalanx (see Fig. 5-14E). The flexor digitorum superficialis inserts on the middle phalanx by dividing into two bundles, with each segment moving around the flexor digitorum profundus tendon. This is best appreciated by imaging in short axis (Fig. 5-15A and B). More proximally over the palm of the hand (Fig. 5-16), the lumbrical and interosseous muscles can be identified, as can the common and proper palmar digital arteries and nerves. The volar metacarpophalangeal and interphalangeal joints are also evaluated in the sagittal plane for volar plate abnormality and joint recess distention from fluid or synovial disorders.

FIGURE 5-14  Volar finger evaluation (longitudinal).

A, Sagittal imaging of the volar finger (B) at the level of the metacarpophalangeal joint, (C) the proximal phalanx, and (D and E) the middle phalanx shows the flexor digitorum profundus (P) and superficialis (S) tendons and volar plates (V). Note (B) the A1 pulley (arrowheads), (C) A2 pulley (arrows), (D) A3 pulley (arrowheads), and A4 pulley (arrows). Note the attachment of the flexor digitorum superficialis (open arrow). DP, distal phalanx; M, metacarpal; MP, middle phalanx; PP, proximal phalanx.

FIGURE 5-15  Volar finger evaluation (transverse).

A, Transverse imaging over the proximal phalanx (B) and middle phalanx (C) shows the flexor digitorum profundus (FDP) and superficialis (FDS) tendons.

FIGURE 5-16  Palmar hand evaluation (longitudinal).

Parasagittal imaging (A) shows the distal aspect (arrow) of a lumbrical muscle (L). Transverse imaging (B) shows the flexor digitorum superficialis (S) and profundus (P) tendons, the common digital arteries (arrows) and nerves (arrowheads), and interosseous muscles (I). MC, metacarpal.

Dorsal

At the dorsal aspect of each digit, the thin, hyperechoic, and fibrillar extensor digitorum tendon extends over the metacarpophalangeal joint in the sagittal plane (Fig. 5-17A and B). At the level of the proximal interphalangeal joint, the central band of the extensor tendon inserts on the middle phalanx (see Fig. 5-17C and D). With movement of the transducer just off midline of the phalanx, the slips of the extensor tendon to the lateral bands can be seen (see Fig. 5-17E), which insert distally on the distal phalanx (see Fig. 5-17F). The ultrasound beam penetrates through the nail and allows visualization of the underlying hypoechoic nail bed, subungual space, and the surface of the distal phalanx (see Fig. 5-17F). At the level of the metacarpophalangeal joint, the transducer is positioned short axis to the extensor tendon, and the finger is flexed to evaluate for subluxation of the tendon, which would indicate extensor hood injury. The joints of each digit are also evaluated for distention from fluid or synovial hypertrophy, where often the dorsal joint recess is pronounced as it extends proximally beneath the extensor tendon. In addition, the hypoechoic hyaline articular cartilage of each joint can be visualized (see Fig. 5-17B), which is accessible with flexion of the digits (see Fig. 5-17G to I). A triangular region of connective tissue is normally found superficial to the metacarpophalangeal joint articulation (see Fig. 5-17H).¹³

FIGURE 5-17  Dorsal finger evaluation.

A, Sagittal imaging of the dorsal finger shows (B) the extensor tendon (arrowheads), also seen in transverse imaging (C). Note the hypoechoic hyaline cartilage covering the metacarpal head and triangle-shaped area of connective tissue (asterisk). Imaging over the proximal interphalangeal joint shows (D) the attachment of the central band (open arrows). Parasagittal imaging (E) shows a lateral band (arrows), attaching distally (F) on the distal phalanx (DP). Note the nail (arrows). With finger flexion (G), ultrasound images show metacarpal articular cartilage (arrowheads) in the (H) sagittal and (I) transverse planes. Note overlying triangle-shaped connective tissue (asterisk) and extensor tendon. MC, metacarpal head; MP, middle phalanx; PP, proximal phalanx.

Ligaments

The collateral ligaments of the digits can also be assessed with ultrasound in the coronal plane around each individual joint. To specifically evaluate the ulnar collateral ligament of the first metacarpophalangeal joint, the hand is placed around a rolled up towel, and the transducer is placed in the coronal plane relative to the first metacarpophalangeal joint (Fig. 5-18A). The ulnar collateral ligament will appear in long axis as hyperechoic with a compact fibrillar echotexture, extending from a broad concavity in the metacarpal to the proximal phalanx (see Fig. 5-18B