DOI: 10.1148/rg.256055028
RadioGraphics 2005;25:1577-1590
© RSNA, 2005
Normal Sonographic Anatomy of the Wrist and Hand1
Justin C. Lee, FRCR and
Jeremiah C. Healy, FRCR
1 From the Department of Radiology, Chelsea and Westminster Hospital, 369 Fulham Rd, London SW10 9NH, England. Recipient of a Certificate of Merit award for an education exhibit at the 2004 RSNA Annual Meeting. Received February 11, 2005; revision requested March 10 and received May 18; accepted May 23. Both authors have no financial relationships to disclose.
Address correspondence to J.C.L. (e-mail: justin.lee{at}chelwest.nhs.uk).
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Abstract
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The advent of ultra-high-frequency sonographic transducers has significantly enhanced our ability to image superficial structures. As a result, sonography now can be used to assess injuries of the tendons in the wrist and hand. A clear understanding of normal sonographic anatomy is required to prevent misdiagnosis and ensure optimal patient care. The anatomy of the wrist and hand is best described by considering the extensor and flexor surfaces separately. The carpal extensor retinaculum divides the dorsal extensor tendons into six separate synovial compartments, which are demarcated by the points of its attachment to the radius and ulna. The course of these tendons from the wrist to the sites of their insertion can be traced by using sonography. The intrinsic wrist ligaments, triangular fibrocartilage, and dorsal finger extensor hood also can be assessed sonographically. The anatomy of the flexor surface of the wrist is defined principally by the flexor retinaculum. The median nerve, which is located deep to the retinaculum in the carpal tunnel, and the ulnar nerve, which is superficial to the retinaculum in the Guyon canal, can be easily detected. The long flexor tendons in the wrist and hand are also clearly depicted at sonography. The flexor annular pulley system is formed by five foci of thickening along the long flexor finger tendon synovial sheath, and the second and fourth annular pulleys can be identified sonographically in most patients. Sonography provides a rapid, cheap, noninvasive, and dynamic method for examination of the soft-tissue structures of the wrist and hand. Familiarity with the appearance of normal anatomic structures is a prerequisite for reliable interpretation of the resultant sonograms.
© RSNA, 2005
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Introduction
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Recent advances in ultrasound transducer technology have led to the development of very-high-frequency probes that allow imaging of superficial structures with exquisite detail. The fine spatial resolution, speed of examination, and dynamic assessment make sonography useful for the evaluation of superficial tendon injuries in the wrist and hand. The long flexor and extensor tendons, many of the major ligaments, and the retinacula of the wrist and hand can be assessed with sonography. However, the successful application of sonography for this purpose requires specific knowledge and experience. The purpose of this article is to familiarize the reader with the normal structures of the wrist and hand that can be reliably identified by using ultra-high-frequency transducers with currently available sonographic systems. An initial comment about sonographic technique is followed by a detailed description and illustration of the extensor tendon surface of the wrist and hand. The extensor retinaculum, which defines the anatomy of the dorsum of the wrist, forms six separate synovial compartments by means of its attachment to the radius and ulna. Sonograms are used in this article to illustrate the contents of each compartment. Images of the scapholunate and lunatotriquetral ligaments, triangular fibrocartilage, ulnar collateral ligament of the thumb, and extensor hood of the finger complete the illustration of the dorsal surface. Next, the flexor surface of the wrist, from the distal radius and ulna to the distal phalanx of the finger, is described, and the sonographic features of the flexor retinaculum, long flexor tendons, median and ulnar nerves, and muscles of the thenar eminence are illustrated with images. Last, an illustrated description of the finger flexor annular pulley system is given.
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Sonographic Technique
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The images included in this article were acquired in 10 healthy volunteers. Individuals with a history of previous trauma, arthritis, or any pathologic condition of the tendons were excluded from study. All images were acquired by using a commercially available sonographic system (Antares Sonoline; Siemens Medical Solutions, Malvern, Pa) with a 5–13-MHz linear-array transducer. Tissue harmonic imaging was not used, although experience in previous studies suggests that it may help to improve tissue contrast and spatial resolution (1).
The volunteer sat on a chair opposite the radiologist, with the hand placed in an appropriate position for imaging of the specific areas of interest. A large standoff mound of gel was used to allow optimal visualization of the most superficial structures. Transverse, longitudinal, and extended-field-of-view images were obtained.
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Structure
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Normal tendon consists of fascicles of type I collagen, which are oriented mainly parallel to the long axis. Sonographically, tendons are echogenic fibrillar structures that consist of multiple parallel lines in longitudinal planes and multiple dotlike echoes in transverse planes (Fig 1). The tendon is surrounded by the paratenon and epitendineum or, where greater latitude in relation to adjacent structures is required for movement (eg, where tendons pass under ligamentous bands, retinacula, fascial slings, and osseofibrous tunnels), by a synovial sheath. On sonograms, the synovial sheath is depicted as a thin echogenic fluid-containing structure that surrounds the echogenic tendon fibers (Fig 2). The synovial fluid is usually but not always anechoic. A small amount of fluid is frequently seen within the extensor tendon sheaths of the wrist or hand in individuals with normal anatomy.
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Figure 1a. Normal sonographic appearance of tendons in the wrist. (a) Longitudinal sonogram of the flexor surface of the wrist depicts the flexor digitorum superficialis (FDS) tendon at its junction with the muscle. Note the typical linear fibrillar appearance of the tendon. (b) Transverse sonogram at the same level as a shows the musculotendinous junctions of the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP). The tendons appear as hypoechoic fibrils in echogenic fascicles surrounded by an echogenic epitendineum.
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Figure 1b. Normal sonographic appearance of tendons in the wrist. (a) Longitudinal sonogram of the flexor surface of the wrist depicts the flexor digitorum superficialis (FDS) tendon at its junction with the muscle. Note the typical linear fibrillar appearance of the tendon. (b) Transverse sonogram at the same level as a shows the musculotendinous junctions of the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP). The tendons appear as hypoechoic fibrils in echogenic fascicles surrounded by an echogenic epitendineum.
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Figure 2. Transverse sonogram shows the extensor surface of the wrist at the level of the distal carpal row, with a normal small volume of anechoic synovial fluid in the tendon sheath between the extensor tendons.
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Ligaments, like tendons, consist mainly of type I collagen fibers, typically oriented in sheets that may be grouped together in bundles. At sonography, ligaments should appear as echogenic fibrillar structures. However, as the ultrasound beam interacts with multiple parallel interfaces such as ligament or tendon fibers, the beams may be reflected away from the transducer if the probe is not held exactly perpendicular to the structure (Fig 3). This effect results in anisotropy artifact (ie, an apparent area of reduced echogenicity in the ligament or tendon on the acquired image), which should not be misinterpreted as a pathologic entity. Rocking the transducer backward and forward over the ligament or tendon in the longitudinal axis (also referred to as heel-toeing) helps depict the normal echogenic fibrillar pattern. Anisotropy artifact is most noticeable in tendons and ligaments, but it also may appear to a lesser extent in muscles and nerves depicted on sonograms.
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Figure 3a. Anisotropy artifact. Transverse sonograms of the extensor surface of the wrist show the extensor digitorum (ED) and extensor pollicis longus (EPL) tendons, clearly and without artifact on the image obtained with the probe held exactly perpendicular to the tendons (a), but with a significant loss of echogenicity on the image obtained with the probe held at an oblique angle to the tendons (b).
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Figure 3b. Anisotropy artifact. Transverse sonograms of the extensor surface of the wrist show the extensor digitorum (ED) and extensor pollicis longus (EPL) tendons, clearly and without artifact on the image obtained with the probe held exactly perpendicular to the tendons (a), but with a significant loss of echogenicity on the image obtained with the probe held at an oblique angle to the tendons (b).
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Nerves are composed of multiple axons that are bundled together in neuronal fascicles. Thin concentric layers of dense connective tissue, the perineurium, surround each fascicle. The fascicles are further grouped and held together by a matrix of loose connective tissue, the epineurium. At sonography, peripheral nerves are depicted as multiple parallel hypoechoic areas (groups of fascicles) surrounded by echogenic perineurium and/or epineurium (Fig 4) (2).
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Figure 4a. Normal sonographic appearance of nerves. (a) Transverse sonogram of the median nerve in the distal forearm shows multiple hypoechoic groups of fascicles surrounded by the echogenic perineurium and epineurium, as well as an unusually prominent but normal median artery. (b) Longitudinal sonogram of the median nerve shows parallel hypoechoic groups of nerve fascicles and the median nerve, which lies deep to the flexor digitorum superficialis (FDS) muscle in the distal forearm.
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Figure 4b. Normal sonographic appearance of nerves. (a) Transverse sonogram of the median nerve in the distal forearm shows multiple hypoechoic groups of fascicles surrounded by the echogenic perineurium and epineurium, as well as an unusually prominent but normal median artery. (b) Longitudinal sonogram of the median nerve shows parallel hypoechoic groups of nerve fascicles and the median nerve, which lies deep to the flexor digitorum superficialis (FDS) muscle in the distal forearm.
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Extensor Surface of the Wrist
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The pronate wrist is placed over a pillow, a small towel, or a bag of fluid to achieve passive flexion of the radiocarpal joint. A large standoff mound of coupling gel is used to achieve clear depiction of the underlying structures. The transducer is placed transversely across the wrist for the initial assessment.
Two key structures define the sonographic anatomy of the extensor surface of the wrist. They are the extensor retinaculum and the dorsal tubercle of the radius (Lister tubercle). The extensor retinaculum is a strong fibrous band that extends obliquely across the dorsum of the wrist. The extensor retinaculum has several deep attachments along its course, and these divide its surface into six separate compartments. With the body in the anatomic position, the medial (ulnar) attachment at the extensor retinaculum is to the triquetrum and pisiform bones. The retinaculum then passes radially across the dorsum of the wrist, to the site of its attachment at the anterior border of the distal radius. Deep to the extensor retinaculum are six tunnels that are formed by its attachments to the radius and ulna. Each tunnel contains a single synovial sheath that surrounds one or more extensor tendons.
Compartment 1
With the body in the anatomic position, this compartment is lateral to the radial styloid process for the extensor pollicis brevis and abductor pollicis longus tendons (Fig 5). The first extensor compartment is involved in de Quervain stenosing tenosynovitis, in which the tendon sheath becomes thickened without any apparent cause and entraps the extensor pollicis brevis and abductor pollicis longus tendons, causing pain in, and restricted movement of, the thumb. A thick fibrous band may separate the extensor pollicis brevis and abductor pollicis longus tendons. It is important that this fibrous band, if present, be recognized at sonography prior to surgery for de Quervain tenosynovitis, because the band must be divided for successful surgical treatment. In addition, for successful treatment with steroid injection, the needle must penetrate both subcompartments (3).
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Figure 5. Transverse sonogram through the anatomic snuffbox shows the tendons of the first extensor compartment: the extensor pollicis brevis (EPB) and the abductor pollicis longus (APL). The slip from the extensor pollicis brevis to the extensor pollicis longus (slip to EPL) is an uncommon but normal variant. Note the location of the radial artery (radial a.) and the accompanying veins (*), deep to the tendons.
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Compartment 2
This compartment is located on the radial side of the Lister tubercle, over the radial styloid process for the extensor carpi radialis longus and extensor carpi radialis brevis tendons (Fig 6). The tendons pass through the second compartment and insert onto the second and third metacarpal bases, respectively.
Compartment 3
This compartment is located on the ulnar side of the Lister tubercle. It contains the extensor pollicis longus tendon, which curves around the Lister tubercle (Fig 7) and crosses the tendons of the second extensor compartment before passing up the thumb to the site of its insertion at the base of the distal phalanx. The extensor pollicis longus tendon forms the lateral border of the anatomic snuffbox, the floor of which contains the radial styloid, scaphoid, trapezium, and thumb metacarpal base. The extensor pollicis longus tendon is at risk of rupture during healing (4–10 weeks) of a distal radius fracture; rupture occurs in approximately 3% of cases (4). The Lister tubercle occasionally is groove shaped, and, in this situation,the extensor pollicis longus tendon may be seated in the groove instead of curving around the tubercle (Fig 7) (5).
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Figure 7. Transverse sonogram shows the third extensor compartment, which contains the extensor pollicis longus (EPL) tendon, and its location between the neighboring extensor digitorum (ED) and extensor carpi radialis brevis (ECRB) tendons.
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Compartment 4
The compartment is situated on the ulnar side of the third compartment, over the distal radius for the extensor digitorum and extensor indicis tendons (Figs 8, 9). Dynamic scanning can help differentiate between the tendons. The extensor retinaculum reaches its maximum thickness over the fourth compartment (Fig 8).
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Figure 8. Transverse sonogram shows the fourth extensor compartment. The extensor retinaculum (ER) is identified as a thick hypoechoic band (arrowheads) above the extensor digitorum (ED) and extensor pollicis longus (EPL) tendons. Note the slip from the extensor retinaculum to the distal radius, which forms the radial border of the fourth compartment.
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Figure 9. Transverse sonogram shows the third through the fifth extensor compartments. Note the position of the extensor digiti minimi (EDM) tendon, above the distal radioulnar (RU) joint. ED = extensor digitorum tendon, EI = extensor indicis tendon, EPL = extensor pollicis longus tendon.
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Compartment 5
The fifth compartment is dorsal to the interval between the radius and the ulna, and it accommodates the extensor digiti minimi tendon (Fig 9).
Compartment 6
The sixth compartment is situated between the head and styloid process of the ulna, for the extensor carpi ulnaris tendon (Fig 10). The extensor carpi ulnaris tendon and synovial sheath are frequently the first parts of the anatomy to become inflamed in rheumatoid arthritis (6). The associated pannus formation may be responsible for the erosion of the styloid process of the ulna, an early radiographic sign of rheumatoid arthritis (7).
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Intrinsic Ligaments of the Wrist
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The two most important intrinsic ligaments of the wrist are the scapholunate and lunatotriquetral ligaments. Disruption of these may result in pain, instability, and carpal dissociation. Injury to the triangular fibrocartilage may occur in association with injuries to these ligaments or in isolation and may be responsible for similar clinical symptoms. These three structures can be visualized, at least in part, by using sonography.
Scapholunate and Lunatotriquetral Ligaments
The forearm is placed prone, and the wrist is positioned over a volar-placed pad or rolled towel to achieve slight flexion for sonography of the ligaments. The scapholunate ligament is depicted on transverse sonograms as a compact triangular echogenic fibrillar structure between the lunate and scaphoid, just distal to the Lister tubercle (Fig 11). The dorsal bundle of the scapholunate ligament is depicted at sonography in approximately 80% of individuals with normal anatomy (8). This ligament is fundamental to carpal stability, and its assessment with sonography is useful if a normal ligament is identified. The absence of a sonographically detectable ligament does not necessarily indicate injury (9).
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Figure 11. Transverse sonogram shows the dorsal aspect of the proximal carpal row, just distal to the level of the Lister tubercle. Note the echogenic fibrillar appearance of the dorsal scapholunate ligament, which underlies the extensor digitorum (ED) tendons.
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With the wrist in the same position, the dorsal lunatotriquetral ligament can be located by passing the transducer slightly to the ulnar side. This ligament has a sonographic appearance similar to that of the scapholunate ligament; it appears as a compact echogenic fibrillar structure between the lunate and triquetrum (Fig 12).
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Figure 12. Transverse sonogram at the same level as Figure 11 but on the ulnar side of the dorsal carpus shows the echogenic dorsal aspect of the lunatotriquetral ligament and, above it, the extensor digiti minimi (EDM) tendon. ECU = extensor carpi ulnaris tendon, ED = extensor digitorum tendon.
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Triangular Fibrocartilage
The triangular fibrocartilage may be optimally visualized with the forearm pronated, the wrist positioned on a volar pad, and the humerus internally rotated at the shoulder. Longitudinal paracoronal sonography performed with the transducer positioned in line with the ulna, just distal to the ulnar styloid, enables depiction of the triangular fibrocartilage (10) (Fig 13). The triangular fibrocartilage appears as a homogeneously echogenic inverted triangular structure deep to the extensor carpi ulnaris tendon. Tears in the triangular fibrocartilage, particularly in the ulnar aspect of the cartilage, also may be depicted at sonography (11).
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Figure 13a. Sonographic examination of the ulnar surface of the wrist. (a) Photograph shows the correct position of the transducer. (b) Longitudinal sonogram shows the echogenic triangular fibrocartilage deep to the extensor carpi ulnaris (ECU) tendon.
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Figure 13b. Sonographic examination of the ulnar surface of the wrist. (a) Photograph shows the correct position of the transducer. (b) Longitudinal sonogram shows the echogenic triangular fibrocartilage deep to the extensor carpi ulnaris (ECU) tendon.
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Ulnar Collateral Ligament of the Thumb
The ulnar collateral ligament runs inferior to the adductor aponeurosis, at an oblique angle from the ulnar side of the metacarpal head to the lateral tubercle of the proximal phalanx of the thumb. The adductor aponeurosis is composed of fibers from the tendon of adductor pollicis and the extensor hood of the thumb. This important ligament may be visualized with sonography by placing the transducer on the ulnar side of the metacarpophalangeal joint at longitudinal scanning (12). The normal ulnar collateral ligament is a convex hypoechoic bandlike structure that overlies the thumb metacarpophalangeal joint and is covered by echogenic subcutaneous soft tissue and the adductor aponeurosis (Fig 14). Tears of this ligament are known as gamekeeper or skier thumb. The adductor aponeurosis may shift into the gap left by a torn ulnar collateral ligament; this condition is known as Stener lesion.
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Figure 14. Longitudinal sonogram of the ulnar aspect of the metacarpophalangeal joint of the thumb shows the convex echolucent ulnar collateral ligament (arrows) with the overlying echogenic adductor aponeurosis (AA).
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Extensor Hood
The extensor hood is a fibrous expansion on the dorsum of the proximal phalanx of each digit. The expansion is triangular, with the base wrapped around the dorsal and collateral aspects of the metacarpophalangeal joint. A tendon of extensor digitorum (or extensor pollicis longus to the thumb) blends with the expansion along its central core, separated from the metacarpophalangeal joint by a small bursa. The interossei and lumbrical muscles attach to the expansion, which is further stabilized by links to the deep transverse metacarpal ligament. Each expansion forms a moveable hood that moves distally during flexion of the metacarpophalangeal joint (13).
At sonography, the extensor hood appears as a thin (<2 mm) echogenic fibrillar structure that overlies the dorsal aspect of the finger (Fig 15). The sonographer should suspect an injury if the body of the hood, or the central tendon, becomes decentered during flexion. Injuries to the extensor hood rarely occur in the absence of rheumatoid arthritis. Tears usually involve the radial sagittal band with ulnar subluxation (14).
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Figure 15a. Sonographic appearance of the dorsal extensor hood of the finger. (a) Line drawing shows the finger extensor hood and its attachments from the radial side. (b) Transverse sonogram depicts the lateral extent of the dorsal extensor expansion or hood (arrows) between its sites of attachment to the proximal phalanx, as well as the extensor digitorum (ED) tendon, which courses through the middle of the hypoechoic hood. (c) Longitudinal sonogram over the dorsum of the proximal phalanx of the middle finger shows hypoechoic thickening over the extensor digitorum (ED) tendon, a finding that represents the extensor hood. MCP jt = metacarpophalangeal joint.
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Figure 15b. Sonographic appearance of the dorsal extensor hood of the finger. (a) Line drawing shows the finger extensor hood and its attachments from the radial side. (b) Transverse sonogram depicts the lateral extent of the dorsal extensor expansion or hood (arrows) between its sites of attachment to the proximal phalanx, as well as the extensor digitorum (ED) tendon, which courses through the middle of the hypoechoic hood. (c) Longitudinal sonogram over the dorsum of the proximal phalanx of the middle finger shows hypoechoic thickening over the extensor digitorum (ED) tendon, a finding that represents the extensor hood. MCP jt = metacarpophalangeal joint.
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Figure 15c. Sonographic appearance of the dorsal extensor hood of the finger. (a) Line drawing shows the finger extensor hood and its attachments from the radial side. (b) Transverse sonogram depicts the lateral extent of the dorsal extensor expansion or hood (arrows) between its sites of attachment to the proximal phalanx, as well as the extensor digitorum (ED) tendon, which courses through the middle of the hypoechoic hood. (c) Longitudinal sonogram over the dorsum of the proximal phalanx of the middle finger shows hypoechoic thickening over the extensor digitorum (ED) tendon, a finding that represents the extensor hood. MCP jt = metacarpophalangeal joint.
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Flexor Surface of the Wrist and Hand
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The wrist is placed on a pillow with the forearm supinated so that the hand is in the anatomic position. The wrist is initially assessed in the transverse plane.
Flexor Retinaculum
The key anatomic structure that defines the anatomy of the flexor surface of the wrist is the flexor retinaculum (Fig 16). This strong fibrous band crosses the front of the carpus and converts its anterior concavity into the carpal tunnel, through which pass the flexor tendons of the digits and the median nerve. The retinaculum is attached medially to the pisiform and the hook of hamate. Laterally, it splits into two laminae, one superficial, which attaches to the tubercles of the scaphoid and trapezium, and one deep, which attaches to the medial lip of the groove on the trapezium. Between the two slips passes the tendon of flexor carpi radialis in its own synovial sheath (Fig 17).
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Figure 16a. Normal sonographic appearances of the carpal tunnel. (a) Transverse sonogram over the carpal tunnel shows the hypoechoic flexor retinaculum (arrowheads) with the median nerve immediately beneath it. The long flexor tendons of flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) are located deep to the nerve. Note the presence of a normal variant median artery (curved arrow) alongside the median nerve. (b) Extended-field-of-view transverse sonogram of the carpal tunnel shows the bones that mark its boundaries. FCR = flexor carpi radialis tendon, FPL = flexor pollicis longus tendon.
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Figure 16b. Normal sonographic appearances of the carpal tunnel. (a) Transverse sonogram over the carpal tunnel shows the hypoechoic flexor retinaculum (arrowheads) with the median nerve immediately beneath it. The long flexor tendons of flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) are located deep to the nerve. Note the presence of a normal variant median artery (curved arrow) alongside the median nerve. (b) Extended-field-of-view transverse sonogram of the carpal tunnel shows the bones that mark its boundaries. FCR = flexor carpi radialis tendon, FPL = flexor pollicis longus tendon.
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Long Flexor Tendons
The flexor pollicis longus tendon, protected by a separate long synovial sheath that also surrounds the long flexor tendons to the fingers, passes through the radial side of the tunnel. The third and fourth superficialis tendons lie superficial to the second and fifth superficialis tendons, and the four profundus tendons are positioned side by side, deep to the second and fifth superficialis tendons. The superficialis tendon is superficial to the profundus tendon in the palm (Fig 18) until it divides at the level of the proximal third of the proximal phalanx. At this point, the two slips of the superficialis tendon pass around the profundus tendon (Fig 19), reunite deep to the profundus tendon, and are attached at a site in the proximal half of the middle phalanx (Fig 20). The profundus tendon passes through the divided superficialis tendon to the site of its insertion at the base of the distal phalanx (Fig 21). The fifth ray flexor tendons often are housed within a sheath that is continuous with the common sheath at the level of the middle palm. The second through fourth flexor tendons are housed in tendon sheaths that extend from the metacarpal necks to the distal interphalangeal joints.
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Figure 18a. Sonographic appearance of the long flexor tendons in the palm. Transverse (a) and longitudinal (b) sonograms of the long flexor tendons in the palm show the flexor digitorum superficialis (FDS) tendon, which lies above the flexor digitorum profundus (FDP) tendon.
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Figure 18b. Sonographic appearance of the long flexor tendons in the palm. Transverse (a) and longitudinal (b) sonograms of the long flexor tendons in the palm show the flexor digitorum superficialis (FDS) tendon, which lies above the flexor digitorum profundus (FDP) tendon.
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Figure 19a. Sonographic appearances of the long tendons of the finger, the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP). (a) Transverse sonogram through the finger flexor tendons at the midpoint of the proximal phalanx shows the division of the superficialis tendon into two slips to surround the profundus tendon, which cannot be seen because of anisotropy. (b) Transverse sonogram in exactly the same position as a but with the transducer held perpendicular to the profundus tendon shows hypoechogenicity in the superficialis tendon slips because of an anisotropy artifact. (c) Transverse sonogram at a more distal location along the proximal phalanx depicts the reunion of the superficialis tendon slips deep to the profundus tendon.
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Figure 19b. Sonographic appearances of the long tendons of the finger, the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP). (a) Transverse sonogram through the finger flexor tendons at the midpoint of the proximal phalanx shows the division of the superficialis tendon into two slips to surround the profundus tendon, which cannot be seen because of anisotropy. (b) Transverse sonogram in exactly the same position as a but with the transducer held perpendicular to the profundus tendon shows hypoechogenicity in the superficialis tendon slips because of an anisotropy artifact. (c) Transverse sonogram at a more distal location along the proximal phalanx depicts the reunion of the superficialis tendon slips deep to the profundus tendon.
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Figure 19c. Sonographic appearances of the long tendons of the finger, the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP). (a) Transverse sonogram through the finger flexor tendons at the midpoint of the proximal phalanx shows the division of the superficialis tendon into two slips to surround the profundus tendon, which cannot be seen because of anisotropy. (b) Transverse sonogram in exactly the same position as a but with the transducer held perpendicular to the profundus tendon shows hypoechogenicity in the superficialis tendon slips because of an anisotropy artifact. (c) Transverse sonogram at a more distal location along the proximal phalanx depicts the reunion of the superficialis tendon slips deep to the profundus tendon.
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Figure 20. Longitudinal sonogram shows the insertion site of the flexor digitorum superficialis tendon at the base of the middle phalanx. Note the thickening of the joint capsule on the volar aspect of the proximal interphalangeal joint, which is also known as the volar plate.
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Figure 21a. (a) Longitudinal sonogram shows the insertion of the flexor digitorum profundus (FDP) tendon onto the base of the terminal phalanx. Note the thickening of the distal interphalangeal joint capsule at the volar plate. (b) Extended-field-of-view image of the distal part of the flexor digitorum profundus tendon shows hypoechoic artifacts (*) caused by tendinous anisotropy.
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Figure 21b. (a) Longitudinal sonogram shows the insertion of the flexor digitorum profundus (FDP) tendon onto the base of the terminal phalanx. Note the thickening of the distal interphalangeal joint capsule at the volar plate. (b) Extended-field-of-view image of the distal part of the flexor digitorum profundus tendon shows hypoechoic artifacts (*) caused by tendinous anisotropy.
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The presence of the flexor pollicis longus tendon, which lies between the abductor pollicis and opponens pollicis muscles, characterizes the thenar eminence at sonography (Fig 22).
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Figure 22a. Transverse (a) and longitudinal (b) sonograms through the thenar eminence show the relationship of the echogenic flexor pollicis longus tendon to the short muscles of the thumb.
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Figure 22b. Transverse (a) and longitudinal (b) sonograms through the thenar eminence show the relationship of the echogenic flexor pollicis longus tendon to the short muscles of the thumb.
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Median Nerve, Ulnar Nerve, and Guyon Canal
The median nerve lies within the carpal tunnel and is often located immediately beneath the retinaculum, just to the radial side of the superficial row of flexor digitorum tendons (Figs 16 , 17). On transverse sonograms, the nerve appears elliptic in outline and seems to become progressively flatter as it passes through the canal. A prominent median artery may accompany the median nerve in the forearm and wrist; this is considered a normal variant of the anatomy (Fig 4). Patients who have a persistent median artery often have a bifid proximal median nerve (15). It is important to be aware of this anomaly when planning carpal tunnel release surgery (16).
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 23).
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Figure 23. Transverse sonogram of the Guyon canal, obtained by using the linear-array transducer in sector mode for a wider field of view, shows the presence of a normal variant accessory muscle that may be associated with compression of the adjacent ulnar nerve.
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Both the median and the ulnar nerves may be involved in nerve entrapment syndromes (carpal tunnel syndrome and Guyon canal syndrome, respectively) due to the strict confines of their fibro-osseous tunnels (17).
Finger Flexor Tendon Pulley System
The flexor synovial sheath extends from the neck of the metacarpal to the distal interphalangeal joint. A series of retinacular structures, which thicken the sheath at five specific points, form the annular pulley system (pulleys A1–A5). Additional fibers that crisscross between the annular pulleys create the cruciate pulley system (pulleys C1–C3) (18). These pulleys combine to prevent excursion of the flexor tendons from the metacarpophalangeal and interphalangeal joints during finger flexion.
The A1 pulley begins in the region of the volar plate of the metacarpophalangeal joint and extends to the level of the base of the proximal phalanx. The A2 pulley arises from the volar aspect of the proximal part of the proximal phalanx and extends to the junction of the middle and distal thirds of the proximal phalanx (Fig 24); the superficialis tendon divides beneath this pulley. The A3 pulley extends for a short distance over the region of the proximal interphalangeal joint. The A4 pulley is in the midportion of the middle phalanx, and the A5 pulley is in the region of the distal interphalangeal joint.
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Figure 24. Transverse sonogram at the level of the proximal part of the proximal phalanx shows the second annular pulley as a hypoechoic thickening of the flexor sheath that extends to the sides of the base of the proximal phalanx.
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Sonography can depict the A2 pulley directly in most cases. The pulley is best depicted in the sagittal plane, in which it appears as focal hyper-echoic thickening of the synovial sheath or a thin (0.3–0.5-mm diameter) hyperechoic line at the level of the proximal third of the proximal phalanx (Fig 25) (19). Sonography also can depict the A4 pulley, which appears as a subtle thickening of the synovial sheath at the level of the midpoint of the middle phalanx in the sagittal plane. The A3 and A5 pulleys are not routinely visible but occasionally can be identified at sonography (Fig 26). The annular pulleys may be traumatically disrupted during finger flexion, with resultant bowstring injury to the tendons (19). Rock climbers are particularly likely to experience this type of injury. Visual comparison of the injured finger with an unaffected finger helps confirm the diagnosis.
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Figure 25. Longitudinal sonogram of the finger at the level of the proximal phalanx shows the second annular pulley as a thin hyperechoic line (arrows) superficial to the long flexor tendons.
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Figure 26. Transverse sonogram of the finger at the level of the head of the middle phalanx shows the fifth annular pulley, which covers the flexor digitorum profundus (FDP) tendon at a point just proximal to the distal interphalangeal joint, as well as several vessels in a location superficial to the pulley. The pulley is infrequently depicted at sonography.
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Conclusions
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Sonography provides a rapid, inexpensive, and dynamic means of imaging the soft-tissue structures of the wrist and hand in fine detail. By using modern machines with ultra-high frequency transducers, we are able to see the majority of soft-tissue structures relevant to radiologic and surgical practice.
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Abstract
Introduction
