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Citation Information : Journal of Ultrasonography. Volume 20, Issue 81, Pages 122-128, DOI: https://doi.org/10.15557/JoU.2020.0020
License : (CC-BY-NC-ND 4.0)
Received Date : 26-March-2019 / Accepted: 07-May-2020 / Published Online: 15-June-2020
Many anatomical details and variants occur in the finger tendons and soft tissue structures. These may lead to misdiagnosis if the radiologist is not well aware of them. We discuss the midhand extensor tendons, dorsal hood, junctura tendinea, conjoint tendons, transverse retinacular ligament, triangular ligament as well as central and distal slip anatomy and ultrasound correlation. The dorsal hood is an important structure to center the tendons at the midportion of the MCP heads, and the sagittal bands are its main components. Two tendons are present at the second digit, and two or more at the fifth digit. The extensor mechanism is anatomically interrelated with the palmar lumbricals and interosseous tendons. At the palmar side, the flexor superficialis and profundus tendons show varying relationships along the finger. The flexor profundus passes through an opening in the flexor superficialis. We also discuss the chiasma crurale, ridges at the flexor superficialis insertions and bifid flexor profundus tendon. Although a typical distribution of annular pulleys can be observed, many variants may be present of which we address some. The volar plate is a midline fibro-cartilaginous meniscus attached proximally to the well-identifiable checkrein ligaments and distally to the base of the phalanges. Knowledge of these details and variations allows for better understanding of the finger and hand ultrasound.
Many variations can be observed in the anatomy of the extensor and flexor tendon system of the hand(1–4). These variations may involve the extensor tendons at the wrist and midhand level. Variations also occur in the tendons and pulleys at the flexor side. These variations present potential pitfalls at ultrasound (US) and radiologists should be aware of them to avoid misdiagnosis. The finger extensor and tendon system is complex and many details are not well known by radiologists. Knowledge of this complex anatomy is also important to correctly interpret ultrasound images. Our aim is to provide an overview of clinically relevant variations, complex areas, and anatomical details that radiologists should be aware of. We address the anatomy and variants at the level of the midhand and dorsal hood. Next, we address the complex extensor system at the finger. At the volar side, we review the complex anatomy of the flexor system, anatomical details and variations of the flexor superficialis and profundus tendons, variations in the pulley system, and the anatomy of the volar plate and checkrein ligaments. Our article is based on ultrasound images obtained in volunteers, patients, and anatomical dissections of fresh cadaveric specimens. Ethical board approval was obtained to study the extensor and flexor system of the hand in the cadavers. Good quality images in musculoskeletal (MSK) US of the hand require a high frequency linear transducer and a dedicated MSK system.
At the dorsal midhand, the extensor tendons of the third and fourth finger appear as broad structures with multiple areas of splitting between the fibers that should not be mistaken for pathological fissures (Fig. 1). At the level of the second finger, 2 tendons are present (Fig. 2). The extensor indicis proprius tendon is sometimes used as a potential graft in surgical procedures. At least 2 tendons are also present at the level of the fifth finger. These tendons arise from the 5th and the 6th compartment (Fig. 3). Proper identification of these multiple tendons in US is important to avoid confusion with longitudinal splits and to confirm the presence of tendons in case they could be used as grafts. The tendons at the midhand are connected by intertendinous bands also termed juncturae intertendinea or connexus(5). These bands are classified as A, B, and C (Fig. 1). Type A is located between the 2nd and 3th finger and is relatively thin and difficult to visualize. Type B is slightly thicker and located between the 3th and 4th finger. Type C is tendon-like and located between the 4th and 5th finger.
The dorsal hood(5) is a complex anatomical structure that serves to keep the extensor tendons centered over the metacarpal heads (Fig. 1). The sagittal bands are the most important components. In the second and fifth fingers, where multiple extensor tendons are present, these tendons are connected to each other. The sagittal bands course alongside the fingers superficial to the collateral ligaments and attach to the palmar plate (Fig. 4). Injuries to the sagittal band usually involve the 3rd finger. The radial band is torn with subluxation of the extensor tendon to the ulnar side. At the 2nd and 5th finger, due to the presence of multiple tendons, sagittal band injury may consist of ruptures of the connections between the tendons. Normal thickness of the sagittal bands has been reported to be less than 1.3 mm in men and less than 0.9 mm in women. In the case of pathology, the sagittal bands may appear thickened. Distal to the sagittal bands, the dorsal hood continues as an aponeurotic expansion.
The extensor anatomy is quite complex(6). The extensor tendon reaches the MCP level, and from this point on, some fibers continue distally and attach to the middle phalanx. Some fibers also descend alongside the finger, where they intermingle with fibers of the interosseous and lumbricalis, coming from the ventral side (Fig. 5A, B, Fig. 6A, B). Some of these fibers continue to the central slip, and some continue alongside the distal phalanx as the conjoint tendons (Fig. 7A, B). The conjoint tendons and the central slip are connected by the transverse retinacular ligament, a system similar to the dorsal hood. The central extensor forms the central slip together with contributions from the interosseous and lumbricals. The conjoint tendons continue distally to form the distal slip. Rupture of the central slip typically leads to boutonniere deformity, and rupture of the distal slip results in a mallet finger. Ultrasound allows visualization of the central and distal slip, as well as conjoint tendons, and allows a dynamic examination. Along the midphalanx, a connection is present between the distal extensor tendons, which is known as the triangular ligament (Fig. 8).
The relationship of the flexor superficialis and flexor profundus varies along the course of these tendons (Fig. 9). The flexor superficialis becomes C-shaped at the level of the proximal phalanx to allow the passage of the flexor profundus. The flexor superficialis then attaches on both sides of the middle phalanx. The flexor profundus then continues distally to insert on the distal phalanx. These anatomical details are well seen on ultrasound, but less on MR imaging. At the insertion of the flexor superficialis on the middle phalanx, ridges may be seen on the bone. These bony ridges(7) represent enthesophytes related to traction of the flexor tendon and the A4 pulley (Fig. 10). The distal continuation of the flexor profundus often shows a central split as a normal variation (Fig. 11). These details are best appreciated on ultrasound and represent a potential pitfall, especially if the tendon is thickened. Before the flexor superficialis inserts on the middle phalanx, connections between both slips arise with an X-shaped appearance (Fig. 12). These connections are thin, but when the tendons are thickened they may become visible and should be identified as such rather than as tears.
On the palmar aspect of the fingers, pulleys are present that keep the tendons in close proximity to the bone(8). The annular pulleys are particularly important (Fig. 13). The A2 and A4 pulleys are mechanically most important. The A2 is located at the level of the proximal phalanx, and the A4 at the level of the distal phalanx. The A1, A3, and A5 pulleys are located at the joint levels. When the pulleys are torn, the tendon moves away from the bone, which is called the bowstring injury. Normally, the pulleys are quite thin. All pulleys can be well visualized with state of the art ultrasound equipment. The pulleys may show variations. The A1 and A2 and A3 pulley may be fused instead of presenting as separate pulleys (Fig. 14, Fig. 15). Other variations include a proximal extension of the A1 pulley, sometimes termed A0 pulley. This is clinically important since in the case of the trigger finger the proximal extension should be assessed. A tendon cyst is a common variant on pulleys. This is a misnomer since the cyst is actually located on the pulley and hence does not move with the tendon during a dynamic study. The A2 pulleys insert on ridges on the palmar side of the proximal phalanx. The ridges become more prominent with age.
The volar plates are fibro-cartilaginous structures having a broad attachment distally. Volar plate avulsion is a common injury (Fig. 16). It appears hyperechoic on ultrasound (Fig. 16). The volar plate contains a central groove (Fig. 17). Proximally, the volar plates attach by thin bands termed the checkrein ligaments. The checkrein ligaments can be better visualized if some joint fluid is present (Fig. 18).