Ultrasound anatomy of the fingers: flexor and extensor system with emphasis on variations and anatomical detail

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.

Fig. 1.

Fig. 2.

Fig. 3.

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.

Fig. 4.

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).

Fig. 5.

Fig. 6.

Fig. 7.

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.

Fig. 9.

Fig. 10.

Fig. 11.

Fig. 12.

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.

Fig. 13.

Fig. 14.

Fig. 15.

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).

Fig. 16.

Fig. 17.

Fig. 18.