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

Abstract

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 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.

Fig. 1.

Distal midhand. Note the broadening of extensor tendons at midhand (arrows) with pseudosplitting. Also, the connexus can be observed (C) between the extensor tendons. At the level of the MCP joints, the extensor tendons are kept in place by the sagittal bands (dorsal hood, black arrows)

Fig. 2.

Dissection of the 2nd row on the level of metacarpals of the left hand dorsally. Note the presence of 2 tendons for this finger (arrow)

Fig. 3.

Dissection of 5th finger of the left hand. Note 4 tendons for this finger (arrow). This is a variation as normally 2 tendons would be expected for this finger

Fig. 4.

Coronal US image. ‘Dorsal hood’ image with extensor tendon in midline. The extensor tendon connects to the sides and is enveloped by the sagittal bands (arrows)

Fig. 5.

A. The anatomy of the extensor system is complex. The central extensor tendon continues distally to insert as central slip (central), but gives off fibers to the side that connect to the fibers from the interosseous muscles. Both these continue as the conjoint tendons to form the distal slip insertion (distal). The fibers of the extensor tendon and interosseous tendons mix alongside the proximal phalanx (arrow). B. Note the anatomical dissection showing the central extensor tendon (ET), the interosseous muscles coming from the palmar aspect (IO). They form an X along the side of the phalanx. The conjoint tendons are made up of a slip of the extensor tendon as well as slips of the interosseous and lumbrical muscles. The central tendon inserts as the central slip (C)

Fig. 6.

A. Cadaveric slice showing the central and distal slip insertion (arrows). B. Sagittal US image. The distal slip insertion is smaller and typically more difficult to see (arrow)

Fig. 7.

A. Drawing at the level of the extensor tendon (E) and conjoint tendons (CT). The conjoint tendons are connected by the transverse retinacular ligament (arrows), similar to the dorsal hood. B. Transverse US image shows the conjoint tendons on the side (C) connecting to the extensor tendon (E) by the transverse retinacular ligament (arrowheads)

Fig. 8.

Transverse image at middle phalanx. The conjoint tendons (arrowheads) continue distally and, before they join, they are connected by the triangular ligament (arrow)

Fig. 9.

Dissection showing relationships between flexor profundus (distal) and superficialis (proximal). Note the opening in the flexor superficialis (arrow). The flexor profundus continues distally through this opening (arrowheads). The vincula containing vessels may be present between tendons and tendons and sheath (not shown).

Fig. 10.

A. CT scan. Note bone ridges (arrows, A) at the side of the middle phalanx indicating enthesophytes, where the flexor superficialis tendons insert. B. Histological slice showing enthesophytes at the insertion of the flexor superficialis tendons (arrow, R, ridges). C. Transverse US image shows flexor superficialis tendon insertions on the phalangeal ridges (arrows)

Fig. 11.

A. Anatomical dissection. Note that the flexor profundus (FDP, arrow) tendon (P, two parts) appears bifid, which should not be mistaken for a tear. PROX, proximal; DIST, distal B. Transverse US image. Note a central cleft in flexor profundus tendon distally (arrow)

Fig. 12.

A. Dissection of the flexor superficialis (s). Note the X-shaped connection between the two tendons deep to the flexor profundus termed the chiasma crurale (c); PROX, proximal, DIST, distal. B. Transverse US shows a connection (arrow) between the flexor superficialis (S, two parts), deep to flexor profundus

Fig. 13.

Dissection of finger. A typical distribution of the annular pulleys (A1 to A5, arrows) is demonstrated

Fig. 14.

Dissection of the palmar aspect of finger. An atypical variant of the pulleys. The A1-2-3 pullies (white arrows show sectioned pulley, black arrows extension) are continuous. S, flexor superficialis; P, flexor profundus

Fig. 15.

Sagittal US. Note a thin hypoechoic band covering tendons compatible with A2 pulley (arrow)

Fig. 16.

Sagittal US. Note a thin hypeochoic band covering tendons corresponding to A1 pully (arrow). The arrow head points to cartilage and the curved arrow points to the volar plate

Fig. 17.

A. Note the meniscoid palmar plate deep to flexor tendons (arrow). B. Dissection of the palmar-volar plate (v). Note the ridge in the midline which was sectioned (arrow). The arrowheads indicate proximal attachments termed checkrein ligaments; P, proximal; D, distal

Fig. 18.

Note the proximal band-like extension on one side of the volar plate (thick arrow) corresponding to checkrein ligament (arrows)