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

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 fibrocartilaginous 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.