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

WHY THE STUDY WAS PERFORMED:

The finger is a small and complex part of our anatomy. Alongside complex anatomy, variants exist within both the flexor and extensor tendons of the finger. Being aware of these variations can prevent misdiagnosis when assessing the finger anatomy with ultrasound.

HOW THE STUDY WAS PERFORMED:

The study was performed by using high frequency ultrasound and a dedicated MSK ultrasound system on both volunteers and patients to obtain images of the anatomy of the finger. They then obtained images of the finger of fresh cadaver specimens with ethics board approval. Both ultrasound images and cadaver specimen images were then used to demonstrate clinically relevant anatomy of the finger including complex structures and anatomical variations.

WHAT THE STUDY FOUND:

Accurate knowledge of the complex anatomy of the finger avoids misdiagnosis of pathology. By using images from cadaver specimens and directly correlating them with the ultrasound appearance of these structures the authors are able to show the variance in anatomy accurately.

The anatomy of the extensor tendons is small and complex and varies between the 2nd and 3rd, 4th fingers. The extensor tendons of the 3rd and 4th fingers are broad with multiple tendon fibres that can be mistaken for pathology. 2 tendons are often seen at the 2nd finger, one of which, the extensor indicis proper used as a graft. The 5th finger has a minimum of 2 extensor tendons. Ultrasound examination of the central slip and distal tendon allows for dynamic assessment of the anatomy.

The sagittal bands are an important structure preventing subluxation of the extensor tendons. Where there are multiple tendons present, injury to the sagittal bands can also impact the connection between the multiple tendons.

Along the course of the finger, the relationship between the flexor digitorum profundus (FDP) and the flexor digitorum superficialis (FDS) varies. The FDS bifurcates and inserts laterally onto the middle phalanx, whilst the FDP inserts onto the distal phalanx. The FDP can show a continuous bifurcation in the tendon ill its insertion which is a normal variation and not a tear of the tendon.

The pulley system is integral in keeping the flexor tendons adjacent to the bones. Whilst normally the A1, A2, and A3 pulley are separate structures, they can be fused. The A1 pulley can also extend down into the hand proximally. This can be called the A0 pulley and is important to be noted when assessing the finger for trigger finger.

RELEVANCE TO CLINICAL PRACTICE:

The study provides detailed analysis of the anatomy of the finger and its anatomical variations. As sonographers we need to be aware of these variants so we can precisely assess the anatomy and pathology. This allows for an accurate diagnosis. n

REVIEWED BY Sophie O’Brien ASA SIG Musculoskeletal

REFERENCE De Maeseneer M, Meng J, Marcelis S, Jager T, Provyn S, Shahabpour M. J Ultrason. 2020 Jul; 20(81): e122–e128.

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... being aware of these variations can prevent misdiagnosis