The A2 pulley and the flexor tendon sheath

Anatomical preparation of a finger specimen is used to explain the anatomy and give an impression of the function and concept of the flexor tendon sheath and its pulley system particularly the A2 pulley. The pulley system, the flexor tendons and the finger bones represent a functional unit which enables the finger to be bent.

The preparation below shows a right middle finger, where soft tissue like skin, vessels, nerves and subcutaneous fat has been removed except of the tip of the finger. The whole flexor tendon sheath is shown and represents a tunnel surrounding the tendons (S). The sheath has the function to keep the tendons next to the bone if they are under tension. Four of the five annular pulleys (A1 - A4) as well as the three cruciate pulleys (C1-C3) are marked.

The next picture shows the situation with the A3 pulley removed, the deep flexor tendon (FDP, flexor digitorum profundus) is lifted by tweezers. It lays at the finger middle joint (PIP, proximal interphalangeal joint) in a superficial position after piercing the superficial flexor tendon (FDS, flexor digitorum superficialis). The FDP tendon inserts at the terminal bone of the finger (distal phalanx) and flexes both the middle (PIP joint) as well as the terminal finger joint (DIP, distal interphalangeal joint). The FDS tendon inserts at the middle bone of the finger and flexes the middle finger joint (PIP joint). In a functional point of view the A2 and the A4 pulleys are of utmost importance concerning strength and range of motion of finger flexion. The cut of the A2 pulley impairs more the range of motion and the cut of the A4 pulley more the strength of finger flexion.

In the extended finger the increased structural thickness of the A1, A2 and the A4 pulleys is well visible. The deep flexor tendon (FDP) lays in the area of the middle finger joint in a superficial position. At the distal (left side) edge of the A2 pulley, the greatest amount of force is diverted during finger flexion. This is the site where during crimp grip position in rock climbers most injuries and overuse syndromes occur.

The picture shows the situation after partial cutting the distal part of the A2 pulley, which is lifted by tweezers. The 1-2 mm thick and very stable structure of the A2 pulley is well visible and shows the fibers which run in a perpendicular direction to the flexor tendons. The inner surface of the pulley is lined by microgrooves which contain cartilage like cells. These are able to bear the high compressive forces occurring during power grip.

The whole A2 and A1 pulley are cut. Again, the perpendicular direction of the fibres of the pulley are seen. During power grip, like the crimp grip position in rock climbers, the microgrooves of the inner lining of the pulley causes considerable friction against the deep flexor tendon (FDP) consisting of 10% of the eccentric grip strength. In bats, this structure is developed in a manner, that such a high friction is generated between the pulleys and the flexor tendons that the tendon is locked in a flexed position. This mechanism enables the bats to hang free on their fingers without any active muscle force. The superficial flexor tendon (FDS) is split just under the A2 pulley and is pierced by the deep flexor tendon (FDP) tendon.

The next preparation step shows the A1 - A4 pulleys cut. The FDP tendon is held apart in a way that the crossing fibers (Camper's Chiasma) of the FDS tendon is well visible.

The FDP and the FDS tendons are lifted. It can be well seen how the FDP tendon pierces the FDS tendon. At the site of the tendon insertion, the so called vinculae (V) are visible. They bring blood vessels into the tendon and are mainly responsible for the nutrition of the flexor tendons.

The FDP tendon has been cut at the middle finger bone and was pulled back through the FDS tendon slip. It is shown how the FDP tendon is surrounded by the FDS tendon and how the fibres are crossing back at the Camper's Chiasma. This FDS sling is contracted during muscular activity and stabilizes the FDP tendon as well as acts as a flexing force in the middle finger joint. It can be seen how the shape of the two tendons fit together.

The A2 pulley again in detail. The fibres insertion into the bone (Sharpey's fibres) are shown. They run perpendicular to the direction of the flexor tendons. The interaction between the A2 pulley and the flexor tendons are of crucial importance during the crimp grip position in rock climbers.