The flexor pulley system of the hand is an intricate structure that coordinates the flexion of the digits. It comprises of long flexor tendons and their associated synovial sheaths, annular pulleys – 5 associated with each finger, 2 associated with the thumb, cruciate pulleys – 3 associated with each finger, and oblique pulley – 1 associated with the thumb. The primary role of the flexor pulley system is to hold the flexor tendons against the phalanges, thus preventing them from pulling away and bowstringing. This allows the long flexor muscles to efficiently flex the individual digits. In this article, we shall look at the anatomy of the flexor pulley system – including the fibrous digital sheaths, annular ligaments and cruciate ligaments.
Long Flexor Tendons
The long flexor tendons of the fingers originate from the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) forearm muscles. These tendons enter the hand via the carpal tunnel, enclosed within a single synovial sheath. Within the hand, the tendons branch out and enter their respective fibrous flexor sheaths. These sheaths are strong ligamentous passageways that are each assigned to a single digit. At the base of the proximal phalanx, the FDS tendon splits into two, allowing the FDP tendon to pass between. The split FDS tendons attach to the base of the middle phalanx, whilst the FDP tendon attaches to the base of the distal phalanx.
Annular and Cruciate Pulleys
The fibrous flexor sheaths contain thickened areas - known as the annular and cruciform pulleys. The annular pulleys (or ligaments) comprise of five areas where the fibrous flexor sheaths are fortified with circular fibres. It is believed that A2 and A4 have the highest importance in preventing bowstringing of the flexor tendons. Each of the five annular pulleys is positioned differently:
The cruciate pulleys are also areas where the fibrous flexor sheaths are reinforced with cruciform fibres and consist of three areas:
Flexor Pulley System of the Thumb
The long flexor tendon of the thumb originates from the flexor pollicis longus (FPL) and passes through the carpal tunnel into the hand. The FPL tendon enters the fibrous flexor sheath of the thumb and attaches to the base of the distal phalanx. The fibrous flexor sheath of the thumb is reinforced by three pulleys - A1, Oblique and A2.
Trigger Finger
Trigger finger is a condition in which the finger or thumb clicks or locks when in flexion, preventing its return to extension. It is assumed to be caused by inflammation of the long flexor tendons, which consequently become thickened or develop a nodule, making it difficult for the tendon to pass through the pulleys. This is especially pertinent with the A1 pulley, which is the most commonly involved ligament in trigger finger.
Management Options
The management of trigger finger consists of splinting of the affected digit, steroid injections and surgical release of the tendon.
The flexor pulley system of the hand is an intricate structure that requires an understanding of its anatomy in order to be able to treat it effectively. This article provides a comprehensive overview of the flexor pulley system, including the fibrous digital sheaths, annular ligaments and cruciate ligaments, as well as the long flexor tendons and the associated management options for trigger finger.
The flexor pulley system of the thumb is a complex anatomical structure reinforced by three pulleys, specifically the A1 pulley, oblique pulley, and A2 pulley. The A1 pulley overlies the metacarpophalangeal joint, the oblique pulley overlies the proximal half of proximal phalanx, and the A2 pulley overlies the distal half of proximal phalanx. This structure plays an important role in normal hand function by helping to transmit the long flexor tendons around the carpaltunnel and modulating their direction of insertion into the phalanges of the fingers. Consisting of both annular and cruciate pulleys, together they create a bowstring mechanism which controls the movement of the long flexors during flexion and extension of the fingers.
The flexor pulley system is also clinically relevant in the etiology of a common condition known as trigger finger. Trigger finger is a condition in which the thumb or finger locks when flexed, preventing the digit from returning to extension. This is thought to be caused by inflammation of the long flexor tendons – often caused by repetitive movements of the hand – which can lead to the development of a thickened nodule. This nodule becomes trapped proximal to the A1 pulley, making it difficult for the tendon to pass back through the pulley when the digit is extended. As a result, the digit remains locked in a flexed position.
Management of trigger finger typically requires a combination of methods, all of which are aimed at relieving the inflammation of the flexor tendon and restoring its movement. These strategies typically include splinting of the affected digit, steroid injections, and surgical release of the tendon.
In conclusion, the flexor pulley system plays an important role in allowing for normal hand function, and damage to this system can have serious implications, namely the development of trigger finger. In order to maintain healthy, pain-free hand and finger function, it is important to be aware of the anatomy of the flexor pulley system and seek timely treatment for any symptoms of trigger finger.
