Outline

This chapter describes the anatomy of the flexor and extensor apparatus in the hand and wrist, supplemented with relevant clinical and biomechanical features. It emphasizes the key importance of the retinacular structures of the wrist and fingers and reviews both normal anatomy and common variations of the flexor and extensor tendons in the hand. Special attention is given to the vincula tendinea and pulleys supporting the flexor tendons in the digital flexor sheaths—in particular, the structure and biomechanics of the pulley system. The key role of the A2 and A4 pulleys is emphasized. Finally, a closer look is taken at the tendons and bands contributing to the extensor mechanism, and their action on the metacarpophalangeal and interphalangeal joints is discussed.

The hand, with the carpus, is the most complex collection of interconnected joints in the human body, and a large assembly of tendons carries out its extensive range of motion. Distinctive groups of muscles execute the actions of these tendons on the hand. Together with numerous aiding structures, the tendons form an ingenious biomechanical system that gave humans the ability to perform complicated tasks and develop dexterous professions that require sophisticated hand and finger movement, like hand surgery. Treatment of hand disorders is similarly delicate and complex; therefore, a thorough understanding is essential to provide adequate treatment.

Flexor Retinaculum

On the volar aspect of the wrist, the carpal bones form a deep excavation over which the flexor retinaculum arches, creating the carpal tunnel. The tendons of the flexor digitorum superficialis (FDS) and profundus (FDP) and the flexor pollicis longus (FPL) pass through this tunnel together with the median nerve.

This retinaculum inserts on the ulnar side into the pisiform bone and the hook of the hamate and is reinforced by the pisiform-hamate ligament. On the radial side, the retinaculum firmly inserts into the volar ridge of the trapezium (trapezial tuberosity) and the tubercle of the scaphoid. Occasionally an insertion into the styloid process of the radius can be seen.^1,²

Controversial terminology surrounds this strong, fibrous band that crosses the volar side of the carpus. According to the Terminologia Anatomica, the term flexor retinaculum carpi should be used. However, in textbooks, the terms flexor retinaculum, transverse carpal ligament and anterior/volar annular ligament are used for the same structure.³

In hand surgery, it is often preferred to distinguish between the different parts of the flexor retinaculum. Stecco et al ³ investigated its histological and anatomical features. Continuous with the antebrachial fascia, a fibrous reinforcement was identified at the volar side of the wrist. After removal of this layer, another fibrous layer was found, with strong lamina and histological similarities to those of a ligament. According to Stecco’s group,³ “ligament derives from Latin ligare (to bind) and means a sheet or band of tough fibrous tissue connecting bones.” They stated that “the role of the pulley for the tendons of the flexor muscles is assumed above all” for this deeper layer covering the carpal tunnel, and therefore the term transverse carpal ligament was suggested to be the most appropriate.

In addition to the thickened continuation of the antebrachial fascia and the transverse carpal ligament, Cobb et al ⁴ described a third, more distal segment composed of an aponeurosis between the thenar and hypothenar muscles. Based on their anatomical findings, they advocate a more extensive release when dividing the carpal ligament.

Tanabe and Okutsu ⁵ endoscopically released the flexor retinaculum in management of carpal tunnel syndrome and found that the mean distance between the ends of the retinaculum increased from 1.3 mm to 6.6 mm if the distal fibers were also divided. They therefore concluded that sectioning of the distal fibers is essential for endoscopic carpal tunnel release.

Extensor Retinaculum

On the dorsal side of the wrist, a thickened part of the deep antebrachial fascia is found, holding the tendons of the extensor muscles in place. It works in continuity with the volar carpal ligament and for the previous stated reasons the term dorsal carpal ligament or posterior annular ligament could also be suggested. However, the term extensor retinaculum is most commonly used.

As studied by Taleisnik et al,⁶ the deep antebrachial fascia begins to thicken proximal to the radiocarpal joint and becomes the dorsal annular ligament, a component of the extensor retinaculum of the wrist. Distally, this supratendinous layer is continuous with the pretendinous fascia of the hand. On the ulnar side, it inserts in three distinct places: proximal around the flexor carpi ulnaris tendon, the middle part into the triquetrum and pisiform bone, and distally onto the fascia of the abductor digiti quinti and base of the fifth metacarpal.

The retinaculum attaches to ridges on the radius, forming septa that divide the space below the extensor retinaculum in six compartments through which the extensor tendons pass. The six compartments are further described in the section about extensor tendons.

Within the supratendinous layer, at the floor of the fourth and fifth compartment, lies the infratendinous layer, from which circular fibers form a tube around the tendons within the compartments. In the sixth compartment, a duplication of the infratendinous layer forms a tube around the extensor carpi ulnaris (ECU) tendon from the ulnar styloid to the triquetrum. This subsheath stabilizes the tendon along the groove in the distal ulna and contributes to the stability of the distal radioulnar joint. Proximally, longitudinal fibers form the linea jugata that reinforces the tendon to prevent subluxation during full supination. The ECU subsheath is frequently injured, typically in young athletes who play racket or stick sports, which can result in recurrent subluxation of the ECU tendon.⁷

Flexor Tendons in the Hand

The tendons in the wrist have a relatively constant arrangement with the median nerve. This arrangement varies mainly when muscle or tendon anomalies are present. It is important to keep these variations in mind, because it can be confusing even for the experienced surgeon. Usually, the median nerve can be found directly under the tendon of the palmaris longus. Sometimes, the nerve lies more superficial, next to the palmaris longus tendon. The median nerve is also frequently observed to split in the forearm.⁸

The palmaris longus tendon lies completely enclosed in the deep fascia of the forearm between the superficial veins and cutaneous nerves. The presence of this tendon is highly variable, as its absence has been reported from 0.6% to 63.9% (either bilateral or unilateral) among different populations all over the world.⁹ In humans, this tendon is seen as rudimentary, and for that reason, and its easy accessibility, the palmaris longus is often used as a tendon graft.

To the radial side of the median nerve, the FPL tendon can be found; to the ulnar side, the superficial flexor tendons. When coursing through the carpal tunnel the superficial flexor tendons are arranged in pairs, the tendons to the middle and ring finger form the superficial pair, and the little and index finger tendons form the deep pair. Dorsal to the superficial tendons lie the flexor profundus tendons.^2,⁸

In the case that the palmaris longus tendon is absent, the tendon of the flexor superficialis indicis can sometimes be found between the superficial flexor tendon to the middle finger and the median nerve. In other variations, the palmaris longus tendon can separate into two or three tendons, the muscle can be doubled, the muscle can be reversed or inverted, and partial or complete insertion in the antebrachial fascia can be seen. Additionally, insertion into the pisiform bone, scaphoid, flexor carpi ulnaris tendon, and muscles of the thenar has been observed.⁸

Accessory slips for the FDS and FDP are frequently observed, especially to the index finger. Slips arise from the ulnar tuberosity to the superficialis tendon of the index and middle finger, and from the annular ligament to the superficialis tendon of the little finger. The superficialis tendon to the little and index finger can be absent and the index finger’s deep flexor tendon can divide into two tendons or have a shared origin with the FPL.⁸

The flexor carpi radialis (FCR) tendon courses through its own synovial sheath that forms a narrow osteofibrous tunnel when passing the trapezium. The radial side of the tunnel is formed by the body of the trapezium and the volar side by the trapezial tuberosity. On the ulnar side, the tunnel is separated from the carpal tunnel by a thick septum. Distally, the FPL tendon pivots around this septum. At the floor of the tunnel, the tendon sheath lies in contact with the volar capsule of the scaphotrapeziotrapezoid (STT) joint. The narrow tunnel induces a higher risk of primary stenosing tenosynovitis; however, because of the close relationship with surrounding structures, tenosynovitis can also occur secondary to traumatic or degenerative changes. Tenosynovitis of the FCR tendon is not an uncommon finding in arthritis of the STT joint, which can cause additional pain to the baseline osteoarthritic pain.^10,¹¹

The FCR tendon is also suggested to play a role in stabilizing the scapholunate joint through its close relationship between the FCR tendon sheath and the ligaments attached to the tuberosity of the scaphoid. The tendon was thought to use the distal pole of the scaphoid as a pulley to increase its mechanical advantage and prevent the scaphoid from rotating into flexion. Unlike in previous hypotheses, Salvà-Coll et al ¹² found that loading the FCR tendon rotated the scaphoid in flexion and supination, while rotating the triquetrum in flexion and pronation. These opposite rotations are attributed to relax the dorsal scapholunate ligament.

The FCR tendon inserts into the base of the second metacarpal and gives off a strong, tendinous slip to the third metacarpal. Frequently a small slip to the trapezial tuberosity can be seen. Other variations are insertions in both the trapezium and scaphoid and in the base of the second, third, and fourth metacarpals. The FCR muscle can even be absent. Insertion in the transverse carpal ligament can also be seen and may cause confusion with the palmaris longus tendon.⁸

The flexor carpi ulnaris (FCU) encloses the pisiform bone, which can be seen as a sesamoid bone for this tendon. The FCU attaches to the base of the fifth metacarpal and hamate, via the pisohamate and pisometacarpal ligaments. The pisohamate ligament creates an additional tunnel, Guyon’s canal, between the hook of the hamate and pisiform bone through which the ulnar artery and nerve pass. Occasionally, the FCU tendon doubles before inserting in the pisiform bone. The FCU can also merge with the flexor superficialis or profundus tendon.⁸

The FPL tendon passes through its own osseo-aponeurotic canal and inserts at the distal phalanx of the thumb. Sometimes an additional tendon to the index finger is found. The FPL tendon can be absent, in combination with a significant smaller thumb. Another variation is splitting of the tendon into two or three slips that insert into the scaphoid or flexor retinaculum.⁸

Flexor Tendon Sheaths

Structure of the Sheath at Wrist and Palm

In the wrist, synovial sheaths allow the flexor tendons to glide through the carpal tunnel. The sheaths begin approximately 2.5 cm proximal to the transverse carpal ligament and extend distally to the neck of the metacarpals. In the fingers, the flexor tendons are also surrounded by sheaths. A considerable level of variation in communicating patterns between these sheaths and the digital flexor sheaths is observed.¹³

As reported by Fussey et al,¹³ the most common pattern shows one common flexor tendon sheath or ulnar bursa surrounding the tendons of the FDS and FDP. A second sheath, forming the radial bursa, envelopes the FPL tendon.

In most cases the little finger flexor sheath works in continuity with the tendons along the phalanges. Likewise, the FPL tendon sheath accompanies the tendon along the thumb as far as its insertion.¹³

Occasionally communications between the common flexor tendon sheath and the second, third, and fourth digital tendon sheaths are observed in the palm. Furthermore, the ulnar and radial bursa can be found to be connected. In case of an infection within the sheath, the communication between the ulnar and radial bursa can result in the “horseshoe abscess.”

In the carpal tunnel, the subsynovial connective tissue (SSCT) connects the flexor tendons with the synovial membrane of the ulnar bursa. It consists of fibrous bundles parallel to the tendons, which in turn are interconnected by smaller microfibrillar fibers. By acting as a sliding unit, the SSCT reduces friction and maintains the integrity of the vascularity around the tendons. Noninflammatory fibrosis of the SSCT is the most common histological finding in carpal tunnel syndrome, which suggests injury to the SSCT to be an important etiologic factor ¹⁴ (Figures 1-1 to 1-4).

Figure 1-1 Nutritional microenvironment of the flexor tendon. In zone 2 the tendons are intrasynovial within the digital flexor sheath. Zone 3, in the palm of the hand, is extrasynovial tendon. In the carpal tunnel, zone 4, the tendon is surrounded by a hybrid of both intrasynovial and extrasynovial tissue. SM, Synovial membrane; SF, synovial fluid; PT, paratenon; SSCT, subsynovial connective tissue.

Figure 1-2 Flexor tendon zones. The zones are numbered from distal to proximal. Zone 1 contains a single tendon, the flexor digitorum profundus tendon. Zone 2 is the critical area of the digital flexor sheath. It contains both the profundus and superficialis tendons, and the area where the profundus tendon penetrates the superficialis at the chiasm of Camper. In the extrasynovial zone 3 the lumbrical muscles arise. Zone 4, the carpal tunnel, contains mixed intra/extrasynovial tissue. Zone 5 is also extrasynovial, without the lumbrical muscles. In the thumb, although we have marked zones 1 to 5, one may consider that zones 2 and 3 do not exist, because there is only a single tendon, and no lumbrical.

Figure 1-3 Sliding unit in the carpal tunnel region has both structures (i.e., paratenon and synovial sliding mechanism). All layers from the flexor retinaculum to the tendon are shown. The tendons in the carpal canal have a subsynovial connective tissue and a bursa, containing synovial fluid between the parietal and visceral synovium (hematoxylin-eosin; original magnification, ×400).

(From Ettema AM, Amadio PC, Zhao C, et al: Changes in the functional structure of the tenosynovium in idiopathic carpal tunnel syndrome: a scanning electron microscope study, Plast Reconstr Surg 118:1413-1422, 2006.)

Figure 1-4 Top, Loose vertical fibers join adjacent layers in the subsynovial connective tissue (scanning electron microscopy; original magnification, ×1100). Bottom, The loose vertical fibers between adjacent layers are stretched during flexor tendon movements (scanning electron microscopy; original magnification, ×1000).

Structure of the Digital Flexor Sheath

The tendons of the FDP and FDS lie deep in the palm of the hand, beneath the superficial palmar arch and the superficial branches of the median and ulnar nerve. There, the tendons are surrounded by several layers of loose connective tissue. Vessels emerging from the palmar arches traverse through this tissue toward the tendons, forming the paratenon.

In a histological study of the flexor tendon sheath by Cohen and Kaplan,¹⁵ two vascular systems are observed to become organized in the paratenon; one adjacent to the tendons, the other more peripheral. The development of a “synovial-lined cleft” between these systems creates a separation into a visceral and a parietal synovium.

The parietal layer lines the wall of the tunnel through which the tendon glides. As the layer continues distally, the layer was found to be uninterrupted and reinforced externally by dense bands of collagen. By maintaining the tendons close to the bone and joint surface, these bands act as a system of pulleys, facilitating efficient finger movement. Between these reinforcements, the thickness of the parietal synovium is greatest and is called the membranous part of the tendon sheath.

The visceral layer or epitenon adheres to the tendon, creating a synovial space between the two layers. It was found to be supported by a thin cover of laminated collagen. All structures entering the tendons are surrounded by epitenon. In the opposite direction along these structures, the visceral layer works in continuity with the parietal layer. The synovial layers extend distally to the insertion of the FDP tendon, where they become confluent.

Vincula Tendinea

On the palmar osseous side of the tunnel, vessels protrude through the synovial layers, forming mesenterial folds or mesotenon. These vascular bands connect the dorsal side of the flexor tendons with the palmar side of the flexor sheath. In consideration of their tendinous structure, these bands are named vincula tendinea (vinculum derives from Latin and means “bond or link”).

Several variations in the vincula tendinea have been observed. Ochiai et al ¹⁶ studied the distribution patterns of the vincula in the fingers of 35 human hands. Their study showed that four transverse communicating branches arising from the digital arteries enter the flexor tendon sheath through openings in the pulleys, traveling toward the vincula. The sites of entry are at the base of the proximal phalanx, the neck of the proximal phalanx, the base of the middle phalanx, and the neck of the middle phalanx (Figure 1-5).