3.1 Collagen

• Flexor digitorum profundus (FDP)

• Flexor pollicis longus (FPL)

• Vascular supply

Tendons

trigger finger [6].

continuous [7].

102

expand and compress with flexion and extension.

2.2 Vascular supply of the extrinsic hand flexors

carrying blood to the tendons [9]:

2.1 Flexor sheath and pulleys

• Flexor sheath and pulley system

The extrinsic flexor tendons of the hand possess true fibro-osseous tunnels in the digits, called the "flexor sheath". Their purpose is to provide very efficient lubrication in an area subject to a change of direction and increase in friction [1]. Proximal to the metacarpophalangeal (MCP) joints, the flexor tendons enter the flexor sheath. This tunnel functions to hold the tendons in close proximity to the phalanges to prevent "bowstringing" and to increase the efficiency of tendon glide [2]. Condensations in the sheath are called pulleys—these almost encircle the flexor tendons to form a fibro-osseous channel that keeps the tendons adjacent to the phalanges [3]. In effect, the pulleys enable the transfer of a translational force generated from the muscle tendon unit into a rotational moment on the phalanges [3]. Pulleys are classified on the basis of their shape—annular or cruciate. There are five annular pulleys (named A1–A5 from proximal to distal) and three cruciate pulleys (named C1–C3 from proximal to distal). The A2 and A4 pulleys insert directly onto the bone over the proximal and middle phalanges, respectively [3]. Traditionally, A2 and A4 are considered to be the pulleys that prevent bowstringing. However, it has now been shown that partial distal excisions of 25% of the A2 pulley, up to 75% of the A4 pulley and 25% of combined A2 and A4 have no significant effect on digit range of motion or work of flexion [4, 5]. The A1, A3 and A5 pulleys are located over the MCP, proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints, respectively [3]. Proximal to the A1 pulley is the palmar aponeurosis (PA) pulley which has been implicated in the aetiology of

The cruciate pulleys lie between the A2–A3, the A3–A4 and the A4–A5 pulleys, respectively [3]. These pulleys function like accordions, allowing the sheath to

Traditionally, it was once thought that the thumb had three pulleys—A1, oblique

and A2 [3]. The A1 pulley lies over the MCP joint, the oblique pulley runs from proximal ulnar to distal radial over the proximal phalanx and the A2 pulley is located over the interphalangeal (IP) joint [3]. A fourth pulley, the variable annular pulley, was first reported in 2012 where it was found to be present in 93% of cadaver specimens [7]. It can have three orientations—transverse, oblique or

Both FDP and FDS tendons in the digits receive dual nutritional supply from vascular perfusion and synovial diffusion [8]. There is some variation in the vascular system, but, generally speaking, the flexor tendons receive their blood supply via two vincula each—a short and long vinculum [9]. Vincula are folds of mesotenon

• The vinculum longus superficialis (VLS) arises from the radial or ulnar side of the base of the proximal phalanx. It receives its blood supply from the

Tendons consist of mostly Type I collagen and elastin embedded in a proteoglycan-water matrix [1]. Collagen contributes 65–85% of the dry mass of the tendon [1]. The collagen, elastin and proteoglycan-water matrix are formed by tenoblasts and tenocytes (refer to Section 3.4). These cells are elongated fibroblasts and fibrocytes which lie between the collagen fibres and are organised in a complex hierarchal scheme to form the tendon proper [10]. Soluble tropocollagen molecules form cross-links to create insoluble collagen molecules, which then aggregate progressively into microfibrils and then into visible units under the electron microscope called collagen fibrils [1].
