1. Introduction

An accurate understanding of the physiology of flexor tendon healing is essential to maximising patient outcomes and justifying the current treatment regimens of surgical repair and postoperative rehabilitation protocols. Our current understanding of flexor tendon healing is a continually evolving area. Therefore, this chapter aims to instruct the reader of the current understanding of flexor tendon basic science, the latest molecular updates, justifications for various surgical and rehabilitation regimens and future research trends. The reader is strongly encouraged to seek alternative resources for more detail regarding flexor tendon anatomy as this will be covered briefly in this chapter. Secondary flexor tendon reconstruction will not be discussed.

### 2. Macroscopic flexor tendon anatomy

This section will focus only on the flexor sheath and vascular supply. The reader is strongly encouraged to seek the vast array of anatomy texts to familiarise themselves with flexor tendon anatomy, paying attention to the:

• Flexor digitorum superficialis (FDS)


### 2.1 Flexor sheath and pulleys

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].

transverse communicating branch of the digital arteries at the base of the

• The vinculum brevis superficialis (VBS) arises from the volar plate of the PIP joint and attached to the decussation of the FDS. The source vessels are from

• The vinculum brevis profundus (VBP) arises along the distal two thirds of the middle phalanx (whose source arteries are from both the interphalangeal and

• The vinculum longus profundus generally originates at the level of insertion of the FDS tendon (through the decussation of the FDS) and attaches to the FDP

distal transverse digital arteries) to insert dorsally on the FDP.

Physiology of Flexor Tendon Healing and Rationale for Treatment Protocols

directly, and its source is the proximal transverse digital artery.

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 micro-

The collagen fibrils in turn aggregate together to form the basic tendon unit the collagen fibre. The collagen fibre is defined as the smallest tendon unit visible using light microscopy [1]. Aggregates of collagen fibres form a primary fibre bundle called a subfascicle, and a group of primary fibre bundles form a secondary fibre bundle called a fascicle. A group of secondary fascicles in turn form a tertiary

Both the fascicles and tertiary tendon bundles show a spiral formation along the course of the tendon [1]. In the resting state, the collagen fibres and fibrils show a wavy configuration that appears as regular bands across the fibre surface [11]. This configuration disappears when the tendon is stretched—here the collagen fibres straighten. When the stretching forces are removed, the tendon resumes its normal wavy appearance. If an acute stress causes an elongation of 8% or more, the tendon

Fibres along the tendon are not only parallel. Jozsa et al. [12] demonstrated that

there are five types of fibre crossings—parallel running fibres, simply crossing fibres, crossing of two fibres with one straight running fibre, a plait formation with three fibres and an up-tying of parallel running fibres with one fibre. The ratio of longitudinal to transverse running fibres ranges between 10:1 and 26:1 [13]. Within one collagen fibre, the fibrils are oriented longitudinally and transversely.

bundle; it is the tertiary bundles that contribute to the full tendon and are

3. Microscopic aspects of flexor tendon anatomy

proximal phalanx.

3.1 Collagen

scope called collagen fibrils [1].

is likely to rupture [1].

103

3.2 Collagen fibres, fibre bundles and fascicles

surrounded by epitenon (refer to Section 3.3).

the proximal transverse digital artery.

DOI: http://dx.doi.org/10.5772/intechopen.86064

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 trigger finger [6].

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 expand and compress with flexion and extension.

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 continuous [7].

### 2.2 Vascular supply of the extrinsic hand flexors

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 carrying blood to the tendons [9]:

• 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

transverse communicating branch of the digital arteries at the base of the proximal phalanx.

