26 Hand Fascia, Retinacula, and Microvacuoles
The term “fascia” has been applied to a large number of very different tissues within the human body and in particular to specialized connective tissue structures in the hand.
These range from organized ligamentous formations such as the longitudinal bands of the palmar fascia, whose aligned type I collagen fibers are visible to the naked eye as parallel silvery bundles or the loose packing tissues that surround all of the moving structures within the hand. In other parts of the body the terms “superficial” and “deep” fascia are often used but these have little application in the hand and fingers.
Fascia provides a number of important roles that can broadly be considered as providing either mechanical function or frictionless gliding motion. The fascial continuum can be considered as a fibrous skeleton or framework channelling and guiding the course of a variety of longitudinal anatomical components as they pass through the palm to the digits. The mechanical, that is retinacular, roles of individual fascial structures include transmission of loads, acting as anchors for the skin, binding together mobile structures thus restraining unwanted motion, tethering, or limiting motion. Overall Benjamin 1 has considered the role of fascia as optimizing the transfer of muscle force.
The most defined connective tissue structures, i.e., tendons and ligaments, are well known and named individually, but the more delicate connective tissue structures also have important functional roles. Furthermore, they have very different specializations on the palmar and dorsal surfaces. The palm is adapted for padding and anchorage whereas the dorsum is particularly developed for gliding. The terms “palmar fascia” or “palmar aponeurosis” deserve special mention as they have often been used clinically as a generic term for all fascia in the hand. These terms are best reserved for the well-developed planes of longitudinal and transverse ligamentous fibers in the central part of the palm (▶Fig. 26.1). The longitudinal fibers represent the distal fascial bundles of palmaris longus when it is present (the fibers are present even when the muscle is absent). These fibers, clinically known as “pretendinous”, are arranged in bundles corresponding to the four digital rays; in addition, there is a variable bundle crossing the thenar eminence toward the radial side of the thumb. This structural arrangement was well known to early anatomists and surgeons (Albinus 2 ; Weitbrecht 3 ; Dupuytren 4 ; many early works have been translated by Stack 5 in his book The Palmar Fascia).
The thin fascial coverings over the thenar and hypothenar muscles have sometimes been considered as lateral and medial parts of the palmar aponeurosis, but these are much thinner and more flexible than the central part. Much has been written about palmar spaces (Chapter 33), particularly in relation to the accumulation of pus in infections, 6 but there are many potential spaces and it is difficult to identify their margins when operating on sepsis.
The fascial structures of the hand are all in continuity; the term continuum is often applied, although certain individual components are well defined. Their structure can best be understood by considering their different functional roles.
26.1 Channelling of Structures in Transit Between Forearm and Digits
When visualized in cross-section, the hand comprises a series of compartments that channel tendons, nerves, and blood vessels. The separating longitudinal septa, illustrated in transverse section as a honeycomb structure by Bojsen-Møller and Schmidt 7 (▶Fig. 26.2), act as spacers between the tendons and neurovascular bundles of the individual digital rays. Fascial septa between the flexor tendons in the palm were described by Legueu and Juvara 8 that seem to be quite filmy structures in the proximal palm and may be merged with the annular pulleys of the flexor sheath distally. This is one of a number of areas where there are diferent interpretations of the fascial structure depending on the anatomical preparation and tissue dissection technique.
26.2 Restraint of Unwanted Motion
The channels are thickened and have a retinacular role, forming sheaths with specialized annular pulleys where tendons must change direction around a concave surface so as to prevent tendon springing away from the underlying skeleton. These pulleys allow the direction of pull in a tendon to change as it rounds a corner, and in doing so the pulley must itself apply a considerable lateral load on the tendon. It must, therefore, have considerable strength and a system of lubrication to prevent frictional resistance to gliding. The pulley in such areas has a more cartilaginous matrix specialization and a different surface morphology. The flexor and extensor retinacula at the wrist have a similar function.
The term “retinaculum” (meaning that which retains or keeps in place) is not restricted to sheaths but is also used for various retaining ligaments of the extensor apparatus in the digits that keep it in place over convex surfaces (▶Fig. 26.3). In this instance, the retinacular ligaments form not pulleys but guy ropes, sufficiently long to allow the extensor apparatus to glide backward and forward.
26.3 Transmission of Loads
When compressive loading is applied to the hand, shock absorption is into the loculi of fat contained within defined fibrous boundaries in palm and digital pulp, such that the shape of each loculus can change when external pressure is applied but not its volume (▶Fig. 26.1 and ▶Fig. 26.4).
Thus, the compliance or deformability of the fascial boundaries between fat compartments determines the amount of shock absorption. The clinical term “turgor” is a measure of this anatomical property together with the vascular supply, since the local blood, and extracellular fluid, volume also has a major influence on tissue compliance. The palm septae has a shock absorber function in addition to the subcutaneous fat loculi, which is due to the larger honeycomb fibrous compartments between skin and skeleton described above (▶Fig. 26.2). No plane of fascia, therefore, has a simple mechanical role; all are involved in distortion of the tissues but whether the tissue is compressed or pulled, the individual collagen bundles are placed under tensional strain; the different fiber orientations in the fascia reflect the various directional vectors of loading. The soft padded parts of the hand are able to conform to the contours of objects being grasped, allowing better interpretation of sensation and better grip.
On a macroscopic scale, the palm of the hand must resist not only compression but tensile loading. Tendons and ligaments are structures particularly suitable for resisting such forces but many parts of the fascial continuum also have a major function in resisting “pulling” forces such as the anchorage system of the palm.
26.4 Anchorage
Skin is retained by fascial ligaments in an ingenious system that allows the hand to flex while retaining the skin in position. The skin folds at palmar and digital creases but there are few deep anchoring fibers located exactly at the point of folding; it is the skin on either side of the crease lines that has the best developed deep anchorage ligaments, perpendicular to the plane of the palm, allowing the unanchored skin between to fold in a repetitive pattern; the palmar creases have been described as skin “joints” which are apparent from early stages of intrauterine development.
A particularly well-developed anchorage system is the insertion of the longitudinal (pretendinous) fibers of the palmar aponeurosis, which represents the distal continuation of palmaris longus. These fibers are well developed even when the tendon is absent, in which case the fibers may merge with other fascial systems in the region of the flexor retinaculum or receive an insertion from flexor carpi ulnaris. 9 This is the fiber system principally involved in Dupuytren’s Contracture and knowledge of its detail will facilitate surgical dissection. There are three distal insertions which can be considered in layers (▶Fig. 26.5). The most superficial longitudinal fibers insert into the dermis of the distal palm, distal to the distal palmar crease, rather than in to the creases per se. This arrangement resists horizontal shearing force in gripping tasks such as holding a golf club, in which it prevents distal skin slippage or degloving of the palm on striking the golf ball. The characteristic blisters on the palms of those unaccustomed to such sports map out the sites of the skin anchorage points. The readers can demonstrate this anchorage system on their own hand by flexing the palm until the skin of the distal palm folds loosely. This loose skin can then be pinched by the thumb and forefinger of the other hand. An attempt to pull the skin distally will reveal the anchoring longitudinal fibers of the palmar aponeurosis.
There are two deeper types of anchorage of the longitudinal fibers (▶Fig. 26.5d). Layer 2 consists of fibers that bifurcate on either side of the A1 flexor tendon sheath pass underneath the superficial transverse metacarpal ligament (natatory ligament) and deep to the neurovascular bundles toward the digits. When this fascial system becomes involved in Dupuytren’s Contracture, it forms a spiral cord as the encircled neurovascular bundle can be displaced, making it vulnerable to surgical injury. The deepest longitudinal fibers (layer 3) pass around the sides of the flexor sheath and metacarpophalangeal (MP) joint to merge with a number of ligamentous structures eventually reaching the retinacular fibers that retain the extensor apparatus over the MP joints (McGrouther 1990, Zancolli 1979). Thus, the longitudinal fibers of the palmar fascia have wide insertions into skin and fascia in the distal hand.
The skin anchorage sytem in the digits is more complex but the overall pattern is arranged to anchor the skin to the axis region of the interphalangeal (IP) joints thus preventing sliding (degloving) of the skin on digital flexion. Cleland’s ligaments are oblique anchors which tether the skin of proximal and middle segments of the digits to the region of the proximal IP joints (▶Fig. 26.4 and ▶Fig. 26.6). There is a particularly well-developed band arising from fascial structures overlying the lateral aspect of the proximal interphalangeal (PIP) joint which then passes distally to reach the lateral digital subcutaneous region in the middle segment of the digit. It is not possible to define a point origin or insertion as there is a fascial continuum merging proximally with flexor sheath and continuous with fascia over the lateral band of the extensor tendon. Distally there is a loose network of lateral subcutaneous fascia, the lateral digital sheet, which can form bands in a number of directions depending on the direction of tension on this structure (▶Fig. 26.3). This rather imprecise anatomy has led to a variety of different anatomical descriptions with some controversy as to whether certain structures exist as defined structures (such as the retrovascular band and lateral digital sheet) or merely as tension lines in a three-dimensional (3D) network of fibers. Cleland’s ligaments are however easily defined, and a second bundle passes proximally from the PIP joint region to the lateral skin area over the proximal segment of the digit (▶Fig. 26.4). A similar pair of ligaments orientated proximally and distally radiates from the distal interphalangeal (DIP) joint region. Cleland’s ligaments lie posterior to the neurovascular bundles, whereas Grayson’s ligaments are skin-retaining ligaments anterior to the bundles (▶Fig. 26.4). 12 , 13 , 14 , 15
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