15 The Forearm Fascia and Retinacula

Alexis Laungani, Alain Carlier, Nirusha Lachman, and Michel Saint-Cyr

15 The Forearm Fascia and Retinacula

15.1 General Considerations for Fascia

At the turn of the century, the term fascia was used in the general sense to describe all connective tissue sheaths and envelopes of the body. ¹ ^– ³ The term fascia is derived from the Latin for a band or bandage. ⁴ Anatomically, it was considered a soft tissue layer that had to be dissected away in order to expose the underlying structures. ³ ^, ⁵ ^, ⁶ Since the actual function of this enveloping fascia was not clearly understood, early descriptions in anatomical studies were often confusing. ⁷ However, as proper understanding of anatomical structure, function, and relations continues to guide surgical technique, the last decade has seen a resurgence of interest in the fascial systems of the body. ³ ^, ⁸ ^, ⁹ While its exact role is still misunderstood, many authors agree that the fascia within the extremities plays an active role in achieving and providing tendon stability during movement. ⁷ ^– ¹²

The description proposed by the Fascia Research Society defines the fascia as “the soft tissue component of the connective tissue system that permeates the human body forming a whole body continuous three dimensional matrix of structural support.” ¹¹ Based on this definition, we explore the concept of the fascias of the human body as organized in “fascial systems” as a representation of the external equivalent of the bony skeleton.

Fascial systems provide critical and unique anatomical support for the underlying musculature and vary in density and thickness based on their location and distribution. The reader should be aware of the ambiguous terminology when referring to fascia. In the literature, the term fascia has been used to describe a wide range of connective tissue, which is confusing. ¹³ This might be explained by the diversity of disciplines involved in the field of “fascia research.” We will refer in this chapter to the terminology proposed by Langevin and Huijing, ³ who avoid using the term fascia, preferring a description of the structure as mentioned in ▶Table 15.1.

**Table 15.1** Recommended use of terms regarding fascial structures
Designation	Description of the tissue
Dense connective tissue	Connective tissue containing closely packed, irregularly arranged (that is, aligned in many directions) collagen fibers.
Non dense (areolar) connective tissue	Connective tissue containing sparse, irregularly arranged collagen fibers.
Superficial fascia	Enveloping layer directly beneath the skin, containing dense and areolar connective tissue and fat.
Deep fascia	Continuous sheet of mostly dense, irregularly arranged connective tissue that limits the changes in shape of underlying tissues. Deep fasciae may be continuous with epimysium and intermuscular septa and may also contain layers of areolar connective tissue.
Intermuscular septa	A thin layer of closely packed bundles of collagen fibers, possibly with several preferential directions predominating, arranged in various layers. The septa separate different, usually antagonistic, muscle groups (for example, flexors and extensors) but may not limit force transmission.
Interosseal membrane	Two bones in a limb segment can be connected by a thin collagen membrane with a structure similar to the intermuscular septa.
Periost	Surrounding each bone and attached to it is a bilayered collagen membrane similar in structure to the epimysium.
Neurovascular tract	The extramuscular collagen fiber reinforcement of blood and lymph vessels and nerves. This complex structure can be quite stiff. The diameter and, presumably, the stiffness of neurovascular tracts decrease along limbs from proximal to distal parts. Their stiffness is related to the angle or angles of the joints that they cross.
Epimysium	A multilayered, irregularly arranged collagen fiber sheet that envelopes muscles and that may contain layers of both dense and areolar connective tissue.
Intra- and Extramuscular aponeurosis	A multilayered structure with densely laid down bundles of collagen with major preferential directions. The epimysium also covers the aponeuroses but is not attached to them. Muscle fibers are attached to intramuscular aponeuroses by their myotendinous junctions.
Perimysium	A dense, multilayered, irregularly arranged collagen fiber sheet that envelopes muscle fascicles. Adjacent fascicles share a wall of the tube (like the cells of a honeycomb).
Endomysium	Fine network of irregularly arranged collagen fibers that form a tube enveloping and connecting each muscle fiber. Adjacent muscle fibers share a wall of the tube (like the cells o f a honeycomb).
Source: Used with permission from Langevin and Huijing. ³

Different classifications of the fascias can be found in the textbooks, some of them referring to the relationship with the skin (superficial and deep) and some referring to the biomechanical and histological properties (“loose” or areolar and dense connective tissues). ³ Nevertheless, it is common to describe the fascial systems of the body as organized into three distinct layers:

The superficial fascia is located under the skin, contains a variable amount of fat, and is sometimes also called “panniculus adiposus.” ⁷ This fatty layer is thicker in upper extremities, on the posterior aspect of the body, and in females. ⁸
The deep fascia forms an intricate network separating muscles, nerves, and vessels and acts as a component-binding structure. ⁸ Some authors therefore describe a “muscular fascia” that enables the gliding of muscles against each other and also against other structures. ⁵ ^, ¹¹
The epimysium is the outermost layer of the muscle and sometimes provides attachments to muscle insertions. The most common example is the “lacertus fibrosus” aponeurosis, which extends from the biceps tendon and then merges with the antebrachial fascia. ⁵ ^, ⁸

Based on this description of the generic structure of the fascial system, it is easier to understand the specificities of each region of the body. The fascial system of the forearm is described in this chapter.

15.2 The Fascial System of the Forearm

Three compartments (the anterior compartment, the posterior compartment, and the mobile wad) are defined by nonextensible intermuscular and intramuscular fascial sheaths that compose the antebrachial fascia, or “fascia antebrachii” ⁵ ^, ⁸ ^, ¹¹ (▶Fig. 15.1). This nonextensibility of the fascia has a clinical relevance, since it is the reason acute compartment syndromes may occur. ¹⁴ ^, ¹⁵ The deep fascias are reinforced around the joints, in the form of retinacula that can be considered as pulleys preventing the bowstringing of the tendons. ⁸

**Fig. 15.1** Aspect of the fascia of the forearm.

15.2.1 Retinacula System

As mentioned previously in the general considerations, there is a whole variety of terms used to describe all types of connective tissues. While some authors consider the terms flexor retinaculum, carpal transverse ligament, volar, or anterior annular ligament as being interchangeable, ¹⁶ ^, ¹⁷ others consider the use of the term flexor retinaculum to be incorrect, believing that it does not correspond to any properly defined anatomical structure. ¹⁶ In this chapter, however, the term flexor retinaculum will be used to describe the fascial band on the volar aspect of the wrist.

The Flexor Retinaculum

The flexor retinaculum is composed of two layers, superficial or deep, depending on its relation to the ulnar nerve and vessels. ¹⁷ ^, ¹⁸

The superficial layer is referred to as the “palmar carpal ligament” and comprises the thickened antebrachial fascia proximally and the palmar fascia distally.

The deep layer is composed of three distinct adjacent segments:

The proximal segment that represents continuity with the deep fascia of the forearm.
The central segment, properly called “transverse carpal ligament” (▶Fig. 15.2), which has attachments to the pisiform and the hamate on the ulnar side and to the scaphoid and the trapezium on the radial side of the wrist. This central segment is often referred to as “flexor retinaculum” in anatomy textbooks. ¹⁶ ^, ¹⁹ It converts the anterior concavity of the carpus into the carpal tunnel, through which the tendons of the flexor digitorum profundus, flexor digitorum superficialis, and median nerve pass. ¹⁷
The distal segment is an aponeurosis lying between the thenar and hypothenar muscles.

**Fig. 15.2** The transverse carpal ligament.

The flexor retinaculum merges distally with the palmar aponeurosis (▶Fig. 15.3). Its proximal boundaries are difficult to define due a proximal thinning as well as a merging with the antebrachial fascia. Moreover, the course of the palmaris longus tendon below the antebrachial fascia and then above its distal third may compromise the visualization of the transition between the antebrachial fascia and the flexor retinaculum. ²⁰ Therefore, Won et al ²¹ described the flexor retinaculum as consisting of proximal and distal parts that actually correspond to the central and distal segments described by Cobb et al. ¹⁷ The flexor retinaculum is vascularized through a deep network from the superficial palmar arch, a superficial network formed by branches of the ulnar artery. ²² The flexor retinaculum, by its position, defines a carpal tunnel underneath. This carpal tunnel leads the way to the flexor tendons of the fingers and to the median nerve.

**Fig. 15.3** Palmar view of the wrist, showing the relation between palmar aponeurosis and flexor retinaculum.

There are two common sites of potential compression of the median nerve at the wrist: proximally at the proximal edge of the transverse carpal ligament and distally at the level of the hook of the hamate, where the canal is narrowest. ¹⁷

The flexor retinaculum has a dorsal equivalent: the extensor retinaculum (▶Fig. 15.4).