Carla Stecco and Antonio Stecco

The deep fascia of the shoulder

The deep fasciae of the shoulder present characteristics that are similar to both the fasciae of the trunk and of the extremities. In particular, the fasciae of the pectoralis major, deltoid, trapezius, and latissimus dorsi muscles form a unique layer, enveloping all of these muscles and passing over the serratus anterior, where it forms a strong fascial lamina. This myofascial arrangement agrees with the description of the trunk as reported by Sato and Hashimoto (1984), who affirm that the pectoralis major, latissimus dorsi, and trapezius muscles form an additional myofascial layer with respect to the muscular planes in the rest of the trunk.

The fasciae of the pectoralis major, deltoid, trapezius, and latissimus dorsi muscles are relatively thin, collagen fiber layers. All of these fasciae adhere firmly to their respective muscles due to a series of intramuscular septa that extend from the internal surface of these fasciae, dividing the muscle itself into many bundles. A true epimysial fascia, or epimysium, is not discernible between this deep fascia layer and the underlying muscle. A number of muscular fibers originate from the inner side of these fasciae, as well as directly from the intramuscular septa.

The pectoral fascia originates from the clavicle, but only the deep layer of the pectoral fascia adheres to the clavicular periosteum, whereas its superficial layer continues upwards with the superficial lamina of the deep cervical fascia, which surrounds the sternocleidomastoid and the trapezius muscles. Medially, the deep layer of the pectoral fascia inserts into the sternal periosteum, while the superficial layer extends beyond the sternum to continue with the pectoral fascia on the other side. Distally, the pectoral fascia is reinforced by some fibrous expansions originating from the rectus abdominis sheath and by the fascia of the contralateral external oblique muscles. In particular, the pectoral fascia presents a mean thickness of 151 μm, and it increases in a craniocaudal direction, to reach a mean thickness of 578 μm in the mammary region. Over the xiphoid process, the pectoral fascia forms a clearly visible, interwoven pattern of fibers (Stecco et al. 2009).

The deltoid fascia appears to be of variable thickness from subject to subject, without any apparent correlation to the size of the underlying muscle mass. The fascia adheres strongly to the muscle and connects the different parts of the deltoid. According to Rispoli et al. (2009), it was consistently possible to distinguish the three portions of the deltoid muscle (anterior, lateral, and posterior), to a varying degree, with each portion continuing with the brachial fascia. The deltoid fascia continues with the fascia covering the trapezius muscle. In particular, the superficial fascial layers are in continuity, while the deep layers insert into the scapular spine and clavicle, in continuity with the periosteum.

On histologic examination, the deltoid and pectoral fasciae appear to be formed by undulated collagen fibers, arranged more or less transversely with respect to the underlying muscles. An elevated number of elastic fibers are evident with van Gieson stain (approximately 15% of all the fibers), forming an irregular mesh. The S100 stain highlights rare nerve terminations, arranged in a homogeneous manner throughout the entire fasciae.

After detaching the superficial muscular layer, the clavipectoral fascia is visible. There is an ample plane of cleavage between the pectoralis major muscle and this fascia due to the presence of loose connective tissue, which allows the deep layer of the pectoral fascia to glide autonomously with respect to the clavipectoral fascia. The latter is a strong connective layer arising from the clavicle and extending distally to enclose the subclavius and pectoralis minor muscles. Laterally, the clavipectoral fascia continues with the axillary fascia and the fascia of the coracobrachialis muscle. The clavipectoral fascia can be divided into two parts: one covering the pectoralis minor muscle and one that forms a triangular shaped layer between the upper border of this muscle and the clavicle, called the coracoclavicular fascia. The anterior thoracic artery and nerve, and the cephalic vein pierce the coracoclavicular fascia. Its thicker, lateral border, which extends from the coracoid process of the scapula to the cartilage of the first rib, is known as the costocoracoid ligament. It separates the cavity of the axilla from the anterior chest wall.

Singer (1935), describes the subscapularis fascia as being the thinnest of the different fasciae surrounding the muscles of the scapula; however, it is a well-defined lamina. Laterally, it continues with the axillary and infraspinatus fasciae, and superiorly, with the supraspinatus fascia.

The infraspinatus fascia covers the infraspinatus and teres major muscles. The deltoid and latissimus dorsi muscles cover part of this infraspinatus fascia, while only the fascia that joins the latissimus dorsi, trapezius, and deltoid muscles covers the part lying in a superficial plane. In this portion, the two fasciae adhere to each other, forming a strong fascial plate.

The supraspinatus fascia covers the supraspinatus muscle and continues with the fascia of the levator scapulae muscle. It varies in thickness, and occasionally contains some adipose tissue. According to Bektas et al. (2003), the spinoglenoid ligament, usually implicated in the compression of the suprascapular nerve, could be evidenced only in 15.6% of cases, while a thickening of the distal third of these two fasciae is always present. It is probable that this thickening could cause dynamic compression of the suprascapular nerve.

The axillary fascia is formed from the union of the superficial fascia and the deep fascia. It continues laterally with the superficial fascia of the arm and the brachial fascia, medially with the pectoralis major fascia and the coracoclavicular fascia, and posteriorly with the fasciae of the latissimus dorsi and subscapularis muscles. The axillary fascia contains numerous lymph nodes and is pierced by numerous nerves and vessels. Hence, it quite similar to the cribriform fascia of the thigh and, likewise, it is filled with a plug of fibrous tissue and fat.

The deep fascia of the arm

The brachial fascia and the antebrachial fascia form the deep fasciae of the arm. The superficial fascia in the arm is clearly evident within the subcutaneous adipose tissue, and it is easily detached from the deep fascia.

The brachial fascia is a strong, semitransparent laminar sheet of connective tissue that covers the arm muscles. It presents a mean thickness of 863 μm (SD  ±  77 μm), being thinner in the anterior region as compared to the posterior region. Collagen fiber bundles, with different directions, are easily identifiable within this fascia. They exhibit a prevalently transverse course with respect to the long axis of the arm, although longitudinal and oblique collagen bundles are present. The brachial fascia is easily separable from the underlying muscles, while it attaches to the lateral and medial intermuscular septa and the epicondyles (Plate 1.4.1). Proximally it is continuous with the axillary fascia, and the fasciae of the pectoralis major, deltoid, and latissimus dorsi muscles.

The antebrachial fascia appears as a thick, whitish layer of connective tissue, sheathing the flexor and extensor muscles compartments and extending septa between them from its internal surface. The mean thickness of the antebrachial fascia is 0.75 mm, yet this increases (mean value 1.19 mm) in the wrist region, forming the flexor and extensor retinacula of the wrist. Fiber bundles running in various directions form the antebrachial fascia. At the wrist, these bundles thicken and are arranged, from proximal to distal, in multiple layers extending in mediolateral and lateromedial directions (Plate 1.4.2). Many muscular fibers insert onto the inner surface of the antebrachial fascia in the proximal portion of the forearm, whereas this same fascia is always easily separable from the underlying muscles in the distal portion of the limb, attaching only to the radial and ulnar styloid processes and the pisiform bone. The tendon of the palmaris longus muscle pierces the antebrachial fascia in the distal third of the forearm, running superficial to the fascia before continuing with the palmar aponeurosis. The flexor carpi radialis and ulnaris muscles lie beneath the antebrachial fascia but, distally, their epitenons fuse with this fascia, so that the latter appears to envelop them at the wrist. The antebrachial fascia also forms the roof of Guyon’s canal, through which the ulnar artery and nerve pass. In the palm, the antebrachial fascia continues laterally and medially with the thenar and hypothenar fasciae, and with the thick, transversal fiber bundles tensed between the eminences. In the mid-palm region, this thickening is continuous with the deep layer of the palmar aponeurosis. Muscular fibers of the thenar and hypothenar muscles also insert onto the inner surface of the fascia.

On histological analysis, three layers of parallel collagen fiber bundles, separated from each other by a thin layer of loose connective tissue, form the deep fasciae of the arm. The alignment of these bundles is parallel in each single layer but differs from layer to layer. Many vessels are present, mostly in the loose connective tissue layers, and are intermingled with the fibrous bundles. The collagen fibers represent less than 20% of the total fiber volume. The elastic fibers form a thin, irregular mesh, and are more evident within the loose connective tissue dividing the collagen layers. The histology of the wrist retinacula presents some differences. In particular, the fiber bundles are more densely packed, there is less loose connective tissue, and the van Gieson stain did not evidence any elastic fibers. Nerves are present throughout the brachial and antebrachial fasciae, although differences can be found according to the area and individual subjects. Small unmyelinated nerves are observed in all specimens, whereas Ruffini, Pacini and Golgi-Mazzoni corpuscles are present only in some, and mainly at the level of the wrist retinacula (Stecco et al. 2007).