Fascia is a type of connective tissue that features irregularly arranged collagen fibers, as opposed to the regular arrangement found in tendons, ligaments, or aponeurotic sheets. The complex fascial system, with its various layers and regional classifications, plays a crucial role in the stability and functionality of the overall musculoskeletal system. Also see closely related article Fibrous Connective Tissues.
Fascia is distinct from other connective tissues, such as tendons, ligaments, and aponeuroses, due to its irregular arrangement of collagen fibers. This unique feature enables fascia to function as a packing tissue, resisting tensional forces in multiple directions. In contrast, tendons, ligaments, and aponeuroses have a more organized arrangement of collagen fibers, allowing them to resist maximum force in specific planes but making them vulnerable to tensional or shear forces in other directions.
Aponeurotic tissue, a flattened tendon consisting of regularly arranged collagenous fiber bundles, differs from fascia according to the Terminologia Anatomica of the Federative Committee on Anatomical Terminology (1998). Retinaculum is a retaining band or ligament composed of dense connective tissue. Bands with irregularly arranged collagenous fibers are classified as fascia, while those with a regular arrangement, such as those around the ankle, are classified as ligaments.
Classification of Fascia
The fascial system can be classified into four main types:
- Pannicular or superficial fascia surrounding the body.
- Deep or investing fascia surrounding the musculoskeletal system, also known as axial or appendicular fascia based on location .
- Meningeal fascia investing the central nervous system.
- Visceral or splanchnic fascia investing body cavities and their contained organs.
These fascial layers can be visualized as a series of concentric tubes. Other regionalized systems of classification have also been used for fascia.
Previously referred to as the panniculus, superficial fascia comprises three sublayers: a superficial adipose layer, a deep adipose layer, and the fascia separating them. This classification serves as a general description of subcutaneous tissue throughout the body.
Deep or Investing Fascia
Deep to the superficial layer lies the investing fascia, also known as deep fascia, which surrounds the musculoskeletal system. This thicker, denser fascia is often bluish-white, typically devoid of fat, and described as 'felt-like' in composition and texture. It surrounds bones, cartilages, muscles, tendons, ligaments, and aponeuroses. The investing fascia seamlessly blends into the periosteum of bone, the epimysium of skeletal muscle, and the peritenon of tendons and ligaments. Although not explicitly named, this investing fascia layer also extends from muscle to any associated aponeuroses. On an aponeurosis, the investing fascia represents the irregular, translucent layer that must be removed, typically through meticulous dissection, to reveal the underlying regularly arranged collagen fibers in the aponeurosis.
In adult vertebrates, the trunk muscles can be generally categorized into two groups: hypaxial muscles, which are situated ventral to the vertebral horizontal septum, and epaxial muscles, positioned dorsal to the septum.
The investing (or deep) fascia can be divided into two forms based on location: axial investing fascia surrounding trunk or torso muscles, and appendicular investing fascia surrounding extremity muscles. Axial investing fascia is regionally divided into hypaxial fascia, which invests muscles developing anterior to the transverse processes of the vertebrae and is innervated by the anterior or ventral primary ramus, and epaxial fascia, which surrounds muscles developing posterior to the transverse processes and receives innervation by branches of the posterior or dorsal primary ramus. Hypaxial investing fascia forms one large cylinder investing the muscles of the thoracoabdominopelvic cavity, while epaxial investing fascia is divided into two longitudinal cylinders by the spinous processes of the vertebrae.
Bridging Muscles and Fascial Sheaths
Muscles spanning from extremity to torso, such as the pectoralis major and minor, rhomboid major and minor, trapezius, latissimus dorsi (LD), serratus anterior, and serratus posterior muscles, are embedded in a common fascial blanket that extends from the limb to wrap around the torso. This blanket reaches from the first rib down to the xiphoid process anteriorly and from the cranial base to the sacrum posteriorly.
These upper extremity-bridging muscles share a common embryological origin, arising from limb bud mesenchyme and growing onto the somatic portion of the body to form a broad expansion that ensheaths the torso. The appendicular fascial sheath is shaped like an inverted cone, fitting over the tapering walls of the thorax to support the upper extremity. Each muscle in the proximal portion of the extremity must find attachment to the torso but cannot penetrate through axial muscles. Thus, the pectoral muscles and serratus anterior attach to the ribs and associated hypaxial fascial membranes covering the hypaxial muscles, while trapezius and rhomboid muscles extend to the midline. The LD wraps around the body to reach the midline in the thoracolumbar region and then extends diagonally, attached to the investing fascia of the epaxial muscles all the way to the iliac crest in some individuals.
Paraspinal Retinaculum and Bridging Muscles
Based on the musculoskeletal system's embryology, it is expected that the paraspinal (epaxial) muscles would be located within an intact fascial sheath (retinaculum), and this sheath should pass from the spinous processes and supraspinous ligament around the lateral border of the muscles to reach the tip of the transverse processes. Moreover, this sheath should extend, uninterrupted, from the cranial base to the sacrum, providing a retinaculum for the paraspinal muscles. Bridging muscles from the extremity will attach to the sheath but not penetrate it.
Upper Extremity Development and Bridging Muscles
Finally, considering the development of the upper extremity, it is expected that the bridging muscles should form an external layer (superficial lamina of the PLF) covering the paraspinal retinaculum. As these muscles originate from limb bud mesenchyme, they grow onto the somatic portion of the body, forming a broad expansion that ensheaths the torso without penetrating the axial muscles. This fascial sheath supports the upper extremity, and the associated muscles must find attachment points on the torso. The pectoral muscles and serratus anterior, for example, form attachments to the ribs and associated hypaxial fascial membranes covering the hypaxial muscles, while trapezius and rhomboid muscles extend to the midline.