Introduction

Several interpretations of the term "fascia" exist within the research community, with no singular agreed-upon definition.[1] However, in simplest terms, fascia can be described as a thin layer of connective tissue that separates muscles and organs from other structures within the body. It supports and protects muscles and internal organs and reduces friction between muscles. Fascia also forms distinct muscular compartments, provides attachments, and improves circulation.[2]

Issues of Concern

Many factors can compromise fascial integrity, including aging, genetics, and surgery. Dysfunctions in fascia may result in clinical conditions due to altered structure and functional properties. There exist two main categories of fascial dysfunction; lack of fascial stiffness and an increase in fascial stiffness. 

Lack of fascial stiffness can be seen in inguinal, abdominal, femoral, and incisional hernias. Increased fascial stiffness can be seen in Dupuytren contracture, adhesive capsulitis, and Peyronie disease. Genetic conditions leading to widespread fascia laxity include Ehlers-Danlos syndrome and Marfan syndrome.[3]

Cellular Level

The histologic composition of fascia differs based on its location within the body. The superficial fascia consists primarily of collagen, elastin fibers, and adipose tissue. Superficial fascia requires the ability to resist tensile forces while also allowing elasticity. Hence, the superficial fascia is composed of many elastic fibers.[4] 

The deep fascia similarly consists of collagen and elastin, but the composition varies. The deep fascia has fewer elastin fibers, and the collagen fibers are aligned in parallel. This composition gives fascia remarkable tensile strength and the ability to stretch and contract with the musculoskeletal system. Other cell types are also present within the deep fascia, including myofibroblasts, fibroblasts, and endothelial cells.[5] Additionally, hyaluronic acid is produced and concentrated within the deep fascia and underlying muscles.[6]

Development

The majority of the fascial system is derived from the mesoderm, except for the fascia of the face and anterior region of the neck, which originates from the ectodermal cranial neural crest.[7]

Organ Systems Involved

There are four fundamental categories of fascia:

Superficial Fascia

• A thin layer of loose connective tissue directly beneath the skin

Deep Fascia

• Surrounds individual muscles and forms fascial compartments
• Is categorized as axial or appendicular fascia based on location within the body

Parietal Fascia

• Layers of connective tissue lining the walls of body cavities which lie outside the parietal layer of the serosa

Visceral Fascia

• Supports organs in their cavities and encloses them in layers of connective tissue

Function

Among the many functions of fascia considered in detail are its ectoskeletal role (acting as a soft tissue skeleton for muscle attachments), its importance for creating osteofascial muscle compartments, assisting venous return in the lower limb, decreasing stress concentration at entheses, and acting as a protective sheet for underlying structures. It is essential to recognize the continuity of fascia between regions and appreciate its vital role in coordinating muscular activity and acting as a body-wide proprioceptive organ.[2] 

Protective Function

In multiple areas of the body, the fascia provides support for the underlying nerves, vessels, and lymphatics. An anatomic relationship that illustrates this principle is seen within the carpal tunnel, as the flexor retinaculum overlies the carpal tunnel protecting the median nerve.

Myofascial Force Transmission

Myofascial force transmission occurs when a muscle produces force toward a tendon and joint and transmits that force to the connective tissue within and around the muscle. Due to the intimate relationship between muscle and fascia, the fascia plays a role in force transmission. When a muscle produces force, the force is transmitted toward the joint's direction and towards the muscle's surface. The connective tissue overlying the surface of the muscle is involved in transmitting this force. The myofascial connections can play a role in up to 30% of force transmission. At the macroscopic level, force is transmitted from the muscle to the surrounding connective tissue. At the microscopic level, cellular changes occur involving the various cells making up the fascia. Fibroblasts, one of the principal cells within the fascia, respond to the increased force by increasing cellular signaling and gene expression. These changes lead to increase cell proliferation and connective tissue remodeling.[8] 

Vascular Support Function

Interestingly, the deep fascia is more abundant within the lower limbs than the upper limbs. It is hypothesized that this abundance of fascia in the lower limbs serves as a "compression device" to increase the venous return from the lower limbs to the heart. This distinct role is seen in specific pathologies such as deep vein thrombosis that can form after long periods of inactivity. Without adequate muscle contraction, the blood becomes stagnant, fulfilling one factor of the Virchow triad, leading to thrombus formation.

Compartment syndrome describes a pathology where fascia restricts blood flow to the muscle leading to ischemia. In cases where a fasciotomy is required, the decreased fascial tension due to surgically incised tissue may lead to chronic venous insufficiency. These pathologies illustrate how fascia contributes to vascular support, specifically in the lower limbs.[9]

Pathophysiology

The list of fascia and musculoskeletal systems diseases is extensive (i.e., frozen shoulder, Dupuytren contracture, hernias, compartment syndrome).

These pathologies are due to several etiologies:

• increased fascia stiffness caused by increased inflammation from overuse or trauma
• systemic disease
• genetic disorders
• a fascial defect or disruption in the fascia
• the tension produced within a body compartment

Adhesive Capsulitis

Patients with adhesive capsulitis (frozen shoulder) have restricted and painful movements at the shoulder joint. The pain is described as constant and severe, leading to decreased passive and active range of motion. Many etiologies have been described, including rotator cuff tears, biceps tendinopathy, and shoulder trauma.[10] Biopsies have revealed an increased density of fibroblasts and myofibroblasts within the shoulder capsule. These cells are responsible for producing type III collagen, which is present in various inflammatory conditions. The initial inflammation produces pain, and the subsequent fibrosis leads to decreased range of motion.[3]

Dupuytren Contracture

Dupuytren contracture is a progressive condition caused by the shortening and thickening of the fibrous tissue of the palmar fascia. This disease tends to affect men older than 40 years, those of Northern European descent, and individuals who smoke, drink alcohol, or have diabetes. Patients typically present with a small, pitted nodule (or multiple nodules) on the palm, which slowly progresses to finger contracture. The disease is caused by increased production of type III collagen by fibroblasts and myofibroblasts within the fascia. Patients typically have difficulty with daily tasks such as hair combing, face washing, and putting their hands in their pockets.

The disease regresses approximately 10 percent of patients without treatment or surgical intervention. Steroid injections into the nodule(s) have been shown to reduce the need for surgery. Surgical intervention should occur when the metacarpophalangeal (MCP) joint contractures exceed 40 degrees or when proximal interphalangeal (PIP) joint contracture exceeds 20 degrees. Percutaneous needle aponeurotomy performed in-office may be an effective alternative to surgery in some cases.[11] 

Hernias

Hernias are a common condition caused by a defect or disruption in the fascia. This increased laxity within the fascial compartment leads to herniation of contents through the fascia. Hernias are classified based on where they occur throughout the body.

• Inguinal hernias
  • Indirect
    • Most common
    • Passes through the inguinal canal
    • Incomplete obliteration of the processus vaginalis during fetal development
    • Lateral to the inferior epigastric vessels
  • Direct
    • An acquired condition caused by the weakening of the transversalis fascia
    • Medial to the inferior epigastric vessels
    • Passes through Hesselbach triangle
    • Lateral to rectus abdominis muscle
  • Femoral
    • More common in females
    • Intraabdominal contents herniate through the femoral ring into the femoral canal
    • Inferior to the inguinal ligament, medial to the femoral vein, and lateral to the pubic tubercle
• Umbilical hernias
  • Congenital umbilical hernia
    • Failed spontaneous closure of the umbilical ring
    • Protrusion through the umbilical orifice
  • Acquired umbilical hernia
    • Caused by persistently elevated intra-abdominal pressure
    • Located adjacent to the umbilical orifice 
• Incisional hernias
  • Herniation of intraabdominal contents through weakened fascia caused by a previous surgery

Compartment Syndrome

Compartment syndrome is a common orthopedic and surgical emergency that involves increasing pressures within a compartment leading to local ischemia. The fascia surrounding a compartment provides tension that compresses the nearby structures. In trauma or other clinical scenarios, swelling within a compartment can occur, leading to increased intraluminal pressures. These pressures grow steadily and eventually become higher than the surrounding vasculature, reducing perfusion. The reduced perfusion leads to surrounding hypoxia, ultimately leading to the death of the muscles and nerves. An emergent fasciotomy is necessary to correct compartment syndrome to release the built-up pressure.[12]

Plantar Fasciopathy 

The origin of the plantar fascia occurs at the posteromedial calcaneal tuberosity, with its insertion occurring at each metatarsal head to form the longitudinal arch of the foot. Plantar fasciopathy is an overuse disorder resulting in degenerative changes at the calcaneal attachment. Histologic inspection of samples taken from patients undergoing plantar fascia release surgery shows myxoid degeneration with fragmentation and degeneration of the plantar fascia. Collectively, these findings support the premise that this condition is a degenerative fasciosis without inflammation, not fasciitis. Therefore, plantar fasciopathy is a more accurate descriptor of the condition. Risk factors for developing plantar fasciopathy include excessive running, obesity, and occupations associated with long periods of standing.[13]

Genetic Disorders

Two specific genetic disorders associated with increased laxity of fascia include Ehlers-Danlos syndrome and Marfan syndrome. Ehlers-Danlos is a group of connective tissue disorders characterized by defective collagen synthesis with symptoms including hyperextensible skin, tissue fragility, and joint hypermobility. Marfan syndrome is a connective tissue disorder of autosomal dominant inheritance that affects elastin and microfibrils in connective tissue throughout the body. These patients typically have tall stature with long extremities, joint hypermobility, subluxation of the eye lens, and are prone to cardiovascular disorders such as mitral valve prolapse, aortic aneurysm, and dissection.[14]

Scleroderma (systemic sclerosis) is a well-known genetic disorder associated with increased rigidity of fascia. Systemic sclerosis is a disease of abnormal connective tissue growth, which leads to diffuse thickening and hardening of the skin and inner organs. Limited systemic sclerosis is associated with symptoms of CREST syndrome, including calcinosis cutis, Raynaud phenomenon, esophageal dysmotility, sclerodactyly, and telangiectasia.[15]

Clinical Significance

As noted above, many clinical manifestations of pain syndromes are associated with fascial tissue. Fascia surrounds and penetrates skeletal muscle, organs, joints, nerves, and vascular beds. Therefore, fascial tissue forms a whole-body structural support system and is of great clinical significance.[3]

Stiffening of connective tissue can be seen in the clinical setting as runner's knee, tennis elbow, golfer's elbow, adhesive capsulitis, and plantar fasciopathy. Recent data suggests fascia as a central participant in the pathogenesis of sport-induced delayed onset muscle soreness and injuries. Unspecific back pain, especially the thoracolumbar fascia, is suggested to be mediated in part by fascial structures. As fascia is rich in nociceptive nerve endings, any stretch, tear, or laxity in the fascial system may be accompanied by irritation and pain. 

Of specific clinical interest is the role of fascia in chronic neck pain. An ultrasound examination of patients with chronic neck pain revealed a greater fascial thickness at the sternal end of the sternocleidomastoid and the lower and upper side of the medial scalene muscle. This thickening of fascia was associated with a reduction in active and passive cervical range of motion.[3]

Out of the vast array of symptoms patients report, pain is the most common and most debilitating. To treat a patient's pain, the etiology must be known and researched. In many different studies, fascia has been elicited as a potential cause of pain and, therefore, is of great clinical importance.