Photobiomodulation in Tendons: Effects in vitro, in vivo, and Clinical Use

<strong>14.1 Photobiomodulation in Tendons</strong> Tendons (structures that connect muscles to bones) are composed of a dense connective tissue containing fibroblasts and extracellular matrix, including glycoproteins, elastin, proteoglycans, multiple saccharides, and collagen fibrils, which is the majority component by weight. In normal conditions, type-I collagen provides tensile strength to the tendon, and type-III collagen is distributed among bundles of type-I collagen. The tensile stiffness of collagen fibrils is due to their geometrical configuration, wherein the alignment of the individual fibrils confers resistance to the mechanical tension exerted by the muscle contraction, transmitting the muscle contraction forces to the bone in order to produce the movement of the limb. However, as a result of injuries combined with different kinds of extraneous factors (obesity, trauma, prolonged use of drugs such as antibiotics and corticosteroids, repetitive stress in work activities, and recreational sports activities), tendon tissue can develop a painful condition called tendinopathy that is characterized by an inflammatory process, biochemical changes in the extracellular matrix, and loss of biomechanical properties, such as lower resistance to tensile forces. Classified as a degenerative disease, tendinopathies generally have a poor ability to spontaneously heal, and the chronic conditions can sometimes lead to the complete rupture of the tendons, which requires surgical repair.