Photobiomodulation in Cartilage: in vitro, in vivo, and Clinical Trials

<strong>13.1 Photobiomodulation in Cartilage</strong> Cartilage is a tissue that develops following the differentiation of mesenchymal cells into chondroblasts, which in turn become chondrocytes that specialize in synthesizing an extracellular matrix rich in proteoglycans, elastin, and collagen. Chondrocytes are embedded within a dense extracellular matrix that is distributed in several cartilage layers classified as superficial, transitional, radial, and calcified zones. Differentiated chondrocytes are responsible for maintaining the homeostasis of the cartilage, adjusting the synthesis and catabolism of the extracellular matrix. However, adult chondrocytes do not divide, and the cell population in cartilage is supplied by new cells arriving as a result of diffusion from outside, limiting the capacity of cartilage to undergo self-regeneration. In this context, cartilage is avascular (contains no blood vessels), which contributes to the poor self repair of this tissue. Because cartilage has only a limited capacity for self healing, any kind of damage can produce major complications, especially if this damage occurs in articular cartilage. Any chondral and osteochondral damage will cause disabilities and poor quality of life in young people and especially in elderly subjects. Moreover, during pathological conditions such as arthritis and osteoarthritis, the homeostasis of the cartilage is disturbed, and a cascade of biochemical and cellular reactions are responsible for the accumulating destruction of this tissue. Inflammation and an imbalance between anabolic and catabolic factors can result in the destruction of cartilage tissue, causing pain and loss of function.