# Laser Reconstruction of Spinal Discs Experiments and Clinic
## 2 Non-Destructive Laser Modification of Cartilage Is the Basis of the New LRD Approach
### Mechanisms of Laser Regeneration  
Laser activation of cell regeneration can occur as a result of the direct action of laser radiation on cells, and indirectly--through the ECM due to the modification of its structure and the formation of temperature and mechanical stress fields. It is known that chondrocytes are sensitive to environmental conditions, in particular, to temperature and mechanical stress [45; 46]. Modulated in space and time, laser radiation causes pulsed-periodic heating, leading to inhomogeneous thermal expansion and inhomogeneous pulsating field of mechanical stresses, which can actively affect the function of chondrocytes, contributing to their proliferation and biosynthetic activity. Compressive stress of a certain frequency (0.1-10 Hz) and amplitude (5-25 MPa) promotes the proliferation of chondrocytes and the activation of the synthetic activity of cells to produce collagen and proteoglycans [47; 48]. Going beyond the indicated boundaries of the vibration frequency range and an excessive increase in the amplitude of mechanical action leads to inhibition of the life of cells and to their apoptosis [48]. Figure 2: Targets for laser radiation and regeneration processes leading to the restoration of IVD. Mechanical forces act on processes in the cells through mechanotransducers PIEZO1 and PIEZO2, which convert external mechanical stresses into electrical signals that control cell metabolism and apoptosis [49; 50]. Another important mechanism of laser regeneration is the formation and distribution of thermo-shock proteins, cytokines, and growth factors, released by a small fraction of cells dying as a result of laser action [51]. According to the data [52; 53] obtained in experiments with cultures of chondrocytes, growth factors and cytokines increase the production of proteoglycans and type II collagen, cause accelerated cell proliferation, and inhibit their apoptotic death. In experiments [54], the induction of chondrogenic differentiation of bone marrow stem cells under the influence of growth factors and glucocorticoids were found. In vivo studies, an increase in IVD mass and proteoglycan content and the appearance of cell clusters similar to clusters of normal hyaline cartilage were found [55]. The third important mechanism of laser regeneration is the modification of the porous structure of the ECM (Figure 3). Loosening of the matrix is observed using an optical or ultrasound microscope [40; 56], and a more precise microscopy (atomic force microscope, structured irradiation microscope, and electron microscope) makes it possible to study specific pores of micron and submicron size [57]. Microropores increase the diffusion of water and nutrients in the cartilage matrix and thus promote regeneration. In this case, there are no macroscopic structural changes and deterioration of the mechanical properties of the cartilaginous tissue [32; 40]. The pore size in ECM is of great importance. Macroscopic pores and defects (ranging in size from hundreds of microns to several millimeters) tend to become overgrown with cells and matrix. Small pores a few microns in size, as a rule, do not overgrow. Such micropores, formed in the cartilaginous tissues of the IVD and joints of animals as a result of laser irradiation, were observed several months after laser exposure [42; 57]. The regeneration processes discussed above are associated with a slight heating of the tissue and can be called thermomechanical mechanisms. Regarding the mechanisms of photobiomodulation, low-level laser therapy (LLLT) does not lead to noticeable heating of the tissue. Various hypotheses have been put forward concerning various chromophores [58; 59; 60; 61; 62]. The effects of laser radiation over living tissues are based on the absorption of its energy and its transduction into a biological process, mainly due to Adenosine triphosphate (ATP) synthesis. LLLT may modulate transcription of several growth factors, including fibroblast growth factors (FGF) and vascular endothelial growth factors (VEGF) [63].