# Laser Reconstruction of Spinal Discs Experiments and Clinic
## 2 Non-Destructive Laser Modification of Cartilage Is the Basis of the New LRD Approach  
### Physicochemical and Biological Processes in the Disc Tissues under Laser Radiation  
When exposed to laser radiation on disc tissue, various processes occur, including optical (absorption and scattering of light), thermal (heating and cooling), mechanical (the formation and propagation of mechanical stresses, changes in the strength characteristics of the tissues), electrical (redistribution of charges, change in electrical conductivity) and physicochemical processes (rupture of chemical bonds, phase transitions and chemical reactions, formation of gas bubbles, denaturation, coagulation, tissue carbonization). At the same time, various biological processes (proliferation and death of cells, transformation of the cartilage matrix, and tissue regeneration) also occur, which can lead to positive or negative medical effects.  
Laser-induced phase transformations and chemical reactions in tissues occur in several stages. The first stage of laser modification of the structure and mechanical properties of cartilage is a change in the state and structure of interstitial water. Depending on the power and duration of laser heating, the subsequent stages of laser exposure can be various processes, such as structural changes in the collagen and proteoglycan subsystems, local mineralization, local melting, and the formation of micropores in the cartilage matrix [40]. The study of collagen stability in AF was carried out by the method of differential scanning calorimetry [41]. It was shown that the violation of the structural organization of the IVD collagen network is determined not only by the maximal temperature of the heated zone, but also by the space-time temperature distribution. Short-term (within 5-10 s) heating of IVD tissues to temperatures of 65-70 \({}^{\circ}\)C does not lead to a noticeable denaturation of CF. At the same time, the non-uniform temperature distribution leads to the expansion of heated zones, the movement of interstitial water and the development of thermal stresses in the affected area. As a result, local foci of violation of the strictly ordered arrangement of CFs and fibrils arise. If the temperature of the tissue in this case turns out to be above 80 \({}^{\circ}\)C, then a noticeable denaturation of collagen occurs, which leads to local shrinkage of the CFs. This process, in turn, stimulates the development of additional tension on adjacent fibers, which can lead to rupture of individual CFs]. Thermomechanical stresses affect cells, the structure and thermal stability of the extracellular matrix (ECM), both in the heat-affected zone and outside it [41]. The results of these experiments can explain the reasons why various currently used physical methods associated with significant heating of AF (including laser, electrothermal, or RF) lead to a decrease in the mechanical and thermal stability of IVD tissues, which, in a number of cases, is the cause of postoperative complications when exposed to intense energy sources with excessively increased or poorly controlled power. Therefore, the main target of laser action in the LRD technology was chosen to be the NP of the disc and the EP region without a significant effect on the AF [32].