# Laser Reconstruction of Spinal Discs Experiments and Clinic
## 3 Laser Reconstruction of the Intervertebral Disc in Animal Experiments
### Safe and Effective Laser Settings. Choice of Optimal Parameters of Laser Radiation  
The transition from optimal irradiation regimes causing IVD restoration in rabbits to the regimes of irradiation of discs of large animals and patients required special studies. It was assumed that the main mechanism of the therapeutic effect is associated with a small short-term and local heating of the disc and the resulting thermomechanical stresses, which are determined by the values of the achieved temperature gradients. Three modes of laser exposure, leading to heating of rabbit discs by (1) 30-35 \({}^{\circ}\)C (radiation power 1.2 W); (2) 20-25 \({}^{\circ}\)C (0.9 W); (3) 7-10 \({}^{\circ}\)C (0.6 W) were tested ex vivo on sheep discs. The radiation of a fiber laser with a wavelength of 1560 nm was applied through a fiber of 0.6 mm in diameter. The pulse duration was 2 s, the interval between the pulses was 1 s, the irradiation was carried out with three series of pulses, 10 pulses in each series, and the interval between the series was 20 s.  
Histological and histochemical examinations of the irradiated discs showed that in the given time regime of irradiation at a power of 0.6 W, the structure of the disc differed little from the initial structure of the unirradiated NP. With an increase in power to 1.2 W, a noticeable denaturation of the disc tissues was observed, the area of coagulation necrosis increased to 10-15%. With a power of 0.9 W, changes in temperature by 20-25 \({}^{\circ}\)C correspond to the final temperature of the disk 55-60 \({}^{\circ}\)C, at which the proportion of coagulation necrosis is insignificant, a very small fraction of cells are damaged, which nevertheless emit signaling molecules that trigger regeneration processes into the surrounding matrix. It is this regime of laser exposure in experiments on rabbits in vivo that led to the predominant growth of hyaline-type cartilage. Therefore, the temperature range of 20-25 \({}^{\circ}\)C was determined as the optimal heating for laser treatment of discs, since at the selected time irradiation mode, such heating does not lead to significant coagulation of the tissue and is sufficient to create the necessary thermal stresses leading to the activation of chondrocytes, to loosening of the ECM, formation of micropores, generation of shock proteins, growth factors and other signaling molecules that promote cartilage regeneration. To achieve optimal heating (by 20-25 \({}^{\circ}\)C) of human discs, the modes of laser exposure were used, which were previously studied in the animal experiments. The radiation power was refined considering the differences in the sizes and optical properties of human and animal discs. Since, with a decrease in the water content, the heat capacity of collagen decreases, which compensates for the change in light absorption and leads to a weak dependence of the optimal radiation power on the water content. A more important parameter affecting temperature was found to be the size of the discs, which are significantly different for humans and most of the animals [73]. For relatively large human discs, the radiation energy spreads by thermal conduction to a larger volume than for the discs of rabbits and sheep; therefore, the optimal power values for human discs should be higher than for these animals. Calculation based on the solution of the thermal conductivity equation shows that in the selected irradiation mode, to heat human discs with a diameter of 4-5 cm by 20 \({}^{\circ}\)C, requires 25-30% more power than for the same heating of a sheep disc with a diameter of 1.5-2 cm.