# Laser Reconstruction of Spinal Discs Experiments and Clinic
## 3 Laser Reconstruction of the Intervertebral Disc in Animal Experiments  
### Morphological Changes in Rabbit Discs as a Result of Ex Vivo and In Vivo Laser Irradiation  
The effect of laser radiation on the processes of IVD tissues regeneration was studied based on theoretical models of temperature fields and thermomechanical stresses arising during laser action on cartilage [69; 70] and experimentally ex vivo and in vivo on rabbits using holmium (2090 nm) and erbium fiber (1560 nm) lasers. To simulate the DDD, the discs were preliminarily mechanically damaged by removing the NP or puncturing the AF with a needle. A month after the mechanical damage of the discs, they were irradiated using an optical fiber of 400 \(\upmu\)m in diameter, which was introduced into the IVD through a puncture needle. The irradiated area was examined by optical coherence tomography and histology methods one and two months after the operation. It was shown that, depending on the laser settings, the damaged tissues are transformed into various types of cartilage (hyaline or fibrous) or bone tissue is formed [30; 31]. Non-ablative laser action modifies the cartilage structure, promotes the growth of the new tissue with a pronounced heterogeneous structure, which causes a useful functional combination of elastic and plastic properties of IVD [32]. It has been shown that radiation with a wavelength of 1560 nm causes changes in areas larger in size (about 1 mm) than radiation of 2090 nm. Temporal and spatial dependences of temperature and thermomechanical stresses in the affected zone were studied in ex vivo experiments [47; 66]. It is shown that when exposed to laser pulses 1 s in duration, with a frequency of 0.5-1 Hz at a radiation power of 1 W, stresses (pressures) from 10 to 25 MPa arise. It is known that mechanical stress is one of the main stimuli for the activation of cartilage cells [43]. Usually, physiological loads that increase the synthetic activity of chondrocytes do not exceed 20-25 MPa. At 30-50 MPa, the formation of shock proteins was observed, and even higher pressures can cause noticeable damage and cell death [46; 71]. Then, in a short (3-day) in vivo experiment, a preliminary assessment of 12 different modes of laser exposure was carried out. It was found that the repetitively pulsed laser radiation with frequencies in the range of 0.3-2 Hz leads to more pronounced regeneration processes than continuous wave radiation. The intensity of cell proliferation and synthesis of ECM components after laser exposure was significantly higher than after mechanical destruction of the inner region of the disc. Three modes of laser radiation (given in Table 1) with a pulse duration of 10, 100, and 1000 ms and a repetition rate of 0.5 and 1 Hz, for which the maximum arising stresses were in the range 15-25 MPa, and the maximum tissue heating was 20-30 \({}^{\circ}\)C within 10 s did not lead to a noticeable denaturation of IVDcollagen. These laser settings were selected for further research. At this stage, the DDD of the rabbit was simulated by puncturing with an 18 G needle. A month later, this led to the development of a degenerative process with the formation of zones of necrosis, which remained in the IVD for a long (at least 6 months) postoperative period. Then, laser treatment was performed to study the most pronounced biological effects leading to cartilage regeneration and IVD restoration. (Figure 4A). _Results of laser exposure in the selected modes._  
Mode 1: On the 30 th day, hyaline EP thickness increased significantly. At the same time, rounded cells with lacunae (mature chondrocytes) prevailed in the outer layer while the cells without lacunae (i.e., proliferating chondroblasts replacing NP cells) occupied the inner layer. In the middle and internal zones of AF, there was a pronounced fibrohyaline transformation of necrotic tissues. In some animals, the normal tissue of the NP with a reticular ECM and clusters of notochordal cells was partially retained.