**2. Therapeutic motivation**

One of the modern tendencies in a low-invasive medical therapy is a medium power (1–10 W) laser treatment of connective tissues. The examples of such technologies are: laser engineering of cartilages (Bagratashvili et al., 2006); puncture multichannel laser decompression of disc (Sandler et al., 2002; Sandler et al., 2004); laser intervention upon osteochondrosis (Chudnovskii & Yusupov, 2008); laser treatment of chronic osteomyelitis (Privalov et al., 2001); endovenous laser ablation (Van den Bos et al., 2009); fractional photothermolysis (Rokhsar & Ciocon, 2009).

Treatment of osteochondrosis, for example, is based on laser-induced (0.97 µm in wavelength and 2–10 W in power) formation of multiple channels inside an intervertebral disc using silica fiber with a carbon coated fiber tip surface, in order to enhance laser light absorption nearby the fiber tip. Osteochondrosis is caused by such partial destruction of

Laser-Induced Hydrodynamics in Water and Biotissues Nearby Optical Fiber Tip 97

As a result, the hernia transforms into a soft sponge, the pressure of hernia on nervous roots decreases, and relevant pain releases. The hernia itself disappears after some period of time, and regenerative processes take place which result (in a few month) in recovery of the disc structure and their main functions (Sandler et al., 2002, 2004; Chudnovskii et al., 2008, 2010a,

Another important example of a medium power laser therapy is a laser treatment of chronic osteomyelitis. Fig. 3а demonstrates the X-ray image of femoral bone of the 14 year old patient heavily affected by osteomyelitis (Privalov et al., 2001). Significant destruction and rarefication of bone structure takes place. Typically, such bone tissue degradation requires amputation of organ. However, application of medium power laser treatment approach in this case gave, as a result, a complete regeneration of affected femoral bone (Fig. 3b), and no amputation was required. Again, therapy was based on a medium power laser-induced formation of channels (similar to that presented at Fig.1) in a bone medullary tissue, which

Fig. 3. X-ray images of right hip of the 14 y.o. patient with chronic osteomyelitis (Privalov et al., 2001). a - destruction and rarefication of bone structure before treatment. b - complete

Strong regenerative potential of medium power laser treatment for different kinds of tissues is already well recognized (Sandler et al., 2002, 2004; Chudnovskii & Yusupov, 2008, Chudnovskii et al., 2010a, 2010b), however the dominant primary physical mechanisms of such regeneration are still the subject of controversy. It is commonly accepted that the effects of medium power laser irradiation result from laser heating of tissues. However in most of cases, the pronounced therapeutic effect cannot be rationalized by laser-induced thermal tissue degradation only. For example, appearance of cavities in the hernia and significant decrease of its density observed immediately after laser manipulation (Fig. 2b), takes place without its heating, since hernia is located quite far from the area of laser-induced channel

stimulate successively the regeneration processes in the bone tissue.

regeneration of bone structure 11 month after laser treatment.

formation, and, thus, heating of hernia is negligible.

2010b).

intervertebral disc, followed by release of nucleus pulposus from disc in the form of hernia, which exerts pressure upon nervous roots thus giving pain. Fig 1а shows the scheme of formation of multiple laser channels inside intervertebral disc in the course of laser treatment of osteochondrosis (Sandler et al., 2002; Sandler et al., 2004; Chudnovskii & Yusupov, 2008). Transport laser delivery fiber passes inside the disc under treatment through a thin needle inserted to the disc (laser puncture procedure). Optical fiber is inserted through a thin needle via a posterolateral percutaneous approach under a local anesthesia. Important, that saline water is permanently introduced into the disc through the needle. Channel is formed by the heated fiber moving forward inside the disc. The fiber forms the channel and is shifted 1 -2 cm per 5 – 10 s inside the disc. Fig. 1b shows the example of such channels in nucleus pulposus of spinal disc formed by a fiber laser in the course of laboratory experiment (Sandler et al., 2004).

Fig. 1. a - Scheme of laser irradiation of spinal disc. b – Laser channel formed in spinal disc through optical fiber in presence of physiological solution (Sandler et al., 2004).

Surprisingly, that such action on herniated disc causes significant effect in some period of time on tissues located out of laser irradiated zone. As one can see, for example, on tomography picture (Fig. 2b), some cavities appear in the hernia, and its density decreases significantly compared with the density of hernia before laser treatment (Fig. 2a).

Fig. 2. Computer tomography pictures of herniated disc area. a– before healing: big sequester of hernia (side view); b - cavity inside hernia, stimulated by laser-induced channel formation in disc; с –three month after laser healing : no hernia.

intervertebral disc, followed by release of nucleus pulposus from disc in the form of hernia, which exerts pressure upon nervous roots thus giving pain. Fig 1а shows the scheme of formation of multiple laser channels inside intervertebral disc in the course of laser treatment of osteochondrosis (Sandler et al., 2002; Sandler et al., 2004; Chudnovskii & Yusupov, 2008). Transport laser delivery fiber passes inside the disc under treatment through a thin needle inserted to the disc (laser puncture procedure). Optical fiber is inserted through a thin needle via a posterolateral percutaneous approach under a local anesthesia. Important, that saline water is permanently introduced into the disc through the needle. Channel is formed by the heated fiber moving forward inside the disc. The fiber forms the channel and is shifted 1 -2 cm per 5 – 10 s inside the disc. Fig. 1b shows the example of such channels in nucleus pulposus of spinal disc formed by a fiber laser in the

Fig. 1. a - Scheme of laser irradiation of spinal disc. b – Laser channel formed in spinal disc

Surprisingly, that such action on herniated disc causes significant effect in some period of time on tissues located out of laser irradiated zone. As one can see, for example, on tomography picture (Fig. 2b), some cavities appear in the hernia, and its density decreases

through optical fiber in presence of physiological solution (Sandler et al., 2004).

significantly compared with the density of hernia before laser treatment (Fig. 2a).

Fig. 2. Computer tomography pictures of herniated disc area. a– before healing: big

formation in disc; с –three month after laser healing : no hernia.

sequester of hernia (side view); b - cavity inside hernia, stimulated by laser-induced channel

course of laboratory experiment (Sandler et al., 2004).

As a result, the hernia transforms into a soft sponge, the pressure of hernia on nervous roots decreases, and relevant pain releases. The hernia itself disappears after some period of time, and regenerative processes take place which result (in a few month) in recovery of the disc structure and their main functions (Sandler et al., 2002, 2004; Chudnovskii et al., 2008, 2010a, 2010b).

Another important example of a medium power laser therapy is a laser treatment of chronic osteomyelitis. Fig. 3а demonstrates the X-ray image of femoral bone of the 14 year old patient heavily affected by osteomyelitis (Privalov et al., 2001). Significant destruction and rarefication of bone structure takes place. Typically, such bone tissue degradation requires amputation of organ. However, application of medium power laser treatment approach in this case gave, as a result, a complete regeneration of affected femoral bone (Fig. 3b), and no amputation was required. Again, therapy was based on a medium power laser-induced formation of channels (similar to that presented at Fig.1) in a bone medullary tissue, which stimulate successively the regeneration processes in the bone tissue.

Fig. 3. X-ray images of right hip of the 14 y.o. patient with chronic osteomyelitis (Privalov et al., 2001). a - destruction and rarefication of bone structure before treatment. b - complete regeneration of bone structure 11 month after laser treatment.

Strong regenerative potential of medium power laser treatment for different kinds of tissues is already well recognized (Sandler et al., 2002, 2004; Chudnovskii & Yusupov, 2008, Chudnovskii et al., 2010a, 2010b), however the dominant primary physical mechanisms of such regeneration are still the subject of controversy. It is commonly accepted that the effects of medium power laser irradiation result from laser heating of tissues. However in most of cases, the pronounced therapeutic effect cannot be rationalized by laser-induced thermal tissue degradation only. For example, appearance of cavities in the hernia and significant decrease of its density observed immediately after laser manipulation (Fig. 2b), takes place without its heating, since hernia is located quite far from the area of laser-induced channel formation, and, thus, heating of hernia is negligible.

Laser-Induced Hydrodynamics in Water and Biotissues Nearby Optical Fiber Tip 99

Fig. 4. Microscope pictures (in scattering mode) of intrusions of Ag nanoparticles in water (outlined with dashed line) stimulated by laser induced hydrodynamics nearby optical fiber tip at 1.0 W of 0.97 µm laser power in 6 s (a), 12 s (b), and 18 s (c) of laser irradiation. Fiber

Energy of incident laser light is partly (10–20%) absorbed by the carbon layer on the blackened fiber, so that the fiber is heated. When laser radiation with a power of greater than 3 W is transmitted by the fiber tip in air, the spectrum of the optical radiation from the fiber tip contains the fundamental line (0.97 µm or 1.56 µm) and the broadband visible and near-IR radiation caused by the heating of the tip surface to relatively high temperatures. When a blackened tip is placed into water, the tip surface is effectively cooled and the absence of the broadband radiation means the substantially lower temperatures of the tip surface. However, a medium power laser radiation (1–5 W) is sufficient for surface heating and generation of vapor-gas bubbles. When water is heated, the dissolved gases are liberated in the vicinity of the tip surface and gas bubbles emerge. Water is evaporated inside the bubbles, so that the bubbles are filled with vapor and, consequently, increase in size. At the lower boundary of the above power interval, the bubbles increase in size residing on the tip surface (Fig. 5a). When a critical size is reached, the bubbles are detached

Water molecules which approach the heated tip surface acquire additional kinetic energy and momentum. The component of the total momentum of vapor molecules that is directed perpendicularly to the tip surface of the fiber towards water appears insufficient for the detachment of the bubble. Figure 5a shows that the bubbles sizes can be close to the diameter of the silica fiber core (400 µm). In the experiments, the bubbles normally emerge at same spots on a tip surface, which correspond to a high temperature areas. Evidently, the presence of such spots is related to the nonuniformity of the carbon layer: the absorbed energy (and, hence, the temperature) is greater for thicker regions. The stabilization (i.e., the

tip is shown by dashed line (Yusupov et al., 2011b).

and move to the surface.

We believe that *effective hydrodynamic processes* play dominant role for the effect of a medium power laser-induced regeneration and healing of connective tissues diseases (intervertebral hernia, osteomyelitis and some other diseases) using laser puncture procedures (Chudnovskii & Yusupov, 2008; Chudnovskii et al., 2010a, 2010b). Main features of these processes will be considered below.
