New steps

1. New steps NEW STEPS TOWARDS AN UNDERSTANDING OF THE MECHANISMS BEHIND PHOTOBIOMODULATION. by Lars Hode Swedish Laser Medical Society

2. Historik By the time when the biostimulation effects from laser light first were identified by Mesters group 1967, it was in the medical society generally believed that the only light sensitive substance in our body is found in our retina. Since then, more than 50 light sensitive molecules and substances have been identified in our cells and tissue.

3. Why not sunlight If it only would be a question of excitation by means of photon energy, it would be enough to spend some time in the sunshine - sunlight contains photons of all energy levels from ultraviolet and up to far infrared. But even though sun bathing can ease some skin problems and make many of us feel better, it is not at all as potent as laser treatment.

4. Many light sources stimulate • But if the sun is not an efficient treatment modality, how come that we have clear biostimulative effects from so many different types of light: • lasers of many types, • light emitting diodes, • polarized broad band light, • strong wide-band non polarized light pulses • ... and even radio waves ! • They all give - and I will use the latest term - • photobiomodulation.

5. So, what is the mechanism In this example, so called non ablative skin rejuvenation is performed by use of a KTP-laser(532 nm, green light). The effects on skin condition are based on biostimulation.

6. NEW STEPS TOWARDS AN UNDERSTANDING OF THE MECHANISMS BEHIND PHOTOBIOMODULATION. So, what is the mechanism behind laser biostimulation? The answer is: we are not looking for one mechanism, we are locking for several.

7. In the following, references will occur, like: Horvath [223]. These numbers refer to the reference list in mine and Jan Tunérs latest book: “The Handbook of Laser Therapy”. More information about that and any of our former books, can be found on the web page www.prima-books.com I can also be helpful through my e-mail: lars@hode.com Information about laser therapy can also be found on Laser-World, www.laser.nu

8. Are the biostimulative effects laser specific?All forms of light affect the living organism. It has been shown that white light in certain doses influences seasonal depression conditions. This is usually named "Bright Light Phototherapy". But first a few words about laser light and coherence.Laser light is primarily characterized by two physical properties: (1) it is very narrow banded and (2) it is much more coherent than light from any other source. The upper picture is showing light of long coherence length – laser light ... ... and ... ... the lower picture shows light with shorter coherence length - monochromatic ”ordinary light”.

9. It is essential to clarify whether or not the biological effects obtained with laser therapy will appear only if the light source is a laser - that is, if the effects are laser specific. This is not just of theoretical interest. If one could just as well use a light bulb with a polarization filter, or ordinary Light Emitting Diodes (LED) of a certain color or infrared light (such as in remote controls) - it would be considerably less expensive to manufacture therapeutic instruments. Due to this, laser light causes easily interference and one example of this is the so called laser speckles. These can be seen if you illuminate a rough surface with visible laser light. Those laser speckles are not only a matter of curiosity - they play an important role in the mechanisms behind photo-biomodulation.

10. What characterizes the light in a laser speckle? The phenomenon of speckles is a form of optical noise. Speckles can be real or virtual. The three-dimensional structure of real speckles is manifest in a patient's tissue during irradiation with laser light. It arises as a result of interference between different beams with a random direction, amplitude and phase. Between areas of high and low light intensity, strong field gradients occur. They can influence electric dipoles, cause pressure and force and cause temperature gradients when the light is absorbed.

11. Possible primary mechanisms The primary mechanisms relate to the interaction between photons and molecules in tissue, while the secondary mechanisms relate to the effect of the chemical changes induced by the primary effects. The fact that the biostimulative effects are dose dependent indicates that there may be thresholds involved in different mechanisms - a certain photon density is needed. There can be many reasons for this, one of which may be multiple photon action. If the effects simply were due to electron excitation and ionization, there would be no thresholds - single photons would do the job. One kind of treatment seen today is the so called ”Non ablative skin rejuvenation” where rather strong lasers or flash lamp instruments are used. The mechanism is biostimulation and mostly it is beleived that it is a question of multi photon action.

12. Polarization effects One important property of light is the polarization. Any kind of light can be made polarized, simply by letting it pass a polarization filter. It is easy to show that when non-coherent polarized light is penetrating and being scattered in tissue, the degree of polarization rapidly decreases as a function of penetration depth. It has been documented that ordinary broadband non-coherent polarized light can give biostimulative effects on superficial problems like wounds and ulcers, but not deeper down in tissue. The so called Bioptron lamp is such a device. This further supports that some of the mechanisms are laser specific.

13. In such laser speckles, where the intensity is higher than the surrounding environment, the light is linearly polarized, or partially polarized because the said higher intensity has come about as a result of constructive interference, which occurs only if the interfering waves have the same polarization. In this way, islands of polarized light appear in the tissue with an average size of one or a few tenths of a millimeter. These islands occur regardless of whether the irradiating laser emits polarized or non-polarized light. The light in laser speckles is polarized In this figure, the direction of the linearly polarized light is shown with arrows.

14. So, what is the mechanism Now, accepting that laser light gives rise to areas of polarized light in tissue, we might also ask ourselves: what is there in the body / tissue / /cells that reacts to the light's polarization? Are there polarization sensitive elements? Yes, there are. It is known that matrix-fixed chromophore molecules (e.g. the body's porphyrins) possess absorption dipoles and both absorb and emit (e.g. through fluorescence) linearly polarized light of a determined polarity. Porphyrins are just one of the elements in the mitochondria's respiratory chain and are the molecules chiefly responsible for the absorption of blue and red light.

15. High peak power. In a speckle field, there is a random addition of beams contributing. Due to the statistics of this optical noise, there is a certain probability for points of high power. In the figure beside, the specle field is exposed to a black and white film plate. Color coding and an intensity threshold is showing in red color, such peaks where the power is between 5 and 10 times higher than the average power level. In the peaks with high power density there is a certain probability for multi photon effects.

16. The effect of heat development in the tissue It has occasionally been asserted that the "possible" effects of laser therapy are due to the laser heating the tissue, and that one could just as well use a blanket, hot shower or a heat-lamp. Heat can of course be valuable many times, but in this context we have to look a bit closer into the matter. Macroscopic heating A heat-lamp has an output of 50-100 watts, while a therapeutic laser often has an output of 5-100 milli-watts. All light that is absorbed by tissue is converted to heat, but it is not the heat itself that is of importance here. A blanket, hot shower or a heat lamp causes macroscopic heating of the skin and tissue - a rather even and smooth temperature distribution. Therapeutic lasers cause usually no perceptible heating, which heat-lamps obviously do and still, there is a clear biostimulative effect, and nota bene, also on chilled tissue! So if it were just a question of heating the tissue, heat-lamps would give just as good or even better therapeutic results! It is true that a GaAlAs laser in the >100 mW range can cause sensations of heat on sensitive areas such as the lip, and on pigmented areas, but more than 95% of the laser therapy described in the literature is performed with lasers in the <100 mW range.

17. The microscopic heat effect However, the uneven, speckled light distribution in tissue causes local temperature differences. These have been calculated by Horvath [223]. Such temperature differences lead to local gradients in certain concentrations of substances, which in turn bring about transport of materials in the tissue in the manner described by Fink's equations. In other words, when tissue is irradiated with laser light, a microcirculation will be initiated, which is not the case during irradiation with non-coherent light sources, such as LEDs for example. Spanner [224] has shown that a temperature difference across a cell membrane of 0.01 °C causes a difference in pressure of 1.32 atmospheres, and this can mean that the distribution pattern of Na+ and K+ can be considerably influenced [225]. The local transient rise in temperature of absorbing biomolecules may cause structural changes and trigger biochemical activity (cellular signaling or secondary dark reactions).

18. Mechanical forces The laser intensity gradients in tissue arise due to the interference of the light scattered by the tissue with the incident light beam (speckle formation). It is known that gradient laser fields may cause spatial modulation of the concentration of particles and increase their “partial temperature”. Interesting experiments has been performed by Rubinov [1417]. He brings about a new approach to the understanding of biological activity caused by low-intensity laser radiation, in which coherence is a factor of paramount importance. His approach is based on the dipole interaction of gradient laser fields with cells, organelles and membranes. Rubinov presents the results of experimental observation of trapping of different types of particles, including human lymphocytes in interference fields of a HeNe laser. A sweep-net effect on particles of different sizes when moving the laser field is demonstrated and crystal-like self-organization of particles in laser gradient field is observed. The influence of gradient laser fields on erythrocyte rouleaus, on the apoptosis of human lymphocytes as well as on their chromosome aberrations is demonstrated. It may be concluded from the experimental studies that the influence of an interference laser field with a correctly chosen period can stimulate the repair system of a cell and increase its viability.

19. Rubinov concludes: “Illumination of biological tissue by coherent laser light unavoidably leads to strong intensity gradients of the radiation in the tissue due to speckle formation. This causes the appearance of inter- and intracellular gradient forces whose action may significantly influence the paths and speeds of biological processes. In contrast to the photochemical action of light, which is accompanied by absorption of quanta and has a specific character (i.e. is characterized by a specific spectrum of action), the action of the gradient field is of non-resonant type. It is not accompanied by photon absorption and has a universal character - it depends weakly on the radiation wavelength, but requires a high degree of coherence.” In the next picture I have shown the most important laser specific primary mechanisms.

20. Laser light penetrates skin or mucosal membrane Diffuse scattering of laser light in tissue gives interference and speckle formation. Laser speckles in a diffuse scat-tering medium, such as tissue, have volumes of polarized light. The three dimensional speckle pattern has small volumes of light with high power density. The speckle pattern has neigh-boring parts with very low and very high light intensities Several cytochrome molecules in cells and tissue (e.g. porphy-rines) absorb linearly polarized light. This stimulates produc-tion of singlet oxygen. In points of high intensity, the probability is higher for multi photon effects. The electric field across the cell membrane creates a dipole moment on e.g. the bar shaped lipids. Local differences in intensity create temperature- and pres-sure gradients across cell mem- branes. They can also cause strong mechanical forces on cells and small particles.

22. All these primary effects are laser specific because they are caused by interference, which in turn depends on coherence. This also explain the universal stimulating biological effect of laser light observed for a wide range of laser wavelengths. The higher degree of coherence, the higher degree of the intensity modulation in a resulting speckle structure. At illumination of the same object (tissue) with non-coherent light e.g. from, light emitting diodes, filtered lamp light or the sun, the speckle structure disappears and the tissue is illuminated uniformly. Gas lasers, such as the HeNe laser have much longer coherence length and this makes it possible to use lower doses. Diode lasers are less coherent, but the light is instead usually polarized. Solid state lasers, such as ruby laser, Nd:YAG, KTP-laser, are highly coherent and their light is usually not polarized. No doubt that the speckle based, laser specific effects are very essential in biostimu-lation. However, they are not the only important effects occurring in laser illuminated tissue. Also photon absorption and other photon energy specific effects occur.

23. Photon absorption. In the following I will show some importent facts about photon absorbtion in tissue. This makes no difference between lasers and other sources, like LED’s. We know for instance that the absorbed light stimulates the cellular ATP production and this is regarded as one of the most important effects in the mechanisms of photobiomodulation. In a study by Mochizuki [1279] the effect of 830 nm laser irradiation on the energy metabolism of the rat brain was observed. A diode laser was applied for 15 min with an irradiance of 4.8 W/cm2. Tissue adenosine tri-phosphate (ATP) content of the irradiated area in the cerebral cortex was 19% higher than that of the non-treated area, whereas the adenosine di-phosphate (ADP) content showed no significant difference. Laser irradiation at the wave-length 652 nm had no effect on either ATP or ADP contents. The temperature of the tissue was increased by 4.4 - 4.7 °C during the irradiation of both wavelengths. These results suggest that the increase in tissue ATP content did not result from the thermal effect, but from a specific effect of the laser with 830 nm wavelength.

24. Primary reactions due to excitation There are several possible primary reactions. When a photon is absorbed, it can transfer its energy to an electron. If the photon energy is high enough, it can change the ener-gy state of the electron, e.g. from level S0 to S1. Also triplet states can be involved. It has also been shown that excitation can occur by means of multiple photon action. The five hypothesis: Changes in redox properties and acceleration of electron transfer, ("Redox properties alteration hypothesis“). Photo-excitation of certain chromophores in the cytochrome-c-oxidase molecule, like CuA and CuB or hemes a and a3, influences the redox state of these centers and, consequently, the rate of electron flow in the molecule. NO release from catalytic center of cytochrome c oxidase. ("NO hypothesis") [1369]. It is thought that laser irradiation and activation of electron flow in the molecule of cytochrome-c-oxidase could reverse the partial inhibition of the catalytic center by NO and in this way increase the O2-binding and respiration rate. Superoxide generation. ("Superoxide anion hypothesis"). It has been suggested that activation of the respiratory chain by irradiation would also increase production of superoxide anions and that the production of -O2 primarily depends on the metabolic state of the mitochondria. Photodynamic action. ("Singlet oxygen hypothesis"). Certain photo-absorbing molecules like porphyrins and flavoproteins (some respiratory-chain components belong to these classes of compounds) can be reversibly converted to photo sensitizers. Changes in biochemical activity induced by local transient heating of chromophores. ("Transient local heating hypothesis"). When electronic states are excited with light, a noticeable fraction of the excitation energy is inevitably converted to heat, which causes a local transient increase in the temperature of absorbing chromophores.

25. The first two processes are of Redox type and the next two give rise to reactive oxygen species (ROS). The belief that only one of these reactions occur when a cell is irradiated and excited electronic states are produced is groundless. Rather, it is likely that more or less all of them take place. The question is, which mechanism is decisive or dominating? Also, it is quite possible that all the mechanisms mentioned above lead to a similar result - a modulation of the redox state of the mitochondria (a shift in the direction of greater oxidation). However, depending on the light dose and intensity used, some of these mechanisms can prevail significantly. Experiments with E. coli provided evidence that, at different laser-light doses, different mechanisms were responsible - a photochemical one at low doses and a thermal one at higher doses. The next page will show absorbtion of a photon in more detail.

26. Photon absorption in the mitochondria Photon Cytochrome-c-oxidase and flavoproteins like NADH in the respiratory chain in the mitochondria can act as photoreceptors. This can cause a short time activation of the respiratory chain and oxidation of NADH pool leading to changes in the redox state of both mitochondria and cytoplasm, changes in pmf (proton motive force), ΔΨ, ΔpH and leading to extra synthesis of ATP. Also a rise of Na+ / H+ antiporter in the cytoplasmic membrane which causes a short term increase in pHi and causing a change in the redox state of the cytoplasm, i.e. the redox state in the whole cell. Primary reactions in mitochondria Secon-dary reac-tions (cellu-lar signal-ling)

27. These processes have been further verified. Yaou Zhang, e.g. used the cDNA micro array technique to investigate the gene expression profiles of human fibroblasts irradiated by low-intensity red light. Proliferation assays showed that the fibroblast HS27 cells responded differently to different doses of low-intensity red light irradi-ation at a wavelength of 628 nm. An optimal dose of 0.88 J per cm2 was chosen for subsequent cDNA micro array experiments. The gene expression profiles revealed that 111 genes were regulated by the red light irradiation and can be grouped into 10 functional categories. Most of these genes directly or indirectly play roles in the enhancement of cell proliferation and the suppression of apoptosis. Two signaling pathways, the p38 mitogen-activated protein kinase signaling pathway and the platelet-derived growth factor signaling pathway, were found to be involved in cell growth induced by irradiation of low-intensity red light. Several genes related to antioxidation and mitochondria energy metabolism were also found to express differentially upon irradiation. This study provides insight into the molecular mechanisms associated with the beneficial effects of red light irradiation in e.g. the acceleration of wound healing by laser therapy. Some persons are of the opinion that the respiratory chain is at the base of all effects that laser therapy might have. However, there are effects of e.g. HeNe laser irradiation on red blood cells in which there are no mitochondria.

28. Some words about the non coherent sources. It has become common to use light emitting diodes (LED) as light source in ”laser” therapeutic instruments. The reason for this is primarily that they are much more easy to power electrically and often also to mount mechanically. They are usually also much cheaper to buy. The mechanisms for the biologic action of non-coherent light is primarily by photon absorbtion (if the light is polarised – which is not the case for LED's – also other mechanisms can occur, such as influence particles in biological system through light induced dipole-dipole interaction). One important question is then: How good are the LED instruments in treatment of e.g. pain and other typical physio therapy problems? Are they better than the true lasers, or are they at least as good? I have worked in practice both with most kind of lasers and also since 1984 with some types of LED‘s and I have seen effects. So, at least they work.

29. But instead of giving you my personal, subjective view, let us look at the literature: There have been quite a number of studies conducted on both people and laboratory animals - even blind studies - in which the effect of laser light was compared with the effect of light from non-coherent sources, such as LED's. In all these investigations, a more obvious (significant) effect was observed with lasers than with the other, light sources. As we know that at least some of the biostimulative effects in vivo are laser specific, it is not a surprise that the literature shows a difference when those light sources are compared. In fact, I have not yet found even one single study indicating that non-coherent light is as efficient as coherent light. This does not mean that non-coherent light is not useful for therapy, only that it is less efficient than lasers. In the following pages I have shown some studies where the effects of coherent and non-coherent light is compared:

30. Literature: Bihari [13] treated three groups of patients with long-standing crural ulcers with HeNe, HeNe/GaAs and non-coherent unpolarized red light, respectively. Groups 1 and 2 demonstrated excellent healing, with group 2 slightly better than group 1, compared to group 3, which had a low effective percentage. Kubota [14] found that a 830 nm GaAlAs laser increased flap survival area in a rat model. Laser treated flaps had better perfusion, a greater num­ber of larger blood vessels and significantly enhanced flow rates. There was no difference between control and LED 840 nm groups. Berki [15] used a HeNe laser to stimulate activation of cells in vitro. These effects (increased phagocytic activity, immunoglobulin secretion) were not seen when irradiating the cell cultures with normal monochromatic light of the same wavelength and doses. Muldiyarov [16] used a HeNe laser on arthritis in rats and found that the laser exerted an evident therapeutic effect. Analysis of the cases where the rats were treated with ordinary red light revealed no essential differences from the control group. Haina [17] compared the effects of HeNe-laser and incoherent light of the same wavelength. Experimental wounds were punched out in the muscle fascias of 249 Wistar rats. In the HeNe groups, the granulation tissue increased 13% at 0.5 J/cm² and 22% at 1.5 J/cm². The increase in the inco­herent group was less than 10%. Rochkind [158] compared five different wavelengths, giving a single transcutaneous irradiation to injured peripheral nerves. HeNe laser pre­vented the drop in functional activity following crush injury. 830 nm laser was less effective, 660 nm incoherent light was even less effective, and 880 and 950 nm incoherent light was completely ineffective. Laakso [253] studied the relationship between laser therapy and opio­ids. In a double blind study, 56 selected patients with chronic pain conditions were treated with 820 nm laser therapy 25 mW, 670 nm laser therapy 10 mW, or 660 nm LED 9.5 mW. ACTH and ß-endorphin levels were significantly ele­vated in the laser therapy groups but not in the LED group. Pöntinen [332] compared the effect of laser light (633 and 670 nm) and light from a LED-source (with 660 nm wavelength) on head skin blood flow in 10 healthy men, using laser doppler technology. Doses were from 0.1 to 1.36 J/cm². Skin blood flow was measured before, immediately after and 30 minutes after each treatment session at 4 sites on the scalp. The conclusion was that 670 nm laser induced a temporary vasodilatation and increased blood flow when the dose given was in the range of 0.12 - 0.36 J/cm². The non-coherent visible monochromatic irradiation with doses between 0.68 and 1.36 J/cm² decreased blood flow for at least 30 minutes after irradiation.

31. Lederer [426] found that "irradiation with coherent HeNe laser light affected leukocytes in migration inhibition assays. Incoherent light of the same wavelength and power density showed no influence." Rosner [493] evaluated the ability of HeNe laser to delay posttraumatic optical nerve degeneration in rats. The optical nerve was crushed and irradiated through the eye. Interestingly enough, irradiation immediately before the injury was as effective as irradiation beginning soon after it. Non-coherent infrared light was ineffective or had an adverse effect. However, the non-coherent light had a wavelength of 905 nm, which makes comparisons difficult. Nicola [511] developed a technique of causing highly reproducible inflammatory lesions on the skin of rats. HeNe laser with a dose of 1 J/cm² produced an acceleration of the healing process. Incoherent light of the same wavelength and dose was less favorable. Onac [659] compared the effect of HeNe laser and monochromatic light at 618 nm. The intact skin of guinea pigs was irradiated with different doses. He not only compared the two different light sources but also com­pared them at different doses (from 0.63 J/cm² and up to 38.1 J/cm²) and came to the following conclusion: Non-coherent monochromatic red light irradiation leads to tegument trophicity at 4.96 J/cm² (but less than a HeNe-laser); lower doses have no effect (the HeNe laser does) whereas higher doses cause focal epidermic hypertrophy. Thus, the therapeutic window seems to be narrower for monochromatic non-coherent light. Nicola [750] investigated the role of polarization and coherence of laser light on wound healing in rats. There were four groups of wounds: #1 was treated with coherent and polarized HeNe laser light (633 nm). #2 was treated with non-polarized, coherent HeNe laser light (633 nm). #3 was treated with polarized, low degree coherent light (633 nm). #4 was untreated and served as control. After the fourth treatment, lesions #1 had healed completely; lesions #2 had not healed completely but showed a more advanced healing process than lesions #3. The lesions #4 showed a poor degree of cicatrisation as compared to lesions #1, #2 and #3. Simunovic [1105] compared the effects of 830 nm laser light with broadband, non-coherent, polarized light in the treatment of epicondylitis. Both groups of patients (n = 20) received 4 J/cm², 12 consecutive treatment sessions, excepting week-ends. 40% of the laser treated patients recovered fully, while in the non-coherent group no patients recovered fully.

32. One investigation that unexpectedly strengthens the hypothesis that most treatments in vivo are laser specific, was published by Zhou [825]. The study concerns PDT (Photo dynamic therapy) using three light sources: a) copper-vapor pumped dye laser, b) HeNe laser, and c) non-coherent red light (filtered from a halogen lamp), when irradiating the liver in normal mice. The mice (each group containing 18 - 20 mice) received hematoporphyrine derivative in a dose of 10 mg/kg intravenously, 24 hours prior to light irradi­ation. The mice livers were directly irradiated with different types of red light at a dose of 5, 10, 25, 50, or 100 J/cm², respectively. Forty-eight hours later the mice were killed and the depth of liver necrosis was measured using a computerized image-analysis system. No necrosis was found in the control liver irradiated with 500 J/cm² alone. The depth of photodynamic necrosis showed a light dose-dependent response. The mean depth of necrosis of all groups was compared statistically. The Cu-dye laser showed the best effect, while the non-coherent light showed the poorest. There were significant dif­ferences between non-coherent light and laser-irradiated groups, but not between Cu-dye and HeNe laser groups. The results indicate that of the light sources examined, the Cu-dye laser is most suitable to photodynamic therapy (PDT) of tumors. However, the halogen lamp with a special filter device may still be occasionally used as a light source in PDT if needed. In a wound healing study by Lowe [851], mouse skin was irradiated with 20 Gy X-ray irradiation. 72 hours later a wound was made on the dor­sum and the area was treated with 890 monochromatic LED light three times weekly. There was no effect on the wound healing using 0.18 J/cm² or 0.54 J/cm² but an inhibiting effect at 1.45 J/cm². In a study by Paolini [940], 99 patients with shoulder tendonitis were divided into three groups. One received HeNe laser irradiation, one LED 660 nm irradiation and one anti-inflammatory medication. 25 sessions with either laser or LED were given. The outcome of the laser group was better than the pharmacological group and much better than the LED group. Antipa [1077] compared the effect of: 1) GaAlAs laser, 720 nm 3 mW, 2) non-coherent light at 750 nm, 9 mW, and 3) placebo irradiation. 74 patients with sciatic problems were treated. The positive results were 66.6% for the laser group, 52% for the non-coherent group and 36.4% for the placebo group. Simunovic and Trobonjaca [1233] compared the efficiency of laser therapy on lateral epicondylitis (tennis elbow) in 120 patients with a) transcutaneous electro-neural stimulation (TENS), b) visible, incoherent polarized light (VIP) and c) placebo "treatment" with a non working "laser" unit. The number or treatment sessions per patient were twelve. The laser dosage was 4 J/point and the VIP-light dosage was 4 J/cm². The results demonstrated that the laser therapy gave the highest percent of pain relief (>45% of lased patients reported 90-100% pain relief), TENS gave the second best pain relief. None of the patients in the VIP group reported 90-100% pain relief. The worst result was reported by the placebo group (<20% of average pain relief).

33. I will underline that the studies above do not indicate that non-coherent light therapy for suitable indications and with sufficient energy densities is inefficient.They only show that whenever compared, coherent light has come out on top. But indeed, the effect of non-coherent light therapy has been verified [1074], even in double blind studies [433, 941, 1232]. However, in these studies, non-coherent light was not compared with coherent light.Some points of view:The more we learn about this, the more we realize that the mechanisms behind laser biostimulation are very complex and I doubt that we will ever know all details. Why are for instance about 10% of humans and animals resistant to laser therapy?What are the mechanisms for the systemic effects?How come that CO2-laser is working, it penetrates less than 1 mm?Lars Hode

34. New steps Thank you for listening Lars Hode Swedish Laser Medical Society