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Laserclass 3 and 4 – What is the difference?

This editorial poses the question: Which laser class is best suited for the PBM? Are there differences in effectiveness and safety?

Low-level laser therapy means light therapy with c o l d, low-energy and coherent light. This light is safe, because it does not heat the tissue. Therefore it belongs to laser class 3B! Here light is discharged in the milliwatt range (up to a maximum of 500 mW).
Yet the term low-level laser therapy is not protected. Some class 4 lasers are also low-energy compared to surgical lasers with a very high power output. They work in a range of up to 30 watt, and their light is w a r m. Therefore they belong to laser class 4, which means they heat tissue and can be dangerous. Thermal effects and skin burns can happen after just a few seconds. Manufacturers of class 4 lasers claim: "Stronger is better. Dangers can be excluded by using specific application techniques."

We do not claim that 4 lasers have no therapeutic effects! With certain orthopaedic indications and when using suitable application techniques, they can no doubt generate positive results. However, the crucial question is: Do we need them? Are these effects stronger than with traditional low-level lasers so that we should put up with the dangers that class 4 lasers entail? Do they expand the application range and hence offer a genuine advantage?
Our answer is: Definitely not. The contrary is true. 

Below we compare the two laser classes with respect to their evidence, their safety, and their effectiveness. Moreover, we comment on the claims made by class 4 laser manufacturers.

During the past 40 years, 99% of research on LLLT has been performed with class 3B lasers. There are more than 300 RCTs and over 3,500 scientific studies on the effectiveness of 3B lasers. Manufacturers of class 4 lasers use the evidence of 3B lasers to promote their products. Hardly any studies are available on the effectiveness of class 4 lasers.

3B lasers are not dangerous, they are safe. Even though a few manufacturers claim the contrary, class 4 lasers are by no means harmless. When applied improperly, they can quickly cause skin burns. Especially in veterinary medicine, in which class 4 lasers are more common, incidents keep happening.

We tested the heating of the tissue by a class 4 laser on a piece of meat (steak).
> Irradiating a Steak with a Class 4 Laser
> Temperature Comparison Class 3 – Class 4
Depending on the place and purpose of the application, the potential dangers of class 4 lasers are more or less serious. With many applications, it must be advised against using these lasers even if they are applied "properly". These include, e.g., applications in the area of the eyes and face, on the ear, mucous membranes, open wounds, for nerve regeneration, in the case of dermatological indications, and conditions which involve inflammations.

Thesis 1: Higher power output = better tissue penetration = more effective

1. Wrong! because:
The modes of action in LLLT are of a biomodulatory nature and therefore depend primarily on the wavelength!
Biomodulatory effects are mostly a consequence of the applied wavelength and its absorption coefficient. After all, the purpose of LLLT is to impact the cell's biochemistry. Different wavelengths have different medium penetration depths in the skin of about 0.3 to 2 mm, which already at a tissue depth of only 6 mm results in penetration differences between 0.000001 and 0.125 mm respectively. This is a factor of 125,000. By contrast, the impact of the power applied is only marginal (e.g., 10,000 for a power difference of 5 mW and 5 W respectively)!
Due to the critical importance of the wavelength, it is therefore also true that a class 4 laser beam with a wavelength between 940 and 980 nm can penetrate the skin less than a 3B laser with a wavelength between 800 and 900 nm.
2. Wrong! because:
In LLLT, the depth effect in the tissue is primarily based on amplification effects!
The depth effect in LLLT is based on the "patient's internal amplification effects" (PASER according to Mary Dason) which are triggered by LLLT. LLLT does not require very high stimuli, as the primary effects in the cell are set off by very low doses, while high doses have an inhibiting effect. The enhancement and transfer of the effect takes place via potentiating effects along the metabolism paths in the downstream and the intercellular communication paths. This is precisely why LLLT is so effective. In short: The depth effect in traditional LLLT with 3B lasers already exists, and this precisely is LLLT!

Thesis 2: More is more helpful!
1. Wrong! because:
Hormesis is a characteristic of biological systems (Arndt-Schulz rule).
Ever since laser therapy has existed, users have been told about the so-called Arndt-Schulz rule, according to which weak stimuli promote physiological processes, while strong stimuli inhibit them. Clinical experience has confirmed this rule for decades and is well documented, e.g. regarding the treatment of open wounds.
Even so, the argument that more is better still stands a good chance to sound convincing today. Partly in response to this indestructible notion, which is largely fed by marketing strategies, the University of Massachusetts founded its own association back in 2005: the International Dose Response Society (see www.dose-response.org). Its purpose is to advance the understanding of low-level effects and the associated phenomenon that low doses stimulate while high doses inhibit. Research into biological dose–effect relationships across the entire range from promotion to destruction has great implications for many sciences. These include medical disciplines such as toxicology, pharmacology, neuroscience, immunology, physiology, and radiation biology.
The research topic is hormesis, from the Greek for "to put in motion, to trigger". Hormesis refers to the positive reaction of biological systems to exposition of very low stimuli (such as toxins or other stressors). They are also called "eustresses". Improvement through small doses and inhibition through large doses can be illustrated by an upside-down U-shaped curve. Its shape resembles that of the therapeutic dose window for LLLT, which is based on clinical experience between stimulating and cumulative negative effects due to low doses (mW range) and high doses (W range) respectively.

2. Wrong! because:
An overdose has negative effects in LLLT.
Numerous studies tell us that cellular metabolism mechanisms are inhibited by excessive radiation doses. Since the beginnings of laser therapy, inhibitory effects have mostly been ascribed to improper application in the form of overdoses.
The reason is that in LLLT, the decisive impact factor is not the radiation dose but the wavelength, on which the impact on the cellular metabolism depends almost exclusively. With the right wavelength, it can be achieved with even minimal doses of 0.1 Joule. Stimulation cannot be increased ad infinitum – a physiological system can work no better than perfectly. The physiological limits make sense, and crossing and challenging them may cause great damage.
Therefore: There exists no evidence for the claim that in LLLT more power means greater effectiveness! There is, however, evidence for the exact opposite!

Take a look at the following example:
Twenty-five RCTs met the inclusion criteria for a systematic review and a meta-analysis on the effectiveness of LLLT in the treatment of tendinopathies conducted in 2010 (Tumilty et al. – http://www.ncbi.nlm.nih.gov/pubmed/19708800). Of these 25 studies, 12 had positive and 13 had negative results. The researchers were able to demonstrate that the positive results were related to the fact that the therapeutic dose windows were maintained (that is to say, the doses recommended for LLLT) while the negative studies could be attributed to overdoses.
Leading authorities on the research of the relationship between dose and effect in LLLT and the greatest experts on LLLT literature, such as Prof. Jan Bjordal, Dr. Jan Tunér and James Caroll, have tirelessly explored these questions for years: How much laser is enough? And how much is too much? How did negative study results come about? And they never stop repeating:
Yes! There is a dose window in LLLT, and heeding it is crucial for its future and its acceptance!

3. Wrong! because:
Thermal effects are no primary effects of LLLT.
There are studies which demonstrate that changes in the redox state of the cell may also be triggered by photothermal (as opposed to photobiomodulatory) effects, which might explain some therapeutic effects of class 4 lasers. According to this explanation, 3B lasers trigger photobiomodulatory processes, and class 4 lasers, photothermal effects. Yet in the latter case, the transition between positive effects that are induced via helpful thermal mechanisms in the cell and inhibiting effects that are a result of overheating are extremely vague. In specific cases, the tipping point depends on so many parameters (such as type, tonus, and physiological age = stress elasticity of the tissue) that it is difficult to conceive how it might be precisely calculated.
By contrast, photobiomodulatory effects generate only minimal thermal effects due to the cellular stimulation of the metabolism and within the natural physiological margin.

Manufacturers of class 4 lasers claim that dangers due to overdoses and skin burns are excluded because
the laser beam is expanded (i.e., not focused) and applied from a distance (which entails a great loss of intensity) and because
the laser beam is constantly moved, so that no excessive intensity can be generated at any one place.
Theoretically, this is true: a class 4 laser is to be handled in such a way that it becomes like a class 3 laser. The great expansion of the beam, treatment from a distance, and constant movement of the laser are to neutralize the dangers class 4 entails while maintaining the alleged advantages that are a result of the greater power (in particular greater tissue penetration). How is that supposed to be possible?
On the one hand the greater power is supposedly more effective (which, as we have seen, is already a basic misconception), and on the other, it is neutralized from the start by the way in which it is to be handled so as to avoid dangers. In plain English: it's a self-contradictory panacea.
Fact is that when a class 4 laser is applied conscientiously, the same results may be expected as with class 3B lasers – at best.
Conscientious application means: you know precisely the distance from which to radiate, the degree by which the focus has to be expanded, and the movement technique and speed that must be applied to remain within the dose window that is effective with LLLT, and hence to avoid both overdose and underdose. For when using this method, underdosing is by no means impossible: by expanding the beam and moving it further away reduces the beam density to such a degree that it is doubtful if the same energy densities in the tissue can be achieved as with 3B lasers.
Assuming that all adjustment measures for class 4 lasers could be defined so precisely that a 3B laser could be simulated, these settings would certainly have to be fixed. This would be possible with the help of fixed scanners whose effective speed of movement and effective distance to the tissue are ensured. Yet this is impossible with a handheld laser unit.

Laser class Class 3B Class 4
Evidence very high Minimal
Please note: The evidence in LLLT literature was generated with 3B lasers
It does n o t apply to class 4 lasers!
Safety very high Danger of burns
  • penetration depth
  • Dose
  • Radiation penetration:
    Comparable for the same wavelength, as the depth effect of LLLT in the tissue depends primarily on the wavelength rather than the power.
  • Depth effectiveness:
    The depth effect in the tissue depends on the body's own amplification effects.
  • Maintaining the dose window leads to positive results.
  • Radiation penetration:
    Zero evidence in the entire literature about class 4 laser light penetrating deeper into the tissue.
  • Depth effectiveness:
    Potentially equally effective in deeper tissue layers, but danger of overdosing in the process.
  • Overdose and underdose are almost impossible to calculate.
Cost-effectiveness   The fibre-optic cables used with laser class 4 are considerably more expensive than those of class 3 lasers and have a tendency to break.

Class 4 lasers are not lasers in the context of LLLT in the sense in which the term has been used in science and actual practice for 50 years.
Class 4 lasers are potentially dangerous without offering advantages either regarding penetration depth in the tissue or regarding depth effectiveness.
Class 4 lasers might be able to achieve similar results in certain instances in the treatment of orthopaedic diseases. However, the results are difficult to calculate. So are therefore the contrary effects due to overdoses and underdoses.


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