Laser therapy dosage


Photomed Laser Surg. 2012 Dec;30(12):688-94. doi: 10.1089/pho.2012.3306. Epub 2012 Oct 1.

Skin penetration time-profiles for continuous 810�nm and Superpulsed 904 nm lasers in a rat model.

Joensen J, Ovsthus K, Reed RK, Hummelsund S, Iversen VV, Lopes-Martins R�, Bjordal JM.

Author information


Objectives: The purpose of this study was to investigate the rat skin penetration abilities of two commercially available low-level laser therapy (LLLT) devices during 150 sec of irradiation.

Background Data: Effective LLLT irradiation typically lasts from 20�sec up to a few minutes, but the LLLT time-profiles for skin penetration of light energy have not yet been investigated.

Methods: Sixty-two skin flaps overlaying rat's gastrocnemius muscles were harvested and immediately irradiated with LLLT devices. Irradiation was performed either with a 810 nm, 200 mW continuous wave laser, or with a 904�nm, 60 mW superpulsed laser, and the amount of penetrating light energy was measured by an optical power meter and registered at seven time points (range, 1-150 sec).

Results: With the continuous wave 810�nm laser probe in skin contact, the amount of penetrating light energy was stable at �20% (SEM±0.6) of the initial optical output during 150�sec irradiation. However, irradiation with the superpulsed 904�nm, 60�mW laser showed a linear increase in penetrating energy from 38% (SEM±1.4) to 58% (SEM±3.5) during 150�sec of exposure. The skin penetration abilities were significantly different (p<0.01) between the two lasers at all measured time points.

Conclusions: LLLT irradiation through rat skin leaves sufficient subdermal light energy to influence pathological processes and tissue repair. The finding that superpulsed 904 nm LLLT light energy penetrates 2-3 easier through the rat skin barrier than 810�nm continuous wave LLLT, corresponds well with results of LLLT dose analyses in systematic reviews of LLLT in musculoskeletal disorders. This may explain why the differentiation between these laser types has been needed in the clinical dosage recommendations of World Association for Laser Therapy.

Photomedicine and Laser Surgery Volume 31, Number 4, 2013

Penetration of Laser Light at 808 and 980nm in Bovine Tissue Samples

Donald E. Hudson, BSEE, Doreen O. Hudson, BS, CET, James M. Wininger, BSEE, and Brian D. Richardson, BA, JD


Objective: The purpose of this study was to compare the penetration of 808 and 980nm laser light through bovine tissue samples 18�95mm thick.

Background Data: Low-level laser therapy (LLLT) is frequently used to treat musculoskeletal pathologies. Some of the therapeutic targets are several centimeters deep.

Methods: Laser light at 808 and 980nm (1 W/cm2) was projected through bovine tissue samples ranging in thickness from 18 to 95 mm. Power density measurements were taken for each wavelength at the various depths.

Results: For 808 nm, 1 mW/cm2 was achieved at 3.4 cm, but for 980 nm, 1 mW/cm2 was achieved at only 2.2 cm depth of tissue.

Conclusions: It was determined that 808nm of light penetrates as much as 54% deeper than 980nm light in bovine tissue.

Skin Penetration Time-Profiles for Continuous 810nm and Superpulsed 904nm Lasers in a Rat Model

Jon Joensen, P.T., M.Sc.,1,2 Knut �vsthus, MIng, Ph.D.,3 Rolf K. Reed, M.D., Ph.D.,4, Steinar Hummelsund, P.T., M.Sc.,1 Vegard V. Iversen, Phys., Ph.D.,5, Rodrigo A´ lvaro Branda�o Lopes-Martins, Ph.D.,6 and Jan Magnus Bjordal, P.T., Ph.D.1,2

Objective: The purpose of this study was to investigate the rat skin penetration abilities of two commercially available low-level laser therapy (LLLT) devices during 150 sec of irradiation.

Background Data: Effective LLLT irradiation typically lasts from 20 sec up to a few minutes, but the LLLT time-profiles for skin penetration of light energy have not yet been investigated.

Materials and Methods: Sixty-two skin flaps overlaying rat�s gastrocnemius muscles were harvested and immediately irradiated with LLLT devices. Irradiation was performed either with a 810 nm, 200mW continuous wave laser, or with a 904 nm, 60mW superpulsed laser, and the amount of penetrating light energy was measured by an optical power meter and registered at seven time points (range, 1�150 sec).

Results: With the continuous wave 810nm laser probe in skin contact, the amount of penetrating light energy was stable at *20% (SEM � 0.6) of the initial optical output during 150 sec irradiation. However, irradiation with the superpulsed 904 nm, 60mW laser showed a linear increase in penetrating energy from 38% (SEM � 1.4) to 58% (SEM � 3.5) during 150 sec of exposure. The skin penetration abilities were significantly different ( p < 0.01) between the two lasers at all measured time points.

Conclusions: LLLT irradiation through rat skin leaves sufficient subdermal light energy to influence pathological processes and tissue repair. The finding that superpulsed 904nm LLLT light energy penetrates 2�3 easier through the rat skin barrier than 810nm continuous wave LLLT, corresponds well with results of LLLT dose analyses in systematic reviews of LLLT in musculoskeletal disorders. This may explain why the differentiation between these laser types has been needed in the clinical dosage recommendations of World Association for Laser Therapy.

The recommended dosage (WALT) for anti inflammatory effect

Laser classes 3 or 3 B, 780 -860nm GaAlAs Lasers. Continuous or pulse output less than 0.5 Watt Energy dose delivered to the skin over the target tendon or synovia

Tendinopathies Points/cm2 Joules Notes
Carpal-tunnel 2-3 12 Minimum 6 Joules per point
Lateral epicondylitis 1-2 4 Maximum 100mW/cm2
Biceps humeri c.l. 1-2 8  
Supraspinatus 2-3 10 Minimum 5 Joules per point
Infraspinatus 2-3 10 Minimum 5 Joules per point
Trochanter major 2-4 10  
Patellartendon 2-3 6  
Tract. Iliotibialis 2-3 3 Maximum 100mW/cm2
Achilles tendon 2-3 8 Maximum 100mW/cm2
Plantar fasciitis 2-3 12 Minimum 6 Joules per point

Arthritis Points/cm2 Joules Notes
Finger PIP or MCP 1-2 6  
Wrist 2-4 10  
Humeroradial joint 1-2 4  
Elbow 2-4 10  
Glenohumeral joint 2-4 15 Minimum 6 Joules per point
Acromioclavicular 1-2 4  
Temporomandibular 1-2 6  
Cervical spine 2-4 15 Minimum 6 Joules per point
Lumbar spine 2-4 40 Minimum 8 Joules per point
Hip 2-4 40 Minimum 8 Joules per point
Knee medial 3-6 20 Minimum 5 Joules per point
Ankle 2-4 15  

Laser classes 3 or 3B, 904 nm GaAs Lasers (Peak pulse output more than 1 Watt) Energy dose delivered to the skin over the target tendon or synovia

Tendinopathies Points/cm2 Joules Notes
Carpal-tunnel 2-3 4 Minimum 2 Joules per point
Lateral epicondylitis 1-2 1 Maximum 100mW/cm2
Biceps humeri c.l. 1-2 2  
Supraspinatus 2-3 3 Minimum 2 Joules per point
Infraspinatus 2-3 3 Minimum 2 Joules per point
Trochanter major 2-3 2  
Patellartendon 2-3 2  
Tract. Iliotibialis 2-3 2 Maximum 100mW/cm2
Achilles tendon 2-3 2 Maximum 100mW/cm2
Plantar fasciitis 2-3 3 Minimum 2 Joules per point

Arthritis Points/cm2 Joules Notes
Finger PIP or MCP 1-2 2  
Wrist 2-3 3  
Humeroradial joint 1-2 2  
Elbow 2-3 3  
Glenohumeral joint 2-3 6 Minimum 2 Joules per point
Acromioclavicular 1-2 2  
Temporomandibular 1-2 2  
Cervical spine 2-3 6 Minimum 2 Joules per point
Lumbar spine 2-3 10 Minimum 4 Joules per point
Hip 2-3 10 Minimum 4 Joules per point
Knee anteromedial 2-4 6 Minimum 2 Joules per point
Ankle 2-4 6  

Daily treatment for 2 weeks or treatment every other day for 3-4 weeks is recommended Irradiation should cover most of the pathological tissue in the tendon/synovia.


Start with energy dose in table, then reduce by 30% when inflammation is under control (Does not apply for carpal tunnel tendo synovitis)

Therapeutic windows range from typically +/-50% of given values Recommended doses are based on ultrasonographic measurements of depths from skin surface and typical volume of pathological tissue and estimated optical penetration for the different laser types in caucasians.

Disclaimer: The list may be subject to change at any time when more research trials are being published. World Association of Laser Therapy is not responsible for the application of laser therapy in patients, which should be performed at the therapist/doctor`s discretion and responsibility

Revised August 2005

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Ying-Ying Huang  Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, MA; Department of Dermatology, Harvard Medical School, Boston, MA; Aesthetic and Plastic Center of Guangxi Medical University, Nanning, P.R. China
Aaron C.-H. Chen  Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, MA; Boston University School of Medicine, Graduate Medical
Sciences, Boston, MA
James D. Carroll  THOR Photomedicine Ltd, 18A East Street, Chesham, HP5
Michael R. Hamblin  Wellman Center for Photomedicine, Massachusetts
General Hospital, Boston, MA; Department of Dermatology, Harvard Medical
School, Boston, MA; Harvard-MIT Division of Health Sciences and Technology,
Cambridge, MA
 The use of low levels of visible or near infrared light for reducing pain, inflammation
and edema, promoting healing of wounds, deeper tissues and nerves, and preventing cell
death and tissue damage has been known for over forty years since the invention of lasers.
Despite many reports of positive findings from experiments conducted in vitro, in animal
models and in randomized controlled clinical trials, LLLT remains controversial in mainstream
medicine. The biochemical mechanisms underlying the positive effects are incompletely
understood, and the complexity of rationally choosing amongst a large number of
illumination parameters such as wavelength, fluence, power density, pulse structure and
treatment timing has led to the publication of a number of negative studies as well as many
positive ones. A biphasic dose response has been frequently observed where low levels of
light have a much better effect on stimulating and repairing tissues than higher levels of
light. The so-called Arndt-Schulz curve is frequently used to describe this biphasic dose
response. This review will cover the molecular and cellular mechanisms in LLLT, and
describe some of our recent results in vitro and in vivo that provide scientific explanations
for this biphasic dose response.
1.1. Brief history
Low level laser therapy (LLLT) is the application of light (usually a low power laser or LED in the range of 1mW – 500mW) to a pathology to
promote tissue regeneration, reduce inflammation and relieve pain. The
light is typically of narrow spectral width in the red or near infrared
Dose-Response (Prepress)
Formerly Nonlinearity in Biology, Toxicology, and Medicine
Copyright © 2009 University of Massachusetts
ISSN: 1559-3258
DOI: 10.2203/dose-response.09-027.Hamblin
Address correspondence to Professor Michael R. Hamblin, BAR 414, Wellman Center for
Photomedicine, Massachusetts General Hospital, 40 Blossom Street, Boston, MA 02114; Phone:
617-726-6182, Fax: 617-726-8566, E-mail: Diese E-Mail-Adresse ist vor Spambots geschützt! Zur Anzeige muss JavaScript eingeschaltet sein!
(NIR) spectrum (600nm – 1000nm), with a power density (irradiance)
between 1mw-5W/cm2. It is typically applied to the injury for a minute or
so, a few times a week for several weeks. Unlike other medical laser procedures,
LLLT is not an ablative or thermal mechanism, but rather a photochemical
effect comparable to photosynthesis in plants whereby the
light is absorbed and exerts a chemical change.
The phenomenon was first published by Endre Mester at Semmelweis
University, Budapest, Hungary in 1967 a few years after the first working
laser was invented (Mester et al. 1967). Mester conducted an experiment
to test if laser radiation might cause cancer in mice. He shaved the hair
off their backs, divided them into two groups and irradiated one group
with a low powered ruby laser (694-nm). The treatment group did not get
cancer and to his surprise, the hair grew back more quickly than the
untreated group. He called this “Laser Biostimulation”.
1.2. Evidence for effectiveness of LLLT
Since 1967 over 100 phase III, randomized, double-blind, placebocontrolled,
clinical trials (RCTs) have been published and supported by
over 1,000 laboratory studies investigating the primary mechanisms and
the cascade of secondary effects that contribute to a range of local tissue
and systemic effects.
RCTs with positive outcomes have been published on pathologies as
diverse as osteoarthritis (Bertolucci and Grey 1995; Ozdemir et al. 2001;
Stelian et al. 1992), tendonopathies (Bjordal et al. 2006b; Stergioulas et al.
2008; Vasseljen et al. 1992), wounds (Caetano et al. 2009; Gupta et al. 1998;
Ozcelik et al. 2008; Schubert et al. 2007), back pain (Basford et al. 1999),
neck pain (Chow et al. 2006; Gur et al. 2004), muscle fatigue (Leal Junior
et al. 2008a; Leal Junior et al. 2008b), peripheral nerve injuries (Rochkind
et al. 2007) and strokes (Lampl et al. 2007; Zivin et al. 2009); nevertheless
results have not always been positive. This failure in certain circumstances
can be attributed to several factors including dosimetry (inadequate or
too much energy delivered, inadequate or too much irradiance, inappropriate
pulse structure, irradiation of insufficient area of the pathology),
inappropriate anatomical treatment location and concurrent patient
medication (such as steroidal and non-steroidal anti-inflammatories
which can inhibit healing) (Aimbire et al. 2006; Goncalves et al. 2007).
1.3. The medicine and the dose
As with other forms of medication, LLLT has its active ingredients or
“medicine” (irradiation parameters) and a “dose” (the irradiation time).
Table 1 lists the key parameters that define the medicine and Table 2
defines the dose. It is beyond the scope of this paper to exhaustively list
and discuss every conceivable aspect of laser radiation or other light
Y.-Y. Huang and others
sources however we believe we have captured the main elements with
some comment on others.
Energy (J) or energy density (J/cm2) is often used as an important
descriptor of LLLT dose, but this neglects the fact that energy has two
components, power and time,
Energy (J) = Power (W) × Time (s)
and it has been demonstrated that there is not necessarily reciprocity
between them; in other words, if the power doubled and the time is
halved then the same energy is delivered but a different biological
response is often observed.
It is our view LLLT is best described as two separate sets of parameters;
(a) The medicine (irradiation parameters)
(b) The dose (time)
This paper will mainly focus on irradiance and time, as it is beyond
the scope of this paper to report in detail on the response to all aspects
Biphasic dose response in low level light therapy
TABLE 1. Parameters involved in determining the LLLT “medicine”
Irradiation Unit of
Parameter measurement Comment
Wavelength nm Light is electromagnetic energy which travels in discrete
packets that also have a wave-like property. Wavelength is
measure in nanometres (nm) and is visible in the 400-700 nm
Irradiance W/cm2 Often called Intensity, or Power Density and is calculated as
Irradiance = Power (W)/Area (cm2)
Pulse structure Peak Power (W) If the beam is pulsed then the Power should be the Average
Pulse freq (Hz) Power and calculated as follows:
Pulse Width (s) Average Power (W) = Peak Power (W) × pulse width (s) ×
Duty cycle (%) pulse frequency (Hz)
Coherence Coherence length Coherent light produces laser speckle, which has been
depends on postulated to play a role in the photobiomodulation
spectral bandwidth interaction with cells and subcellular organelles.
Polarisation Linear polarized Polarized light may have different effects than otherwise
or circular identical non-polarized light (or even 90-degree rotated
polarized polarized light). However, it is known that polarized light is
rapidly scrambled in highly scattering media such as tissue
(probably in the first few hundred μm).
laser radiation listed in the “medicine” table; however there is evidence to
show that different wavelengths, pulses, coherence, polarization have
some effect on the magnitude of biomodulation (see sections 3 and 4).
2.1. Cellular Chromophores and First Law of Photobiology
The first law of photobiology states that for low power visible light to
have any effect on a living biological system, the photons must be
absorbed by electronic absorption bands belonging to some molecular
photoacceptors, or chromophores (Sutherland 2002). A chromophore is
a molecule (or part of a molecule) which imparts some decided color to
the compound of which it is an ingredient. Chromophores almost always
occur in one of two forms: conjugated pi electron systems and metal complexes.
Examples of such chromophores can be seen in chlorophyll (used
by plants for photosynthesis), hemoglobin, cytochrome c oxidase (Cox),
myoglobin, flavins, flavoproteins and porphyrins (Karu 1999). Figure 1
illustrates the general concept of LLLT.
2.2. Action Spectrum and Tissue Optics
One important consideration should involve the optical properties of
tissue. There is a so-called “optical window” in tissue, where the effective
Y.-Y. Huang and others
TABLE 2. Parameters involved in determining the LLLT “dose”
Irradiation Unit of
Parameter measurement Comment
Energy (Joules) J Calculated as:
Energy (J) = Power (W) x time (s)
This mixes medicine and dose into a single expression and
ignores Irradiance. Using Joules as an expression of dose is
potentially unreliable as it assumes reciprocity (the inverse
relationship between power and time).
Energy Density J/cm2 Common expression of LLLT “dose” is Energy Density
This expression of dose again mixes medicine and dose into
a single expression and is potentially unreliable as it assumes
a reciprocity relationship between irradiance and time.
Irradiation s In our view the safest way to record and prescribe LLLT is to
Time define the four parameters of the medicine (see table 1.) and
then define the irradiation time as “dose”.
Treatment Hours, days or The effects of different treatment interval is underexplored
interval weeks at this time though there is sufficient evidence to suggest that
this is an important parameter.
tissue penetration of light is maximized. This optical window runs
approximately from 650 nm to 1200 nm. (Figure 2). The absorption and
scattering of light in tissue are both much higher in the blue region of the
spectrum than the red, because the principle tissue chromophores
(hemoglobin and melanin) have high absorption bands at shorter wavelengths,
tissue scattering of light is higher at shorter wavelengths, and furthermore
water strongly absorbs infrared light at wavelengths greater
than 1100-nm. Therefore the use of LLLT in animals and patients almost
exclusively involves red and near-infrared light (600-1100-nm) (Karu and
Afanas’eva 1995).
Phototherapy is characterized by its ability to induce photobiological
processes in cells. Exact action spectra are needed for determination of
photoacceptors as well as for further investigations into cellular mechanisms
of phototherapy. The action spectrum shows which specific wavelength
of light is most effectively used in a specific chemical reaction (Karu
and Kolyakov 2005). The fact that defined action spectra can be constructed
for various cellular responses confirms the first law of photobiology
described above (light absorption by specific molecular chromophores).
2.3. Mitochondrial Respiration and ATP
Current research about the mechanism of LLLT effects inevitably
involves mitochondria. Mitochondria play an important role in energy
generation and metabolism. Mitochondria are sometimes described as
Biphasic dose response in low level light therapy
FIGURE 1. Schematic diagram showing the absorption of red and NIR light by specific cellular chromophores
or photoacceptors localized in the mitochondrial respiratory chain
“cellular power plants”, because they convert food molecules into energy
in the form of ATP via the process of oxidative phosphorylation (see
Figure 3 for an illustartion of the mitochondrial respiratory chain).
The mechanism of LLLT at the cellular level has been attributed to
the absorption of monochromatic visible and NIR radiation by components
of the cellular respiratory chain (Karu 1989). Several pieces of evidence
suggest that mitochondria are responsible for the cellular response
to red visible and NIR light. The effects of HeNe laser and other illumination
on mitochondria isolated from rat liver, have included increased
proton electrochemical potential, more ATP synthesis (Passarella et al.
1984), increased RNA and protein synthesis (Greco et al. 1989) and
increases in oxygen consumption, membrane potential, and enhanced
synthesis of NADH and ATP.
2.4. Cytochrome c oxidase and nitric oxide release
Absorption spectra obtained for cytochrome c oxidase (Cox) in different
oxidation states were recorded and found to be very similar to the
action spectra for biological responses to light (Karu and Kolyakov 2005).
Therefore it was proposed that Cox is the primary photoacceptor for the
red-NIR range in mammalian cells (Karu and Kolyakov 2005).
Nitric oxide produced in the mitochondria can inhibit respiration by
binding to Cox and competitively displacing oxygen, especially in stressed
Y.-Y. Huang and others
FIGURE 2. Absorption spectra of the main chromophores in living tissue on a log scale showing the
optical window where visible and NIR light can penetrate deepest into tissue.
or hypoxic cells (Brown 2001). Increased nitric oxide (NO) concentrations
can sometimes be measured in cell culture or in animals after LLLT
due to its photo release from the mitochondria and Cox. It has been proposed
that LLLT might work by photodissociating NO from Cox, thereby
reversing the mitochondrial inhibition of respiration due to excessive NO
binding (Lane 2006). Figure 4 illustrates the photodissociation of NO
from its binding sites on the heme iron and copper centers where it
cometively inhibits oxygen binding and reduces necessary enzymic activity,
thus allowing an immediate influx of oxygen and resumption of respiration
and generation of reactive oxygen species.
2.5. NO signaling
In addition to NO being photodissociated from Cox as described, it
may also be photo-released from other intracellular stores such as nitrosylated
hemoglobin and nitrosylated myoglobin (Shiva and Gladwin 2009).
Light mediated vasodilation was first described in 1968 by R F Furchgott,
in his nitric oxide research that lead to his receipt of a Nobel Prize thirty
years later in 1998 (Mitka 1998). Later studies conducted by other
researchers confirmed and extended Furchgott’s early work and demonstrated
the ability of light to influence the localized production or release
of NO and stimulate vasodilation through the effect NO on cyclic guanine
Biphasic dose response in low level light therapy
FIGURE 3. Mitochondrial respiratory chain consisting of contains five complexes of integral membrane
proteins: NADH dehydrogenase (Complex I), succinate dehydrogenase (Complex II),
cytochrome c reductase (Complex III), cytochrome c oxidase (Complex IV), and ATP synthase
monophosphate (cGMP). This finding suggested that properly designed
illumination devices may be effective, noninvasive therapeutic agents for
patients who would benefit from increased localized NO availability
2.6. Reactive oxygen species and gene transcription
Reactive oxygen species (ROS) and reactive nitrogen species (RNS)
are involved in the signaling pathways from mitochondria to nuclei.
Reactive oxygen species (ROS) are very small molecules that include oxygen
ions such as superoxide, free radicals such as hydroxyl radical, and
hydrogen peroxide, and organic peroxides. They are highly with biological
molecules such as proteins, nucleic acids and unsaturated lipids. ROS
form as a natural by-product of the normal metabolism of oxygen and
have important roles in cell signaling (Storz 2007), regulating nucleic
acid synthesis, protein synthesis, enzyme activation and cell cycle progression
(Brondon et al. 2005). LLLT was reported to produce a shift in
overall cell redox potential in the direction of greater oxidation (Karu
1999) and increased ROS generation and cell redox activity have been
demonstrated (Alexandratou et al. 2002; Chen et al. 2009b; Grossman et
al. 1998; Lavi et al. 2003; Lubart et al. 2005; Pal et al. 2007; Zhang et al.
2008). These cytosolic responses may in turn induce transcriptional
changes. Several transcription factors are regulated by changes in cellular
redox state. But the most important one is nuclear factor B (NF-B).
Figure 5 illustrates the effect of redox-sensitive transcription factor NF-κB
Y.-Y. Huang and others
FIGURE 4. When NO is released from its binding to heme iron and copper centers in cytochrome c
oxidase by the action of light, oxygen is allowed to rebind to these sites and respiration is restored to
its former level leading to increased ATP synthesis.
activated after LLLT and is instrumental in causing transcription of protective
and stimulatory gene products.
2.7. Downstream cellular response
Although the underlying mechanism of LLLT are still not completely
understood, in vitro studies, animal experiments and clinical studies have
all tended to indicate that LLLT delivered at low doses may produce a better
result when compared to the same light delivered at high doses. LLLT
can prevent cell apoptosis and improve cell proliferation, migration and
adhesion at low levels of red/NIR light illumination (see Figure 6).
LLLT at low doses has been shown to enhance cell proliferation in
vitro in several types of cells: fibroblasts (Lubart et al. 1992; Yu et al. 1994),
keratinocytes (Grossman et al. 1998), endothelial cells (Moore et al.
2005), and lymphocytes (Agaiby et al. 2000; Stadler et al. 2000). The
mechanism of proliferation was proposed to involve photostimulatory
effects in mitochondria processes, which enhanced growth factor release,
and ultimately led to cell proliferation (Bjordal et al. 2007). Kreisler et al
showed (Kreisler et al. 2003) that the attachment and proliferation of
human gingival fibroblasts were enhanced by LLLT in a dose-dependent
manner. LLLT modulated matrix metalloproteinase activity and gene
expression in porcine aortic smooth muscle cells (Gavish et al. 2006).
Shefer at el. showed (Shefer et al. 2002) that LLLT could activate skeletal
Biphasic dose response in low level light therapy
FIGURE 5. Reactive oxygen species (ROS) formed as a result of LLLT effects in mitochondria may
activate the redox-sensitive transcription factor NF-κB (relA-p50) via protein kinase D (PKD).
muscle satellite cells, enhancing their proliferation, inhibiting differentiation
and regulating protein synthesis.
2.8. Downstream tissue response
There have been a large number of both animal model and clinical
studies that demonstrated highly beneficial LLLT effects on a variety of
diseases, injuries, and has been widely used in both chronic and acute
conditions (see Figure 7). LLLT may enhance neovascularisation, promote
angiogenesis and increase collagen synthesis to promote healing of
acute (Hopkins et al. 2004) and chronic wounds (Yu et al. 1997). LLLT
provided acceleration of cutaneous wound healing in rats with a biphasic
dose response favoring lower doses (Corazza et al. 2007). LLLT can also
stimulate healing of deeper structures such as nerves (Gigo-Benato et al.
2004), tendons (Fillipin et al. 2005), cartilage (Morrone et al. 2000),
bones (Weber et al. 2006) and even internal organs (Shao et al. 2005).
LLLT can reduce pain (Bjordal et al. 2006a), inflammation (Bjordal et al.
2006b) and swelling (Carati et al. 2003) caused by injuries, degenerative
diseases or autoimmune diseases. Oron reported beneficial effect of
LLLT on repair processes after injury or ischemia in skeletal and heart
muscles in multiple animal models in vivo (Ad and Oron 2001; Oron et
al. 2001a; Oron et al. 2001b; Yaakobi et al. 2001). LLLT has been used to
mitigate damage after strokes (in both animals (Lapchak et al. 2008) and
humans (Lampl et al. 2007)), after traumatic brain injury (Oron et al.
2007) and after spinal cord injury (Wu et al. 2009).
Y.-Y. Huang and others
FIGURE 6. The downstream cellular effects of LLLT signaling include increases in cell proliferation,
migration and adhesion molecules. Cell survival is increased and cell death reduced by expression of
proteins that inhibit apoptosis.
3.1. Dose dependence and dose rate effects—the biphasic curve
A biphasic response has been demonstrated many times in LLLT
research (Lanzafame et al. 2007; Oron et al. 2001a) and the “Arndt-Schulz
Law” is frequently quoted as a suitable model to describe dose dependent
effects of LLLT (Chow et al. 2006; Hawkins and Abrahamse 2006a;
Hawkins and Abrahamse 2006b; Lubart et al. 2006; Sommer et al. 2001).
The concept of the Arndt-Schulz Law dates from the years around the
end of the nineteenth century, when H. Schulz published a series of
papers that examined the activity of various kinds of poisons (iodine,
bromine, mercuric chloride, arsenious acid, etc.) on yeast, showing that
almost all these agents have a slightly stimulatory effect on the yeast
metabolism when given in low doses (Schulz 1877; Schulz 1888). He then
came into contact with the psychiatrist R. Arndt and together they developed
a principle that later became known as the ‘Arndt-Schulz law’, stating
that weak stimuli slightly accelerate vital activity, stronger stimuli raise
it further, but a peak is reached and even stronger stimuli suppress it,
until a negative response is finally achieved (Martius 1923). In 1960
Townsend and Luckey surveyed the field of classic medical pharmacology
and published a list of 100 substances known to be capable of causing
an inhibition at high concentrations and stimulation at low concentrations
and termed the phenomenon “hormoligosis” (Townsend and
Luckey 1960). The modern term “hormesis” was first used by Stebbing in
1982 (Stebbing 1982) and has been thoroughly reviewed by Calabrese
(Calabrese 2001b; Calabrese 2002; Calabrese 2004a; Calabrese 2004b;
Calabrese 2005).
Biphasic dose response in low level light therapy
FIGURE 7. Beneficial tissue effects of LLLT can include almost all the tissues and organs of the body.
In the context of LLLT the increasing “stimulus” may be irradiation
time or increased irradiance. This non-linear effect contradicts the
Bunsen-Roscoe rule of reciprocity (which was originally formulated for
visual detection of light by photoreceptors (Brindley 1952)), which predicts
that if the products of exposure time in seconds and irradiance in
mW/cm2 are equal, i.e. the energy density is the same, then the changes
in biological endpoint will be equal. This inverse linear relationship
between irradiance and time has frequently failed in LLLT research
(Karu and Kolyakov 2005; Lubart et al. 2006).
A “biphasic” curve can be used to illustrate the expected dose
response to light at a subcellular, cellular, tissue or clinical level. Simply
put, it suggests that if insufficient energy is applied there will be no
response (because the minimum threshold has not been met), if more
energy is applied the then a threshold is crossed and biostimulation is
achieved but when too much energy is applied then the stimulation disappears
and is replaced by bioinhibition instead. An idealized illustration
(Figure 8) similar to that suggested by Sommer (Sommer et al. 2001)
helps understand the concept.
3.2. Biphasic Response—irradiance
As early as 1978 Endre Mester observed a “threshold phenomenon”
after laser irradiation of lymphocytes in vitro (Mester et al. 1978). Peter
Bolton in 1991 irradiating macrophages with two different irradiances
(W/cm2) but the same energy density (J/cm2) recorded different results
(Bolton et al. 1991). Karu (Karu and Kolyakov 2005) found a dependence
of stimulation of DNA synthesis rate on light intensity at a constant energy
density 0.1 J/cm2 with a clear maximum at 0.8 mW/cm2. In another
study (Karu et al. 1997) the same group found no less than seven maxima
in the dose vs. biological effect curves using a pulsed 810-nm diode laser.
Y.-Y. Huang and others
FIGURE 8. Idealized biphasic dose response curve (often termed Arndt-Schulz curve) typically
reported in LLLT studies.
Four different biological models were used: luminol-amplified chemiluminescence
measured in nucleated cells of murine spleen (splenocytes),
bone marrow (karyocytes), and murine blood and adhesion of HeLa cells
cultivated in vitro. The peaks coincided for all four models. Anders conducted
the widest ranging in-vitro study (on normal human neural progenitor
cells) with four different energy density groups, each group tested
across a range of six different irradiance parameters (Anders et al.
2007) Table 3.
In 1979 Ginsbach found that laser stimulation of wound closure had
“no reciprocity relation”. His controlled experiments on rats with He-Ne
laser at an energy density of 4 J/cm2 found stimulation at an irradiance
of 45 mW/cm2 but not at 12.4 mW/cm2 (Ginsbach 1979). Uri Oron
(Oron et al. 2001a) showed different reductions of infarct size after
induced heart attacks in rats. Keeping energy density constant and varying
the irradiance he found that the beneficial effects were maximumal
at 5 mW/cm2 and significantly less effect both at lower irradiances (2.5
mW/cm2) and also at higher irradiances (25 mW/cm2). Ray Lanzafame
(Lanzafame et al. 2007) conducted a study varying irradiance and interval
on laser-induced healing of pressure ulcers in mice. Energy density (5
J/cm2) was fixed but four different irradiance (0.7 – 40 mW/cm2) parameters
were tested with a significant improvement only occurring for 8
We know of only one human clinical trial which varied irradiance but
this trial kept treatment time the same so energy density (J/cm2) did not
remain the same. This RCT by Hashimoto on the treatment of the stellate
ganglion to reduce pain in patients with post herpetic neuralgia of the
facial type. This study compared the effects of 830-nm lasers delivering 60
mW, 150 mW and placebo, each applied for 3 minutes to the anterior
aspect of the lateral process of the 7th cervical vertebrae. Each patient
Biphasic dose response in low level light therapy
TABLE 3. Comparison of different irradiances and fluences of 810-nm laser on differentiation of
normal human neural progenitor cells. Cells received light once a day for three days and neurite
outgrowth was measured.
Anders et al. 2007
Average Summed Neurite Length Parameters
1 mW/cm2 5 mW/cm2 15 mW/cm2 19 mW/cm2 25 mW/cm2 50 mW/cm2
0.01 J/cm2 NS NS — NS — —
0.05 J/cm2 NS NS NS S S NS
0.2 J/cm2 NS NS NS S S S
1 J/cm2 NS NS NS NS NS S
NS: No statistical difference.
S: Groups significantly greater than Factors group.
(One way ANOVA *p<0.01, **p<0.001)
had three treatments (one treatment, three consecutive days), each treatment
was with a different laser or placebo. The study was properly blinded
and randomized. There was a significant difference in skin temperature
of the forehead and in recorded pain scores. The greatest improvements
were for the 150mW laser (Hashimoto et al. 1997).
There have been several systematic reviews and meta analyses of RCTs
and these have revealed some irradiance dependant effects: Bjordal published
a review of LLLT for chronic joint disorders and identified 14 RCTs
of suitable methodological quality, 4 of which failed to report a significant
effect because the irradiance was either too high or too low, and/or delivered
insufficient energy, the remaining eight studies all produced positive
effects (Bjordal et al. 2003). Tumilty reviewed 25 LLLT RCTs of
tendinopathies,13 of which (55%) failed to produce a positive outcome,
all of these negative/inconclusive studies that recorded irradiance (or
could subsequently be established) had delivered an irradiance in excess
of the guidelines set by the World Association for Laser Therapy
( (Tumilty et al. 2009).
3.3. Biphasic Response—time or energy density
Again, Peter Bolton’s study mentioned in 3.2 above had an energy
density aspect showing a different response for each of the irradiances
used. For the 400mW/cm2 study he found increasing energy density from
2.4 J/cm2 to 7.2 J/cm2 increased fibroblast proliferation, in the 800
mW/cm2 group increasing energy density from 2.4 J/cm2 to 7.2 J/cm2
decreased fibroblast proliferation (Bolton et al. 1991). Anders’ study also
mentioned in 3.2 above looked at four energy density groups, and for the
irradiance parameters that produced significant results increasing energy
density increased neurite length (Anders et al. 2007) Table 3. Yamaura and
colleagues found a biphasic dose response in MTT activity in rheumatoid
arthritis synoviocytes after 810-nm laser with a peak at 8 J/cm2 and less
effect at lower and higher fluences (Yamaura et al. 2009). Loevschall measured
human oral mucosal fibroblast cell proliferation by incorporation of
tritiated thymidine after varying fluences of 812-nm laser delivered at 4.5
mW/cm2 and found a biphasic dose response with a distinct peak at 0.45
J/cm2 (Loevschall and Arenholt-Bindslev 1994). Another study (al-Watban
and Andres 2001) looked at chinese hamster ovary and human fibroblast
proliferation after various fluences of He-Ne laser delivered at a constant
irradiance of 1.25 mW/cm2. Again they found a clear biphasic dose
response with a peak at 0.18 J/cm2. Zhang et al (Zhang et al. 2003) found
a biphasic dose response in human fibroblast cell numbers after treatment
with varying fluences of 628-nm light, with a maximum increase of 30%
after 0.88 J/cm2 and an actual reduction appearing at 9 J/cm2. Brondon
and colleagues (Brondon et al. 2005) found that two treatments per day
Y.-Y. Huang and others
caused a bigger increase than 1 or 4 treatments per day measuring proliferation
index in human HEP-2 and murine L-929 cell lines. They used a
670 nm light emitting diode device with an irradiance of 10 mW /cm2 and
each single treatment was 5 J/cm2 and the course was stopped after 50
J/cm2 had been given (at 10, 5 or 2.5 days).
Lopes-Martins showed a biphasic response to LLLT on the number of
mononuclear cells that accumulate in pleural cavity after carrageenan
injection. The results showed neutrophil influx mice treated with three
different laser fluencies at 1, 2.5 & 5 J/cm2) with 2.5 having the greatest
effect (Lopes-Martins et al. 2005).
As stated in 3.2 above, Hashimoto reported on the laser treatment of
the stellate ganglion to reduce pain in patients with post herpetic neuralgia
of the facial type. The study compared the effects of 830-nm lasers
delivering 60mW, 150mW and placebo, The greatest improvements were
for the 150mW laser (Hashimoto et al. 1997). Again as stated in 3.2 above,
there have been several systematic reviews and meta analyses of RCTs and
these revealed some energy density dependant effects (Bjordal et al. 2003;
Tumilty et al. 2009).
3.4. Beam measurement reporting errors
One notable aspect of the dose rate (W/cm2) studies is the wide variation
of “optimal” irradiances in vitro studies as they range from 1-800
mW/cm2 in just the few papers referenced in this review. If the primary
photo acceptor is cytochrome C oxidase as postulated here, then why
would so many authors arrive at different conclusions for optimal parameters
in vitro, should it not be the same for all of them?
Explanations may include, the slightly different wavelengths used or
sensitivity due the redox state of mitochondia in the target cells (Tafur
and Mills 2008), but we consider that the greater contributor may be laser
beam measurement problems. It may be a surprise to non-physicists that
diode laser beams are not inherently round, and even if circularizing
lenses are used to correct this, then the beam intensity distribution is not
homogeneous. Laser beams are brighter (higher irradiance) in the middle
and weaker towards the edge. Cells in the centre of a culture well will
be exposed to considerably higher irradiances than those on the periphery.
Because the edge of a laser beam is hard to define and find this could
mean that irradiance calculations are significantly different between
research centers. Agreement on beam measurement and reporting of
intensity distribution is needed to reduce these inconsistencies. This is
important not only for in vitro studies but also in vivo and clinical trials as
reporting of irradiance is just as important though we accept that tissue
scattering diffuses the beam probably making non-homogenous sources
less critical to clinical effectiveness.
Biphasic dose response in low level light therapy
4.1. In vitro activation of NF-κB
We developed the hypothesis (Chen et al. 2009a) that NIR light (810-
nm laser) would activate the transcription factor NF-κB by generating
reactive oxygen species from the mitochondria (see section 2.5). We tested
this in mouse embryonic fibroblasts that had been genetically engineered
to synthesize luciferase in response to NF-κB activation (Chen et al.
2009a). We used a wide range (four orders of magnitude) of delivered fluences
by adjusting the laser power so that the illumination time was kept
constant at 5 minutes. As shown in Figure 9 there was a biphasic dose
dependent activation of NF-κB as measured by luciferase assay 10 hours
after the illumination was completed. There was no significant increase at
0.003 J/cm2 compared to the dark control, a small increase at 0.03 J/cm2,,
the maximum activation was observed at 0.3 J/cm2, while at 3 J/cm2 and
even more so at 30 J/cm2 there was a decrease in NF-κB activation, but the
level was still higher than that found at 0.03 J/cm2. The level of luciferase
expression was also measured in the presence of cycloheximide (CHI) as
a control. CHI is a protein synthesis inhibitor that removes even the background
level of luciferase seen in dark control cells, as well as all the
increases seen with the different fluences of 810-nm light.
We tested the hypothesis that the activation of NF-κB by LLLT was
mediated by generation of ROS because NF-κB is known to be a redoxsensitive
transcription factor (Schreck et al. 1992) and moreover ROS
have previously been shown to be generated during LLLT (Alexandratou
Y.-Y. Huang and others
FIGURE 9. Biphasic dose response of NF-κB activation (measured by bioluminescence reporter
assay) in mouse embryonic fibroblasts 10 hours after different fluences of 810-nm laser light. CHI is
control where all protein synthesis has been inhibited.
et al. 2002; Lubart et al. 2005; Pal et al. 2007). We used dichlorodihydrofluorescein
diacetate (DCHF-DA) which is taken up into cells, hydrolyzed
and oxidized to a fluorescent form by most species of ROS probably via
lipid peroxides (Diaz et al. 2003). As can be seen in Figure 10 even the low
fluence of 0.003 J/cm2 produced detectable levels of ROS, greater at 0.03
J/cm2 and maximum at 0.3 J/cm2 with a slight decrease observed at 3
J/cm2. The maximum level observed at 0.3 J/cm2 was only slightly less
than that observed inside the cells after addition of hydrogen peroxide to
the extracellular medium.
4.2. Mouse wound healing
In an in vivo study (Demidova-Rice et al. 2007) we used a set of fluences
of 635-nm (+/–15-nm) light delivered from a filtered lamp. The
model was a full thickness dorsal excisional wound in BALB/c mice treated
with a single exposure to light 30 minutes after wounding. These fluences
were 1, 2, 10 and 50 J/cm2 delivered at constant fluence rate of 100
mW/cm2 and taking 10, 20, 100 and 500 seconds respectively. In this
model the untreated wound tends to expand for 2-3 days after it was
made, but even a brief exposure to light soon after wounding, reduces or
stops the expansion of the wound and the integrated time course of the
wound size can therefore be significantly reduced. Our hypothesis is that
fibroblasts in the edge of the wounded dermis can be transformed into
myofibroblasts, and the contractile nature of these cells with their smooth
muscle actin fibers prevents the wound expanding. It should be noted
that the fibroblast-myofibroblast transition can be mediated by NF-κB
activation (Watson et al. 2008). As shown in Figure 11 there was a bipha-
Biphasic dose response in low level light therapy
FIGURE 10. Biphasic dose response in generation of ROS as detected by fluorescence probe under
same conditions as Fig 9 but measured at 5 minutes post-irradiation.
sic dose response with positive effects (difference in integrated area
under the curve of time course of wound size compared to no treatment
control) seen in low doses with a clear maximum seen at 2 J/cm2, and the
high dose of 50 J/cm2 actually gave a worsening of the wound healing
time curve i.e. there was a greater expansion of the wound compared with
non-treated controls.
4.3. Rat arthritis
In another in vivo study (Castano et al. 2007) we investigated whether
LLLT using an 810-nm laser could have a therapeutic effect in a rat
model of inflammatory arthritis caused by zymosan injected into their
knee joints. In this model the severity of the arthritis is quantified by
measuring the diameter of the swollen joint every day and plotting a time
course for each joint. We compared illumination regimens consisting of
a high and low fluence (3 and 30 J/cm2), delivered at high and low irradiance
(5 and 50 mW/cm2) using 810-nm laser light daily for 5 days, with
the positive control of conventional corticosteroid (dexamethasone)
As shown in Figure 12 three of the illumination regimens were effective
in reducing the mean integrated knee swelling almost as much as the
positive control of the powerful steroid, dexamathasone; these were 3
J/cm2 delivered at 5 mW/cm2 and 30 J/cm2 delivered at 50 mW/cm2
both of which took 10 minutes, and 30 J/cm2 delivered at 5 mW/cm2
which took 100 minutes. The only ineffective dose regimen was 3 J/cm2
Y.-Y. Huang and others
FIGURE 11. Biphasic dose response in measured difference in integrated area under the curve of
time course of wound size compared to no treatment control, with a clear maximum seen at 2 J/cm2,
and the high dose of 50 J/cm2 gave a worsening of the wound healing time curve.
delivered at 50 mW/cm2 which took the comparatively short time of 1
minute to deliver. This observation led us to propose that the illumination
time was an important parameter in some LLLT applications.
The repeated observations that have been made of the biphasic dose
response phenomenon in LLLT require some explanation. The natural
assumption that is frequently made is, that if a small dose of red or nearinfrared
light produces a significant therapeutic effect, then a larger dose
should produce an even more beneficial effect. This natural assumption
is frequently not the case. We here propose three possible explanations
for the existence of the biphasic dose response based upon mechanistic
considerations outlined in section 2.
5.1. Excessive ROS
As discussed in 2.5 the light mediated generation of reactive oxygen
species has been observed in many in vitro studies and has been proposed
to account for the cellular changes observed after LLLT via activation of
redox sensitive transcription factors (Chen et al. 2009a). The evidence of
Biphasic dose response in low level light therapy
FIGURE 12. Dose response of illumination time found in a study of 810-nm laser to treat zymosaninduced
arthritis in rats. Integrated curves of knee circumference versus time were compared. Three
LLLT regimens were equally successful where the illumination time was either 100 minutes or 10
minutes, but the ineffective regimen only had a 1 minute illumination time.
ROS mediated activation of NF-κB in MEF cells presented in 4.1 provides
additional support for this hypothesis (Chen et al. 2009a). It is well-accepted
that ROS can have both beneficial and harmful effects (Huang and
Zheng 2006). Hydrogen peroxide is often used to kill cells in vitro (Imlay
2008). Other ROS such as singlet oxygen (Klotz et al. 2003) and hydroxyl
radicals (Pryor et al. 2006) are thought to be harmful even at low concentrations.
The concept of biphasic dose response in fact is well established
in the field of oxidative stress (Day and Suzuki 2005). If the generation
of ROS can be shown to be dose dependent on the delivered energy
fluence this may provide an explanation for the stimulation and inhibition
observed with low and high light fluences.
5.2. Excessive NO
The other mechanistic hypothesis that is put forward to explain the
cellular effects of LLLT relates to the photolysis of nitrosylated proteins
that releases free NO (see section 2.6). Again the literature has many
papers that discuss the so-called two-faced or “Janus” molecule NO
(Anggard 1994; Lane and Gross 1999). NO can be either protective or
harmful depending on the dose and particularly on the cell or tissue type
where it is generated (Calabrese 2001a).
5.3. Activation of a cytotoxic pathway
The third hypothesis to explain the biphasic dose response of LLLT
is the idea that the protective and stimulatory effects of light occur at low
doses, but there is an additional pathway that leads to damaging effects of
light that only occurs at high doses, and effectively overwhelms the beneficial
effects of low doses of light. Work from South China Normal
University provides some support for this hypothesis. Low doses of LLLT
were found to phosphorylate hepatocyte growth factor receptor (c-Met),
and initiate signaling via cyclic AMP and Jun kinase and Src (Gao and
Xing 2009). By contrast, high dose LLLT was found to induce apoptosis
via a mitochondrial caspase-3 pathway and cytochrome c release was
attributed to opening of the mitochondrial permeability transition pore
caused by high-level intracellular reactive oxygen species (ROS) generation
(Wu et al. 2009). A secondary signaling pathway through Bax activation
was observed (Wu et al. 2009).
LLLT delivered at low doses tends to work better than the same wavelength
delivered at high levels, which illustrates the basic concept of biphasic
dose response or hormesis (Calabrese 2001b). In general, fluences of
red or NIR as low as 3 or 5 J/cm2 will be beneficial in vivo, but a large dose
Y.-Y. Huang and others
like 50 or 100 J/cm2 will lose the beneficial effect and may even become
detrimental. The molecular and cellular mechanisms LLLT suggest that
photons are absorbed by the mitochondria; they stimulate more ATP production
and low levels of ROS, which then activates transcription factors,
such as NF-κB, to induce many gene transcript products responsible for
the beneficial effects of LLLT. ROS are well known to stimulate cellular
proliferation of low levels, but inhibit proliferation and kill cells at high
levels. Nitric oxide is also involved in LLLT, and may be photo-released
from its binding sites in the respiratory chain and elsewhere. It is possible
that NO release in low amounts by low dose light may be beneficial, while
high levels released by high dose LLLT may be damaging. The third possibility
is that LLLT may activate transcription factors, upregulating protective
proteins which are anti-apoptotic, and generally promote cell survival.
In contrast, it is entirely possible that different transcription factors
and cell-signaling pathways, that promote apoptosis, could be activated
after higher light exposure. We believe that further advances in the mechanistic
understanding of LLLT will continue to be made in the near
future. These advances will lead to greater acceptance of LLLT in mainstream
medicine and may lead to LLLT being used for serious diseases
such as stroke, heart attack and degenerative brain diseases. Nevertheless
the concept of biphasic dose response or LLLT hormesis (low levels of
light are good for you, while high levels are bad for you) will remain.
Research in the Hamblin laboratory is supported by the US NIH
(grants R01CA/AI838801 and R01AI050875 to MRH).
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