Light is a form of energy that behaves
like a wave and also as a stream of particles called photons. The
development of monochromatic light sources with single or a narrow
spectra of wavelengths paved the way for studies, which continue
to show that appropriate doses and wavelengths of light are therapeutically
beneficial in tissue repair and pain control. Evidence indicates
that cells absorb photons and transform their energy into adenosine
triphosphate (ATP), the form of energy that cells utilize. The resulting
ATP is then used to power metabolic processes; synthesize DNA, RNA,
proteins, enzymes, and other products needed to repair or regenerate
cell components; foster mitosis or cell proliferation; and restore
homeostasis.
Other reported mechanisms of light-induced beneficial
effects include modulation of prostaglandin levels, alteration of
somatosensory evoked potential and nerve conduction velocity, and
hyperemia of treated tissues. The resultant clinical benefits include
pain relief in conditions such as carpal tunnel syndrome (CTS),
bursitis, tendonitis, ankle sprain and temporomandibular joint (TMJ)
dysfunction, shoulder and neck pain, arthritis, and post-herpetic
neuralgia, as well as tissue repair in cases of diabetic ulcer,
venous ulcer, bedsore, mouth ulcer, fractures, tendon rupture, ligamentous
tear, torn cartilage, and nerve injury. Suggested contraindications
include treatment of cancer; direct irradiation of the eye, the
fetus, and the thyroid gland; and patients with idiopathic photophobia.
The Nature of Light
It is common knowledge that sunny days are exciting and dull ones,
depressing. Not so well known is the fact that light -even in small
amounts- produces a multitude of clinical benefits, including tissue
repair and pain control. This article discusses the nature of light
energy, encapsulates the evidence supporting its effects on tissue
repair and pain control, summarizes the mechanisms involved, and
outlines the clinical conditions that benefit from therapeutic light.
Each wakeful moment we use sunlight or man-made light
to see the world around us, yet it is not so well known that what
we perceive as light is actually a form of energy that behaves like
a wave and also as a stream of particles called photons. Photons
behave differently from conventional particles. They have no mass
and are not limited to a specific volume in space or time.
Figure 1. The electromagnetic spectrum showing the range of wavelengths
and categories of light waves. Note that the spectrum of visible
light is very narrow compared to the invisible spectrum, which includes
gamma rays, x-rays, UV rays, infrared radiation, and radio waves.
Each photon gyrates and bounces at a unique frequency
and exhibits electrical and magnetic properties. As a result, their
waves are called electromagnetic (EM) waves. Not all photons are
visible to the human eye. As shown in Figure 1, what we see as light
is only a minute range of the spectrum of EM waves associated with
photons. The entire spectrum includes radio waves, infrared radiation,
visible light, ultraviolet rays, x-rays, gamma rays, and cosmic
radiation. The photons of different regions of the EM spectrum vibrate
differently and have different amounts of energy.
Thus, even though radio waves, infrared radiation,
visible light, ultraviolet rays, x-rays, and gamma rays are photons,
ie, light, they vibrate at different rates and differ in photon
energy. Their waves have different wavelengths as well. A wavelength
is the interval between two peaks of a wave (Figure 2), and relates
to the color of visible light. For example, blue, green, red, and
violet light have different wavelengths. This difference becomes
clearer when one compares red and infrared light. Red light is visible;
infrared is not.
Figure 2. Illustration of the wave nature of light. Light is transmitted
as sinusoidal wave. A plot of the amplitude and time is shown.
Light For Therapy
Since the photons of different regions of the EM spectrum differ
in energy and vibration frequency, they produce differing effects
on humans. For example, gamma rays, x-rays, and UV rays tend to
ionize matter and damage tissue because their photons have high
energy. In comparison, radio waves have much lower energy and longer
wavelengths, and are relatively innocuous. Infrared and visible
light fall somewhere in between. The evidence shows that red and
near infrared (NIR) light have therapeutic benefits; as a result,
most of the equipment being sold today have either red, NIR, or
a combination of red and NIR light.
The development of single color (monochromatic) light
sources with unique wavelengths enabled scientists to study the
effects of various colors of light on tissues. This event occurred
in 1960 when Theodore Maiman-using a technique earlier proposed
by two teams of scientists, Charles H. Townes and Arthur L Schawlow
of the United States and Alekxandr Prokhorov and Nikolay Basov of
Russia-developed a device that produced red light with a unique
wavelength. The device was called LASER, because it was produced
using a technique known as Light Amplification by Stimulated Emission
of Radiation. Early research on this new form of light focused on
high power (> 500 mW) lasers, resulting in the development of
weapons grade lasers and the type of lasers used for surgery today.
As detailed below, serendipity, not a deliberate attempt, opened
the field of therapeutic low power lasers.
Beginning from the late 1960s, Endre Mester, a Hungarian
physician, began a series of experiments with monochromatic light.
Like others of his era, Mester attempted to use "high power" laser
to destroy tumors. Early in his experiments, he implanted tumor
cells beneath the skin of laboratory rats and zapped them with a
customized ruby laser-red light. To his surprise, the tumor cells
were not destroyed by doses of what was presumed to be high-power
laser. Instead, he observed that in many cases the skin incisions
he made to implant the recalcitrant cells appeared to heal faster
in treated animals compared to incisions of control animals that
were not treated with light.
This casual observation led him to design an experiment
to ascertain his suspicion that treatment with red light accelerated
healing of the surgical skin incisions he made to implant the cells.
The experiment was successful as it showed that treatment with red
light indeed produced faster healing of the skin wounds. Baffled
but fascinated by this development, he carried out other experiments
in which he showed that experimental skin defects, burns, and human
cases of ulcers arising from diabetes, venous insufficiency, infected
wounds, and bedsores also healed faster in response to his laser
treatment.1-3 How could a device that was intended to destroy tumor
cells promote tissue repair? It turned out that Mester's custom-designed
ruby laser was weak and was not as powerful as he thought it to
be. Instead of being photo-destructive, the low power light had
no effect on the tumor. Indeed, it stimulated the skin to heal faster-just
as sunlight may be beneficial in small amounts but destructive in
high amounts. This fortuitous encounter opened the field of monochromatic
light treatment.
Tissue Repair
Since Mester first uncovered the therapeutic value of red light,
different wavelengths of light have been shown to promote healing
of skin, muscle, nerve, tendon, cartilage, bone, and dental and
periodontal tissues.4-15 When healing appears to be impaired, these
tissues respond positively to the appropriate doses of light, especially
light that is within 600 to 1,000 nm wavelengths.12,16-19 The evidence
suggests that low energy light speeds many stages of healing. It
accelerates inflammation,4 promotes fibroblast proliferation,5,6,20,21
enhances chondroplasia,6 upregulates the synthesis of type I and
type III procollagen mRNA,23 quickens bone repair and remodeling,8
fosters revascularization of wounds,8 and overall accelerates tissue
repair in experimental and clinical models.4-15,19 The exact energy
density (energy per unit area) necessary to optimize healing continues
to be explored for each tissue.
However, there is emerging consensus that accelerated
healing can be accomplished with doses ranging from 1.0 to 6.0 Jcm-2.16-19,24
Indeed, recent studies of human cases of healing-resistant ulcers
suggest that this dose range results in healing of 55% to 68% of
ulcers that did not respond to any other known treatment.25-33
In our recent (unpublished) clinical study, we used
a double-blind randomized crossover experiment to examine the effects
of 3.0 Jcm-2 dose of 830 nm light applied twice weekly on slow-healing
diabetic leg ulcers in patients that, for at least 4 weeks, did
not respond to conventional treatment. Treatment was carried out
for 10 weeks; 5 weeks of one treatment (sham or real), followed
by 5 weeks of the other treatment (sham or real) that was not given
during the initial 5 weeks. The sham treatment consisted of a standard
ulcer care protocol followed by sham (fake) light treatment, while
the actual treatment was carried out in the same manner but with
real infrared 830 nm light.
Figure 3: Graphs showing some of the cases treated with light. In
these graphs, ulcer size is plotted on the Y-axis while the number
of treatments given is shown on the X-axis. Plots [A] and [C] illustrate
two ulcers that healed completely in 5 weeks without crossover,
[B] shows an ulcer that was treated with fake 830 nm light before
being treated with actual 830 nm infrared light. Note that complete
healing was achieved only after crossover to actual treatment. Plot
[D] shows an ulcer that did not respond to fake or actual treatment.
Four of the seven cases treated (57%) responded positively
with total healing of the ulcers achieved within 5 to 10 weeks (Figure
3). The remaining three did not respond at all, suggesting that
not all ulcers respond positively to this form of treatment. Two
of these patients healed within the first 5 weeks, making crossover
unnecessary. None of the ulcers healed with the sham treatment.
This case study suggests that light therapy may be beneficial in
treating healing-resistant ulcers that fail to respond to other
known treatments.
Overall, the literature indicates that more than 50%
of patients with ulcers that do not respond to any known treatments
heal rapidly with low energy densities of light therapy.27,38,30-33
This noninvasive treatment could save hospitals and the nation the
billions of dollars spent in treating chronic healing-resistant
wounds each year.34 Twenty-seven percent of patients with chronic
leg ulcers have diabetes mellitus.35 In 84% of these patients, ulcers
resistant to healing are cited as the cause of lower limb amputation,36
which in turn produces varying levels of disability.
Treating a patient with light adds energy to the target
tissue. The amount of energy added to the tissue depends on factors,
such as the power of the light source and the duration of treatment,
in the same manner as the amount of energy used in one's home depends
on how powerful the light bulbs and other home equipment are, and
how long the lights and equipment are left on.
Light, at appropriate doses and wavelengths, is absorbed
by chromophores such as cytochrome c, porphyrins, flavins, and other
light-absorbing entities within the mitochondria and cell membranes
of cells.37 Once absorbed, the energy is stored as ATP, the form
of energy that cells can use. A small amount of free radicals or
reactive oxygen species-also known to be beneficial-is produced
as a part of this process, and ca++ and the enzymes of the respiratory
chain play vital roles as well.38
Figure 4. Schematic showing how light is absorbed by cells and the
cascade of events resulting from light absorption. ATP is produced
in this process and used to synthesize needed proteins, enzymes,
and other tissue components.
The ATP produced may be used to power metabolic processes;
synthesize DNA, RNA, proteins, enzymes, and other biological materials
needed to repair or regenerate cell and tissue components;39 foster
mitosis or cell proliferation; and/or restore homeostasis. The result
is that the absorbed energy is used to repair the tissue, reduce
pain, and/or restore normalcy to an otherwise impaired biological
process (see Figure 4).
Pain Control
The evidence that low power light modulates pain dates back to the
early 1970s, when Friedrich Plog of Canada first reported pain relief
in patients treated with low power light. But during this period
the mood was neither right nor were minds ready to accept the idea
that a technology that was being developed for destructive purposes-one
that can cut, vaporize, and otherwise destroy tissue-could have
beneficial medical effects. Thus, like Mester's findings, Plog's
results were met with skepticism, particularly in the United States,
where until the early part of 2002, the Food and Drug Administration
(FDA) repeatedly declined to endorse low power light devices for
patient care.
Works by other groups in Russia, Austria, Germany,
Japan, Italy, Canada, and, more recently, Argentina, Israel, Brazil,
Northern Ireland, Spain, the United Kingdom, and, of late, the United
States, have produced a preponderance of evidence supporting the
original findings of Plog by showing that appropriate doses and
wavelengths of low power light promote pain relief.40-54 More recent
reports include studies that indicate that 77% to 91% of patients
respond positively to light therapy when treated thrice weekly over
a period of 4 to 5 weeks.42-45 Not surprisingly, CTS is one of the
first conditions for which the FDA granted approval of low power
light therapy.
In addition to the mechanism detailed above, reports
indicate that light therapy can modulate pain through its direct
effect on peripheral nerves as evidenced by measurements of nerve
conduction velocity and somatosensory evoked potential.43-55 Other
reports indicate that light therapy modulates the levels of prostaglandin
in inflammatory conditions, such as osteoarthritis, rheumatoid arthritis,
and soft tissue trauma.56,57 Furthermore, works from the laboratories
of Drs Shimon Rochkind of Tel-Aviv, Israel, and Juanita Anders of
Bethesda, Md, indicate that specific energy fluences of light promote
nerve regeneration, including regeneration of the spinal cord-a
part of the central nervous system once considered inert to healing.58-59
The combination of these and other mechanisms perhaps accounts for
the overall promotion of recovery from inflammatory conditions such
as CTS43-45 and arthritis.48,49,56,57
Clinical Considerations
Light technology has come a long way since the innovative development
of lasers more than 40 years ago. Other monochromatic light sources
with narrow spectra and the same therapeutic value as lasers-if
not better in some cases-are now available. These include light
emitting diodes (LEDs) and superluminous diodes (SLDs). As the name
suggests, SLDs are generally brighter than LEDs; they are increasingly
becoming the light source of choice for manufacturers and researchers
alike. The light source does not have to be a laser in order to
have a therapeutic effect. It just has to be light of the right
wavelength. Lasers, LEDs, SLDs, and other monochromatic light sources
produce the same beneficial effects. Simply stated, light is light.
The dose and wavelengths are critical. At present, it is believed
that appropriate doses of 600 to 1,000 nm light promote tissue repair
and modulate pain.
Indications and Contraindications
Indications: The FDA has approved light therapy for the treatment
of head and neck pain, as well as pain associated with CTS. In addition
to these conditions, the literature indicates that light therapy
may be beneficial in three general areas:
Inflammatory conditions (eg, bursitis, tendonitis,
arthritis, etc).
Wound care and tissue repair (eg, diabetic ulcers, venous ulcers,
bedsores, mouth ulcer, fractures, tendon ruptures, ligamentous tear,
torn cartilage, etc).
Pain control (eg, low back pain, neck pain, and pain associated
with inflammatory conditions-carpal tunnel syndrome, arthritis,
tennis elbow, golfer's elbow, post-herpetic neuralgia, etc).
Contraindications: There is a dearth of scientific
evidence that light therapy, when used at appropriate doses, is
contraindicated for any condition. However, experience and prudence
suggest the following:
Cancer (tumors or cancerous areas)
Direct irradiation of eyes
Treatment of patients with idiopathic photophobia or abnormally
high sensitivity to light.
Patients who have been pretreated with one or more photosensitivity
enhancing agents, as for example, patients undergoing photodynamic
therapy (PDT).
Direct irradiation over the fetus or the uterus during pregnancy.
Direct irradiation of the thyroid gland.
Light can be destructive at high doses but therapeutic
at appropriately low doses. Therefore, it is of paramount importance
to use the right dose (fluence or energy per unit area treated),
and frequency of treatment appropriate for each condition. A detailed
description of methods of treatment, doses suitable for the multitude
of ailments that respond well to light treatment, and the rationale
for each treatment is beyond the scope of this article but can be
found in our recent publication.
Conclusions
Since the late 1960s when Endre Mester first demonstrated the beneficial
effects of monochromatic light, accumulating evidence indicates
that light therapy relieves pain and promotes healing of skin nerve,
bone, muscle, tendon, cartilage, and ligament.
It has been shown that light energy is absorbed by
endogenous chromo-phores-notably in the mitochondria-and used to
synthesize ATP. The resulting ATP is then used to power metabolic
processes; synthesize DNA, RNA, proteins, enzymes, and other biological
materials needed to repair or regenerate cell and tissue components;
foster mitosis or cell proliferation; and restore homeostasis. Other
reported mechanisms of light-induced tissue repair and pain control
include modulation of prostaglandin, alteration of nerve conduction
velocity and somatosensory evoked potential, and hyperemia of treated
tissues. The clinical benefits resulting from these demonstrated
effects are pain control and tissue repair in the multitude of circumstances
described in clinical studies.
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Chukuka S. Enwemeka, PT, PhD, FACSM, is professor and dean, School
of Health Professions, Behavioral and Life Sciences, at the New
York Institute of Technology, Old Westbury, NY.
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