Low-level laser therapy has been studied for over 40 years, with mixed outcomes reported. The applications have been extensive, involving dermatology, respiratory conditions, arthritic conditions, soft tissue and bone healing, pain, and nerve lesions, to name a few. Conclusions on efficacy are difficult to ascertain because many of the reported outcomes are in non-peer-reviewed literature, anecdotal reports, uncontrolled studies, or published abstracts; or controlled studies have poorly described methodology or show contradictory outcomes. The FDA relies on clinical data to allow an expansion in the application of LLLT. Ideal studies are from multiclinical sites and are randomized, blinded, placebo-controlled studies that verify safety and efficacy. As far as the FDA is concerned, the safety considerations have been largely satisfied but further research is needed to determine efficacy with medical conditions other than carpal tunnel syndrome and neck pain.
Of the methodological flaws noted, dosage has been and remains a significant pitfall in research. Frequently, studies used very low doses of laser. The treatment parameters that should be documented to allow consistency in further research include the following:
- Laser model
- Laser type and wavelength
- Probe description (single/cluster)
- Output power
- Pulsing/pulsing duration
- Pulse frequency
- Power density (intensity)
- Treatment technique (distance)
- Treatment time
- Treatment frequency
The potential use of LLLT in sports medicine would be to enhance wound healing and pain management after injury. It would be helpful to find a modality that could be applied acutely and one that would expedite the return of an athlete to competition by providing these outcomes. Low-level laser therapy has been reported to expedite the inflammatory process, decrease pain, and promote tissue healing. Studies have suggested that lasers promote fibroblast proliferation, promote the synthesis of Type I and III procollagen mRNA (Abergel et al. 1984), hasten bone healing (Ozawa et al. 1995), and help in the revascularization of wounds (Kovacs, Mester, and Gorog 1974).
The proposed mechanism of action of laser therapy is associated with the ability of the cell to absorb the photon and transform the energy into adenosine triphosphate (ATP). The ATP is a form of energy that the cell uses to function. The cell must absorb the light energy for this process to occur. We know that certain cells have the capacity to absorb light energy, as in the skin reacting to sunlight. These light-absorbing components of the cells are termed chromophores or photoacceptors and are contained within the mitochondria and cell membrane. Cell components such as cytochrome c, porphyrins, and flavins also have a light absorbing capability (Karu 1987).
Production of ATP is essential to cell function. Normally ATP is produced by the mitochondria, using oxygen as the primary fuel. Laser stimulation has been shown to enhance the production of ATP by forming singlet oxygen, reactive oxygen species (ROS), or nitric oxide, all of which influence the normal formation of ATP (Derr and Fine 1965; Lubart et al. 1990). The increased ATP prompts homeostatic function of the cells to resume. Furthermore, the ATP energy may drive the messenger RNA to foster cell mitosis and proliferation.
The proposal that laser energy merely promotes normal cellular function rather than changing cell function explains why injured tissues respond to laser therapy while there is little effect on noninjured cells. This is in contrast to the application of high levels of laser energy or excessive doses that cause damage to cells. Again, the effect of the laser is dependent on the intensity of the energy, the exposure time, and the irradiated area. The wavelength of the laser affects the depth of penetration of the energy.
The effect of LLLT on inflammation has been reported to be pro-inflammatory rather than anti-inflammatory (Kana et al. 1981). Laser has been shown to enhance the degranulation of mast cells that results in histamine production. Histamine, a powerful chemical mediator, tends to accelerate the inflammatory cascade. As the inflammatory process progresses more rapidly, the proliferative phase of healing begins sooner, subsequently enhancing the wound healing process. The effect of laser on inflammation suggests that LLLT may begin early in the injury process and may be combined with RICE (rest, ice, compression, elevation) as an initial intervention.
The FDA is allowing the marketing of approved LLLT devices for the treatment of symptoms associated with carpal tunnel syndrome and for adjunctive use in providing temporary relief of minor chronic neck and shoulder pain of musculoskeletal origin. The effect on other painful conditions has been reported, but the effectiveness is equivocal. Numerous studies have shown that LLLT is effective in reducing pain, but the exact mechanisms are still to be determined. The production of endogenous opioids, nitric oxide, serotonin, and acetycholine has been reported to be a source of analgesic effects elicited from laser radiation (Laakso et al. 1994; Choi, Srikantha, and Wu 1986). These mechanisms need further study. Another proposed mechanism for pain reduction is a direct effect on nerve conduction velocity and somatosensory evoked potentials. These changes have been measured after the application of LLLT, but their ability to influence pain is not well understood.
Wound healing has been enhanced with the application of laser energy (Woodruff et al. 2004; Enwemeka et al. 2004). The most promising investigations have involved using laser to promote the healing of ulcers and other injuries to the skin. Research outcomes have varied for different reasons, including the use of different wavelengths and dosages and the use of healthy animal models.
Laser radiation results in biomodulation, meaning that it can stimulate or inhibit. This is analogized to sunlight and tanning. Some energy is effective in stimulating melatonin, but excessive light results in damage (suntan vs. sunburn). Low-dosage laser would be ineffective, while excessive energy may inhibit rather than stimulate healing. Acute injuries can be treated more frequently (daily) than chronic wounds, which should be treated only two or three times per week. Chronic wounds do not respond to aggressive interventions.
Laser energy is more effective in treating pathological states; therefore, when healthy subjects are used, the outcomes may be subdued. Although tissue healing is accelerated, no hyperplastic effects have been reported (Bosatra, Jucca, and Olliaro 1984). During the course of healing, lased wounds had more collagen and had a higher tensile strength than the controls, but by day 14 the wounds were similar (Abergel et al. 1987; Kana et al. 1981; Surinchak et al. 1983). This shows that laser energy catalyzes normalization rather than creating a supernormal effect.
Systemic effects from laser therapy have been reported (Mester et al. 1971; Kana et al. 1981). This is why research using laser treatments on one body part and using another site on the same subject or animal as the control may give misleading results. These systemic effects are not always observed in the research but should be considered.
Other modalities in addition to LLLT can be beneficial, although thermal devices should be used after the laser treatment. Blood, specifically the hemoglobin, absorbs laser energy, so any modality that increases blood flow could make LLLT less effective. Generally it is recommended that tissues be cooled before laser treatment and heated afterward if these therapies are indicated. When combining laser treatment with ultrasound, it was felt that the individual therapies obtained the best outcomes and the clinician should choose the most appropriate modality rather than combining them (Gum et al. 1997).
Medications may have an effect on laser efficacy, although more research on this issue is needed. Medications such as nonsteroidal anti-inflammatory drugs, steroids, and calcium channel blockers, to list a few, are thought to block membrane channels and pigment receptors (Meersman 1999), which are important for laser actions, and therefore reduce laser effectiveness. Other medications such as procaine, certain antibiotics, and copper-based local substances may enhance the effectiveness of laser energy by enhancing the receptor sites. Researchers should be sensitive to the presence of medications with subject selection when designing laser studies.