During the past 20 years, lasers have become easier to use, less expensive, and more available to the medical community than in the past. The high-power applications (10 to 100+ W) for surgical procedures and hemostasis are well known; however, less dramatic but potentially important biologic effects at powers as low as fractions of milliwatts have also been reported.
Although the medical applications of low-power lasers remain controversial, the clinical use of these devices for a variety of analgesic and wound-healing applications is steadily increasing.
1
, 2
, 3
Early eastern European research4
, 5
, 6
has been supplemented by many subsequent investigations. Numerous biologic and physiologic effects have been reported and are summarized in Table 1. In the United States, low-power laser treatment tends to be limited to muscular strain and overuse syndromes, degenerative arthritis, rheumatoid arthritis, neuropathic pain, and healing of wounds.2
In other countries, treatment indications are broader and include such exotic applications as some forms of baldness, “facial toning,” and “bowed tendons” in racehorses.1
, 3
Table 1Reported Biochemical and Physiologic Effects of Low-Energy Laser Irradiation
| Reported effect | Animal model | References |
|---|---|---|
| Cellular processes | ||
| Stimulated production of collagen and fibroblast activity | Human, rat, mouse | 7 , 8 , 9 |
| Altered prostaglandin content | Human | 7 |
| Increased tissue concentrations of succinic dehydrogenase and lactic dehydrogenase | Human | 7 |
| Altered rates of DNA synthesis | Guinea pig, Chinese hamster | 6 , 10 |
| Cytoplasmic histologic changes | Human | 7 |
| Accelerated cellular proliferation | Rat | 8 |
| Physiologic changes | ||
| Stimulated neuronal activity | Mollusk (Aplysia), guinea pig | 5 , 11 |
| Improved function and repair of damaged peripheral nerves | Rat | 12 , Rochkind S, Bartal A, Razon N, Nissan M, Schwarts M: The long term effect of He-Ne laser irradiation on reparative processes in peripheral nerve and denervated tissue in normal and crushed sciatic nerve in the rat (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 78 13 |
| Improved function and repair of damaged optic nerves | Rabbit | 14 Schwartz M, Lavie V, Doron A, Rochkind S, Belkin M, Bartal A, Ben-Basat J: Indications that low energy He-Ne laser radiation prevents post-traumatic degeneration of adult rabbit optic nerve (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 78 |
| Altered distal nerve latencies (results contradictory) | Human | 15 , 16 |
| Increased superficial vascularity | Human | 7 |
| Improved wound healing | Human, mouse | 7 , 9 |
| Accelerated fracture healing | Human, mouse | 17 |
| Humoral and immune system effects | ||
| Increased concentrations of mast cells | Rat | 6 |
| Decreased concentrations of immune complexes in patients with rheumatoid arthritis | Human | 7 , 18 |
| “Systemic” effects (at sites distant from the treated areas) | Human, rat | 7 , 8 , 18 |
North American interest in these lasers has increased rapidly during the past several years. In particular, several investigative groups in the United States have reported on a variety of laser studies of analgesia
19
, 20
, 21
, 22
and wound healing.23
, 24
, 25
, 26
The results have been inconsistent but have shown substantial enough benefits to prompt US medical laser manufacturers to begin the clinical trials necessary for obtaining approval from the Food and Drug Administration.3
Background
In essence, a laser is a simple device, the theoretical basis of which was established by 1920. The delay in construction of the first operational laser until 1960 probably resulted from theoretical possibilities exceeding technologic capabilities. The first laser used an optically “pumped” (activated) ruby rod and could produce only a red light. Today, however, these devices can produce both fixed and variable frequencies at different wavelengths and virtually any power.
The transparency of biologic systems, as well as the structures and pigments within them, varies widely with the wavelength of incident irradiation. Thus, carbon dioxide laser irradiation (10.6 /μm) is rapidly absorbed in water, and this laser is widely used in surgical procedures because of its limited penetration. The argon laser produces irradiation (0.488 to 0.545 μm) that is rapidly absorbed by hemoglobin. This laser is frequently used in dermatology and ophthalmology. The neodymium:yttrium-aluminum-garnet laser has a wavelength (1.06 μm) that is relatively poorly absorbed by water and hemoglobin and can therefore penetrate deeply to allow tissue vaporization and deep thermal coagulation.
27
The infrared (0.830-μm) gallium-aluminum-arsenide and the visible (0.633-μm) helium-neon lasers are the most frequently used low-power lasers in the United States. The reason for this preference seems to be a combination of ease of use, broader experimental background, low cost, and availability. Although no compelling evidence is available, a survey of the literature tends to suggest that the 0.633-μm irradiation of the helium-neon laser may be more effective for wound healing and analgesia than other wavelengths.
In the United States, low-energy laser treatments usually consist of exposures of about 60 seconds and powers of approximately 1 mW. These energies do not produce appreciable changes in the temperatures of the tissues being treated.
5
, 11
, 15
Thus, any efficacy must be derived from a nonthermal mechanism based on interaction with, or absorption in, specific tissue substructures rather than the generalized heating of a tissue as a whole. Furthermore, if laser irradiation is postulated to be more effective than other light sources of the same frequency and intensity, this effectiveness should result from properties unique to lasers—that is, minimal beam divergence, coherence (wavelengths in phase), and monochromaticity (single wavelength). In practice, fiber-optics are often used to deliver laser irradiation to the treatment sites. These systems may diverge the beam at the fiber-air or fiber-tissue interfaces. Thus, the “narrow beam” capabilities of lasers are often not available, and only the characteristics of coherence and monochromaticity can contribute to the biologic effects. Although high-beam intensities and pure polarizations are produced by other sources, lasers produce beams that are more intense and polarized than are normally available.In Europe, somewhat higher laser powers are frequently used (for example, 10 to 70 mW). At these energies, temperature changes of as much as 0.3 to 0.62°C have been reported.
28
These changes, however, are minimal inasmuch as the conventional heating modalities of hot packs, ultrasound, and shortwave diathermy can produce changes of 5°C or more.29
Thus, even at the higher end of the low-power laser spectrum, any unique effects must be based on a nonthermal mechanism.Discussion
Low-Power Laser Therapy for Pain
Although the substantiation of results has varied greatly in detail and quality, many investigators have described successful treatment of a wide variety of painful musculoskeletal, rheumatologic, and neurologic conditions with low-energy lasers. For example, Walker,
20
Kroetlinger,30
and Calderhead and associates31
reported dramatic effects from laser treatment of osteoarthritis. Walker's study20
was controlled and involved 36 patients with chronic pain, of whom only 8 had osteoarthritis. She found that although the sham group showed no improvement, four of five treated patients with osteoarthritis experienced improvement and reported an average of “70%” relief. In contrast to Walker's study (which used direct helium-neon laser irradiation over peripheral nerves and joints), the studies reported by Kroetlinger30
and Calderhead and associates31
used an acupuncture approach in larger groups of patients (about 50) but without control subjects. Despite the differences in techniques and number of patients, all three investigations revealed that 70 to 80% of the treated patients reported substantial benefits. Similar results, although reported in other studies,32
, 33
are not invariably found; a double-blind crossover study21
of 66 patients with osteoarthritis showed no benefit from gallium-aluminum-arsenide laser irradiation of the affected knees.Several investigators have studied the effects of laser treatment of neck
1
, 30
and back1
, 30
, 31
, 34
pain. These studies were not controlled but involved relatively large patient groups (from about 30 to almost 300). Although the approaches varied, 70 to 80% of the patients reported appreciable or complete relief from pain. Some anecdotal reports1
, 2
, 31
have described patients with chronic, refractory pain who obtained rapid relief with laser treatment and were restored to an active and useful life. Although remarkable, these reports are anecdotal and can only highlight the findings of more detailed scientific studies.Several investigators
2
, 30
, 31
have also studied the potential benefits of low-energy laser treatments for tension and vascular headaches. Most of these investigations were done in conjunction with the aforementioned studies of neck and back pain. As was true in those studies, controls were poor or nonexistent, and from 70 to 80% of the patients obtained “excellent” or “good” relief.Goldman and associates
19
reported the results of a trial of laser treatment in a group of 30 patients with rheumatoid arthritis. The study had a double-blind design, in which only one hand of each patient was treated. Evaluation involved a physician's examination, a functional evaluation by an occupational therapist, serial roentgenograms of the hands, and numerous laboratory tests (complete blood cell counts, Westergren erythrocyte sedimentation rates, blood chemistries, antinuclear antibodies, rheumatoid factors, and a test of platelet aggregation by immune complexes). Although it is unclear whether the double-blind design of the study was maintained for all patients, the study was ambitious and apparently conducted carefully. At the conclusion, the investigators noted substantial diminution in joint warmth, pain, and erythema and improvement in hand function. Although improvement was bilateral, it was most pronounced in the hand that had been treated. Results of laboratory studies and x-ray findings were unchanged except for a decrease in platelet aggregation.The study by Goldman and associates
19
showed functional, physical, and laboratory changes after a course of laser treatment. It may be important, however, that those investigators used a Q-switched neodymium:yttrium-aluminum-garnet laser that delivered 15 to 25 joules/cm2 in a 30-ns pulse. Although the investigators thought that joint temperatures were not significantly elevated, these energies are much higher than those used in most low-energy experiments. Although it is not obvious whether these results can be extrapolated to those with use of low-energy lasers, Oyamada and Izu35
reported similar, encouraging results with use of 5- to 8-mW helium-neon lasers in patients with rheumatoid arthritis.Interesting published reports have described effective laser treatment of shoulder pain,
31
tendinitis,2
and periarthritis.31
In these studies, however, the numbers of patients involved have been small and no control subjects have been included.Laser therapy for neuropathic pain is another area of active investigation. In a double-blind study by Walker,
20
seven of nine patients with trigeminal neuralgia had an estimated 85% improvement, two of five patients with postherpetic neuralgia had 60% improvement, five of six patients with radiculopathy had 70% improvement, and the one patient with diabetes had approximately 80% improvement. Control patients noted no substantial improvement during the treatment period. Calderhead and associates31
and Kroetlinger30
reported similar results in less controlled studies.Other studies of peripheral nerve effects of laser treatment have been reported. One controlled study in which a helium-neon laser was used revealed a notable increase in the distal latencies of superficial radial nerves.
16
Other controlled studies in which gallium-aluminum-arsenide lasers were used, however, found no change in nerve conductions after treatment15
and no benefit in the treatment of carpal tunnel syndrome.22
Many studies of analgesia and wound healing have found a “systemic effect” from laser treatment—that is, pain and inflammation have decreased and healing has improved at sites distant from the treated areas.
7
, 8
, 18
, 19
These observations may be indirectly supported by Walker's report20
of increased excretion of urinary 5-hydroxy-indoleacetic acid (a product of metabolism of serotonin) and the finding by Goldman and associates19
of decreased platelet aggregation after laser treatment. Systemic findings are not invariably present; some investigators have found no such effect in wound-healing experiments in pigs24
, 26
or in perception of pain in the contralateral extremity after laser irradiation of the ulnar nerve.6
Low-Power Laser Treatment of Wounds
Most US physicians and physical therapists are unconvinced that low-energy laser irradiation accelerates wound healing. Nevertheless, laser treatment is available,
2
, 3
and a body of experimental evidence supports the procedure.Considerable improvements in wound healing (particularly in the early phases) after laser therapy have been reported in most studies of rabbits and rodents.
8
, 9
, 25
, 26
, 36
In studies of humans, some evaluations of wound healing of cervical erosions10
and a variety of skin ulcers7
, 30
, 37
have reported remarkable results. These studies, however, tend to be uncontrolled or incompletely described. In addition, other experiments that have involved swine with skin wounds have shown no benefit from laser treatment.23
, 24
Species-specific effects may be important. In particular, in pig and human skin, healing by wound contraction is much less important than it is in rodents and other loose-skinned animals. Other similarities between pig and human integument include skin thickness and turnover times.
23
, 38
, 39
Thus, perhaps species-specific effects are important, and the potentially nonresponding swine may prove to be a better model than rabbits and rodents for prediction of results in humans.It is important to remember that the literature on low-energy laser studies is uneven and disorganized. Future work may show that results now in apparent conflict are actually different aspects of the same problem. For example, it seems reasonable that various tissues with dissimilar absorption spectra could respond differently to diverse stimulating frequencies. In addition, discrepancies in energy dosages, treatment techniques (for example, irradiating a single point or sweeping), and treatment schedules may be important enough to complicate evaluation.
6
, 7
Side Effects
With the exception of prolonged, direct exposure of the eyes to the laser beam, the safety of these low-energy devices does not seem to be a major issue. Although the question of safety has been raised,
8
lasers with much higher energies have been used for many years, and the side effects have been remarkably absent.40
In addition, most40
, 41
but not all42
cell culture studies have shown minimal or no mutagenic effects.Conclusion
Research studies of the effects of low-energy laser irradiation on biologic function are growing in number and scope. Although many experiments show alleviation of pain, the quality of the investigations, the number of subjects, and the varied techniques frequently preclude statistical verification. Wound-healing experiments tend to show acceleration of healing, but these findings are often concentrated in the early phases of the healing process and are most pronounced in rodents and rabbits. The evidence in pigs and humans is, at best, unconvincing. Although some investigators have claimed to find “systemic” rather than simply “local” effects, many studies fail to show either local or systemic benefit. Currently, no universally accepted theory has explained the mechanism of either “laser analgesia” or “laser biostimulation.” Although a theoretical understanding is unnecessary to establish benefit, the lack of knowledge complicates the evaluation of conflicting results.
Even though numerous conditions have been treated with these devices, well-proven benefits at the milliwatt levels usually used in the United States have not been published. Currently, the wavelengths, dosage schedules, and appropriate conditions to be treated are not established. Thus, the question of whether these treatments offer hope, or are merely hype and hokum, is unanswered. Because of the large number of positive reports and the innocuous nature of the treatments, further clinical evaluation of laser therapy is warranted.
REFERENCES
- Stimulation therapy with laser beams (trans).Phys Med Rehabil. 1977; 18: 426-445
- Laser: a preliminary report on its use in physical therapy.Clin Manag Phys Ther. 1982; 2: 30-32
- Fact sheet: laser biostimulation. Division of Consumer Affairs, 1984
- The effect of laser radiation on wound healing and collagen synthesis.Stud Biophys. 1973; 35: 227-230
- Acetylcholine releasing effect of laser irradiation on Auerbach's plexus in guinea-pig ileum.J Neural Transm. 1977; 40: 305-308
- Animal experiments in light-induced woundhealing.Laser Basic Biomed Res. 1982; 22: 1-3
- The biostimulative effect of laserbeam.Laser Basic Biomed Res. 1982; 22: 4-7
- Effect of low-power density laser radiation on healing of open skin wounds in rats.Arch Surg. 1981; 116: 293-296
Lyons R, Abergel RP, Dwyer R, Uitto J: Biostimulation of wound healing by low energy laser (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 77
- Experimental investigation of photostimulation effect of low energy He-Ne laser radiation.Laser Basic Biomed Res. 1982; 22: 14-16
- Laser stimulation of nerve cells in Aplysia.Science. 1971; 171: 907-908
Rochkind S, Bartal A, Razon N, Nissan M, Schwarts M: The long term effect of He-Ne laser irradiation on reparative processes in peripheral nerve and denervated tissue in normal and crushed sciatic nerve in the rat (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 78
McKibbin LS, Cheng RS, Paraschak A, Sugerman G, Ratte Y: A preliminary report on the use of a helium-neon laser to cause collateral axon sprouting in denerved tissue (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 79
Schwartz M, Lavie V, Doron A, Rochkind S, Belkin M, Bartal A, Ben-Basat J: Indications that low energy He-Ne laser radiation prevents post-traumatic degeneration of adult rabbit optic nerve (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 78
- Effects of clinical infrared laser on superficial radial nerve conduction.Phys Ther. 1985; 65: 1184-1187
- The effect of helium-neon laser on latency of sensory nerve (abstract).Phys Ther. 1985; 65: 737
Trelles MA, Mayayo E: Experimental bone fracture consolidates faster by low power laser irradiation (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 80
- Investigative studies of laser technology in rheumatology and immunology.in: Goldman L The Biomedical Laser: Technology and Clinical Applications. Springer-Verlag, New York1981: 293-311
- Laser therapy of rheumatoid arthritis.Lasers Surg Med. 1980; 1: 93-101
- Relief from chronic pain by low power laser irradiation.Neurosci Lett. 1983; 43: 339-344
- Soft laser: a treatment for osteoarthritis of the knee? (abstract).Arch Phys Med Rehabil. 1985; 66: 553-554
- Effects of low power infra-red laser stimulation on carpal tunnel syndrome: a double-blind study (abstract).Arch Phys Med Rehabil. 1985; 66: 577
- Effects of low energy laser on wound healing in a porcine model.Lasers Surg Med. 1984; 3: 285-290
Basford JR, Hallman HO, Sheffield CG, Mackey GL: Comparison of the effects of cold quartz ultraviolet, low energy laser, and occlusion on wound healing in a swine model. Arch Phys Med Rehabil (in press)
- The stimulating effect of low power laser rays: experimental examinations in otorhinolaryngology.Rev Laryngol Otol Rhinol (Bord). 1981; 102: 531-533
- The effect of low energy (He-Ne) laser irradiation on healing dermal wounds in an animal model (abstract).Phys Ther. 1985; 65: 737
- Lasers in surgery and medicine.Phys Med Biol. 1984; 29: 619-641
Boussignac G, Vieilledent C, Geschwind H: Thermal effects of semi-conductor lasers in men (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 77
- Diathermy and superficial heat and cold therapy.in: Kottke FJ Stillwell GK Lehmann JF Krusen's Handbook of Physical Medicine and Rehabilitation. Third edition. WB Saunders Company, Philadelphia1982: 275-350
- On the use of the laser in acupuncture.Int J Acupuncture Electrother Res. 1980; 5: 297-311
- The Nd YAG and Ga Al As lasers: a comparative analysis in pain therapy.Laser Acupuncture. 1982; 21: 1-4
Abe Y, Nakajima M, Chinzei T, Mabuchi K, Motomura K, Fujimasa I, Atsumi K: Evaluation of the biological effect by a semiconductor laser irradiation (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 77
Kamikawa K: Studies on low power laser therapy of pain (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 79
Ohshiro T: Lumbago and thermography (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 79
Oyamada Y, Izu S: Application of low energy laser in chronic rheumatic arthritis and related rheumatoid diseases (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 80
- Effects of low-level energy lasers on the healing of full-thickness skin defects.Lasers Surg Med. 1983; 2: 267-274
Goujon C, Divol J, Moulin G: Preliminary results of midlaser treatment of chronic ulcerations of the legs (abstract). In Proceedings of the 6th Congress of the International Society for Laser Surgery and Medicine, 1985, p 78
- The skin.in: Swenson MJ Dukes' Physiology of Domestic Animals. Ninth edition. Cornell University Press, Ithaca, New York1977: 493-503
- Microscopic anatomy of the integument of adult swine.Am J Vet Res. 1966; 27: 765-772
- Study of carcinogenic effects of in vitro argon laser exposure of fibroblasts.Plast Reconstr Surg. 1983; 71: 92-97
- Carcinogenic potential of in vitro carbon dioxide laser exposure of fibroblast.Obstet Gynecol. 1983; 61: 493-496
- Cytogenic effects of argon laser irradiation on Chinese hamster cells.Radiat Res. 1983; 93: 598-608
Article Info
Identification
Copyright
© 1986 Mayo Foundation for Medical Education and Research. Published by Elsevier Inc. All rights reserved.
