The term laser stands for light amplification stimulated by emission of radiation.
The term laser stands for light amplification stimulated by emission of radiation. The energy emitted from a laser has a fixed wavelength and is very coherent, that is, each photon emitted is in the same phase. This makes laser light different from the light emitted by a light bulb, for example, and the energy produced can be collimated to cut tissue as with surgical lasers or the beam can be made wide enough to not cut but warm the tissues, as with therapeutic lasers.
There are several types of therapeutic lasers including defocused CO2, defocused Ruby, HeNe, InGaAIP, GaAIA, and GaA. Each has a different mechanism of energy storage and wavelength of laser light emitted. The most commonly used veterinary therapeutic laser is the GaAIA, or Gallium-aluminium-arsenide laser. The wavelength used is 800-830 nm which is invisible; however, most units have a visible light in the probe to visualize what tissue is undergoing biostimulation.
All lasers have classification for safety, including their ability to cause retinal damage. Lasers capable of damaging the eye have a parallel light beam of small diameter and have higher (increased wattage) power settings. Radiation from the laser above 1400 nm is absorbed by the lens and above 2500 nm is absorbed by the cornea, and therefore does not reach the retina. Lasers in the visible light range such as laser pointers cause a blink reflex thus also preventing a long exposure time to the eye, whereas lasers in the invisible light range are inherently more dangerous.
Laser pointers are also far too low powered to cause eye damage. Five classes of lasers have been established with classes 1-3A considered safe for eyes while 3B carries some risk of eye injury and class 4 definitely causes eye injury. Class 4 lasers are used in manufacturing, surgical cutting and now in therapeutic biostimulation. Class 4 lasers produce light in the 800-1400 nm range which focus on the retina and therefore cause damage there.
Hair and pigmented skin absorb more of the radiation from the laser and therefore using the laser in these areas can cause burned hair or skin, especially if the laser is left focused on one area for an increased amount of time, such as more than one second.
Why use a class 4 laser for therapeutic applications when you certainly do not what to cut the tissue? For better penetration to deeper tissues. With a 60 to 100-mWatt class 4 laser, you can treat tissues 2 to 3 cm deep to the surface of the skin. This allows for treatment of acryl lick granulomas to coxofemoral osteoarthritis.
Mechanism of action
The primary mechanism of lasers is through the interaction of photons with cells, proteins, cell membranes and DNA/RNA. Porphyrins react with polarized light generated in the tissue from the laser and the cytochromes in mitochondria react to form ATP as well as reactive oxygen species. The Cytochrome C Oxidase enzyme in the mitochondrial chain becomes more efficient at moving an electron down the chain and synthesizing ATP. By changing the Redox state of the cell to oxidation, more ATP is generated and the pH of the cell increases. In addition, heat is generated in areas where polarized light is generated, causing changes in microcirculation and Na+ and K+ gradients. Finally, the laser creates an environment that favors cellular repair. This may be due to changing the charge or electrical gradient of cellular membranes and making them more permeable.
In addition, hemoglobin in red blood cells accepts photons from the laser when it is bound to oxygen resulting in a change in the redox state of the molecule and inducing oxygen release. In that way, when the oxygenated blood reaches the lasered tissues oxygen is released more efficiently to the tissue. Warming of the tissue also occurs with laser therapy which results in vasodilation and increased circulation to the area.
The secondary mechanism of action is the resulting cellular and tissue responses following interaction with photons and polarized light. Cellular production of DNA and RNA synthesis increases as well as ATP, and up to 110 genes are regulated by laser irradiation. Most of these genetic effects work to increase cell proliferation and decrease apoptosis. Both the p38 mitogen activated protein kinase signaling pathway and platelet derived growth factor signaling pathway are upregulated by therapeutic (low level) laser therapy. In addition, tissues are in various stages of electrical stimulation are capable of acting as a laser themselves, that is they can generate a photon as the laser photon hits the cell and thus produce deeper tissue biostimulation than the original photon from the laser itself is capable of producing.
It is important to note that 10% of animals and humans are resistant to laser therapy.
Uses of therapeutic lasers
GaAiA lasers can inhibit pain sensation by blocking c-fiber transmission in damaged tissue. Lasers also increase nitric oxide locally, which also inhibits pain sensation. The temperature of tissue is increased by laser therapy and is due in part to an increase in blood flow. Temperatures rise 0.4-1°C in the tissues for up to 30 minutes following cessation of laser therapy.
Lasers also stimulate immunity by activating lymphocytes, mast cells and macrophages in tissue and whole blood so that even wounds in another part of the body that was not treated respond with faster healing times.
Healing is also stimulated by an increase in procollagen synthesis by fibroblasts and cellular replication of endothelial cells and keratinocytes.
By increasing cell membrane permeability, there is an increase in seratonin and superoxide dismutase levels which accelerates the inflammatory process. This can be beneficial or detrimental and somewhat controlled by the duration, timing and wavelength of low level laser therapy used.
Horses are more sensitive to the effects of laser therapy and the laser can be used to identify areas of pain/inflammation. Horses are also more sensitive to pulsed frequency laser therapy and slower pulses, lower frequencies may be necessary. In addition, laser therapy may make a lame horse sound but that is through alleviation of pain and the primary injury still exists, therefore do not allow the horse to work until the injury has resolved. Defocused CO2 laser therapy has been used successfully in acute traumatic synovitis in horses.
Both horses and dogs have coats that can be impenetrable to light, and while you can part the hair and work in areas, I require my small animal patients have the area clipped and free of hair.
Laser therapy has been used successfully by veterinarians for acral lick dermatitis, chronic degenerative joint and disk disease, wounds- surgical or septic, tendon, ligament and muscle injuries (although laser therapy has not been proven effective in horses with tendonitis), inflammation and edema from blunt trauma, and even epilepsy in dogs. Do not use a laser that has a pulsed visible light on epileptics as this may cause a seizure.
Current investigations into the use of low level laser therapy include bone and meniscal healing, and wound healing times in blinded, placebo controlled experiments in dogs. Our preliminary findings have shown that dogs receiving a single laser therapy treatment prior to TPLO surgery for cruciate ligament repair had improved outcome following surgery with reduced lameness and increased bone healing 8 weeks postoperatively.
Laser therapy has been shown to speed fracture healing, improve tendon repair, and induce chronic wound healing in dogs.
Too much of a good thing
The thyroid gland should not be irradiated with a therapeutic laser since hypothyroidism can result.
Treatment of neoplasia with a therapeutic laser is not recommended, nor is treating a patient with a therapeutic laser that has a malignancy distant to the site of therapy. Current research is ongoing in the use of lasers with chemotherapy to enhance the effects of chemotherapy but much information about the mechanisms still has to be learned. In the future, laser therapy may be used to dilate the vasculature to the tumor so that intravenous chemotherapeutics can attain higher doses in the parenchyma of the tumor with lower overall systemic doses given. Laser therapy has not been documented to cause neoplasia.
While reactive oxygen species are required for mitochondrial function, over use of therapeutic lasers can increase reactive oxygen species to a level greater than the mitochondria and cellular enzymes such as superoxide dismutase and catalase can handle resulting in depletion of ATP, influx of Ca++ into the cell and disruption of cellular processes. This could result in delayed healing.
With treatment many patients will feel tired or even painful the day after, these symptoms are not evidence of an overdose and are most likely the result of stimulation of the tissues locally and systemically.
Dosage and wavelength
Laser therapy is governed by the arndt-Schulz law in that weak stimulation with the laser increases cellular activity and strong stimulation inhibits it. With laser therapy the longer the laser is on the area being treated the more likely you are to over stimulate the tissue and retard physiological activity. The optimum dose for open wounds is thought to be 0.5-1.0 J/cm2 and for tissues with overlying skin 2.0-4.0 J/cm2. However, there is some individual variation with each patient and one should start with a low dosage to prevent inhibition of cellular activity.
Another complication is the power of the laser. More powerful (watts) lasers will give a better response and therefore, also must be used for shorter periods of time. Most therapy lasers come with standard protocols for veterinary, small animal and equine uses such as open wounds, osteoarthritis, and bone healing.
For the GaAIAs laser unit the following formula can be used to determine the time of treatment required: t=DxA/P x (1+d)
D is the dose wanted in J/cm2, A is the area treated in cm2, P is the power in watts, and d is the depth of 1 to 4 cm. One also must remember that deep tissues can only be reached and the outer layers penetrated by using infrared (wavelength) laser beams. Most therapy lasers use red (wavelength 635-650 nm) or infrared (790-980 nm), however the deepest penetration is achieved with the infrared wavelength. Higher wavelengths are highly absorbed and cause water to vaporize within cells, thus surgical cutting lasers are in these higher wavelengths. Another consideration is what effect does the laser therapist want to achieve? Pain relief requires higher doses than, for example, wound healing. By taking these points into consideration the formula above is still a guideline and not an absolute maximum or minimum dose to use.
There are no definitively defined intervals to use between doses administered but moderate doses 3 to 4 times per week are more effective than higher doses in a shorter time period. As a general guideline, acute conditions including postoperative inflammation and pain can be treated with fewer doses in closer time intervals, whereas chronic conditions respond more to treatments that are spread out by one to several days.
Generally we start out at 3 treatments per week, then go to 2 treatments per week and finally one treatment for one to several weeks. In some cases, following no response with 4 to 5 treatments, discontinuing therapy for several weeks and then beginning again can gain a response. In some cases laser therapy prior to the trauma, for example surgery, can be just as beneficial as performing laser therapy after the insult. Investigations studying the effects of lasers used preoperatively are being conducted.
Lasers are often used in the first 48 hours postoperatively to open calcium channels, increase cellular proliferation, and stimulate wound healing. Macrophages will increase their release of growth factors when stimulated with a therapeutic laser. Dosages used in small animals for acute injuries (less than 72 hours old) are usually less than 2 J/cm2, whereas subacute injuries often receive 3 to 4J/cm2. Tendonitis, osteoarthritis and muscles spasms can be administered higher doses of 5 to 8 J/cm2.
In some investigations administration of antioxidants inhibits or can reverse the beneficial effects of laser therapy. In contrast, antioxidants can be used if overdose of laser therapy is suspected.
Laser therapy in veterinary medicine is still in its infancy, despite the many investigations that have been performed on laboratory animals and clinical patients. More blinded, placebo controlled clinical trials need to be performed before advocating its use routinely in practice.
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