element, characterized by a specific wave- length and frequency. Photons of different wavelengths have different energy levels, which are organized into the “electromag- netic spectrum.” While this spectrum ranges from radio waves to gamma rays, for our purpose, the light used in laser therapy involves infrared, ultraviolet ra- diation and visible light. Specifically, LLLT has a therapeutic window between 630 nm and 1000 nm in wavelength. A common question is “How can rela- tively weak electromagnetic radiation promote cellular changes?” Certain cell functions are stimulated by laser, especial- ly those impaired by injury. Specifically, treated tissue has accumulated reactive ox- ygen species that influence ATP formation at a local level. Higher energy is not uti- lized because these levels of energy would become ionizing and damage the tissue. DEFINING THE PARAMETERS FOR LASER THERAPY Laser therapy remains a complex device to analyze, largely because it has a mul- titude of parameters that make it difficult to maintain homogeneity among research studies. However, it is still very important that LLLT parameters are well understood by health-care providers, as they greatly determine the laser’s effect on the condi- tion being treated. In general, laser light has two main properties that differentiate it from a conventional light source: its nar- row bandwidth and high degree of coher- ence. These parameters, among others, are defi ned and explained below: Output Power: This determines the rate at which a given dose is delivered, and is measured in watts (W) or milliwatts (mW). Higher output power allows the de- sired dose to be delivered in shorter time duration. Therefore, higher power settings reduce the patient’s treatment time, while also delivering a higher amount of energy at a greater depth. Power Density: This is the light output power emitted per unit area of tis- sue illuminated by the laser light and is measured in mW/cm² or W/cm². Dose: This represents the amount of energy delivered to a surface area of tissue, and is measured in Joules/cm². One Joule is equal to one Watt/second. Duty Cycle: This is the amount of time the laser is delivering a dose during a given period of time. Essentially, this is used to describe a pulsed light source, www.canadianchiropractor.ca Med-X: the probe, has three LDs with a total power of 200 mW, the cluster has 49 633 and 870 nm SLDs delivering a total of 500mW of power. which may vary between maximum (con- tinuous emission) and zero (light source off). ing beams that are almost parallel with one another. For example, if the duty cycle is 50 per cent, it indicates that the laser is on 50 per cent of the time. In relation to laser pulse, longer pulse duration causes an increase in the dose delivered to the patients, resulting in an increased duty cycle. Superluminous Diodes Versus Laser Diodes: When comparing and contrast- ing the various laser devices available on the market, there are mainly two types of diodes used, which are superluminous diodes (SLD) and laser diodes. Lasers are monochromatic – i.e., producing light at a single wavelength – and are coherent – i.e., their waves are in phase. In contrast, SLDs produce a band of wavelengths, which spreads the light over a broad treatment area. The laser is also collimated, produc- This collimation increases the risk of eye damage if the light shines into one’s eyes. Furthermore, the true laser has a smaller spot size of around 3 mm² while SLDs are approximately 20 mm². In re- gards to power density, laser diodes have a greater power density (800-4,000 W/cm²) than SLD diodes (50-75 mW/cm²). In summary, SLDs are selected for the treatment of a large surface area, as they are less powerful than laser diodes and rely upon a cluster of diodes to deliver a dose over a larger area. Laser diodes have high- er power output and less divergent beam, making them suited as “laser probes” for deeper penetration over areas of localized pathology. In terms of coherence, research studies have shown that light with the same wavelength, intensity, and irradiation time produce the same biological effects, CANADIAN CHIROPRACTOR | FEBRUARY 2008 • 17
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