The main characteristic of the laser’s electromagnetic waves is the wavelength, which is measured in nanometers. For clinical purposes, a second characteristic of lasers that is very relevant is the average power, which is measured in watts.
As with many other types of waves, laser energy can be reflected, transmitted, absorbed, scattered or refracted. A chemical reaction occurs only when cells absorb light. When absorbed by cells, laser light converts into either heat or biochemical energy. Different wavelengths affect the conversion in different proportions. For example, a wavelength such as 1,064 nm will interact with soft tissues to optimally convert into heat for an ablative effect. Another wavelength such as 2,400 nm will interact more effectively with bone. The amount of light energy that converts into biochemical energy is minimal, which ensures maximum ablative efficiency.
Most human tissues are poor absorbers of lasers with wavelengths of 600 nm and 1000 nm, which would produce less conversion to heat and would allow for deeper tissue penetration. As all of the therapeutic lasers in the market today have wavelengths in the therapeutic window (between 600 and 1,000 nm), they all meet the first criterion to be able to deliver light energy into the tissue. It is for these reasons that the Nexus line of lasers have either 810 nm or 980 nm wavelengths.
The power of the laser device is the second factor for effective delivery of light energy into the target tissue for absorption. A laser device could have the appropriate wavelength and still be unable to drive the light energy to the tissue that needs treatment. This is not unlike standard radiography equipment. Therapeutic lasers need to have the appropriate wavelength and power to produce the desired therapeutic effects in the tissue one is treating.
Numerous researchers have stated that therapeutic lasers do not provide positive clinical effects and provide no negative side effects.2,3 By analyzing the articles that reported no clinical effects, one easily finds a pattern: most of these researchers in these studies used low doses in their clinical trials and this was usually due to using a low-powered laser instrument.
The dose in laser therapy is the amount of light energy, measured in joules, delivered to a given unit area during a treatment session. Simply stated, 1 J of energy is delivered by a 1-W laser emitter for one second or other combinations of the two parameters laser power (in watts) and time (in seconds). Therefore, energy density is the energy per cm2 (J/cm2).
Power density is the amount of power (watts) delivered to 1 cm2 of tissue area. One determines this by the size of the treatment applicator and the emitted power. One can conclude that the larger the applicator, the lower the power density because the treated area is larger. Likewise, the lower the average power of the device, the lower the power density because the beam is not as intense. The same results are present in lasers with multiple diodes with the same average power. The power density of a laser with multiple diodes is lower than lasers with a single diode. Research has determined that power density plays a major role in the therapeutic process.4
Tuner and Hode demonstrated that the optimum dose necessary to obtain therapeutic effects at the treated tissue should be at least 4 J per cm2.5 To estimate the energy reaching the target tissue, one must consider the depth of the treated area and the composition of the layers of tissues between the laser applicator and the treated tissue.
A typical laser device in the United States emits approximately 7 milliwatts of power using a 635-nm laser diode (red light). As a comparison, a laser pointer commonly used for presentations typically emits 3 to 5 milliwatts of power in the 660-nm range. On a per-milliwatt basis, the cost comparison between them is staggering. In terms of energy density, the same typical therapeutic laser, as reported by the manufacturer, delivers 0.0002 to 0.0003 J per minute/cm2. As an illustrative example, to deliver the minimum necessary energy at a skin target tissue (no tissue penetration needed) to obtain therapeutic value would take approximately 2,500 minutes.