Basics of Lasers in Dermatology

Selective thermolysis is critical for understanding modern laser use. To fully comprehend its meaning, one must first understand that the interaction of a laser with the skin depends on a number of factors, including the power density of the laser itself (the beam characteristics), the length of time of exposure, and the physical properties of the targeted molecules. Although there is some modulation of laser function via the wavelength of the laser (eg, higher wavelengths penetrate deeper), the properties of the target molecule can allow for precise control of the laser’s action. The framework for understanding this principle was outlined by Anderson and Parrish¹¹ in 1983. Fundamentally, a laser causes damage to a target molecule via application of large amounts of energy; however, the laser beam does not discriminate between different molecules in its path. Rather the size and other properties of the molecule play a critical role in determining the amount of energy it is able to absorb before dissipating the excess energy as heat. This excess heat energy is what causes damage to surrounding tissues, or collateral damage. Conceptually, being able to target a molecule without damaging surrounding tissues is our goal as practitioners when using lasers in dermatology. It is accomplished by heating a molecule to just under its thermal relaxation time (ie, the time needed for a molecule to dissipate half of the energy applied), thus allowing for acceptable results with regard to efficacy balanced with side effects.¹²

Conclusion

Lasers are an important treatment modality, and their use in dermatology is becoming widespread for many possible indications; however, lasers are complex mechanical devices that have the potential to cause great harm when used incorrectly. By gaining a thorough understanding of the basic physics of lasers, the different types of lasers that are available, and critical concepts regarding the cutaneous application of lasers, physicians can better understand these devices and approach their use confidently and safely.