The first medical lasers to be developed were continuous wave lasers that produced a continuous beam of radiation that was subsequently absorbed by a target. Although this constant laser light could effectively treat certain dermatologic conditions, its use was limited by the fact that the laser energy not only altered the target but also “spilled over” into adjacent tissues, causing unwanted collateral damage and scarring. As our understanding of the interplay between living tissue and laser physics evolved, however, so did our ability to restrict laser damage to a specific target. The concept of selective photothermolysis developed by Anderson and Parrish in 1983 gave us the tools necessary to be more precise and safer with laser energy.

Selective photothermolysis states that a specific chromophore or target can be selectively destroyed with minimal collateral thermal tissue damage if the laser wavelength matches that absorbed by the chromophore, and if the target is exposed to the laser energy for an interval less than its thermal relaxation time. The thermal relaxation time is the time it takes a given target chromophore to lose 50% of its absorbed heat energy.

Selective photothermolysis revolutionized laser technology and paved the way for a new generation of lasers that are designed to deliver a set wavelength for a precise duration, resulting in greater specificity and safety. The pulsed, quality Q-switched, and scanned systems are examples of such laser technology. Other so-called quasi-continuous laser systems also attempt to adhere to the theory of selective photothermolysis by limiting pulse durations from a continuous beam source through shuttering or chopping of the emitted laser beam. The usefulness of these systems is often limited owing to their high repetition rates or moderately long pulse durations, causing the target to experience the laser’s energy as if it were a continuous wave.

Lasers emit a coherent and monochromatic light beam, whereas pulsed lights produce a polychromatic light whose bandwidth is selected by adapted filters. The skin’s chromophores are made up of water, hemoglobin, and melanin, to which must be added the exogenous pigments of tattoos. Each chromophore has its specific absorption spectrum. Lasers’ main mechanisms of action are the photothermal effect and the photomechanical effect.

With the previously mentioned concepts in mind, the side-effect profile of a specific laser can be predicted in general terms, based on its wavelength and mode of operation. As a group, continuous wave and quasi-continuous lasers have a higher risk of scarring and textural changes through thermal buildup and heat diffusion to normal skin structures. Lasers designed on the theory of selective photothermolysis are more specific and have a lower risk profile.

Depending on the wavelength and pulse durations delivered, pigmentary changes, epidermal cell injury, textural changes, and crusting and tissue splatter can potentially occur. It is important to remember that even the safest lasers can cause injury if used incorrectly. Repetitive or overlapping pulses, excessive energy or power settings, and improper patient selection can potentially result in a high rate of morbidity with the use of any medical laser.

Complications might be encountered with any currently available laser systems, however, today’s laser technology is universally accepted as very safe for the patient.