To date, there is no agreed-upon definition for fractional resurfacing. For the purpose of this discussion, we will define this as the destruction or removal of a fraction of the skin, including the full thickness of the epidermis and portions of the dermis, where the depth of the injury is greater than the width and ratios of treated to nontreated tissue >10% and <90%. The principle behind fractional photothermolysis is the formation of isolated and microscopic thermal wounds that are surrounded by zones of spared, viable tissue in a geometrical pattern that does not depend on chromophore distribution [9]. Fractional photothermolysis is distinct from selective photothermolysis, which was first described in the early 1980s. With selective photothermolysis, certain wavelengths are chosen that are specifically absorbed by target structures. The laser energy is then converted into heat, destroying these target tissues, while leaving the surrounding tissues undisturbed.
At mid-infrared wavelengths, initially chosen for nonablative fractional photothermolysis, the target chromophore is water, which is uniformly distributed throughout the tissue.
Therefore, damage is confined by using an optically focused beam, which allows microscopic columns of thermal damage to be created in the epidermis and dermis. By keeping the beam tightly focused, high local irradiance is achieved while minimizing the dispersion of energy to surrounding tissues to avoid bulk thermal damage.
Both selective photothermolysis and fractional photothermolysis produce an adjustable three-dimensional microscopic thermal injury with the width and depth of the lesion dependent upon the parameters selected [9]. In fractional photothermolysis, using an infrared wavelength, these microscopic tissue injuries are also referred to as microscopic thermal zones or MTZs. The MTZs are columns of coagulated tissue that extend from the epidermis down to the mid-dermis with sections of noncoagulated viable tissue between the columns. Spared interlesional tissue with fractional treatment of the epidermis appears to stimulate prompt re-epithelialization of damaged tissue [10,11]. With pan-surface ablative procedures, re-epithelialization is prolonged because there is no viable epidermis remaining to participate in the wound-healing process [10,12]. In contrast, complete protection of the entire epidermis in traditional nonablative resurfacing precludes rapid epidermal turnover, reducing the efficacy of the treatment [10]. Fractional photothermolysis affords an excellent compromise by treating a part of the epidermis while allowing spared portions to contribute to the healing process. The stratum corneum also remains intact, following nonablative fractional treatment. This allows the skin to maintain its barrier function to defend against microbial infection [10,11] and minimizes the risk of other potential side effects, such as oozing and crusting. Therefore, the skin also appears visually intact to the human eye due to the preserved stratum corneum and undetectable microscopic injuries [13].
Since the introduction of the first infrared wavelength fractional systems, there have been a series of other “fractional” systems using a variety of parameters. In light of this, it is important to further define fractional resurfacing in terms of the extent of tissue injury, which is dependent upon multiple parameters of the fractional device.