The parameters that influence the extent of fractional injury include wavelength, fluence, spot size, surface density of the microscopic thermal injuries, delivery method of the microbeams (i. e. scanned, stamped), and cooling capabilities. Other important procedural considerations include the number of treatments, interval between treatments, number of passes, interval between passes, and the need for topical applications and anesthetics [14].
Wavelengths between 1400-1600 nm, in the near to mid-infrared range, are ideal for coag – ulative fractional resurfacing because they can penetrate deep within the dermis [15]. Due to their high water absorption, lasers with wavelengths of 2940 and10600 nm are used for ablative fractional resurfacing. These devices can remove or ablate a column of tissue with a small rim of coagulated tissue surrounding it. This type of injury can lead to a slightly longer healing time than mid-infrared lasers, but studies suggest a greater clinical response.
It is important to note that the wavelength is not the only parameter that is important in determining the width and depth of the tissue injury. Fluence levels of each microbeam also affect the depth of penetration, as well as the nature of the beam itself and how it is delivered. There are fractional lasers that produce a single, focused beam that is pulsed in temporal succession as it scans across the surface (Fig. 11.1a), or lasers that produce multiple, focused, micro-beams emitted simultaneously in a stamping fashion (Fig. 11.1b). The fractional
damage pattern and surface coverage area is therefore defined by the scanning or stamping characteristics of each device [14].
Consideration should also be given to the diameter of the thermal injury columns, which should be kept small enough to allow for easy migration of keratinocytes and other biological cell mediators from the surrounding tissues [10,16]. These dimensions have not been specifically defined, but studies have found that the distance between microscopic zone centers should be greater than 125 pm to avoid bulk tissue damage and significant side effects [9].
Cooling may also affect the dimensions of the microscopic columns. Integrated contact cooling devices or an external cooling device, such as the SynerCool™ (Syneron Medical, LTD) or Zimmer Cryo 5® (Zimmer Medizin Systems), are used concurrently during treatment, and have been shown to increase patient comfort to a great extent [17]. However, cooling may also significantly decrease the width of the thermal damage zones with less impact on depth [18,19]. When the skin temperature is decreased from body temperature to 20°C, there is a corresponding 40% decrease in the MTZ area at parameters tested (1550 nm, 10 mJ, 250 MTZ/cm2) [19]. How this affects the clinical outcome is yet to be determined.
Certain parameters, such as the number of passes completed, depend on the density of the microscopic thermal injuries per pass. With this consideration in mind, there should be sufficient passes to allow for approximately 10-90% of the treatment surface area to be covered. Therefore, patients may need more than one treatment to cover most of the skin surface. The time between successive passes should be > 20 s to allow for excess heat to dissipate, and minimize bulk thermal injury [20].
It is also important for individual treatments to be spaced far enough apart to allow for the resolution of clinical swelling and flaking. For most mid-infrared fractional devices, this time is approximately 2-4 weeks.