title: The effects of multiple passes on the epidermal thermal damage pattern in nonablative fractional resurfacing.
abstract: OBJECTIVE
  Nonablative fractional resurfacing is a concept of cutaneous re-modeling whereby laser-induced microscopic treatment zones (MTZs) are surrounded by normal viable tissue. Such thermal damage pattern with a small diameter of individual lesions allows fast re-epithelialization with minimal side effects. The purpose of this in vitro study was to determine the fraction of thermal injury per unit surface area (fill factor) and lesion size in relation to pulse energy and number of passes.
  
  
  METHODS
  Full thickness abdominal skin samples were exposed ex vivo to the Fraxel SR 750 laser (Reliant Technologies, Mountain View, CA). One set of exposures was performed for pulse energies in the range of 8 to 40 mJ for a single pass at 250 MTZ/cm(2). A second set of exposures was performed at 10 mJ with number of passes from 1 to 30. The thermal damage pattern was assessed by incubation of epidermal sheets with NitroBlueTetrazoliumChloride (NBTC) stain. Size of individual MTZ and fill factor were determined by image analysis (ImageJ, NIH, Bethesda, MD) of digital micrographs.
  
  
  RESULTS
  Width of the thermal injury zone was directly related to the pulse energy used. The fill factor did not have a uniform relationship with the number of passes. Due to the stochastic placement of individual MTZs, even for greater number of passes, some residual undamaged tissue was found. Due to formation of thermal damage clusters, defined as overlapping individual MTZs, the size of the resulting clustering lesions which we defined as microscopic treatment cluster (MTC) increased linearly as a function of the number of passes.
  
  
  CONCLUSIONS
  We have described the fill factor as it relates to the number of passes and have demonstrated that the average size of individual lesions depends on the number of passes. Clustering of MTZs lead to the development of MTC, the average size of which increased with the number of passes. The clinical implications of these findings are contingent on further studies.
authors.affiliation: Wellman Center for Photomedicine, Department of Dermatology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114, USA. dmanstein@partners.org
authors.forename: Dieter
authors.initials: D
authors.lastname: Manstein
authors.forename: David
authors.initials: D
authors.lastname: Zurakowski
authors.forename: Siremon
authors.initials: S
authors.lastname: Thongsima
authors.forename: Hans
authors.initials: H
authors.lastname: Laubach
authors.forename: Henry H
authors.initials: HH
authors.lastname: Chan
mesh_terms: D055592:Biophysical Phenomena; D004817:Epidermis / Q000293:injuries*; D006801:Humans; D066298:In Vitro Techniques; D053685:Laser Therapy / Q000009:adverse effects* / Q000379:methods*