Analysis of Depth of Ablation, Thermal Damage, Wound Healing, and Wound Contraction With Erbium YAG Laser in a Yorkshire Pig Model

November 2015 | Volume 14 | Issue 11 | Original Article | 1245 | Copyright © 2015

Salman M.S. Alsaad MD,a E.Victor Ross MD,b Wiley J.Smith MD,c and Damian P.DeRienzo MDd

aDermatology Department, college of medicine, King Saud University, Saudi Arabia
bCosmetic and Laser Dermatology, Scripps Clinic, San Diego, CA
cDepartment of Dermatology, Naval Medical Center, San Diego, CA
dDepartment of Pathology, Scripps Clinic, San Diego, CA

Abstract

BACKGROUND AND OBJECTIVES: The erbium YAG laser is commonly used for skin resurfacing. It is known that varying the pulse duration can influence residual thermal damage and wound healing. Our study used a porcine model to evaluate a broad range of settings in a comparison of depth of ablation, depth of residual thermal damage (RTD), and wound contraction employing both a full coverage and fractional hand piece with an erbium YAG laser.
MATERIALS AND METHODS: The laser delivered an ablative pulse followed by a heating pulse of variable duration using either the full coverage or fractional hand piece. Pulse durations for specific coagulation depths were selected based on existing heat transfer models. The bilateral flanks of a single Yorkshire pig were irradiated. There were 14 treatment groups. 3 sites were treated per group for a total of 42 sites. Two of the 3 sites were for observational assessments and the 3rd site served as a reservoir for biopsies. Biopsy specimens were collected on days 0, 1, 3, 7, 14, and 28. Bleeding, erythema, wound healing, and wound contraction (in the fractional hand piece groups) were assessed.
CONCLUSION: Wound healing is faster for fractional laser skin resurfacing compared with traditional contiguous resurfacing as demonstrated by textural changes and degree of erythema. The laser operator can be confident that the depth of ablation displayed on this system accurately reflects what is occurring in vivo for both confluent and fractional modes. Likewise, the measured degree of coagulation was consistent with panel display settings for the confluent mode. However, the degree of coagulation, as measured by the thickness of residual thermal damage, did not vary significantly between the fractional groups. In other words, the pulse duration of the second (heating) pulse did not impact the degree of coagulation in the fractional mode. There was a 2.3% wound contraction between some groups and a 6.5% wound contraction between other groups. A two way analysis of variance found a statistically significant difference in wound contraction based on ablation depth (P=0.012) but the degree of coagulation did not prove to be statistically significant for wound contraction (P=0.66).

J Drugs Dermatol. 2015;14(11):1245-1252.

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INTRODUCTION

The traditional short pulsed erbium YAG laser delivers 250-350 microseconds pulses, and with each pass the laser ablates 20 to 25 μm of tissue at 5 Joules/cm2 (about 3-4 μm per J/cm2).1,2 Residual thermal damage (RTD) has been shown to be only 10-30 μm after a series of passes in non-fractional (full-field) applications.2,3 Thus erbium YAG lasers provide greater depth control when compared to carbon dioxide lasers. However, the erbium laser’s ability to vaporize tissue with minimal residual heating can reduce hemostasis.4 Newer erbium YAG systems can generate longer pulse duration trains of up to 32 milliseconds. By using a pulse train, it is possible to achieve ablation while increasing the zone of RTD.

The use of fractional lasers has allowed for deeper skin injuries with decreased risks of adverse events. The goal of fractional laser treatment is to damage small areas of skin separated by large zones of un-irradiated tissue. The untreated tissue allows for rapid healing with decreased patient discomfort and shorter “down time”.5 We used a long pulsed erbium YAG laser utilizing both a full coverage and fractionated hand piece to evaluate the effects of ablation depth and pulse width (a.k.a. coagulation settings) on erythema, wound healing, and wound contraction.

METHODS

Animals

A single Yokshore pig (female gender and 4 months of age) was used in the study. The protocol was approved by the Institutional Animal Care and Use Committee of Naval Medical Center San Diego. Anesthesia was induced with a mixture

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