Characteristics of Non‐Ablative Resurfacing of Soft Tissues by Repetitive Er:YAG Laser Pulse Irradiation

Abstract

Background and Objectives Recently, several minimally invasive gynecological, ENT and esthetic procedures have been introduced that are based on delivering “smooth” sequences of Er:YAG laser pulses to cutaneous or mucosal tissue at moderate cumulative fluences that are not only below the ablation threshold but typically also do not require local anesthesia. To explain the observed clinical results using “smooth‐resurfacing,” it has been suggested that in addition to the direct heat injury to deeper‐lying connective tissues, there is an additional mechanism based on indirect triggering of tissue regeneration through short‐exposure, intense heat shocking of epithelia. The goal of this study is to improve understanding of the complex dynamics of the exposure of tissues to a series of short Er:YAG laser pulses, during which the thermal exposure times transition from extremely short to long durations. Study Design/Materials and Methods A physical model of laser‐tissue interaction was used to calculate the temperature evolution at the irradiated surface and deeper within the tissue, in combination with a chemical model of tissue response based on the recently introduced variable heat shock (VHS) model, which assumes that the tissue damage represents a combined effect of two limiting Arrhenius′ processes, defining cell viability at extremely long and short exposure times. Superficial tissue temperature evolution was measured during smooth‐resurfacing of cutaneous and mucosal tissue, and compared with the model. Two modalities of non‐ablative resurfacing were explored: a standard “sub‐resurfacing” modality with cumulative fluences near the ablation threshold, and the “smooth‐resurfacing” modality with fluences below the patient′s pain threshold. An exemplary skin tightening clinical situation was explored by measuring pain tolerance threshold fluences for treatments on abdominal skin with and without topical anesthesia. The obtained temperature data and pain thresholds were then used to study the influence of Er:YAG laser sequence parameters on the superficial (triggering) and deep (coagulative) tissue response. Results The simulations show that for the sub‐resurfacing modality, the parameter range where no excessive damage to the tissue will occur is very narrow. On the other hand, using pain tolerance as an indicator, the smooth‐resurfacing treatments can be performed more safely and without sacrificing the treatment efficacy. Two preferred smooth‐resurfacing treatment modalities were identified. One involves using optimally long pulse sequence durations (≈1–3 seconds) with an optimal number of pulses ( N ≈ 10–30), resulting in a maximal short‐exposure superficial tissue response and moderate coagulation depths. And for deeper coagulation, without significant superficial heat shocking, very long pulse sequences (>5 seconds) with a large number of delivered pulses are to be used in combination with topical anesthesia...

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“…These fast shocks initiate the wound healing process, as tissue cells recognize them as wounds producing ones, although - being short enough, they do not produce any injuries. Effects of such wound healing process were detected even 2 mm deep in the mucosa [ 29 , 30 ].…”

Influence of tissue desiccation on critical temperature for thermal damage during Er:YAG laser skin treatments

“…Studies of tissue damage dynamics have shown that when considering extremely short and long exposure times, cell viability cannot be described by a single biochemical process of Equation (1). 26 At extremely short exposure times (commonly present during Er:YAG laser treatments), 18,[27][28][29] the critical temperature exhibits a shift to temperatures, which are much higher than what would be expected from a single biochemical process characterizing the damage dynamics at long exposure times. 25 In this study, the variable heat shock (VHS) tissue response model will be applied, 26 which takes into account the observed deviation by describing the cell viability as a combined effect of two biochemical processes that dominate cell survival characteristics at very short and very long exposure times (see Figure 1).…”

“…Pulsed mid-infrared Er:YAG laser is by now an accepted tool for performing ablative [1][2][3] as well as nonablative medical procedures. [4][5][6][7][8] This is because the Er:YAG laser wavelength (λ = 2.94 μm) coincides with the major absorption peak of the water molecule, and is thus highly absorbed in virtually all biological tissues. 9,10 There are three basic phases in tissue heating upon exposure to Er:YAG laser irradiation.…”

“…3 The tissue is first heated directly within the optical absorption depth δ (direct heating). The Er:YAG wavelength is very weakly absorbed in dry tissue, 11 and therefore the optical penetration depth of Er:YAG depends predominantly on the water content of the treated tissue, with the penetration depth in pure water being equal to about δ = 0.8 μm. 9,10 Very roughly, water is present in body tissues as both free and bound water.…”

“…Micro-explosive removal of tissue starts when the pressure exceeds the ultimate tensile strength of the tissue, which for soft tissue occurs at water vapor temperatures of about 250°C. 15,17,18 The above process describes what happens with the bound water. On the other hand, the free water can evaporate from the tissue sooner during the heating phase, that is, before the tissue reaches the ablation temperature.…”