Lately, several minimally invasive gynecological, ENT, and aesthetic procedures have been introduced that focus on providing “smooth” sequences of Er: YAG laser pulses to cutaneous or mucosal tissue at moderate cumulative fluences that are below the ablation threshold and also typically do not need local anesthesia. To illustrate the observed clinical results using “smooth‐resurfacing”, it has been proposed that in addition to the direct heat injury to deep-seated connective tissues, there is an additional mechanism based on the indirect triggering of tissue regeneration through short‐exposure, intense heat shocking of epithelia. 

A recent study explained the complex dynamics of the exposure of tissues to a series of short Er: YAG laser pulses, during which the thermal exposure timed transition from extremely short to long durations.

A physical model of laser‐tissue interaction was employed to estimate the temperature evolution at the irradiated surface and deeper within the tissue, in combination with a chemical model of tissue response relying on the newly introduced variable heat shock (VHS) model, which believes that the tissue damage denotes a blended effect of two limiting Arrhenius′ processes, defining cell viability at extremely long and short exposure times. 

Superficial tissue temperature growth was calculated during smooth‐resurfacing of cutaneous and mucosal tissue and compared with the model. Two modalities of nonablative resurfacing were investigated: a standard “sub‐resurfacing” modality with cumulative fluences around the ablation threshold, and the “smooth‐resurfacing” modality with fluences lesser than the patient′s pain threshold. A remarkable skin tightening clinical situation was investigated by calculating pain tolerance threshold fluences for therapies on abdominal skin with and without topical anesthesia. The acquired temperature data and pain thresholds were then used to examine the influence of Er: YAG laser sequence parameters on the superficial (triggering) and deep (coagulative) tissue responses.

The simulations showed that for the sub‐resurfacing modality, the parameter range where no extreme damage to the tissue will occur is very limited. While utilizing pain tolerance as an indicator, the smooth‐resurfacing treatments can be conducted more safely and without affecting the treatment efficacy. Two favored smooth‐resurfacing treatment modalities were determined. One involves the use of optimally long pulse sequence durations (≈1-3 seconds) with an optimal number of pulses (N ≈ 10-30), causing a maximal short‐exposure superficial tissue reaction and moderate coagulation depths. For deeper coagulation, without marked superficial heat shocking, very long pulse sequences (>5 seconds) with a large number of delivered must be used along with topical anesthesia.

Comparing the simulations with the documented smooth‐resurfacing clinical protocols in gynecology, ENT and esthetics indicate that the clinical protocols have been optimized for the maximal superficial heat shock triggering effect. Further research is required to achieve a more satisfactory understanding of the proposed role of heat shock triggering in the clinically observed regeneration of cutaneous, vaginal, and oral tissues following Er: YAG laser smooth‐resurfacing. 

Source: Lukac M, Zorman A, Lukac N, et al. Characteristics of non-ablative resurfacing of soft tissues by repetitive Er: YAG laser pulse irradiation. Lasers Surg Med. 2021;53(9):1266-78.