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

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. CONCLUSIONS: A comparison of the simulations with the established smooth-resurfacing clinical protocols in gynecology, ENT, and esthetics suggests that, through clinical experience, the clinical protocols have been optimized for the maximal superficial heat shock triggering effect. Further research is needed to gain a better 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. Lasers Surg. Med. © 2021 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.

Variable heat shock response model for medical laser procedures.

According to the standard Arrhenius relation, tissue damage is linearly dependent on the duration of exposure to elevated temperatures and exponentially dependent on the temperature itself. However, recently published measurements of damage threshold temperatures at extremely short exposure times (commonly present during laser treatments) exhibit a shift to temperatures that are higher than what would normally be expected from a single-process Arrhenius model. A novel variable heat shock (VHS) response model was developed that takes into account the observed deviation from the single-process Arrhenius relation, by assuming that the cell viability can be described as the combined effect of two biochemical processes that dominate cell survival characteristics at very short and very long exposure times. The potential implications of the VHS model are explored theoretically through an example of non-ablative laser resurfacing. The VHS model shows that under the appropriate conditions, very high temperature heat shocks can be generated within the superficial epithelium tissue layer without causing irreversible tissue damage. A mechanism of action for tissue regeneration by means of non-ablative resurfacing with the Er:YAG laser is proposed, which involves indirect triggering of tissue regeneration through intense heat shock to the epithelia, in addition to the tissue regeneration mechanism by means of direct thermal injury to deeper lying connective tissues.

Minimally invasive, non-ablative Er:YAG laser treatment of stress urinary incontinence in women--a pilot study.

The study presents an assessment of mechanism of action and a pilot clinical study of efficacy and safety of the Er:YAG laser for the treatment of stress urinary incontinence (SUI). The subject of this study is a treatment of SUI with a 2940 nm Er:YAG laser, operating in a special SMOOTH mode designed to increase temperature of the vaginal mucosa up to maximally 60-65 °C without ablating the epidermis. Numerical modelling of the temperature distribution within mucosa tissue following an irradiation with the SMOOTH mode Er:YAG laser was performed in order to determine the appropriate range of laser parameters. The laser treatment parameters were further confirmed by measuring in vivo temperatures of the vaginal mucosa using a thermal camera. To investigate the clinical efficacy and safety of the SMOOTH mode Er:YAG laser SUI treatment, a pilot clinical study was performed. The study recruited 31 female patients suffering from SUI. Follow-ups were scheduled at 1, 2, and 6 months post treatment. ICIQ-UI questionnaires were collected as a primary trial endpoint. Secondary endpoints included perineometry and residual urine volume measurements at baseline and all follow-ups. Thermal camera measurements have shown the optimal increase in temperature of the vaginal mucosa following treatment of SUI with a SMOOTH mode Er:YAG laser. Primary endpoint, the change in ICIQ-UI score, showed clinically relevant and statistically significant improvement after all follow-ups compared to baseline scores. There was also improvement in the secondary endpoints. Only mild and transient adverse events and no serious adverse events were reported. The results indicate that non-ablative Er:YAG laser therapy is a promising minimally invasive non-surgical option for treating women with SUI symptoms.

Method for optodynamic source localization during Er:YAG laser ablation.

We present an improved optodynamic (OD) method which enables measurement of the distance between the OD source on the ablated surface and a piezoelectric sensor above it, with a relative error of about 1%. The method is based on the point explosion model and allows determination of the distance to the OD source and the released energy for each detected OD signal. We estimate the distance and released energy on the basis of two measured OD signal characteristics: the time of flight and the duration of the compressive phase. We show that the finite aperture of the sensor needs to be taken into account to improve measurement accuracy. We present experimental validation of the method using an Er:YAG laser and water as a tissue phantom. We observe an excellent agreement between the measured and theoretical OD signals and between the measured and estimated distances. The method opens the way to practicable implementations of on-line OD monitoring of laser ablation in surgery and medicine.

Ablative potential of four different pulses of Er:YAG lasers and low-speed hand piece.

OBJECTIVE: The aim of the study was to evaluate the ablation rate of caries in dentin with fluorescence-feedback controlled Er:YAG, Variable Square Pulse technology (VSPt) based Er:YAG working in different pulse durations, and steel bur. MATERIALS AND METHODS: Sixty human molar teeth with caries in dentin, extracted for periodontal reasons, were selected for this study. All selected teeth were randomly divided into five groups, each containing twelve specimens: (1) group FFC, fluorescence feedback-controlled Er:YAG laser; (2) group SSP (super short pulse: 50 µs); (3) group MSP (medium short pulse; 100 µs); (4) group SP (short pulse; 300 µs); and (5) group SB, steel bur in a slow-speed hand piece. A profilometer was used to determine the volume of the ablated caries in dentin. The clinical and real ablation rates of caries in dentin were calculated. Two specimens from each experimental group were selected randomly and subjected to SEM examination. RESULTS: The volumes of the ablated caries in dentin in the SSP and SB groups were statistically significantly different in comparison with other experimental groups (p<0.05). The clinical and real ablation rate of caries in dentin was the highest for the SSP group (0.15±0.05 and 0.17±0.05 mm(3)/sec, respectively) and statistically significantly different in comparison with the MSP and SB groups (p<0.05). The SB and FFC groups revealed a dentin surface with a smear layer and closed dentinal tubules on SEM micrographs. In the SSP, MSP, and SP groups, an irregular surface without a smear layer was found. CONCLUSIONS: Taking into consideration the experimental conditions of the present study, SSP was the most efficient in ablation of caries in dentin, providing a smear layer-free surface with open dentinal tubules.

Ablative potential of the erbium-doped yttrium aluminium garnet laser and conventional handpieces: a comparative study.

OBJECTIVES: To compare the ability of the Variable Square Pulse technology (VSP-technology)-based erbium-doped yttrium aluminium garnet (Er:YAG) laser working in maximum speed (MAX) mode (1000 mJ, 300 micros, 20 Hz) and the high-speed handpiece to ablate enamel, to compare the ability of for the VSP-technology-based Er:YAG laser working in MAX mode and the low-speed handpiece to ablate dentin, and to analyze in vitro effects of Er:YAG ablation of enamel and dentine surfaces using scanning electron microscopy (SEM). BACKGROUND DATA: A VSP-technology-based Er:YAG laser operating in MAX mode should be appropriate for achieving the maximum ablating speed in hard dental tissues. MATERIALS AND METHODS: The experiment was conducted on extracted, cleaned, sterilized human molar teeth. Cavity preparations were made in hard dental tissues using the VSP-technology Er:YAG laser in MAX mode, in enamel using the high-speed handpiece, and in dentin using the low-speed handpiece at different time intervals. A laser triangulation profilometer was used to determine cavity volumes. The cavity surfaces of five specimens were examined using SEM. RESULTS: The Er:YAG laser removed a volume of enamel that was 3.3 times as large as that removed by the high-speed handpiece in the same period of time. In dentin, the Er:YAG laser removed 8 to 18 times as much volume as the steel bur in the same period of time. The Er:YAG ablation rate in dentin was faster than in enamel. SEM of laser prepared cavities showed a well-defined surface, free of the smear layer. CONCLUSIONS: The VSP-technology-based Er:YAG laser, working in MAX mode, is more efficient than mechanical drills for enamel and dentin ablation.