Nd:YAG fourth harmonic (266-nm) generation for corneal reshaping procedure: An ex-vivo experimental study.

Corneal reshaping is a common medical procedure utilized for the correction of different vision disorders relying on the ablation effect of the UV pulsed lasers, especially excimer lasers (ArF) at 193 nm. This wavelength is preferred in such medical procedures since laser radiation at 193 nm exhibits an optimum absorption by corneal tissue. However, it is also significantly absorbed by the water content of the cornea resulting in an unpredictability in the clinical results, as well as the high service and operation cost of the commercial ArF excimer laser device. Consequently, other types of solid-state UV pulsed lasers have been introduced. The present work investigates the ablation effect of solid-state laser at 266 nm in order to be utilized in corneal reshaping procedures. Different number of pulses has been applied to Polymethyl Methacrylate (PMMA) and ex-vivo rabbit cornea to evaluate the ablation effect of the produced laser radiation. PMMA target experienced ellipse-like ablated areas with a conical shape in the depth. The results revealed an almost constant ablation area regardless the number of laser pulses, which indicates the stability of the produced laser beam, whereas the ablation depth increases only with increasing the number of laser pulses. Examination of the ex-vivo cornea showed a significant tissue undulation, minimal thermal damage, and relatively smooth ablation surfaces. Accordingly, the obtained 266-nm laser specifications provide promising alternative to the traditional 193-nm excimer laser in corneal reshaping procedure.

Quality of life impact of refractive correction (QIRC) results three years after SMILE and FS-LASIK.

BACKGROUND: This study aimed to compare long-term postoperative quality of life and satisfaction differences between SMILE and FS-LASIK for myopia correction. METHODS: This cross-sectional study enrolled patients under the age of 39 years, who chose to undergo SMILE or FS-LASIK surgery to both eyes 3 years previously. Patients completed a common vision test and Quality of Life Impact of Refractive Correction (QIRC) questionnaire, together with the surgical satisfaction, adverse symptoms subjective survey. Patients with preoperative corrected distance visual acuity and postoperative uncorrected distance visual acuity of 20/20 or greater were included. Propensity score matching (PSM) was used to match the preoperative and postoperative spherical equivalent, age, and designed optical zones of the left and right eyes between the two groups. RESULTS: Forty-nine patients were included in each group after PSM from 188 patients. No significant difference in the total QIRC score was found between the SMILE and FS-LASIK groups (45.89 ± 5.91 vs 45.09 ± 5.65, p = 0.492). There were no differences in surgical satisfaction between the groups (p = 0.178). Compared to the SMILE group, the FS-LASIK group had more glare (2.12 ± 2.25 vs 3.22 ± 2.54, p = 0.026) and severe dryness (1.80 ± 1.98 vs 2.79 ± 2.19, p = 0.021). CONCLUSION: Postoperative quality of life is similar after SMILE or FS-LASIK. Dry eye symptoms and glare were milder in the SMILE group than in the FS-LASIK group.

[Comparative analysis of corneal aberrations after intrastromal segments and MyoRing implantation using femtosecond laser in patients with keratoconus]. / Sravnitel'nyi analiz rogovichnykh aberratsii posle femtolazernoi implantatsii intrastromal'nykh segmentov i kolets pri keratokonuse.

AIM: to comparatively analyze corneal aberrations after intrastromal corneal segments (ICS) and MyoRing implantation using femtosecond laser in patients with stage II-III keratoconus (KC) by Pentacam examination. MATERIAL AND METHODS: We have operated on 135 patients (145 eyes) with stage II or III KC. Depending on surgical technique, all patients were divided into two groups. Group I consisted of patients with KC, who received optimized femtosecond laser-assisted MyoRing implantation, group II - of those, who received femtosecond laser-assisted ICS implantation. Depending on the stage of the disease, each group was further divided into two subgroups, for stage II and stage III KC, respectively. Before and after surgery, all patients underwent visometry, biomicroscopy, corneal topography, and corneal aberrations measurement under photopic and mesopic conditions on Pentacam. The follow-up period averaged 30 months (from 6 to 34 months). RESULTS: In both groups corneal aberrations significantly decreased after surgery, which was confirmed by an improvement in parameters of refraction and keratometry readings. CONCLUSION: Implantation of intrastromal MyoRings leads to a greater reduction in the total corneal aberration, higher-order aberrations under photopic and mesopic conditions, and spherical aberration under photopic conditions as compared to ICS in patients with stage III KC; a comparable reduction in the total corneal, higher-order, and spherical aberrations under photopic conditions as compared to ICS in patients with stage II KC; a comparable increase in spherical aberration under mesopic conditions as compared to ICS in patients with stage III KC.

Penetrating and Intrastromal Corneal Arcuate Incisions in Rabbit and Human Cadaver Eyes: Manual Diamond Blade and Femtosecond Laser-Created Incisions.

OBJECTIVES: To compare morphologic differences between freehand diamond or femtosecond laser-assisted penetrating and intrastromal arcuate incisions. METHODS: Freehand diamond blade, corneal arcuate incisions (180° apart, 60° arc lengths) and 150 kHz femtosecond laser (80% scheimpflug pachymetry depth corneal thickness) arcuate incisions were performed in rabbits. Intrastromal arcuate incisions (100 µm above Descemet's membrane, 100 µm below epithelium) were performed in rabbit corneas (energy 1.2 µJ, spot line separation 3 × 3 µm, 90° side cut angle). Eyes were examined by slit lamp and light microscopy up to 47 days post-procedure. Freehand diamond blade penetrating incisions, and femtosecond laser penetrating and intrastromal arcuate incisions (energy 1.8 µJ, spot line separation 2 × 2 µm) were performed in cadaver eyes. Optical coherence tomography was performed immediately after surgery and the corneas were fixed for light scanning and transmission electron microscopy. RESULTS: The rabbit model showed anterior stromal inflammation with epithelial hyperplasia in penetrating blade and laser penetrating wounds. The laser intrastromal and penetrating incisions showed localized constriction of the stromal layers of the cornea near the wound. In cadaver eyes, penetrating wound morphology was similar between blade and laser whereas intrastromal wounds did not affect the cornea above or below incisions. CONCLUSION: Penetrating femtosecond laser arcuate incisions have more predictable and controlled outcomes shown by less post-operative scarring than incisions performed with a diamond blade. Intrastromal incisions do not affect uncut corneal layers as demonstrated by histopathology. The femtosecond laser has significant advantages in its ability to make intrastromal incisions which are not achievable by traditional freehand or mechanical diamond blades.

Lasers in corneal surgery.

New laser methods have introduced new possibilities and partly replaced traditional methods in corneal surgery. Femtosecond lasers have traditionally been used to replace the surgeons' knife in corneal refractive surgery and corneal transplantation. Excimer laser has been used from the beginning of 1990 for the reshaping of cornea. With the development of excimer laser, traditional photorefractive corneal surgery has improved and new methods have been found such as PRK, PTK, epi-LASIK, and LASEK. Today LASIK, the most commonly used refractive surgery, uses both femtosecond and excimer lasers. SMILE, the most recent method in corneal refractive surgery, is solely based on the use of femtosecond laser.

[Femtosecond laser-assisted cataract surgery]. / Chirurgie de la cataracte assistée par laser.

Femtosecond laser-assisted cataract surgery is a major technological innovation. The femtosecond laser, during a pretreatment step, helps to prepare the patient's eye for the surgery proper by creating corneal incisions, anterior capsulotomy and lens fragmentation in an automated fashion. Thus, these steps can be performed with precision and reproducibility, and lens fragmentation reduces the amount of ultrasound required during surgery. Drawbacks of this technology are a longer operating time, a more demanding surgical procedure and a much higher cost for patients and surgical centers. New models of organization in the operating room, patient flow, and financial systems have to be designed to adapt this procedure to our practice. The benefits of this technology should make it an essential tool in the future, provided that cataract surgery can be reconsidered logistically and economically.

New instruments for lenticule extraction in small incision lenticule extraction (SMILE).

Small incision lenticule extraction (SMILE) is an alternative to Laser-Assisted in situ Keratomileusis (LASIK) for correction of myopia. In cases where surgeons inadvertently dissect the posterior surface first, identification of the anterior surface and subsequent removal become difficult since the anterior surface of the lenticule is compacted against the anterior stromal surface. This may result in incomplete lenticule removal, and a remnant of intrastromal lenticule in SMILE may lead to visual sequelae. In order to aid surgeons in lenticule removal, we have designed and developed 5 novel SMILE lenticule strippers to locate and extract the lenticules more easily. The aim of this study was to investigate and compare the efficacy and quality of these lenticule strippers in assisting SMILE. Thirty porcine eyes were used. The ease of extraction and removal of the lenticule with different strippers was graded by an experienced SMILE surgeon, the extracted lenticule circularity was evaluated by calculating the lenticule circularity, and the intactness of the extracted lenticule edge was assessed using scanning electron microscopy. We found these novel strippers can be of great help to improve the safety and quality of SMILE surgery, particularly in those cases of difficult lenticule extraction.

[Importance of wavelength for ultrashort laser pulses in healthy and pathological corneas]. / Bedeutung der Wellenlänge für ultrakurze Laserpulse in der normalen und pathologischen Hornhaut.

BACKGROUND: A study on the role of laser wavelength in keratoplasty assisted by ultrashort pulse lasers is presented. MATERIAL AND METHODS: This article gives a summary of the principal physical mechanisms contributing to the transparency of healthy corneas and presents transparency measurements as well as laboratory experiments on tissue with lasers at different wavelengths. RESULTS: The transparency of a healthy cornea is strongly related to its regular structure at micrometer and nanometer length scales. Many indications for keratoplasty are associated with a perturbation of this structure and therefore with a sometimes strongly reduced tissue transparency. This explains the often unsatisfactory results obtained when using ultrashort pulse lasers for the procedure. Theoretical considerations and laboratory experiments show that the light scattering processes responsible for the loss in laser beam quality depend strongly on wavelength and the use of wavelengths longer than those presently used allows these processes to be almost completely eliminated. The use of a spectral transparency window close to 1.65 µm is suggested. CONCLUSION: The use of laser wavelengths close to 1.65 µm represents an interesting alternative for the improvement of keratoplasty assisted by ultrashort pulse lasers.

[Innovations in refractive laser surgery 2014]. / Innovationen in der refraktiven Laserchirurgie 2014.

BACKGROUND: Within less than 30 years refractive laser surgery has been developed from an experimental technique to one of the most frequently applied procedures in ophthalmology worldwide. RESULTS: Regarding the success rate of refractive results, myopia correction has reached a level (95% within ± 0.5 D) that is comparable to manifest refraction so that there is not much room for improvement. The most recently developed technique is femtosecond laser lenticule extraction which was introduced in 2007 in Germany. Early clinical results were very promising but the awaited bilateral comparative studies are still lacking. Although new laser types that will be able to provide improved profiles are on the horizon, the problem of reoperations has not yet been solved. Topography-guided ablation is notoriously plagued by undercorrection so that topography-guided treatment is planned as a two step-procedure. The reason for the undercorrection is the leveling effect of the epithelium. PERSPECTIVES: The evolution of refractive surgery has slowed down during the last years, however, some important innovations are at the advent.

Comparison of visual acuity and higher-order aberrations after femtosecond lenticule extraction and small-incision lenticule extraction.

PURPOSE: To compare postoperative visual acuity and higher-order aberrations (HOAs) after femtosecond lenticule extraction (FLEX) and after small-incision lenticule extraction (SMILE). METHODS: Medical records of refractive lenticule extraction patients were retrospectively reviewed. Twenty patients were treated with FLEX. A comparable group of 20 SMILE patients were retrospectively identified. Only one eye of each patient was randomly chosen for the study. Visual acuity, subjective manifest refraction and corneal topography before and 6 months after the surgery were analyzed for both groups. Total HOAs, spherical aberrations, coma and trefoil were calculated from topography data over the 4- and 6-mm-diameter central corneal zone. RESULTS: The mean preoperative SE was -4.03 ± 1.61 in the SMILE group and -4.46 ± 1.61 in the FLEX group. One year after surgery, the mean SE was -0.33 ± 0.25 in the SMILE group and -0.31 ± 0.41 in the FLEX group (p=0.86). In the SMILE group a greater number of eyes were within ±0.50D of the target refraction (95% versus 75%); however, the difference was not statistically significant (p=0.18). Furthermore, 80% of FLEX eyes and 95% of SMILE eyes had an uncorrected distance visual acuity of 20/25 or better (p=0.34). Total HOAs, spherical aberration, coma and trefoil increased postoperatively in both groups. However, there was no statistically significant difference between the groups preoperatively and postoperatively. CONCLUSION: FLEX and SMILE result in comparable refractive results. In addition, corneal aberrations induced by different techniques of lenticule extraction seemed similar to each other.

Improving the ablation efficiency of excimer laser systems with higher repetition rates through enhanced debris removal and optimized spot pattern.

PURPOSE: To evaluate the reasons for the required increased radiant exposure for higher-repetition-rate excimer lasers and determine experimentally possible compensations to achieve equivalent ablation profiles maintaining the same single-pulse energies and radiant exposures for laser repetition rates ranging from 430 to 1000 Hz. SETTING: Schwind eye-tech-solutions GmbH and Co. KG, Kleinostheim, Germany. DESIGN: Experimental study. METHODS: Poly(methyl methacrylate) (PMMA) plates were photoablated. The pulse laser energy was maintained during all experiments; the effects of the flow of the debris removal, the shot pattern for the correction, and precooling the PMMA plates were evaluated in terms of achieved ablation versus repetition rate. RESULTS: The mean ablation performance ranged from 88% to 100%; the variability between the profile measurements ranged from 1.4% to 6.2%. Increasing the laser repetition rate from 430 Hz to 1000 Hz reduced the mean ablation performance from 98% to 91% and worsened the variability from 1.9% to 4.3%. Increasing the flow of the debris removal, precooling the PMMA plates to -18°C, and adapting the shot pattern for the thermal response of PMMA to excimer ablation helped stabilize the variability. Only adapting the shot pattern for the thermal response of PMMA to excimer ablation helped stabilize the mean ablation performance. CONCLUSIONS: The ablation performance of higher-repetition-rate excimer lasers on PMMA improved with improvements in the debris removal systems and shot pattern. More powerful debris removal systems and smart shot patterns in terms of thermal response improved the performance of these excimer lasers.

Visual and refractive outcomes of femtosecond lenticule extraction and small-incision lenticule extraction for myopia.

PURPOSE: To compare the visual and refractive outcomes of femtosecond lenticule extraction (FLEx) and small-incision lenticule extraction (SMILE) in eyes with myopia. DESIGN: Prospective, randomized, masked clinical trial with intraindividual comparison. METHODS: This study evaluated 52 eyes of 26 consecutive patients with spherical equivalents of -4.19 ± 1.65 diopters (D) (mean ± standard deviation) who underwent FLEx in 1 eye and SMILE in the other eye by randomized assignment. Before surgery, and 1 week and 1, 3, and 6 months after surgery, we assessed safety, efficacy, predictability, stability, and adverse events of the 2 surgical techniques. RESULTS: LogMAR uncorrected and corrected distance visual acuity was, respectively, -0.17 ± 0.10, -0.20 ± 0.07 in the FLEx group and -0.15 ± 0.10, -0.19 ± 0.07 in the SMILE group 6 months postoperatively. In the FLEx and SMILE groups 6 months postoperatively, 96% and 100% of eyes, respectively, were within 0.5 D of the targeted spherical equivalent correction. Changes of -0.02 ± 0.39 D and 0.00 ± 0.30 D occurred in manifest refraction from 1 week to 6 months in the FLEx and SMILE groups, respectively. No clinically significant complications occurred in the FLEx or the SMILE group. CONCLUSIONS: Both FLEx and SMILE performed well in the correction of myopia throughout the 6-month observation period. FLEx may be essentially equivalent to SMILE in terms of safety, efficacy, predictability, and stability, suggesting that the presence or absence of lifting the flap does not significantly affect these visual and refractive outcomes.

[Reproducibility of femtosecond laser incisions for posterior lamellar keratoplasty in vitro]. / Reproduzierbarkeit der Femtosekundenlaserschnitte bei der posterioren lamellären Keratoplastik in vitro.

PURPOSE: In the last decade surgeons have tried to use posterior lamellar keratoplasty (PLAK) in a way that maintains the biomechanical stability of the intact cornea. Femtosecond lasers can be used to cut a lenticle out of the posterior part of the stroma. The purpose of this study was to assess the reproducibility of the generated lenticles. METHODS: The FEMTEC-Laser (20/10 Perfect Vision, Heidelberg, Germany) was used to perform posterior lamellar dissections. The goal was to assess the real thickness of the graft in comparison to the intended thickness predefined with the laser. The histological preparations were viewed using light microscopy (Olympus microscope). Main outcome measures included difference of intended versus real distance of the horizontal plane from Bowman's layer and quantification of the created bubbles. RESULTS: The intended distance was reached with a precision of 115-160 µm and was constant over the whole length of the lenticle depending on the depth of the laser action. The number and size of the generated bubbles was equal in the different layers. At least one zone of stromal condensation could be found in 88.9% of the corneas and 63% of the corneas showed a partial perforation or minimal lesions of Descemet's membrane. The distance of the two closest bubbles to Descemet's membrane increased in thinner lenticles, i.e. a greater distance from Bowman's layer. No damage occurred due to compression or heat. CONCLUSIONS: As the laser generates a reproducible number of bubbles of the same size, precise incisions can be achieved. Thus the femtosecond laser proved to be an accurate tool for PLAK. In the majority of cases a lesion of Descemet's membrane could be found which seems to make the removal of the lenticles easier. A disadvantage is a decreasing probability of a lesion of Descemet's membrane as well as the decreasing precision in thinner lenticles.

Femtosecond laser technology in corneal refractive surgery: a review.

PURPOSE: To discuss current applications and advantages of femtosecond laser technology over traditional manual techniques and related unique complications in corneal refractive surgical procedures, including LASIK flap creation, intracorneal ring segment implantation, astigmatic keratotomy, presbyopic treatments, and intrastromal lenticule procedures. METHODS: Literature review. RESULTS: From its first clinical use in 2001 for LASIK flap creation, femtosecond lasers have steadily made a place as the dominant flap-making technology worldwide. Newer applications are being evaluated and are increasing in their frequency of use. CONCLUSIONS: Femtosecond laser technology is rapidly becoming a heavily utilized tool in corneal refractive surgical procedures due to its reproducibility, safety, precision, and versatility.

Initial surface temperature of PMMA plates used for daily laser calibration affects the predictability of corneal refractive surgery.

PURPOSE: To investigate the relevance of initial temperature of the polymethylmethacrylate (PMMA) plates used as a target for photoablation during calibration of excimer lasers performed in daily clinical routine. METHODS: An experimental argon fluoride excimer laser with a repetition rate of 1050 Hz, a radiant exposure of 500 mJ/cm², and single pulse energy of 2.1 mJ was used for photoablation of PMMA plates. The initial plate temperature varied from 10.1°C to 75.7°C. The initial temperature was measured with an infrared camera and the central ablation depth of a myopic ablation of -9.00 diopters (D) with an optical zone of 6.5 mm was measured by means of a surface profiling system. RESULTS: The ablation depth increased linearly from 73.9 to 96.3 µm within a temperature increase from 10.1°C to 75.7°C (increase rate of 0.3192 µm/K). The linear correlation was found to be significant (P<.05) with a coefficient of determination of R²=0.95. Based on these results and assuming a standard room temperature of 20°C, optimal plate temperature was calculated to be 15°C to 25°C to maintain an ablation within 0.25 D. CONCLUSIONS: The temperature of PMMA plates for clinical laser calibration should be controlled ideally within a range of approximately ±5°C, to avoid visually significant refractive error due to calibration error. Further experimental investigations are required to determine the influence of different initial corneal temperatures on the refractive outcome.

[Intrastromal corneal segments implantation in keratoconus].

Up-to-date models of intrastromal corneal segments are reviewed. Different techniques of up-to-date models of intrastromal corneal segments implantation in keratoconus are described. Mechanical and laser techniques of forming of corneal tunnels are analyzed. Information about potential complications of intrastromal corneal segment implantation is given.

[Experimental corneal imaging and corneal surgery with non-amplified femtosecond laser pulses]. / Experimentelle Hornhautbildgebung und Hornhautchirurgie mit nicht verstärkten Femtosekundenlaserpulsen.

BACKGROUND: Non-amplified femtosecond laser was used to induce multiphoton effects for corneal tissue imaging and for tissue ablation. MATERIAL AND METHODS: A non-amplified titanium-sapphire laser was coupled to a laser scanning microscope in order to examine human and porcine cornea. Tissue was subjected to imaging and lesions were created using identical optical pathways at pulse energies below 2 nJ. RESULTS: Cellular components and the extracellular matrix were selectively imaged by applying autofluorescence and second harmonic generation at submicron resolution. Intrastromal linear scanning at higher power resulted in luminescent plasma along the scanning line. Lesion width decreased with increasing tissue depth and increased with increasing laser power at the target. Light microscopy showed intact stromal tissue around the area of the lesion. CONCLUSIONS: High-resolution images as well as high precision tissue lesions were created in the cornea using low energy femtosecond laser pulses. Easy switching between tissue imaging and ablation seems to be suitable for diagnostic and therapeutic applications.