Silvia Schumacher

The Efficacy of Corneal Cross-Linking Shows a Sudden Decrease with Very High Intensity UV Light and Short Treatment Time

PURPOSE Standard treatment in cases of progressive keratectasia is UV-triggered corneal cross-linking. For irradiances larger than 10 mW/cm(2) and treatment times below 10 minutes, the scientific proof of a biomechanical strengthening effect is insufficient. The authors investigated the biomechanical strengthening of ex vivo corneal tissue treated with irradiances between 3 mW/cm(2) and 90 mW/cm(2) and illumination times from 30 minutes to 1 minute, respectively. METHODS A total of 100 porcine eyes received riboflavin + UV treatment (constant irradiation dose of 5.4 J/cm(2)) with different intensities and illumination times and were randomly assigned into 10 groups. A control group (80 eyes) was not irradiated but underwent the same treatment otherwise. Youngs modulus at 10% strain was determined for each strip after uniaxial stress-strain measurement. A Kruskal-Wallis test was used for statistical analysis. RESULTS A statistically significant difference (α = 0.01) was found between the median value of Youngs modulus of the treatment groups up to 45 mW/cm(2) (illumination times from 30 minutes to 2 minutes) compared with the control group. There was no statistically significant difference between the treatment groups from 50 mW/cm(2) up to 90 mW/cm(2) (illumination times of less than 2 minutes) and the control group. CONCLUSIONS The ex vivo results of corneal cross-linking performed in porcine corneas show that the Bunsen-Roscoe reciprocity law is only valid for illumination intensities up to 40 to 50 mW/cm(2) and illumination times of more than 2 minutes. Further experiments are necessary to validate these results for in vivo human corneal tissue. Additionally, safety aspects at high intensities must be investigated.

fs-Laser induced elasticity changes to improve presbyopic lens accommodation

BackgroundAccording to the Helmholtz theory of accommodation, one of the major reasons for the development of presbyopia is the progressive sclerosis of the crystalline lens. However, both the ciliary muscle and the lens capsule stay active and elastic. Thus, the concept for regaining the deformation-ability of the crystalline lens is to create microincisions inside lens tissue to achieve gliding planes.MethodsFor the preparation of the microincisions, near-infrared femtosecond laser pulses are used, generating laser-induced optical breakdowns. Different cutting patterns were performed, and the elasticity regain of the lenses were measured with Fisher’s spinning test for thickness determination.ResultsThe creation of gliding planes inside lens tissue shows very good results in terms of increasing the deformation-ability. The optimization of laser parameters leads to a minimally invasive surgery with no remarkable side effects like residual gas bubbles. Furthermore, ex vivo elasticity measurements of untreated and treated pig lenses show an improvement in the flexibility of the lens. The deformation-ability increases up to 26% with a very low standard deviation (1.6%) and a high significance (p < 0.05).ConclusionGenerating particular cutting patterns inside lens tissue can increase the deformation-ability of the crystalline lens. Thus, it might be one possible way to treat presbyopia.

Femtosecond laser induced flexibility change of human donor lenses.

BACKGROUND According to the Helmholtz theory of accommodation the loss of accommodation amplitude is caused by the growing sclerosis of the crystalline lens, whereas the ciliary muscle and the lens capsule are mainly uneffected by age. A permanent treatment method for presbyopia which offers a dynamic accommodation ability is a recent field of study. The concept followed in this paper uses femtosecond laser pulses to potentially overcome the loss of deformation ability of the crystalline lens by creating gliding planes inside the lens tissue to improve its flexibility. METHODS The aim of the study is to show that the flexibility of human donor lenses can be increased by applying tightly focused near infrared femtosecond laser pulses into the lens tissue. Thereby the tissue is separated by the photodisruption effect. A certain pattern of gliding planes is cut inside the tissue of 41 human donor lenses and the deformation ability of the lenses are compared using the Fisher spinning test before and after laser treatment. RESULTS The laser treatment results in an increased deformation ability of the crystalline lens. The lens a-p thickness increases on average by 97 microm+/-14 microm after the treatment. The Fisher spinning test shows an increase of 16% in deformation ability of the lens at a rotational speed of 1620 rpm. CONCLUSION The creation of gliding planes with a fs laser inside the crystalline lens tissue can change the deformation ability of the lens. This might be an indication for a possible method to treat presbyopia in future.

Femtosecond lentotomy: generating gliding planes inside the crystalline lens to regain accommodation ability

Based on Helmholtz Theory for accommodation the increasing sclerosis of lens nucleus and cortex is the main cause for the developments of presbyopia. Existing therapies, however, do not reverse the stiffness of the crystalline lens and thus do not regain real accommodation ability. A new approach to restore the flexibility of the lens could be realized by photodisruption using ultrafast laser pulses. This process, known as fs-lentotomy, was used to create micro-incisions which act as gliding planes inside the crystalline lens without opening the eye globe.

Interaction dynamics of spatially separated cavitation bubbles in water

We present a high-speed photographic analysis of the interaction of cavitation bubbles generated in two spatially separated regions by femtosecond laser-induced optical breakdown in water. Depending on the relative energies of the femtosecond laser pulses and their spatial separation, different kinds of interactions, such as a flattening and deformation of the bubbles, asymmetric water flows, and jet formation were observed. The results presented have a strong impact on understanding and optimizing the cutting effect of modern femtosecond lasers with high repetition rates (>1 MHz).

Optical ray tracing-guided laser in situ keratomileusis for moderate to high myopic astigmatism.

PURPOSE: To assess the efficacy, safety, and predictability of an individualized laser in situ keratomileusis (LASIK) ablation profile based on an optical ray‐tracing algorithm to treat moderate to high myopic astigmatism. SETTING: Ophthalmology centers in Zurich, Switzerland; Dublin, Ireland; and, Cologne, Germany. DESIGN: Multicenter clinical trail. METHODS: This 3‐center study enrolled eyes with a manifest refraction sphere ranging from 0.50 to −10.25 diopters (D) and/or astigmatism ranging from 0.00 to −4.50 D. The intended outcome was plano in most eyes, undercorrection of 0.50 D in 1 eye, and undercorrection of 0.25 D in 2 eyes. Refractive outcomes were analyzed preoperatively and postoperatively at 1 day and 1 and 3 months. RESULTS: The study enrolled 127 eyes (71 patients). The mean manifest refraction was −5.92 D ± 1.72 (SD). By 3 months postoperatively, 15 eyes had been lost to follow‐up and 1 eye was excluded from analysis because of early retreatment. Of the remaining 111 eyes, 93 (83.8%) had an uncorrected distance visual acuity (UDVA) of 20/20 or better. The mean manifest refraction spherical equivalent (MRSE) in all eyes was 0.03 ± 0.30 D. In 97 (87.4%), the MRSE was within ±0.50 D and in 107 (96.4%), within ±1.00 D. No eye lost 2 or more lines of corrected distance visual acuity. CONCLUSIONS: The new optical ray‐tracing algorithm for individualized LASIK ablation profiles to treat moderate to high myopic astigmatism was efficacious, safe, and predictable. The UDVA in eyes with high myopic astigmatism was better than in those treated with wavefront‐guided LASIK. Financial Disclosure: Drs. Schumacher and Mrochen are paid scientific consultants to and Drs. Seiler and Cummings and Mr. Maus are members of the medical advisory board of Wavelight, Erlangen, Germany.

Visualization of femtosecond laser pulse–induced microincisions inside crystalline lens tissue

PURPOSE: To evaluate a new method for visualizing femtosecond laser pulse–induced microincisions inside crystalline lens tissue. SETTING: Laser Zentrum Hannover e.V., Hannover, Germany. METHOD: Lenses removed from porcine eyes were modified ex vivo by femtosecond laser pulses (wavelength 1040 nm, pulse duration 306 femtoseconds, pulse energy 1.0 to 2.5 μJ, repetition rate 100 kHz) to create defined planes at which lens fibers separate. The femtosecond laser pulses were delivered by a 3‐dimension (3‐D) scanning unit and transmitted by focusing optics (numerical aperture 0.18) into the lens tissue. Lens fiber orientation and femtosecond laser–induced microincisions were examined using a confocal laser scanning microscope (CLSM) based on a Rostock Cornea Module attached to a Heidelberg Retina Tomograph II. Optical sections were analyzed in 3‐D using Amira software (version 4.1.1). RESULTS: Normal lens fibers showed a parallel pattern with diameters between 3 μm and 9 μm, depending on scanning location. Microincision visualization showed different cutting effects depending on pulse energy of the femtosecond laser. The effects ranged from altered tissue‐scattering properties with all fibers intact to definite fiber separation by a wide gap. Pulse energies that were too high or overlapped too tightly produced an incomplete cutting plane due to extensive microbubble generation. CONCLUSIONS: The 3‐D CLSM method permitted visualization and analysis of femtosecond laser pulse–induced microincisions inside crystalline lens tissue. Thus, 3‐D CLSM may help optimize femtosecond laser–based procedures in the treatment of presbyopia.

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.

Investigation of retinal damage during refractive eye surgery

Ultrashort laser pulses are increasingly used in refractive eye surgery to cut inside transparent corneal tissue. This is exploited by the fs-LASIK procedure which affords the opportunity to correct ametropia without any mechanical effects. The cutting process is caused by the optical breakdown occurring in the laser focus. During this process only a certain amount of the pulse energy is deposited into the tissue. The remaining pulse energy propagates further through the eye and interacts with the retina and the strong absorbing tissue layers behind. Therefore this investigation shall clarify if the intensity of the remaining laser pulse and the resulting temperature field can damage the retina and the surrounding tissue. Threshold values of the retinal tissue and theoretical calculations of the temperature field will be presented.

Fs-laser induced flexibility increase in the crystalline lens

Presbyopia is one age related effect every human is suffering beginning at the age of about 45 years. Reading glasses are the conventional treatment so far. According to the Helmholtz theory the loss of accommodation in age is due to the hardening and the resulting loss of elasticity of the crystalline lens. However the ciliary muscle and the lens capsule stay active, respectively. Therefore a possible treatment concept is to regain the flexibility by inducing gliding planes in form of microcuts inside the lens. The increase of flexibility in young porcine lenses by different cutting patterns was shown by Ripken et al.1, 2 who verified the increase in flexibility by the spinning test introduced by Fisher.3 We will present our first measurements of flexibility increase of human donor lenses. Furthermore the influence of the laser cuts into the lens on the accommodation amplitude will be shown in a three dimensional finite-element simulation.