Thomas Bende

Side effects in excimer corneal surgery. DNA damage as a result of 193 nm excimer laser radiation.

UV radiation is known to cause actinic damage to the DNA. Excimer laser light, possibly used for keratorefractive surgery, should not produce this damage, as the penetration depth is far less than the diameter of a cell. However, photoreactivation experiments with yeast cells show a significant amount of DNA repair after excimer irradiation. The zone of influence of a small slitlike exposure has a diameter of 2 cm. Consequently, the limbus, the critical location of epithelial neoplasia, always lies within the sphere of actinic damage. Radiation damage is induced by secondary radiation rather than by direct interaction.

Side effects in excimer corneal surgery. Corneal thermal gradients.

Thermal loading is a significant side effect during excimer laser surgery of the cornea. The maximal temperature rise, which is dependent on fluence and repetition rate of the radiation, amounts to 20° C in excised corneas and 7.5° C in the in situ cornea. The temperature decreases exponentially with increasing distance from the incision with a half-value distance of 610 ± 100 μm. Exposure parameters are recommended to avoid temperature rises greater than 11° C.

Excimer laser keratectomy for correction of astigmatism.

We treated 13 eyes (12 patients) with excimer laser surgery for correction of astigmatism using linear corneal T-excisions. All eyes were followed up for a minimum of three months. We used a newly developed delivery system and special contact masks to deliver the 193-nm excimer light. Astigmatic corrections of up to 4.16 diopters were obtained. The actual corrections corresponded well with the intended values as predicted by a biomechanical theory. The refractive change over time was different than that observed after knife incisions, suggesting different repair mechanisms. An epithelial plug filling the whole T-excision persisted for over one year in all eyes.

Does Bowman's Layer Determine the Biomechanical Properties of the Cornea?

BACKGROUND Bowmans layer is believed to be the stabilizing element of corneal curvature due to its assumed mechanical stiffness. METHODS Uniaxial stress-strain analysis was performed in paired corneal strips to compare the contribution from Bowmans layer. Two central strips were taken from each cornea and Bowmans layer was removed from one of them with the excimer laser. RESULTS Pairwise comparison yielded no statistical difference in elastic or viscoelastic properties according to presence or absence of Bowmans layer. At a strain of 2%, the stress was measured to be (5.06 +/- 2.01) x 10(3) N/m2 with Bowmans layer and (4.72 +/- 1.3) x 10(3) N/m2 without Bowmans layer. Also, the two relaxation times did not differ significantly. CONCLUSIONS These findings imply that Bowmans layer does not contribute significantly to mechanical stability within the cornea.

Ablation Rate of Human Corneal Epithelium and Bowman's Layer With the Excimer Laser (193 nm)

Laser keratomileusis is a laser-specific procedure whereby a layer of corneal tissue as thin as 10 microns or more is removed from the anterior surface. In most cases, the laser ablates not only Bowmans layer but also portions of the anterior stroma. The histologic evaluation presented shows that the ablation behavior of these two layers is not uniform: at a fluence of 205 mJ/cm2 in Bowmans layer, the ablation rate was 0.38 +/- 0.05 microns per pulse, whereas in stroma it amounted to 0.55 +/- 0.1 microns per pulse. In epithelium, the ablation rate was 0.68 +/- 0.15 microns per pulse, but decreased with deeper excisions. We discuss the consequences of these different ablation rates on the procedure of laser keratomileusis.

Mid-IR Laser Applications in Medicine

This chapter reviews medical applications of a variety of mid-infrared lasers. These applications are based on strong absorption of laser light in human tissue due to the presence of naturally occurring chromophores, specific and unspecific absorbers. Medically relevant laser-tissue interactions are described. Experimental data, obtained with free electron lasers describe photoablation quantitatively in the mid-IR as well as collateral adverse effects. Feedback technologies for online therapy control are presented; they enhance the selectivity of the laser—tissue interaction. Typical medical and surgical applications in gynecology, otorhinolaryngology, neurosurgery, dermatology, urology, dental surgery, ophthalmology and cardiovascular surgery are briefly summarized.

Mesopic vision in myopia corrected by photorefractive keratectomy, soft contact lenses, and spectacles

Purpose: To evaluate contrast vision and glare sensitivity under mesopic conditions in eyes having uncomplicated excimer laser photorefractive keratectomy (PRK) for myopia and in eyes corrected by disposable soft contact lenses, soft contact lenses, and spectacles. Setting: Division of Experimental Ophthalmic Surgery, University of Tübingen, Germany. Methods: The Mesoptometer II test was used to evaluate mesopic vision (glare sensitivity and contrast vision) in 28 eyes of 14 patients wearing disposable soft contact lenses, 20 eyes of 10 patients wearing soft contact lenses, 39 eyes of 20 patients wearing spectacles, 30 eyes of 15 emmetropic patients, and 33 eyes of 22 patients after PRK with a 5.0 mm optical zone. Follow‐up was between 15 and 60 months after PRK (mean 34.5 months). Results: The guidelines of the German Ophthalmologic Society state that patients must recognize Mesoptometer II contrast levels of 1:5 or better with and without glare to meet the minimum legal night‐driving standards for private cars. All eyes with disposable soft contact lenses and soft contact lenses, all emmetropic eyes, and 38 eyes corrected by spectacles recognized contrast levels of 1:5 or better without glare. In contrast, 18 eyes in the PRK group were unable to recognize contrast level 1:5 without glare. With glare, 1 eye in the disposable soft contact lens group, 1 in the soft contact lens group, and 7 with spectacles were unable to recognize the 1:5 contrast level. All emmetropic eyes recognized contrast levels of 1:5 or better; 22 PRK eyes were unable to recognize contrast level 1:5 with glare. Conclusion: Myopic PRK may lead to long‐term impairment of mesopic vision, while soft contact lens use does not seem to markedly influence mesopic vision in eyes with low to moderate myopia.

Laser Thermal Keratoplasty Using a Continuous Wave Diode Laser

BACKGROUND Laser thermal keratoplasty is currently performed with a pulsed Ho:YAG laser at 2.07 microns wavelength. Long-term stability depends critically on the coagulation depth of each cone and thus on emission wavelength (absorption in corneal tissue) and focusing, all contributing to controlled stable collagen shrinkage. To achieve this, a temperature range of 65 degrees to 90 degrees C is needed. A continuous wave laser source meets the coagulation requirements more effectively by avoiding tissue cooling by thermal diffusion as well as the peak temperatures of pulsed lasers, which counteracts the intended central corneal steepening. METHODS A continuous wave diode laser was developed, emitting at 1.885 microns with a maximal energy output of 450 mW. In a contact focusing application, the absorption depth in water as a function of wavelength was measured. Using laser parameters, comparable to those used for a pulsed Ho:YAG laser in contact mode, coagulation spots in human cornea were applied for the continuous wave diode laser. RESULTS The macroscopic and microscopic effects of the diode laser coagulation on corneas in vitro and in situ were comparable to those of the Ho:YAG laser, if a comparable amount of total energy per spot was applied. CONCLUSION Due to better optimized laser-collagen interaction, higher corrections and more stable clinical refractive effects appear achievable using the continuous wave diode laser.

Customized aspheric intraocular lenses calculated with real ray tracing.

PURPOSE: To calculate the exact geometry of custom intraocular lenses (IOLs) for pseudophakic eyes and theoretically predict the residual wavefront error by real ray tracing based on Snells law. SETTING: Centre for Ophthalmology, University Hospital, Tübingen, Germany. METHODS: Individual computer models were constructed based on measurements, including corneal topography and axial length. The geometry of custom spherical, aspheric, toric, and toric aspheric IOLs was calculated in an optimization process with real ray tracing to provide the minimum root mean square wavefront error. The geometric optical properties in terms of residual wavefront error was simulated and approximated by Zernike polynomials. RESULTS: Data from 45 pseudophakic eyes were used to construct the models. Defocus was almost completely corrected by the spherical IOL and astigmatism, by the toric IOL. The aspheric IOL strongly reduced spherical aberration but only slightly reduced total higher‐order aberrations (HOAs); both theoretical predictions corresponded to clinical investigations of wavefront measurements in pseudophakic eyes with a spherical or aspheric IOL. CONCLUSIONS: Real ray tracing calculated the exact geometry of custom IOLs to provide the minimum wavefront error, going beyond simple diopter information. Results show spherical aberration can be significantly reduced with aspheric IOLs. However, the limited possible reduction of total HOAs, even perfectly positioned custom aspheric IOLs, may be a reason for the unclear results in studies assessing the potential benefit to visual performance of currently used aspheric IOLs.

Thermal collateral damage in porcine corneas after photoablation with free electron laser

BACKGROUND The study describes a quantitative and systematic investigation of the collateral thermal damage during infrared photoablation as a function of wavelength between 2.7 and 6.7 microns. METHODS Using the tunable Free Electron Laser at Vanderbilt University, Nashville, Tenn, 60 freshly removed porcine cadaver eyes were irradiated at wavelengths between 2.7 and 6.7 microns, at a fluence of 1.3 J/cm2. For wavelengths where no photoablation occurred, fluence was increased to 3.5 J/cm2; pulse length (macropulse) was 4 microseconds, consisting of a train of micropulses (pulse duration 2 ps at a 2.9 GHz repetition rate). The corneal buttons were removed and stained with hematoxylin and eosin (H&E) and analyzed by histologic micrometry. RESULTS Two thermal damage zones in the remaining tissue were observed: zone 1 showed superficial carbonization and measured between 2 and 4 microns; beyond, the eosinophilic zone 2 measured between 10 and 100 microns. The extent of zone 2 was inversely related to the absorption spectra of the cornea; it was minimal at the 3- and 6-micrometer water absorption bands and maximal at minimal target absorption. CONCLUSION The results correlated well with a model of the ablation process. The study provides a systematic and predictive element for the determination of collateral thermal adverse effects; it does not yet include pulse length variation as a determining factor.