[Improvement of power and illumination source of the indirect binocular ophthalmoscope designed by Foerster]. / Optimierung von Strom- und Beleuchtungsquelle des indirekten binokularen Brillenophthalmoskops nach Foerster.

BACKGROUND: Since 1982, the indirect binocular ophthalmoscope designed by Foerster has been in use in ophthalmology. The option to implement a new illumination technique using a light-emitting diode (LED) and a new power source should be evaluated in terms of technical feasibility and patient safety. METHODS: The cooling element was redesigned to accommodate the new LED electronics and their components, including an option for a variable brightness control. A more compact rechargeable battery was utilized with variable fixation at the headband or elsewhere. Photometric measurements of light intensity and the operating time were planned. Furthermore, a review of the new lighting technology in terms of EN ISO 15004-2 and EN ISO 10943 was necessary. RESULTS: Technical adjustments to accommodate the LED inside the cooling element could be realised. The power source was a modern rechargeable lithium-ion battery with variable fixation. The luminous intensity of the LED is superior to that of the halogen lamp and the operating time was increased to 520 minutes. The required limits according to DIN EN ISO 15004-2 for ophthalmic devices were met by our measurements. CONCLUSION: The optimisation of the indirect binocular ophthalmoscope brings improvements in illumination intensity and operating time. A conversion for models already in use is possible. A certified appraisal for compliance with the appropriate standards is the next step.

[Placido topography combined with optical biometry--first results]. / Placido-Hornhauttopografie kombiniert mit optischer Biometrie - erste Ergebnisse.

INTRODUCTION: Optical biometry for calculation of intraocular lenses (IOL) was introduced by Zeiss with the IOLMaster. A newly developed system combines both video topography and optical coherence biometry (BioTop). This allows not only the conventional calculation of IOLs but also enables the design of customised IOLs based on ray-tracing. This paper presents first results regarding the axial length measurement of the new device compared to the IOLMaster. METHODS: The accuracy of the combination system (BioTop) is proven by a well known reference (20,010 ± 0,004) mm. To compare the new system BioTop with the IOLMaster (two different versions 3 and 5) are used. Measurements are taken at the known reference and at 47 eyes with different stages of cataract. RESULTS: The mean geometrical length for 50 measurements of the known reference was AL = (20.011 ± 0.001) mm. For the standard conditions (5 single measurements) the length results in AL = (20.010 ± 0.001) mm. For the IOLMaster these numbers are: AL = (20.00 ± 0,01) mm for version 3 and AL = (20.02 ± 0.01) mm for version 5 also based on 5 single measurements. The measurable axial lengths for the cataract eyes (43 for IOLMaster and 37 for the BioTop) show a mean difference of - 0.015 mm for the two systems. One outlier was detected. CONCLUSION: The measurements of the know reference showed a high accuracy for all systems. The differences in length measurements on the cataract eyes are clinically not relevant. The combination of topography and optical biometry allows new options for calculations of intraocular lenses.

[Current status of infrared photoablation of the cornea]. / Zum Stand der infraroten Photoablation der Hornhaut.

BACKGROUND: IR-photothermal ablation has, for some time, been described as an alternative to UV photoablation with Excimer lasers. Both procedures are based on different laser-tissue-interactions. IR solid state lasers have cost advantages over UV laser technology and allow a broader clinical use. New beam shaping principals had to be developed. IN-VITRO: Systematic biophysical investigations, recently using tunable Free Electron Lasers, have described the ablation efficiency, the collateral thermal adverse effects, the surface properties and particle size, and the dynamic behaviour of the particle ejection, all as a function of wavelengths (and thus of different absorbers in the corneal tissue), of energy and of different pulselengths. The free running Er:YAG laser is suitable and well investigated, the optic parametrical oscillator (OPO) with supershort pulses a more recent alternative. IN VIVO: First in vivo photoablation with different Er:YAG lasers has proved the principle feasibility of IR photoablation. Histology of enucleated human eyes indicates that the wound healing and repair after IR photoablation is not principally different from UV-PRK. The evaluation of the achievable optical quality of the cornea in vivo as well as data on the stability resp. regression of the ablation profiles are still pending.

[Dielectric spectroscopy for noninvasive examination of corneal tissue]. / Dielektrische Spektroskopie zur nichtinvasiven Untersuchung von Hornhautgewebe.

Dielectric spectroscopy is a non-invasive contact technique that permits the in vivo measurement of the specific electrical properties of biological tissue induced by an external electrical field. Permittivity, relaxation time and specific conductivity as a function of corneal hydration (wet weight/dry weight) and temperature were measured in 10 porcine corneas. Variation of tissue hydration has a minor influence on the signal, with a significant variation of the signal being detectable only for relatively dry tissue. A much greater influence was found for temperature, in particular on relaxation times. Dielectric spectroscopy provides us with an opportunity to detect structural, in particular temperature-induced, changes in living tissue. In the frequency range investigated, hydration has only a small influence on the dielectric properties of the tissue.

[Impairment of mesopic vision following photorefractive keratectomy of myopia]. / Beeinflussung des mesopischen Sehens durch die photorefraktive Keratektomie (PRK) zur Myopiekorrektur.

PURPOSE: Halos, glare disability and problems during night driving are significant side effects of photorefractive keratectomy, caused by increasing optical aberrations under reduced conditions of luminiscence. In a previously performed retrospective study an impairment of mesopic vision in most of the investigated eyes was found. To prove these results, a prospective study was conducted. METHODS AND PATIENTS: In a prospective, consecutive study mesopic function was investigated in 41 myopic patients, who were interested in PRK. 26 patients underwent surgery (Summit ExciMed UV200LA, optical zone 5 mm), which was performed in 30 eyes. Mesopic function (contrast vision, glare sensitivity) was measured by using the Mesoptometer II test in all 30 eyes preoperatively and 6 to 9 months postoperatively. Additionally, 21 eyes were examined after 12 to 15 months of follow up. RESULTS: There was a statistically significant reduction of contrast vision and increase of glare sensitivity after 6 to 9 and 12 to 15 months. The Mesoptometer II contrast level of 1:5 (with and without glare) can be used as a critical parameter for the decision, whether mesopic function is markedly reduced or not. Before surgery, contrast level of 1:5 without glare was not recognized in 30%, with glare in 40%. After 6 to 9 months, this contrast level was not recognized in 73% of the 30 eyes without glare, with glare in 80%. Out of these 30 eyes, 21 eyes were followed over 12 to 15 months. Whereas 38% of these eyes (with glare 48%) did not recognized the critical contrast level before surgery, this rate increased to 67% (with glare 81%) 12 to 15 months after surgery. CONCLUSIONS: Impairment of mesopic function has to be considered as a possible side effect of PRK (optical zone 5 mm). Although further investigations are needed, the indication for PRK in special professions (pilots, professional drivers) should be very critical discussed.

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.

Holmium:YAG thermokeratoplasty: treatment parameters for astigmatism induction based upon spherical enucleated human eyes.

BACKGROUND: The refractive outcome of thermokeratoplasty for astigmatism correction depends upon the optical zone diameter (OZD) and coagulation angle (CA; distance between two coagulation spots around the flat meridian). METHODS: Astigmatism was induced in 36 spherical human eye-bank eyes (Ho:YAG laser, 15 Hz, 20 mJ/pulse, 25 pulses per coagulation spot) with different OZDs and CAs. Thirty eyes received free-hand laser application (marked positions). Six eyes were treated using a suction mask, providing a constant OZD of 8.1 mm and an angle of 22.5 degrees between the laser spots. To compare the results, silicone replicas of the eyes were made pre- and postoperatively and analyzed by video-topometery. RESULTS: With an average decrease of 4 D/mm the refractive change is inversely linear to the OZD. Changes of the CA showed significant variations between 22.5 degrees and 45 degrees, only. Mask guidance leads to the same amount of average refractive change. The deviation is +/- 3.4 D for the freehand application but only +/- 0.8 D for mask guidance. CONCLUSION: The data obtained in this in vitro study are transferred to a treatment nomogram describing the immediate postoperative refractive change, not identical with the final refraction. The results of the study are limited by the fact that astigmatism was induced in spherical corneas, rather than correcting preexisting ones.

Automated compensation of defocusing errors in videokeratography.

PURPOSE: The reproducibility of videokeratography measurements is mainly dependent on the accuracy of manual adjustment in the focal plane. Videokeratoscopes having small Placido cones show a considerable amount of error when the required working distance between cornea and keratoscope is not maintained. The advantages of small cones (optimal illumination and the reduction of anatomically caused shadows) are in no proportion to the disadvantage-poor depth of focus, resulting in poor reproducibility. METHODS: The Tubingen Color Ellipsoid Topometer compensates with software and hardware for defocusing errors by means of a triangulation measurement. An 8.0 mm sphere (42 D) was measured in a distance range of +/- 1.5 mm around the focal plane with and without compensation. Five measurements were acquired at each position to evaluate the reproducibility of the calculated data. RESULTS: Defocusation in the range of +/-1.5 mm leads to deviation smaller than 0.05 mm (0.25 D). Without compensation, the maximal deviation is increased up to 0.7 mm (3.0 D), and thus is considerably greater. DISCUSSION: Automated compensation of defocusing errors in videokeratography actively compensates for defocusation errors, enhancing precision and avoiding measuring artifacts.

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 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.

Dynamic mechanical spectroscopy of the cornea for measurement of its viscoelastic properties in vitro.

Cylindric samples (3 mm in diameter) of explanted human corneas were biomechanically characterized in the frequency range from 0.1 mHz to 100 Hz with a specially modified viscoelasticity spectrometer. Such spectra of the shear compliance J (the reciprocal value of the shear modulus G) were measured as a function of corneal hydration and temperature. Variation of the hydration from 0.20- to 1.00-mm sample thickness (5-fold) changed the shear compliance 600-fold. Such a strong effect means that the shear-compliance spectra are highly sensitive to changes in the biomechanical properties of the cornea. This is demonstrated by three examples. A myopic cornea (-3D) was significantly softer (by a factor of 7 at lower frequencies and a factor of 4 at higher frequencies) than an emmetropic cornea. An increasing post mortem interval decreased the shear-compliance values (stiffening) obtained at higher frequencies (by a factor of 0.7 per day), whereas the values obtained at lower frequencies were reproduced. The biomechanics of thermal coagulation were studied in detail. The temperature was increased step by step, and at 48 degrees C a first irreversible decrease in the compliance was recorded at lower frequencies (0.8-fold). A further stiffening by a factor of 1/6 at lower frequencies and 1/5 at higher frequencies ended at 64 degrees C with a minimum in the compliance. To provide a new diagnostic tool, the biomechanical spectra of the cornea have to be measured in vivo. Equipment suitable for the megahertz range has been developed.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

[Moist ablation of the corneal surface with the Er:YAG laser. Results of optimizing ablation]. / Feuchte Hornhautflächenablation mit dem Er: YAG-Laser. Ergebnisse zur Abtragsoptimierung.

The Er:YAG laser, emitting light at 2.94 microns, may be an alternative to the 193 nm excimer laser for photorefractive keratectomy. Compared to the excimer laser, the ablation rate is very high. Surface roughness is also more pronounced than for the excimer laser. Using a precorneal liquid film, these two factors can be reduced, as shown in ablation experiments performed on porcine corneas. Thermal damage of the remaining corneal tissue is another side effect. There is no significant decrease in the amount of thermal damage with this new technique,--not even when the pulse length is reduced.

Photoablation of gelatin with the free-electron laser between 2.7 and 6.7 microns.

BACKGROUND: Photoablation in the infrared (IR) is an option for future refractive and corneal surgery; its basic principles have not yet been investigated systematically. For the first time, the free electron laser allows the dynamic study of photoablation over a wide range of wavelengths with variable combinations of pulselength and energy. The goal of this study is to use the free electron laser as a tool to describe photoablation in the IR quantitatively. We studied the function of wavelength as it is related to target material spectroscopy and the effects of corneal hydration and the pulse repetition rate. METHODS: Surface absorption spectroscopy of the human cornea and of gelatin as a proven model of the cornea was performed between 2.7 and 6.7 microns. Gelatin probes of well-defined thickness (140 +/- 5 microns) and controlled hydration (wet/dry weight 1 to 4.5) served as target material. Photoablation was performed with the Vanderbilt University free electron laser (Nashville, Tenn) in September 1992 at a fluence of 1.27 J/cm2, and a macropulse of 4 microseconds, composed of 2 ps micropulses at a 2.9 GHz pulse repetition rate. Wavelength was tunable between 2.7 and 6.7 microns at stable beam profiles. Ablation experiments were performed as a function of energy, hydration, and pulse repetition rate. Ablation rates were assessed by a) perforation experiments, and b) direct measurements using confocal laser topometry (UBM, Ettlingen, FRG). RESULTS: Ablation rate, assessed by perforation experiments and topometry, correlated well with the corresponding measured absorbencies of the target material: maximal ablation rate at maximal target absorption, around the 3- and 6-micrometer water absorption bands. The ablation threshold at 6.2 microns was 0.7 +/- 0.05 J/cm2 (perforation) and 0.55 +/- 0.08 J/cm2 for depth measurements. Ablation rate as a function of hydration increased to 2.3 (wet/dry weight) with a decrease for higher hydrations. Ablation rate as a function of the pulse repetition rate showed an increase of up to 20 Hz, where it was found to be 60% higher. CONCLUSION: The first systematic use of free electron laser technology positively correlated ablation efficiency with target material absorption, thus identifying a "new" promising wavelength at around 6.2 microns for materials with a high water content such as corneal tissue.

Silicone replica technique and automatic confocal topometry for determination of corneal surface roughness.

No method for the objective measurement of corneal surface roughness has thus far been available for experimental and clinical investigations. The surface roughness of photoablated porcine corneas was investigated with the silicone-cast method and automatic confocal topometry. Photoablation was performed with the 193-nm excimer laser. Surface roughness was determined as a function of the repetition rate (2-60 Hz; fluence, 180 mJ/cm2; beam diameter, 2 mm; 300 laser pulses/experiment). Immediately after photoablation, a silicone replica of the ablated surface was made and analyzed later using confocal topometry. The surface roughness for excimer laser ablation was found to be frequency-dependent: 0.4-0.6 +/- 0.08 micron (2-30 Hz) and 1.2 +/- 0.12 micron (60 Hz). The silicone cast of the cornea of a healthy eye gave an exact image of the profile and curvature: the difference between radius measurements of the cornea (Zeiss ophthalmometer) and of the silicone cast (laser topometer) was about 0.5%. As the silicone-cast method can also be used for in vivo animal experiments, the correlation between surface roughness and laser repetition rate can be studied with regard to photorefractive keratectomy.

Noncontact photoacoustic spectroscopy during photoablation with a 193-nm excimer laser.

Photoablation with the 193-nm excimer laser in organic and inorganic material generates an audible sound wave; in a clinical context, this helps to determine when effective photoablation is taking place. We investigated whether the acoustic phenomenon would qualify as a quantitative parameter of photoablation during non-contact photoacoustic spectroscopy (NCPAS). Photoablation was performed with a 193-nm excimer laser at a fluence of 80-330 mJ/cm2 and ablation-zone diameters ranging between 1.2 and 5.13 mm. A capacitor microphone (Broel & Kjaer) emitted a time signal that was processed for Fourier transformation (FT, spectrum of frequency). The following elements of the FT-signal were studied: the integral, the amplitude, the half-value, and the location of the center frequency. The integral and the amplitude were correlated linearly with the fluence. Variation of the ablation-zone diameter between 1.2 and 5.13 mm was found to produce a squared increase of the integral and the amplitude (fluence, 180 mJ/cm2). The half-value and the center of frequency of the FT signal were constant for the same target material but varied between materials. NCPAS may qualify for the future control of various photoablation and laser parameters such as incision depth, ablation-zone diameter, fluence, and target-material identification and discrimination. Its advantage over competing optical methods is its independence from optical target properties (e.g., fluorescence, surface roughness, reflectivity).

[Silicone impression procedure. Principles for determining ablation and healing parameters in vitro and in vivo]. / Silikonabgussverfahren. Grundlage zur Bestimmung von Ablations- und Heilungsparametern in vitro und in vivo.

Topometry and measurement of photoablation patterns are key questions for keratorefractive photoablation. So far, ablation rates have been determined either by tissue perforation or histological micrometry. A cast of irradiated cornea was made by using a 2-component silicon that polymerizes within minutes, thus preserving the corneal topography immediately after photoablation. The silicon surface is extremely smooth and the accuracy of the cast better than 1 micron. Reproducibility and long-term stability were proven for casts of photoablated PMMA. Thus ablation rates and profile, volumetry and topometry can be determined at any time. The method was applied for 193 nm excimer and 2.94 microns Er:YAG laser in vitro irradiation of the human cornea. Ablation rates in Bowman's layer and stroma were observed for various radiant energies and distinct pulse numbers. The average ablation rates are in agreement with published data. It could be demonstrated that there was an incubation effect for the first pulses. The method feasibility for in vivo measurement was also proven in an animal model. After excimer photoablation in rabbit eyes (beam diameter 3.5 mm, radiant energy 185 mJ/cm3, ablation rate 0.3 micron/pulse), wound healing was recorded: for the right eye silicon casts were molded at three different moments. For the left eye only photographs were taken. There was no difference in the time course of wound healing, so the silicon does not seem to interfere with repair mechanisms of the corneal epithelium. No side effects were observed.

[Effect of pulse duration of the Er:YAG laser on photoablation in ocular tissue (cornea and sclera)]. / Einfluss der Pulslänge des Er:YAG Lasers auf die Photoablation in okulärem Gewebe (Kornea und Sklera).

After photoablation in corneal and scleral tissue with the Er:YAG laser, two zones of thermal damage can be detected: The first zone (about 3 microns thick) consists of thermally denatured debris. The second zone (about 30 microns thick) shows a slightly more intense staining (H&E) compared to the non-influenced tissue. Variation of laser pulse duration between 100 microseconds and 1300 microseconds does not influence the extension of these two thermal damage zones. Furthermore, ablation rate and ablation volume remain nearly constant within this range of pulse durations. These data were obtained by histology (H&E staining) and with a recently published silicon cast replica method. All experiments have been performed on corneas and sclera of pig eyes.