[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.

[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.

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.

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.