Techniques for In Vivo Assessment of Corneal Biomechanics: Brillouin Spectroscopy and Hydration State - Quo Vadis? / Technologien zur In-vivo-Untersuchung der Biomechanik der Hornhaut: Brillouin-Spektroskopie und Hydratationszustand ­ quo vadis?

To assess the structural integrity of the cornea, non-invasive methods are needed for the local measurement of its mechanical properties. Among a number of established techniques and their associated advantages and disadvantages, Brillouin spectroscopy is still a relatively new technique, capable of determining the compressive modulus of biological tissue, specifically the cornea, in vivo. In the present paper, these various existing and developing technologies for corneal biomechanics are discussed and correlated.

[Imaging Properties of Spectacle Lenses and their Influence on the Tolerance of Spectacle Correction]. / Abbildungseigenschaften von Brillengläsern.

It is not uncommon for patients to complain about incompatibility with the glasses made, although an eyeglass determination has been performed with utmost precision. The causes for this can vary. Thus, the specification of the corneal vertex distance (HSA) is mandatory when prescribing spectacles in order to ensure the exact conversion of the measured values into the values in use. In addition, eye movements behind the lenses may result in a deviation from refractive accuracy and/or astigmatic deviation. Perceived colour fringes by the spectacle wearer are largely due to the choice of lens material. In preparation for a spectacle prescription, the correct choice of near addition can make reading at near physiologically tolerable. For this reason, the patient's individual remaining maximum amplitude of accommodation should always be determined. Especially when prescribing progressive-power lenses, it should be checked whether there is anisomotropy in the vertical direction. Asthenopic discomfort during near vision through the spectacle lenses can thus be avoided.

[Effect of the Age-Related Corneal Elasticity on Applanation Tonometry]. / Auswirkungen des altersbezogenen kornealen Elastizitätsmoduls auf die Applanationstonometrie.

Elevated intraocular pressure (IOP) is accepted to be one important criterion for glaucoma and is usually measured by applanation or rebound tonometry. The individual uncertainty due to central cornea thickness (CCT) is thereby corrected, while the error induced by age-related elastic modulus (EM) change of the cornea is ignored. To investigate its influence on IOP measurement, we derive a model including also the elastic modulus. Our approach is based on known equations from experimental physics and several assumptions being justified in this paper. Our correction values are in good agreement with the Dresden correction table for low CCT values up to 650 µm using a mean EM of 0.29 MPa. An EM variation from 0.2 to 0.5 MPa, which relates to ages from infancy to 90 years, results in an IOP error of up to 10 mmHg. A variation of the cornea curvature from 7.4 mm to 8.0 mm results in a total IOP change of about 3 mmHg, which is usually neglected. The derived model shows that established correction formulas can be insufficient for a reliable IOP determination. In many cases, the conventionally measured IOP may be precise enough, but the uncertainty in IOP determination due to CCT and EM influence are almost in the same range. Measuring the IOP using applanation methods with established correction formulas should not be overestimated without to respect the EM of the cornea.

Visualization of IOL Material-Induced Changes in Retinal Color Stimulus.

Purpose. Different IOL materials, particularly blue-light filtering materials, have different spectral transmittance characteristics. The color stimuli, which influence retinal receptors objectively, have consequently implications for color perception. We report on the quantitative determination of IOL-specific transmittance characteristics and present a method visualizing the resultant changes in color stimulus. Methods. A setup was realized to quantify IOL-absorption in a range of 390-780 nm. To visualize the influence of the different spectral transmittance characteristics an algorithm was developed, which converts RGB-pixel values of images into spectra, which performs the corresponding transmittance correction, reconverts to RGB, and reconstructs the image. IOLs of hydrophobic acrylate and hydrophilic acrylate with a hydrophobic surface in each case with/without blue-light filter were examined. Results. Assessment of the reference images verifies the suitability of the pipeline. Evaluation of the transmittance spectra reveals differences of material- and manufacturer-specifics, which are capable of inducing considerable changes in color perception, particularly in the blue color range and mixed colors involving blue. Conclusions. The developed technique provides an approach for determining IOL-specific transmittance behavior and subsequently its influence on the retinal color stimulus. Problems of altered color perception are occasionally reported after cataract surgery and these become obvious with the visualization procedure developed here.

Spectral behavior of second harmonic signals from organic and non-organic materials in multiphoton microscopy.

Multimodal nonlinear microscopy allows imaging of highly ordered biological tissue due to spectral separation of nonlinear signals. This requires certain knowledge about the spectral distribution of the different nonlinear signals. In contrast to several publications we demonstrate a factor of [Formula: see text] relating the full width at half maximum of a gaussian laser pulse spectrum to the corresponding second harmonic pulse spectrum in the spatial domain by using a simple theoretical model. Experiments on monopotassium phosphate crystals (KDP-crystals) and on porcine corneal tissue support our theoretical predictions. Furthermore, no differences in spectral width were found for epi- and trans-detection of the second harmonic signal. Overall, these results may help to build an optimized multiphoton setup for spectral separation of nonlinear signals.

Ex vivo measurement of postmortem tissue changes in the crystalline lens by Brillouin spectroscopy and confocal reflectance microscopy.

Use of Brillouin spectroscopy in ophthalmology enables noninvasive, spatially resolved determination of the rheological properties of crystalline lens tissue. Furthermore, the Brillouin shift correlates with the protein concentration inside the lens. In vitro measurements on extracted porcine lenses demonstrate that results obtained with Brillouin spectroscopy depend strongly on time after death. The intensity of the Brillouin signal decreases significantly as early as 5 h postmortem. Moreover, the fluctuation of the Brillouin frequency shift inside the lens increases with postmortem time. Images of lens tissue taken with a confocal reflectance microscope between measurements reveal a degenerative aging process. These tissue changes correlate with our results from Brillouin spectroscopy. It is concluded that only in vivo measurements appropriately reflect the rheological properties of the eye lens and its protein concentration.

Spatially resolved Brillouin spectroscopy to determine the rheological properties of the eye lens.

Presbyopia is closely associated with the loss of accommodation, and hence with a decline in the viscoelastic properties of the human eye lens. In this article we describe a method for obtaining spatially resolved in vivo measurements of the rheological properties of the eye lens, based on the spectroscopic analysis of spontaneous Brillouin scattering using a virtually imaged phased array (VIPA). The multi-pass configuration enhances resolution to the extent that measurements are possible in elastic biological tissue characterized by intense scattering. We also present spatially resolved measurements obtained in extracted animal eyes and lenses. The results yield entirely new insights into the aging process of the eye lens.