CXL causes corneal dehydration that can be detected immediately after the procedure. This phenomenon may contribute to increased mechanical stiffness of the cornea. A change in stiffness by means of compressibility could not be detected in porcine corneas.

Purpose: To investigate in vitro the accuracy of dynamic contour tonometry, Goldmann applanation tonometry, and Tono-Pen XL in edematous corneas. Methods: Experimental study included 20 freshly enucleated porcine eyes. Epithelium was debrided, and eyes were divided in four groups. Groups were immersed in 35%, 40%, 50%, and 60% glycerin solutions for 3 hours. Subsequently, globes were mounted in a special holder, and their intraocular pressure was hydrostatically adjusted. Intraocular pressure was measured by means of dynamic contour tonometry, Goldmann applanation tonometry, and Tono-Pen XL while adjusting true intraocular pressure to 17, 33, and 50 mm Hg. Ultrasound pachymetry was performed. Results: Mean corneal thickness was 914.5 ± 33.3 μm (730–1015 μm). In true intraocular pressure of 33 mm Hg, Goldmann applanation tonometry and dynamic contour tonometry significantly underestimated true intraocular pressure (mean Goldmann applanation tonometry: 14.7 ± 4.8 mm Hg, p < 0.001, mean dynamic contour tonometry: 21.6 ± 6.8, p < 0.001). Tono-Pen XL also underestimated, but difference was not statistically significant (Tono-Pen XL: 27.9 ± 9.7, p = 0.064). In true intraocular pressure of 50 mm Hg, all three methods significantly underestimated (Goldmann applanation tonometry: 17.6 ± 5.3 mm Hg, p < 0.001, dynamic contour tonometry: 26.8 ± 6.3 mm Hg, p < 0.001, Tono-Pen XL: 35.6 ± 8.4 mm Hg, p < 0.001). The error in measured intraocular pressure for each method (true minus measured intraocular pressure) was significantly correlated to true intraocular pressure ( p < 0.001). The intraocular pressure measurements of each eye taken under true intraocular pressure of 17 and 33 mm Hg with the three methods were correlated to each other. Measurements taken under intraocular pressure of 50 mmHg were not correlated to each other. Corneal thickness was not correlated to intraocular pressure measurement. Conclusion: Goldmann applanation tonometry, dynamic contour tonometry, and Tono-Pen XL underestimate intraocular pressure when measured under edematous conditions. Tono-Pen XL showed better accuracy, especially in lower true intraocular pressure. The measurement error increases when true intraocular pressure increases in all three methods.

Corneal haze following excimer laser ablation is an adverse after-effect of photorefractive keratectomy (PRK) and is associated with the development of subepithelial opacities. The present work pertains to the measurement of light scattering in rabbit corneas following excimer laser treatment; to the microscopic analysis of the light-scattering corneal structures; and to the development of a mathematical model of light propagation through the post-laser treatment cornea. Photorefractive keratectomy (PRK-6D, 6 mm optical zone) followed by standard postoperative pharmaceutical treatment was performed on rabbit eyes. Animals were examined clinically on a weekly basis and sacrificed after the tenth postoperative week. Confocal microscope image sequences were acquired immediately before animal sacrifice. After the scatter measurement, the corneas were prepared for histopathological evaluation. The subepithelial structures observed using the confocal microscope correspond to refractive index (and therefore optical path difference (OPD) variation. This OPD distribution can be approximated with a fractal surface, band-pass filtered in the Fourier domain. The angular distribution of scattered light is characterised by a narrow forward peak of the order of 0.5 degrees full-width at half maximum (FWHM) in accordance with the sizes of the subepithelial structures (5-150 microm).The intensity of scattered light is correlated with the thickness of the subepithelial scar-tissue layer.

Purpose In terms of excimer laser ablation, the cornea is water bound on an organic matrix. Corneal hydration might affect the excimer laser ablation rate, which could affect the accuracy of correction. It was the purpose of the study to investigate the use of Dextran solutions of varying concentrations to control corneal hydration in rabbit corneas. Methods The corneal epithelium was removed by means of a rotating brush from both eyes of 10 anesthetized pigmented rabbits . The Dextran 70 solutions,with concentrations ranging from 1 to 7%, was topically applied to the eyes for two minutes. The thickness of the cornea in each eye was determined by ultrasound pachymetry after epithelial removal and immediately after rinsing. The Dextran concentration corresponding to zero change in pachymetry was determined by linear regression. In a different series of experiments, rabbit corneas were either overhydrated by means of rinsing with saline or left exposed to dehydrate. Following this artificial change in hydration the corneas were rinsed for 2 minutes with the previously determined iso‐osmotic concentration. Results A strong negative correlation between Dextran concentration and change in pachymetry was observed. Rinsing with the isotonic concentration (2.6%) regulated pachymetry whithin 2% of the original value. Conclusion Corneal hydration can be regulated intraoperatively by means of appropriate Dextran solutions. This may prove useful in cases that hydration is potentially different from normal such as in the case of extensive surgical manipulations prior to laser ablation.

Purpose To evaluate the primary effect of corneal collagen crosslinking on corneal hydration. Methods Twenty corneal buttons from freshly enucleated porcine eyes where immersed in riboflavin 0.1% in dextran 20% dilution for three hours in order for their hydration to reach an equilibrium. Corneal buttons where divided in two groups; the first group was stored in dark conditions while the other group was irradiated with UV radiation (390nm) for 30 minutes to stimulate collagen cross‐linking according to the clinically applied protocol. After irradiation all corneas were immersed in dextran 20% solution for 30 additional minutes and subsequently weighted. Finally all corneal buttons were dehydrated for 48 hours in a desiccating oven set at 62Co and weighted again to obtain their dry mass. Hydration (%) of each button was calculated. Results were analyzed with non parametric tests. Results Irradiated group of corneas had statistically significant lower hydration than the non irradiated group. (p<0.05, Mann‐Whitney Test). Mean hydration in the irradiated group was 71% and in the non‐irradiated 75%. Conclusion Collagen crosslinking causes corneal dehydration that can be detected immediately after the procedure. This phenomenon may contribute to increased mechanical stiffness of the cornea.