Modelling corneal transparency with reference to stromal architecture

The arrangement of corneal collagen fibrils within lamellae was investigated by comparing fibril positions obtained from electron microscopy with distorted hexagonal, quasi-random and aperiodic arrays. By calculating the wavelength dependence and Fourier transforms of these various arrays it was determined that an aperiodic array based on the sunflower seed head is the most compatible with corneal ultrastructure. An investigation of corneal light scattering away from the central axis was undertaken for the first time. Experimentally it was shown that corneal transmission decreases peripherally, particularly in the far periphery near the limbus. This was shown to be theoretically compatible with calculated positional changes in refractive index and fibril radius, by calculating transmission using the direct summation of scattered fields method. In swollen human corneas, it was determined that there was a notable change in wavelength dependence in the peripheral regions, possibly suggesting an increase in the size and relative number fibril free voids. . Corneal infrared transmission is poorly studied. In this part of the spectrum, the cornea acts as an absorber of incident radiation. It was hypothesised that there should be a systematic variation between corneal hydration and infrared light transmission. Experimentally, by Fourier transform infrared spectroscopy a convenient linear relationship between hydration and transmission was found. Riboflavin-UVA crosslinking is used to treat keratoconus, a degenerative corneal disorder. A swelling experiment was performed on porcine corneas in order to elucidate whether the crosslinking mechanism is intra- or inter-fibrillar. Swelling rates for the treated and untreated tissue were not statistically significant, excluding interfibrillar crosslinking. The penetration depth of the riboflavin molecule into corneal stroma was also examined by visible spectroscopy of thin segments of tissue. It was demonstrated that when riboflavin infiltrates a full thickness cornea, the highest concentration of riboflavin is present in the anterior tissue segment.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Modelling corneal transparency with reference to stromal architecture

The arrangement of corneal collagen fibrils within lamellae was investigated by comparing fibril positions obtained from electron microscopy with distorted hexagonal, quasi-random and aperiodic arrays. By calculating the wavelength dependence and Fourier transforms of these various arrays it was determined that an aperiodic array based on the sunflower seed head is the most compatible with corneal ultrastructure. An investigation of corneal light scattering away from the central axis was undertaken for the first time. Experimentally it was shown that corneal transmission decreases peripherally, particularly in the far periphery near the limbus. This was shown to be theoretically compatible with calculated positional changes in refractive index and fibril radius, by calculating transmission using the direct summation of scattered fields method. In swollen human corneas, it was determined that there was a notable change in wavelength dependence in the peripheral regions, possibly suggesting an increase in the size and relative number fibril free voids. . Corneal infrared transmission is poorly studied. In this part of the spectrum, the cornea acts as an absorber of incident radiation. It was hypothesised that there should be a systematic variation between corneal hydration and infrared light transmission. Experimentally, by Fourier transform infrared spectroscopy a convenient linear relationship between hydration and transmission was found. Riboflavin-UVA crosslinking is used to treat keratoconus, a degenerative corneal disorder. A swelling experiment was performed on porcine corneas in order to elucidate whether the crosslinking mechanism is intra- or inter-fibrillar. Swelling rates for the treated and untreated tissue were not statistically significant, excluding interfibrillar crosslinking. The penetration depth of the riboflavin molecule into corneal stroma was also examined by visible spectroscopy of thin segments of tissue. It was demonstrated that when riboflavin infiltrates a full thickness cornea, the highest concentration of riboflavin is present in the anterior tissue segment

The variation in transparency of amniotic membrane used in ocular surface regeneration

Background/aims: Scant consideration has been given to the variation in structure of the human amniotic membrane (AM) at source or to the significance such differences might have on its clinical transparency. Therefore, we applied our experience of quantifying corneal transparency to AM. Methods: Following elective caesarean, AM from areas of the fetal sac distal and proximal (ie, adjacent) to the placenta was compared with freeze-dried AM. The transmission of light through the AM samples was quantified spectrophotometrically; also, tissue thickness was measured by light microscopy and refractive index by refractometry. Results: Freeze-dried and freeze-thawed AM samples distal and proximal to the placenta differed significantly in thickness, percentage transmission of visible light and refractive index. The thinnest tissue (freeze-dried AM) had the highest transmission spectra. The thickest tissue (freeze-thawed AM proximal to the placenta) had the highest refractive index. Using the direct summation of fields method to predict transparency from an equivalent thickness of corneal tissue, AM was found to be up to 85% as transparent as human cornea. Conclusion: When preparing AM for ocular surface reconstruction within the visual field, consideration should be given to its original location from within the fetal sac and its method of preservation, as either can influence corneal transparency

Effect of complete epithelial debridement before riboflavin–ultraviolet-A corneal collagen crosslinking therapy

Purpose: To evaluate the importance of complete epithelial removal before riboflavin–ultraviolet-A (UVA) corneal collagen crosslinking therapy. Setting: School of Optometry and Vision Sciences, Cardiff University, Wales, United Kingdom. Methods: Riboflavin eyedrops were applied at 5-minute intervals for 35 minutes to the anterior corneal surface of 36 porcine eyes (12 with no epithelial trauma but treated with tetracaine eyedrops, 12 with superficial epithelial trauma but with an intact basal epithelium, and 12 with a fully removed epithelium). The corneal surface of 6 tetracaine-treated eyes, 6 eyes with superficial epithelial trauma, and 6 eyes with a fully removed epithelium was exposed to UVA light for 30 minutes during riboflavin administration. The light transmission spectra of the enucleated corneas were analyzed with a spectrophotometer and compared with those of 9 untreated porcine corneas. Results: Corneas with a fully removed epithelium treated with riboflavin showed an abnormal dip in the transmission spectrum between 400 nm and 510 nm (P<.01). This was attributed to the presence of riboflavin in the corneal stroma. The spectra of riboflavin-treated corneas with no epithelial trauma but tetracaine administration and those with superficial epithelial trauma did not differ from those of the non–riboflavin-treated controls. Exposure to UVA following riboflavin administration did not alter corneal light transmission. Conclusions: Complete removal of the corneal epithelium is an essential component of riboflavin–UVA crosslinking therapy as superficial epithelial trauma and tetracaine administration alone are not sufficient to permit the penetration of riboflavin into the corneal stroma. Failure to achieve adequate stromal absorption of riboflavin may impair the efficacy of the crosslinking process

Effect of Epithelial Retention and Removal on Riboflavin Absorption in Porcine Corneas

PURPOSE To compare stromal riboflavin absorption after 20% alcohol application and partial or complete epithelial removal by analyzing light transmission properties of porcine corneas after riboflavin/ultraviolet A (UVA) corneal collagen cross-linking. METHODS Riboflavin 0.13% eye drops were applied to 18 porcine eyes (6 in which 20% alcohol solution had been applied for 40 seconds, 6 eyes with a grid pattern of full-thickness epithelial trauma, and 6 with the central epithelium fully removed) at 5-minute intervals for 35 minutes. In all eyes, the corneal surface was exposed to UVA light for 30 minutes during riboflavin administration. The light transmission spectra of the corneas were analyzed with a spectrophotometer and compared to those of 9 untreated controls (4 corneas with epithelium and 5 without) and to the spectra of riboflavin 0.13% solution. RESULTS The spectra of riboflavin-treated corneas in the alcohol group were similar to controls. Those with grid-pattern epithelial trauma showed a dip in light transmission between 400 and 490 nm, but this was significantly less than that in eyes for which epithelial removal was complete, where the spectrum was similar to that of riboflavin 0.13% solution. CONCLUSIONS Complete removal of the corneal epithelium appears to be necessary to allow sufficient riboflavin absorption into the stroma to alter the normal light transmission properties of the porcine cornea. Although partial grid-pattern epithelial removal allows some riboflavin penetration, uptake is limited and non-homogeneous, which may affect the efficacy of the cross-linking process. [J Refract Surg. 2009;25:771-775