Second-Harmonic Reflection Imaging of Normal and Accelerated Corneal Crosslinking Using Porcine Corneas and the Role of Intraocular Pressure

*Advanced Optical Imaging Group, School of Physics, University College Dublin, Dublin, Ireland
Cornea (Impact Factor: 2.36). 12/2013; 33(2). DOI: 10.1097/ICO.0000000000000015
Source: PubMed

ABSTRACT The aim of this study was to investigate morphological changes, using second-harmonic (SH) optical imaging, in the corneal stroma after normal- and high-intensity collagen crosslinking in postmortem enucleated porcine eyes at controlled intraocular pressures (IOPs).
Reflection-mode SH optical imaging of the stroma was realized after standard collagen cross-linking (CXL) and accelerated crosslinking (AXL) of porcine corneas, and the results were compared with the results for untreated controls. Ultraviolet-X lamps (365 nm) were used during riboflavin crosslinking with intensities of 3, 10, 30, and 100 mW/cm for constant 5.4-J/cm exposure doses. The IOP was varied using saline solution injected through the optic nerve and monitored using a Schiotz tonometer. SH optical imaging was realized with an in-house build multiphoton microscope using an ultrafast dispersion-compensated Ti-sapphire laser.
SH reflection images of CXL and AXL porcine corneas obtained at IOPs of 8, 11, 16, and 26 mm Hg showed a similar fibrillar structure of collagen lamellae. All crosslinked corneas showed an increased fibrillar contrast in comparison with untreated baseline images. At low IOPs, strong variations in the scattering were observed that reduced with an increase in the IOP, when fibrils tended to straighten out. At low and normal IOPs, no significant difference between CXL and AXL results could be observed. At very elevated IOPs, however, the impact of AXL was found to alter the fibrillar structure of the collagen becoming less apparent in SH images when compared with that of CXL.
We found a strong influence of the IOP on SH reflection imaging of postmortem porcine corneal stroma. CXL and AXL led to similar SH images indicative of a similar tensile strength. Only at very elevated IOPs (26 mm Hg) did the results for AXL deviate from those of CXL, suggesting an IOP-related threshold for reliable applications of AXL.

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    ABSTRACT: Purpose:To document the first presented report in December 2008 of high irradiance riboflavin/ultraviolet A (UVA) corneal cross-linking in comparison with that of standard irradiance and of fractionated exposure to increase the time for oxygen diffusion into the cornea.Methods:After in vitro studies of oxygen depletion and cross-linking density using type 1 human collagen gels, 36 ex vivo porcine globes were deepithelialized and exposed to 0.1% riboflavin drops in carboxymethylcellulose solution every 5 min for 3 initial doses and then throughout irradiation afterward. Six eyes each were irradiated with 370-nm UVA light at 2, 3, 9, and 15 mW/cm(2) continuously and 15 mW/cm(2) fractionated (with alternate cycles of 30 s ON and 30 s OFF exposure) using an equivalent radiant exposure of 5.4 mJ/cm. The final six eyes received no UVA exposure as a control. The exposed corneas were then dissected and subjected to extensiometry. Analysis of variance with Bonferroni post hoc test was performed between groups.Results:The stress required to induce a 10% strain for the control eyes (no UVA) was 100.620.9x10(3) N/m(2) in comparison with the stress of 3 mW/cm(2) (standard irradiation) at 146.7 +/- 17.6x10(3) N/m(2) (P=0.009). The stress at the other equidose irradiances of 2, 9, 15 continuously, and 15 mW/cm(2) fractionated were 140 +/- 21.9, 162.8 +/- 70, 154.1 +/- 70, and 163.0 +/- 64x10(3) N/m(2), respectively. When comparing the irradiances of 15 mW/cm(2) continuously and fractionated to the standard irradiation, the stress was not statistically different (P=0.799 and 0.643), respectively.Conclusion:High irradiance riboflavin/UVA cross-linking with equivalent energy exposure demonstrates comparable efficacy in stiffening corneal collagen with standard irradiance, but with considerably less exposure time. Over the past 6 years, since this report was first presented, the use of high irradiance cross-linking has been gaining popularity.
    Eye & Contact Lens Science & Clinical Practice 11/2014; 40(6):353-7. DOI:10.1097/ICL.0000000000000095 · 1.68 Impact Factor


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Nov 5, 2014