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B. Ralla, D. Kampik, S. Keller, G. Geerling; Influence of Collagen Cross-Linking on LASIK Surgery With Various Thicknesses of Flaps. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2447.
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With Riboflavin/ UV-light crosslinking, it is possible to modify and stabilize the mechanical properties of the cornea by increasing the corneal stiffness. This new therapeutic procedure can prevent the progression of keratokonus. The objective of this study was to determine whether the crosslinking procedure may influence the flap thickness and the result of a myopic LASIK.
In fresh enucleated porcine eyes (2-9h post mortem) the corneal epithelium was removed, crosslinking with Riboflavin and UV-A and a mikrokeratome-flap created at 150 and 130 µm depth with a Carriazo-Pendular in cross-linked (group 1) and normal (group 2) eyes. A myopic laser keratectomy (Esiris, Schwind) with a diameter of 6 mm and 6D, 4D and 2D correction were performed (n=20 each group). The corneal flap thickness was measured by means of optical coherence tomography (OCP) and a micrometer measuring instrument. Refraction was measured before and after ablation by means of Placido based topography system. During all steps the intraocular pressure was kept at 20 mmHg.
Mean flap thickness after a 150µm microkeratome cut as measured by means of OCP and a micrometer measuring instrument was 135,8µm±3,0 µm (164,7±2µm) in group 1 and 107,5 ±3,2µm (132,6±2,4µm) in group 2 (p=0,00). Mean change of corneal refraction, using a 150µm microkeratome head and 6D, 4D and 2D myopic correction, was 4,0D±1,8D (5,5D±1,6D) (p=.005), 3,0D± 1,2D (4,1D±2D) (p=.038) and 1,7±1,1D (2,5D±1,8D) (p=.057). Using a 130µm microkeratome head and 6D and 4D myopic correction, the mean change was 4,6D±0,9D (5,5D±1,4D) (p=.016) and 3,0D±1,5D (3,5D±1,3D) (p=.324).
Crosslinking of enucleated pig eyes resulted in a significantly higher flap thickness. Flap thickness using OCP was significantly lower than the intended and the results obtained from micrometer measuring instrument. The crosslinking procedure significantly reduced the amount of myopic correction - compared to not cross linked cornea - except for -2D / 150 µm and -4D / 130 µm. This may be due to the higher stiffness or thickness of the flap. This ex-vivo study showed that for the use in cross linked corneal tissue microkeratome and laser paramters would require establishing a new nomogram.
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