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J.O. Hjortdal, T. Møller–Pedersen, A. Ivarsen; Biomechanical Changes in the Human Cornea After PRK and LASIK . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2735.
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Purpose: To compare the short–, medium–, and long–term changes in corneal optical power and corneal aberrations, central corneal thickness, and corneal ‘‘stiffness’’ assessed by pneumotonometry readings in patients having laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) for myopia. Methods: One eye in each of 45 patients with myopia ranging from –6.00 to –8.00 diopters (D) (spherical equivalent spectacle refraction [SER]) was randomized to LASIK (n = 25) or PRK (n = 20). Data were collected prospectively before, and 1, 3, 6, 12, and 36 months after surgery. Measurements included corneal topography (TMS–1, Tomey), corneal thickness (ultrasound pachymetry), and apparent intraocular pressure (IOP) (pneumotonometry). Retreatments were not performed during the first year, and retreated eyes were excluded from the 3–year follow–up. Changes in corneal power and aberrations, thickness, and apparent IOP were calculated in a pair–wise manner for three time periods: short term (preoperative to 1 month after surgery), medium term (1 to 12 months after surgery), and long term (1 to 3 years after surgery). Results: In the short term, corneal power decreased equally in LASIK and PRK eyes. Spherical aberrations and coma–like aberrations increased also similarly, while corneal thickness decreased significantly less in LASIK eyes than in PRK eyes. The apparent IOP decreased significantly more in LASIK eyes than in PRK eyes. In the medium term, corneal power increased significantly in both groups. Spherical aberrations decreased significantly in PRK eyes but not in LASIK eyes. From 1 to 12 months, corneal thickness increased more in PRK eyes than in LASIK eyes. During this period, the apparent IOP increased significantly in LASIK eyes. In the long term, corneal power and corneal aberrations did not change significantly in either group. Corneal thickness increased slightly but significantly in both groups. The apparent IOP increased significantly more in PRK eyes. Conclusions: Differences between LASIK and PRK related to time–dependent events affecting corneal shape and structural integrity were present. Peripheral changes in flap hydration in LASIK eyes and epithelial and/or stromal thickening in PRK eyes appeared to be the most important factors in optical power changes during the first year after treatment. The changes in apparent IOP suggest that some interlamellar healing occurred during the first year after LASIK. Three years after LASIK and PRK, corneal bending stiffness seemed permanently decreased, although some restiffening possibly occurred in PRK eyes over the first years after surgery.
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