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M. Beha, L. Nagy, J. Wang, I. Cox, S. MacRae, K.R. Huxlin; Biomechanical Substrates of Hyperopic Shifts in the Defocus Term After PTK . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2720.
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Purpose: Phototherapeutic keratectomy (PTK) is a form of refractive surgery designed to remove corneal scars and opacities without changing refractive error by ablating a constant thickness of corneal stroma. However, PTK induces significant hyperopic shifts, which in the cat, seem primarily due to altered corneal biomechanics. To better understand the nature of this biomechanical reaction, we measured changes in anterior corneal curvature, corneal hysteresis (a measure of rigidity), intraocular pressure (IOP) and the relative thickness of central corneal stroma and epithelium following PTK in adult cats. Methods: Three cats (felis cattus) underwent 4 sequential, spatially overlapping PTKs using a VISX laser at 14 week intervals. The epithelium was removed prior to each treatment, which ablated ∼100 microns of stroma over a 6mm optic zone. Defocus change was measured using a Shack–Hartmann wavefront sensor. A Reichert Ocular Response Analyzer was used to measure hysteresis and IOP. A custom built optical coherence tomographer (OCT) was used to measure the thickness of the different corneal layers. All tests were carried out pre–operatively and 2, 4, 6, 8, 10 and 12 weeks after each PTK. Individual Zernike terms were computed through the 5th order over a 6 mm pupil. Results: Sequential PTKs induced progressively greater hyperopic shifts ranging from ∼3D to 7D 12 weeks after the second PTK. These shifts were accompanied by a significant increase in the anterior corneal radius of curvature with each PTK. Corneal hysteresis in cats was lower than that in humans, averaging 3.37 + 0.6 mmHg. PTK further decreased hysteresis to 1.99 + 0.06 while leaving IOP unchanged at 10.3 + 2.5 mmHg. Finally, total central corneal thickness did not decrease beyond the first PTK. Instead, decreases in stromal thickness were compensated by epithelial hyperplasia. In intact corneas, the epithelium represented about 11% of the total corneal thickness, increasing to 26% after four PTKs. Conclusions: PTK induces a flattening of the anterior corneal surface which may contribute significantly to the observed hyperopic shift in the defocus term. Such flattening may be potentiated by changes in the viscoelastic properties of the cornea which suggest that it becomes less rigid after PTK. This softening occured in the absence of alterations in IOP or total central corneal thickness. Therefore, corneal epithelium, which fills in the space vacated by the ablated stroma, differs significantly from stromal tissue in its ability to maintain corneal shape.
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