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K.R. Huxlin, L. Nagy, M. Wyble, M. Beha, J. Wang, I. Cox, S. MacRae; Short–Term Optical and Biomechanical Reaction of Experimentally Thinned Corneas After Laser Refractive Surgery . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1327.
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© ARVO (1962-2015); The Authors (2016-present)
Our long–term goal is to determine whether a central stromal thickness of <250µm is critical to induce ectasia after laser refractive surgery. The present study examines the initial optical and biological reaction of the cornea to thinning of its central stroma below this presumed critical limit.
We studied two eyes from two normal adult cats that had previously received spatially overlapping phototherapeutic keratectomies (PTKs), reducing the mean central corneal thickness to 440µm and the stromal bed thickness to 340µm (Beha et al., ARVO 2005). Both eyes underwent a final 200µm PTK to attain a mean residual stromal bed thickness of 140µm, verified intra–operatively with ultrasonic pachymetry. Ocular wave aberrations were measured in the awake–behaving state with a Shack–Hartmann wavefront sensor. Individual Zernike terms were computed through the 5th order over a 6mm pupil. A Reichert Ocular Response Analyzer was used to measure corneal rigidity (hysteresis) and intraocular pressure (IOP); optical coherence tomography (OCT) was used to measure the thickness of different corneal layers. All measures were carried out pre–operatively and every 2 weeks for 12 weeks post–op.
After the final 200µm PTK, the two residual stromal bed thicknesses were 115+11µm and 181+7µm. Once the epithelium healed (by 2 weeks post–op), central stromal thickness remained at 221+17µm and 213+9µm respectively over the ensuing 8–12 weeks. Although the eye with the thinnest initial stroma underwent a small myopic shift during the first weeks post–op, by 6 weeks post–op, both eyes exhibited a marked (∼1.5D) hyperopic shift in the defocus term. However, this regressed towards pre–op values over the ensuing 6 weeks. The 200µm PTKs decreased corneal rigidity to less than half pre–op values, while IOP remained unchanged at ∼13mmHg.
Experimental thinning of the cat corneal stroma to 115–181µm with PTK induced only mild stromal remodeling that kept central stromal thickness at less than 250µm for 3 months. This relatively quiescent stromal reaction was accompanied by a temporary shift in defocus towards hyperopia and a decrease in corneal rigidity in the presence of normal IOP. Monitoring of these corneas over the next 12 months will determine whether they develop ectasia. If so, we will be able to characterize both optical and biomechanical hallmarks of ectasia. If ectasia never develops, this experiment will prove that a thin cornea alone is not critical to induce this pathological condition.
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