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David Varssano, Liza Minkev, Tzahi Sela; Visual Acuity in Flat Corneas. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5774.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the relation between low corneal curvature after PRK (actual and predicted), and final best corrected visual acuity.
Setting: Refractive surgery facility (Care Vision, Tel Aviv, Israel), utilizing the Allegretto 200Hz excimer laser (WaveLight GmbH, Erlangen, Germany). A retrospective EMR based data analysis was performed. All procedures performed Jan 1, 2006 to Nov 1, 2010 were evaluated. Inclusion criteria were: Patients undergoing myopic PRK treatment; Minimal follow-up of 75 days. Exclusion criteria were: Any retreatment/enhancement; any other refractive procedure. Actual measured final keratometry was collected when available. Expected final keratometry was calculated from pre-operative data. Comparison was made for corneas with flat meridians ≥35 diopters and <35 diopters (steep and flat corneas respectively).
Measured and expected final flat keratometry correlated well (pearson's coefficient 0.541, p<0.001). Lowest measured and expected final flat keratometry were 32.10 and 31.72 diopters respectively. Data of 5614 eyes that had myopic PRK was evaluated. Average follow-up time was 325±326 days (mean±S.D.), range 75 to 1774 days. Final best corrected LogMAR was 0.0038±0.06704 for expected steep corneas, and 0.0456±0.08562 for expected flat corneas (p<0.001). Loss of best corrected LogMAR was 0.0059±0.07385 and -0.0085±0.07393 respectively (p<0.037). Actual measured final keratometry, when recorded, was used for the same comparisons. Final best corrected LogMAR was 0.0035±0.06167 for steep corneas, and 0.0411±0.08052 for flat corneas (p<0.001). Loss of best corrected LogMAR was -0.0033±0.06523 and -0.0064±0.07100 respectively (p<0.747).
Low post operative keratometry reading is considered deletorious to visual acuity. The value of 35 diopters is regarded as a line that should not be crossed. This common belief dates back to the early days of refractive surgery. It has not been tested in modern lasers utilizing advanced ablation profiles and large treatment zones. The above results apply to the laser platform used and to the data range of the cohort. Calculated final corneal curvature is predictive of the final visual acuity to a significant level (mean difference of 0.0418), steep corneas seeing better. However, comparison of the change in visual acuity indicates that finally steeper corneas slightly lost acuity (by 0.0059), while finally flatter corneas slightly gained acuity (by 0.0085).Calculated final flat keratometry between 32 and 35 diopters does not predict loss of best corrected visual acuity.
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