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N. Nayeb-Hashemi, I. W. Porter, J. Hallak, R. Vasaiwala, D. T. Azar; Delayed Efficacy and Predictability Outcome Measures in Epi-LASIK and LASIK: A Cox Proportional Hazard Model for the Assessment of Related Risk Factors. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4193.
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To analyze the risk factors associated with delayed efficacy and predictability outcome measures in wavefront-guided EpiLASIK and LASIK.
Cox Proportional Hazard analysis was performed on 67 eyes undergoing wavefront-guided LASIK or EpiLASIK for myopia. Patients with at least 3 follow up time points within the first postoperative year were included. Hyperopes, patients with significant underlying ocular disease, and patients with less than 3 months of follow up were excluded from the analysis. Time to event was determined for each eye. Stability of refractive outcomes was defined as achieving two consecutive consistent outcome measures (without subsequent regression). Time to stable outcome measures (UCVA of 20/40 or better, UCVA 20/20 or better, +/- 1.00D and +/- 0.5D of intended postoperative refractive error) was determined and separately analyzed for LASIK and EpiLASIK. Cox Proportional Hazard models were constructed to assess the following risk factors: high myopia (≥-6.00D), keratometric refractive astigmatism (≥1.25D), dry eye (≤10mm Schirmer), pupil diameter (≥7.5mm), corneal thickness (≤540um), LASIK flap thickness (<115um), and age (≥40 years).
Sixty seven eyes were analyzed. Average patient age was 34.78+1.27 (mean+SEM) and average myopic refractive error was -4.03+0.31D for LASIK and -5.35+0.42D for EpiLASIK. UCVA of ≥20/40 was 6.4+3.5days and 14.1+3.8days for LASIK and EpiLASIK, respectively. Time to ≥20/20 was 18.3+6.8days and 62.1+15.5days for LASIK and EpiLASIK, respectively. Cox Proportional Hazard models showed that high myopia was a significant risk factor for delayed time to stable UCVA of ≥20/20 (beta=-1.9; p<0.05) and dry eye was marginally significant (beta=-2.65; p=0.05) for delayed time to achieving +/- 1.00D predictability in LASIK. For EpiLASIK, high myopia was a marginally significant risk factor (beta=-1.591; p=0.06) and astigmatism (beta= -2.025; p<0.05) a significant risk factor for delayed time to stable UCVA of ≥20/20.
Cox Proportional Hazard is an effective tool to evaluate time-dependent outcomes and related risk factors in refractive surgery. Low schirmer score and preoperative refractive errors are important risk factors for delayed outcomes.
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