The effect of postoperative keratometry on visual acuity after corneal refractive laser surgery
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PURPOSE: To determine if there is a relationship between eyes with flat corneas (as defined by calculated postoperative keratometry values of <38D) undergoing either LASIK (Laser-assisted in Situ Keratomileusis), LASEK (Laser-assisted Subepithelial Keratectomy), or PRK (Photorefractive Keratectomy) corneal refractive surgery and loss of 1 or more lines of postoperative BCVA, and if there is an advantage to undergoing either LASIK or ASA in eyes meeting flat cornea criteria. METHODS: A retrospective analysis of 191 candidate eyes with calculated postoperative keratometry values <38D were identified and matched by manifest refraction and surgery type to 191 control eyes with calculated postoperative keratometry values ≥38D. Both candidate groups and control groups were further stratified into subgroups based on degree of calculated postoperative keratometry. Candidate subgroups: Subgroup 1a (K<35D), Subgroup 2a (K=35-35.99D), Subgroup 3a (K=36-36.99D), and Subgroup 4a (K=37-37.99D). Control subgroups: Subgroup 1b (K=38-38.99D), Subgroup 2b (K=39-39.99D), Subgroup 3b (K=40-40.99D) and Subgroup 4b (K≥41D). All patients had undergone corneal refractive eye surgery procedures LASIK, LASEK, or PRK at Boston Eye Group/Boston Laser in Brookline MA between December 2008 and November 2016. All LASIK flaps were created using the femtosecond laser IntraLase iFS60 Laser (Abbott Medical Optics Inc.). All surface ablation procedures were performed using the excimer lasers VISX STAR S4 IR Excimer Laser System (Abbot Medical Optics Inc.) or WaveLight EX500 Excimer Laser (Alcon Laboratories Inc.). Visual acuity outcomes measuring preoperative and postoperative BCVA and loss of BCVA were recorded as part of the patient’s medical chart and were statistically analyzed to determine correlations. RESULTS: Our data showed no significant differences between overall candidate (K<38D) and control (K≥38D) group mean preoperative BCVA (p<0.23) or mean postoperative BCVA (p<0.13). A total of 15 out of 191 (7.9%) candidate eyes lost 1 or more lines of BCVA in comparison to 23 total control eyes (12.0%) that lost 1 or more lines of BCVA postoperatively. When evaluating subgroup data, Candidate Subgroup 1a (K<35D) showed a significant (p<0.02) decrease in BCVA when compared to other candidate subgroups. Additionally, Control Subgroup 1b (K=38=38.99D) and Control Subgroup 2b (39-39.99D) showed a significant (p<0.001 and p<0.02 respectively) decrease in BCVA compared to other control subgroups. A total of 231 total candidate and control eyes underwent LASIK and a total of 151 total candidate and control eyes underwent ASA. Overall, 17 out of the 231 (7.4%) eyes undergoing LASIK lost BCVA compared to the 21 out of 151 (13.9%) eyes undergoing ASA that lost BCVA which was significant (p<0.04). CONCLUSION: This study did not find evidence to support that the overall flat cornea group (K<38D) lost postoperative BCVA when compared to a control group of eyes with normal keratometry values. However, our data indicated that when the candidate group was stratified by degree of corneal curvature, patients with very flat corneas (K<35D) may be at increased risk of losing BCVA though further studies are needed. Additionally, eyes undergoing ASA may be at increased risk of losing BCVA though further studies are needed.
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