Effect of the Vectorial Difference Between Manifest Refractive Astigmatism and Anterior Corneal Astigmatism on Topography-Guided LASIK Outcomes

PURPOSE: To characterize the preoperative vectorial difference between manifest refractive astigmatism and anterior corneal astigmatism, termed ocular residual astigmatism (ORA), and to investigate its influence on topography-guided laser in situ keratomileusis (LASIK) outcomes. METHODS: Comparative retrospective analysis of 21,581 consecutive eyes treated on the manifest refractive astigmatism. Standard outcomes of the 7,180 eyes with the lowest ORA (first tercile: 0.35 ± 0.13 diopters [D]) were compared to the 7,208 eyes with the highest ORA (last tercile: 1.13 ± 0.25 D). RESULTS: The ORA followed a right-skewed normal distribution ( R 2 = 0.99) with a mean ± standard deviation of 0.73 ± 0.36 D. The efficacy index of eyes with low versus high ORA was identical (0.98 ± 0.07 vs 0.98 ± 0.08; P = .99), with a similar percentage having a spherical equivalent within ±0.50 D of the intended target (94.7% vs 94.1%; P = .11). The safety index (1.00 ± 0.04 vs 1.00 ± 0.04; P = .99) and Alpins correction index (1.01 ± 0.37 vs 1.00 ± 0.43; P = .10) were identical. A greater number of eyes with high versus low ORA had postoperative residual astigmatism of 0.75 D or greater (6.1% vs 3.9%). Eyes with very high ORA (ORA ⩾ 1.50 D; 2.5% of the population) marginally reduced the efficacy index from 0.98 to 0.97 ( P < .001). CONCLUSIONS: The contribution of ORA to topography-guided clinical outcomes in most virgin eyes is negligible, with excellent efficacy, accuracy, and safety in both low ORA and high ORA groups. Myopic eyes with high ORA treated on the manifest refraction should not be excluded from topography-guided LASIK. [ J Refract Surg . 2020;36(7):449–458.]