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Ryan McNabb, Sina Farsiu, Sandra Stinnett, Joseph Izatt, Anthony Kuo; Measured Corneal Power and Astigmatism After LASIK with Spectral Domain Optical Coherence Tomography (SDOCT) Reflects Change in Manifest Refraction. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2596.
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© ARVO (1962-2015); The Authors (2016-present)
Corneal power after laser refractive surgery is difficult to measure. By imaging both corneal surfaces, tomographic imaging techniques like OCT may better characterize corneal power than keratometry or topography. In this study, we compared the ability of OCT, Scheimpflug photography, and topography to characterize the spherical and astigmatic refractive change in eyes due to laser refractive surgery.
After providing informed consent under an IRB approved protocol, subjects undergoing LASIK had corneal imaging prior to and 3 months after LASIK. The corneal imaging consisted of topography (Atlas 995; Carl Zeiss), Scheimpflug photography (Pentacam; Oculus), and distributed scanning OCT (DSOCT). DSOCT is a software modification to commercially available SDOCT (λ0=840nm, Δλ=50nm, 10 kHz A-scan rate; Bioptigen Inc.) to reduce the effects of patient motion . The measured change in pre to post-operative spherical and astigmatic  corneal powers obtained by the three imaging modalities was then compared to the manifest refraction change due to LASIK using generalized estimating equations to account for the use of both eyes of most subjects. Intraclass correlations (ICC) were computed.
39 eyes of 22 subjects (8 M/14 F from 22 - 53 years old) were imaged. The mean spherical equivalent and astigmatic pre to post-LASIK change in manifest refraction at the cornea were 3.34 D and 1.11 D respectively. Table 1 compares the mean pairwise differences between manifest refraction and the three imaging modalities.
Compared with topography and Scheimpflug photography, OCT better reflects the change in spherical equivalent corneal refractive change due to LASIK and has less variability. For astigmatic change, all three modalities were statistically different from the manifest change. However, OCT had the highest correlation and was the only modality with a mean difference less than 0.25 D. OCT provides an improved method to characterize spherical and astigmatic corneal refractive power - as would be needed for cataract surgery - in the difficult to measure post LASIK population.  RP McNabb, F LaRocca, S Farsiu, AN Kuo, and JA Izatt, Biomed. Opt. Express 3, 2050-2065 (2012).  N.S Jaffe and HM Clayman, Trans Am Acad Ophthalmol Otolaryngol 79, OP615-OP630 (1975)
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