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Session Title: Surface Ablation II
Session Date/Time: Tuesday 08/10/2013 | 16:30-18:00
Paper Time: 16:30
Venue: Main Lecture Hall (Ground Floor)
First Author: : Y.Liu CHINA
Co Author(s): : Z. Wang Y. Wang
To evaluate the ability of corneal power obtained from a reference coordinate intrinsic to the cornea to assess the refractive change induced by myopic laser surgery.
Institute of Modern Optics,Nankai University, Weijin Rd 94#. Tianjin, China Tianjin Eye Hospital & Eye Institute, Tianjin Key Lab of Ophthalmology and Visual Science, Tianjin Medical University, Tianjin, China?
Twenty-nine eyes that had laser refractive surgery for myopia (laser in situ keratomileusis, LASIK) were included in the study. The manifest refractions and elevation maps of the anterior corneal surface from the Pentacam HR were collected. Surface fitting was conducted to get the radii of curvature with respect to the reference coordinate intrinsic to the cornea. Subsequently, the corneal powers with respect to the intrinsic coordinate could be obtained. The corneal powers are calculated in two ways: the corneal power K-value is calculated by the keratometric index of 1.3375; the corneal power R-value is calculated by the corneal real index of 1.376. The refractive changes were evaluated at corneal plane and spectacle plane respectively. The correlation between the corneal power change reference to the intrinsic coordinate and refractive change was compared with the correlation between corneal power change from the Pentacam and the refractive change. In addition, the prediction errors of the surgically induced refractive change with Pentacam data and with intrinsic data were compared.
The intrinsic radius of curvature is lower (p<0.01) than that from Pentacam before surgery, whereas it is higher (p<0.01) after surgery, resulting a higher corneal power before surgery and a lower corneal power after surgery in comparison with the corneal power from Pentacam. The corneal power changes from Pentacam (K-values and R-values) underestimate the surgically induced refractive change. The best estimation of the refractive changes at the corneal plane is obtained by the R-values changes from Pentacam (slope= -1.11, r2=0.99, p<0.01). For the intrinsic data, the R-value changes overestimate the refractive change both at the spectacle plane (slope= -0.98, r2=0.98, p<0.01) and at the corneal plane (slope= -0.9, r2=0.98, p<0.01). However, K-value provided by the intrinsic data gives the closest estimation of the surgically refractive change at corneal plane (slope=1.00, r2=0.98, p<0.001), with an underestimation as low as 0.03D. For Pentacam data, the highest prediction error is 23.65%, whereas the lowest prediction error is 8.69%. For the intrinsic data, the highest prediction error is 13.05%, and the lowest prediction error is 1.51%, using K-value to estimate refractive change at the corneal plane.
(1) The using of the radius of curvature with respect to a coordinate intrinsic to the cornea reduces the discrepancies between the corneal power changes and refractive induced refractive changes. The corneal power changes provided by the intrinsic data correlated highly with the changes of surgically induced refractive change after LASIK for myopia. (2) K-value provided by the data reference to the coordinate intrinsic to the corneal gives the closest estimation of the surgically refractive changes at the corneal plane.
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