Comparison of Corneal Power and Astigmatism between Simulated Keratometry, True Net Power, and Total Corneal Refractive Power before and after SMILE Surgery.

Yishan Qian,Yan Liu,Rajeev Krishnan Naidu,Xingtao Zhou

doi:10.1155/2017/9659481

Abstract

Purpose. To compare the mean corneal power (Km) and total astigmatism (Ka) estimated by three methods: simulated keratometry (simK), true net power (TNP), and total corneal refractive power (TCRP) before and after femtosecond laser small incision lenticule extraction (SMILE) surgery. Methods. A retrospective, cross-sectional study. SimK, TNP, and TCRP from a Scheimpflug analyzer were obtained from 144 patients before and 6 months after SMILE surgery. Km and Ka were recorded as the mean of individual paracentral rings of 1.0 to 8.0 mm (R1 to R8). The surgically induced changes in Km (delta-simK, delta-TNP, and delta-TCRP) and Ka (delta-simKa, delta-TNPa, and delta-TCRPa) were compared to the changes in spherical equivalent of the cycloplegic refraction (delta-SE) and astigmatism (delta-RA). Results. Preoperatively, astigmatism values were greatest with simKa from R1 to R5 and greatest with TCRPa from R6 to R8. Astigmatism values were smallest with TNPa from R1 to R7. Postoperatively, astigmatism values were greatest with simKa from R1 to R5 and greatest with TCRPa from R6 to R8. Delta-TCRP3 and Delta-TCRP4 matched delta-SE most closely, and delta-TCRPa3 matched delta-RA most closely. Conclusions. TCRP proved to be the most accurate method in estimating corneal power and astigmatism both before and after SMILE surgery.

Highlights

With advancements in technology, the ability to measure corneal power and other corneal parameters is becoming more and more precise
Total corneal power was calculated by three methods: simulated keratometry (simK), True net power (TNP), and total corneal refractive power (TCRP)
In the central cornea prior to corneal refractive surgery, simK estimated the greatest values of corneal power while TNP estimated the smallest of the three measures. This has been confirmed in several studies in which TCRP was found to be most accurate, while simK overestimated and TNP underestimated the central corneal power [2,3,4]

Summary

Introduction

The ability to measure corneal power and other corneal parameters is becoming more and more precise. With an increase in the number of cases of refractive surgery occurring each year, the need for accurate and reliable measures of corneal power will continue to become more apparent. Conventional keratometry (simulated keratometry) measures the paracentral anterior surface of the cornea (usually 3 mm in diameter) and calculates corneal power using a standard keratometric index of 1.3375. The standard keratometric index is based on the assumption that the corneal thickness is constant and the ratio between the curvatures of the anterior and posterior surfaces is constant. TNP measures both the anterior and posterior surfaces of the cornea and uses the real refractive index values of air, cornea, and aqueous. The Gaussian optics formula is a simplified formula that uses a paraxial approximation and assumes that the rays propagating through the posterior surface of the cornea are parallel.

Objectives

Methods

Results

Conclusion