Evaluation of Zonal Equivalent Keratometry Readings After LASIK

1. Evaluation of Zonal Equivalent Keratometry Readings After LASIK Timmy Kovoor, MD Orkun Muftuoglu, MD V.Vinod Mootha, MD Steven Verity, MD R. Wayne Bowman, MD H. Dwight Cavanagh, MD, PhD James P. McCulley, MD

2. Financial Disclosure • Department Of Ophthalmology University of Texas Southwestern Medical Center Dallas, TX. • The authors have no financial interests in any of the products or topics mentioned. • Acknowledgements: Supported in part by an unrestricted research grant from Research to Prevent Blindness, Inc., New York, New York.

3. Introduction • Laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and laser-assisted subepithelial keratectomy (LASEK) correct myopia by decreasing the anterior corneal surface curvature. • The conventional keratometric index of refraction (usually 1.3375) used by most topographers and keratometers to convert the measured radius into diopters is invalid because the natural ratio between the anterior and posterior corneal curvatures no longer exists. • These instruments thus cannot correctly calculate the corneal power and usually give a measurement that is higher than the actual value. • Overestimating corneal power leads to postoperative hyperopia for eyes that will have cataract surgery.

4. Introduction • Scheimpflug camera imaging (Pentacam, Oculus, Wetzlar, Germany) evaluates the anterior and posterior corneal surfaces. • It measures the true net corneal power, estimated ketometric readings, posterior corneal radius, anterior corneal radius, and corneal thickness. • The computer software uses the correct indices of refraction to calculate the total corneal power. • This measurement of the total corneal power is called the true net power (true net K) which is different from the corneal vertex power measured by manual, automated, or simulated keratometry.

5. Purpose To compare pre & post myopic LASIK keratometric measurements performed with Scheimpflug camera imaging with the values obtained using the clinical history method and simulated keratometry.

6. Methods • All procedures were performed at The Laser Vision Correction Center, University of Texas Southwestern Medical Center at Dallas. • The study was performed with the approval of the University of Texas Southwestern Medical Center Institutional Review Board and in accordance with the Declaration of Helsinki guidelines for human research and the Health Insurance Portability and Accountability Act (HIPAA). • The Intralase femtosecond laser (AMO Inc, Irvine, CA) was used to create the flap in all eyes that underwent LASIK. Femtosecond laser flaps were programmed with the following settings: 120 µm thickness, 9.0 mm diameter, with a 60 KHz repetition rate. • All eyes underwent wavefront-guided LASIKwith VISX S4 CustomVue with iris registration, (VISX Inc., Santa Ana, CA) forthe correction ofmyopia or myopic astigmatism.

7. Methods • 36 eyes of 19 patients were included in the study • The minimum required follow-up was 6 months after LASIK • Each eye was evaluated by videokeratography (TMS,Tomey, Phoenix, AZ) and Scheimpflug camera imaging (Pentacam, Wetzlar, Germany). • Thesurgically induce refractive correction andcorneal power were calculated according to the clinical history method. • To avoid miscalculations due to poor videokeratography quality, both preoperative and postoperative examinations were performed immediately after blinking and were carefully inspected before being included in the study. • The following values were analyzed and compared with those obtained with the clinical history method: mean simulated keratometry (K), mean true net power (ie, corneal power calculated with the Gaussian optics formula using the anterior and posterior corneal radii and the corneal thickness), and equivalent K reading (shown in the Holladay report.) • For each eye, only one good-quality Scheimpflug image (determined when the quality specification provided by the instrument was ‘‘OK’’) was used.

8. Methods • Statistical analyses were performed using SPSS (SPSS Inc. Chicago, IL). A • One-way analysis of variance (ANOVA) for repeated measures with Bonferroni multiple comparisons were used to compare all corneal power measurements. • Preliminary analysis showed that all assumptions required by the ANOVA were assessed by Kolmogorov-Smirnov test

9. Results The mean age of the patients was 41.7 ± 9.5 years.

10. Results

11. Discussion • The mean simulated K given by TMS was higher than the mean clinical history method value. • This is consistent with the fact that the simulated K is calculated using the standard keratometric index (1.3375), which is known to overestimate corneal power after refractive surgery. • Our results agree with those of Savini et al1. • The true net power in our sample was significantly lower than the value obtained with the clinical history method. • This discrepancy is likely the result of the different refractive indices used by the 2 methods; the clinical history method is still based on the conventional value of 1.3375, whereas the Gaussian optics formula adopts the true refractive indices of air (1.0), the cornea (1.376), and aqueous humor (1.336). Savini et al. Corneal power measurements with the Pentacam Scheimpflug camera after myopic excimer laser surgery.J Cataract Refract Surg. 2008 May;34(5):809-13.

12. Discussion • The mean equivalent K readings at 1.0 mm, 2.0 mm, and 3.0 mm were not statistically significantly different from the values derived with the clinical history method. • The 3.0 mm reading was the closest to the benchmark value, however with high variability