1 Minnesota Eye Consultants, Minneapolis, Minnesota

1. Sherman W. Reeves, MD, MPH1,3,4; Jacob A. Kozisek, OD1,2; Noumia Cloutier-Gill, OD1,2; David R. Hardten, MD1,2,3,4 Accuracy of Scheimpflug Imaging Holladay Equivalent Keratometry Values for IOL Power Calculation in Post Laser Keratorefractive Surgery Eyes 1 Minnesota Eye Consultants, Minneapolis, Minnesota 2 Illinois College of Optometry, Chicago, Illinois3 Department of Ophthalmology, University of Minnesota, Minneapolis, Minnesota4 Department of Ophthalmology, Regions Medical Center, St. Paul, Minnesota **Financial disclosures: DRH – prior speaker for Oculus and Zeiss-Humphery; Minnesota Eye Consultants has received research funding from Bausch+Lomb for unrelated studies**

2. Background • Traditional keratometry measurements, such as manual keratometry, inadequately measure corneal refractive power in eyes that have undergone prior laser keratorefactive surgery (LKS) such as laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK).1,2 • Holladay Equivalent Keratometry Readings (EKR), available with the Pentacam Scheimpflug imaging system’s Holladay Report software package, are purported to provide accurate keratometry measurements in post LKS eyes.3 • Analysis of corneal power across a 4.5mm optical zone is the default measurement of the EKR software. • A recent study suggested that EKR measurements at 4.5 mm zone may overestimate the true keratometric power in post LKS eyes.4

3. Purpose To determine the accuracy of Pentacam Scheimpflug system Holladay Equivalent Keratometry Values for intraocular lens calculation prior to cataract extraction in eyes that have previously undergone excimer laser corneal refractive surgery.

4. Methods • Design • Retrospective cohort • IRB approval granted by Chesapeake Research Review, Inc. • Setting • Minnesota Eye Consultants (MEC), a private group practice with a large cataract and refractive surgery patient base. • Subjects • Identified through a search of computerized billing database. • Eligibility: • Eyes with prior hyperopic or myopic LASIK or PRK that subsequently underwent cataract surgery at MEC from Jan. 2006 to Dec. 2009 and with Pentacam measurements obtained preoperatively for cataract surgery. • Exclusions: • Presence of visually significant ocular pathology unrelated to cataract. • Eyes with intraoperative complications during cataract extraction.

5. Methods • Chart Review • Clinical data collected • Preoperative • Axial length • Manual keratometry values • Pentacam EKR values • Extrapolated at 4.5mm, 3.0mm and 2.0mm optical zones • Power and model of intraocular lens implanted • Postoperative • Stable spherical equivalent refractive error obtained between 2 to 12 weeks after cataract extraction.

6. Methods • Analysis • The back-calculated keratometry value (BackCalc K) was generated for each patient with the Holladay IOL Consultant program, Version 2.50.3129a (Holladay Consulting, Inc. Bellaire, TX), using the postoperative refraction, axial length, lens power and lens constant of the lens implanted. • The BackCalc K represents the “ideal” keratometry value for an eye which, if entered into the IOL calculation formula preoperatively, would have generated and emmetropic spherical equivalent postoperative refractive outcome.5 • Manual keratometry and Pentacam EKRs at the 4.5mm, 3.0mm, and 2.0mm zones obtained immediately prior to cataract extraction were then compared to the BackCalc K values for each patient. • Paired t-tests were use to compare the BackCalc K to manual and EKR Ks • Subset analysis was performed on eyes status post myopic laser with a monofocal IOL implanted. Wilcoxan rank-sum testing was used for subset comparisons due to small group numbers.

7. Results • Population • 30 eyes of 25 patients met the inclusion & exclusion criteria • 29 eyes status post Lasik • 23 eyes post myopic LASIK • 6 eyes post hyperopic LASIK • 1 eye status post PRK • Unknown prior refractive error • IOL models implanted included • Tecnis Z9002 and Z9003 • AR40e • Clariflex • Crystalens AT-45, 5.0 and HD • ReZoom • 16 eyes status post myopic LASIK with a monofocal IOL implanted

8. Results ALL Eyes (n = 30) Subset: Eyes s/p myopic lasik with monofocal IOL (n = 16)

9. Results – ALL Eyes (n=30)

10. Results – Myopic LASIK/ Monofocal IOL Subset (n=16) Difference between means = -2.29 ± 1.83 (p<0.001)

11. Conclusions • Pentacam Equivalent Keratometry Readings in the 4.5mm zone overestimate the keratometry value in post LASIK and PRK eyes undergoing cataract extraction when compared to an ideal K based on post surgical outcomes data. • Shrinking the EKR optical zone produced flatter EKRs, which at the 3.0mm and 2.0mm zones were statistically indistinguishable from the ideal K when all eyes in the study were considered. • A 2.0mm EKR gave a more accurate estimate of the actual keratometry than larger zones in the subset of eyes status post myopic LASIK and with a monofocal IOL implanted. • Use of the Pentacam EKR measurements in the 2.0mm and/or 3.0mm zones may provide improved accuracy of Pentacam Equivalent K measurements in these eyes.

12. References • Maeda N, Klyce SD, Smolek MK, McDonald MB. Disparity between keratometry-style readings and corneal power within the pupil after refractive surgery for myopia. Cornea 1997;16:517-524. • Hamilton DR, Hardten DR. Cataract surgery in patients with prior refractive surgery. Curr Opin Ophthalmol. 2003;14:44-53. • Holladay JT, Hill WE, Steinmueller A. Corneal power measurements using Scheimpflug imaging in eyes with prior corneal refractive surgery. J Refract Surg 2009; 25:862–868. • Tang Q, MD, Hoffer KJ, Olson MD, Miller KM. Accuracy of Scheimpflug Holladay equivalent keratometry readings after corneal refractive surgery. J Cataract Refract Surg 2009; 35:1198–1203 • Randleman JB, Loupe DN, Song D, Waring GO, Stulting RD. Intraocular Lens Power Calculations After Laser In Situ Keratomileusis. Cornea 2002; 21(8): 751–755.