EyeWorld India March 2017 Issue

EWAP FEATURE 21 March 2017 Views from Asia-Pacific Sabong SRIVANNABOON, MD Professor of Ophthalmology, Siriraj Hospital, Mahidol University 14th floor, Siammintra Building, Siriraj Hospital, 2 Prannok Road, Bangkok 10600 Tel. no. +662-419-8033 sabong@gmail.com K eratometry values serve two major roles in IOL power calculation, first for the calculation of the IOL spherical power 1 and second for the determination of the IOL toricity 2 (both magnitude and axis). For the spherical power calculation, the accuracy of the measurement is actually less relevant than the repeatability of the measurement. In general, the keratometric measurement should be done two or three times within the same device to check for this repeatability. If the keratometric value is not repeatable, the cause (such as ocular surface disease (OSD), irregular astigmatism, etc.) must be identified and corneal topography is required in these cases. In normal cornea, one device should give the repeatable keratometric measurement but the value may differ from the others. To achieve the same IOL spherical power, IOL constant optimization is needed to compensate for this disparity. One MUST optimize their own IOL constant for each lens to achieve the best outcome. This optimized IOL constant has to be specific with the method of biometry especially the device being used for the measurement as well as the specific surgical technique. For the determination of IOL toricity, it is known that the primary objective of IOL toricity is to counter postoperative corneal astigmatism. Similar to the effective lens position (ELP), postoperative corneal astigmatism can only be estimated preoperatively using several parameters. Different toric calculators have their own algorithms for this calculation. It is the vector summation of anterior corneal astigmatism, posterior corneal astigmatism, and surgically induced astigmatism (SIA), and then vergence up to the IOL plane. One of the key elements for accurate IOL toricity calculation is the correct SIA, particularly for low astigmatic corrections. It is suggested that the mean centroid value be used for the calculation; however, it may not accurately represent the real postoperative value if the standard deviation (SD) is quite large. That would mean that the real SIA is quite scattered from one patient to another and cannot be accurately predicted. This could be the result of inconsistency in surgical technique within the same surgeon. Surgeons should always look at the SD of their own SIA and adjust the surgical technique to be more consistent to achieve the lowest possible SIA SD. This is to ensure the most accurate calculation for the IOL toricity. References 1. Srivannaboon S, Chirapapaisan C, Chonpimai P, Koodkaew S. Comparison of ocular biometry and intraocular lens power using a new biometer and a standard biometer. J Cataract Refract Surg . 2014 May;40(5):709-15. 2. Srivannaboon S, Chirapapaisan C, Chonpimai P, Koodkaew S. Comparison of corneal astigmatism measurements of 2 optical biometer models for toric intraocular lens selection. J Cataract Refract Surg . 2015 Feb;41(2):364-71. Editors’ note: Dr. Srivannaboon declared no relevant financial interests. determine a possible IOL power preop. However, among patients who have had previous refractive corneal surgery, he always employs intraop aberrometry. has been eliminated, but the results are not significantly improved due to the variability in healing.” Dr. Rubenstein also generally prefers toric lens implants over peripheral corneal relaxing incisions for most patients. However, cases where he would use such incisions include patients with a very low degree of astigmatism— less than 1 D—who receive one nasal relaxing incision or two short paired incisions to reduce astigmatism, which avoids potential overcorrection with a toric IOL. “If a patient has a very large degree of astigmatism, I sometimes combine a toric lens implant with a relaxing incision to provide extra effect,” Dr. Rubenstein said. “If the patient cannot afford to pay for a toric lens implant, I may correct the astigmatism with a peripheral corneal relaxing incision.” Additionally, Dr. Rubenstein often adds peripheral corneal relaxing incisions when performing femtosecond laser-assisted cataract surgery “to add a slight extra effect that helps to fine tune my results.” Dr. Rubenstein bases such decisions on a modified Nichamin or Donnenfeld nomogram. EWAP References 1. Behndig A, et al. Aiming for emmetropia after cataract surgery: Swedish National Cataract Register study. J Cataract Refract Surg . 2012;38:1181–6. 2. Maeda N, et al. Disparity of keratometry- style readings and corneal power within the pupil after refractive surgery for myopia. Cornea . 1997;16:517–524. Editors’ note: Dr. Holladay has financial interests with AcuFocus (Irvine, California), Alcon (Fort Worth, Texas), Abbott Medical Optics (Abbott Park, Illinois), Calhoun Vision (Pasadena, California), and Carl Zeiss Meditec. Dr. Klyce has financial interests with NIDEK (Fremont, California). Drs. Tipperman and Rubenstein have financial interests with Alcon. Contact information Holladay: holladay@docholladay.com Klyce: sklyce@klyce.com Rubenstein: Jonathan_Rubenstein@rush.edu Tipperman: rtipperman@mindspring.com “I still obtain preop corneal measurements in the usual fashion, however, I realize that most of the corneal testing will overestimate corneal power,” Dr. Rubenstein said. Surgically induced astigmatism The measurement of surgically induced astigmatism (SIA) is essential in predicting postop astigmatic results. Jack Holladay, MD , clinical professor of ophthalmology, Baylor College of Medicine, Houston, has found the best way to measure SIA is the vector difference between the preop Ks and the postop refraction. For example, if the Ks are spherical and the postop refraction is 0.50 D against-the-rule (ATR), then the SIA is 0.50 at 180, not zero. “Using the preop Ks and postop refraction includes everything: anterior corneal astigmatism, posterior corneal astigmatism, systematic IOL tilt/ decentration, and ATR wound fade with time,” Dr. Holladay said. Dr. Rubenstein noted that the posterior cornea has an important role in overall corneal power. “For patients who have with- the-rule astigmatism, the posterior cornea adds about 0.5 D of effect,” Dr. Rubenstein said. “Therefore, for patients who have with-the-rule astigmatism, you want to decrease the power of a toric lens implant by approximately 0.5 D. For patients with against-the-rule astigmatism, increase the power of the toric IOL by approximately 0.3 D.” The approach requires that patients have approximately 1.7 D of with-the-rule astigmatism before a toric lens is used and as little as 0.7 D of against-the-rule astigmatism in order to qualify for a toric lens that is placed against- the-rule. Incision use Dr. Holladay limits his use of limbal relaxing incisions (LRIs) to addressing very small amounts of preoperative astigmatism. “Toric IOLs are much more precise due to the variability in wound healing with LRIs,” Dr. Holladay said. “With femtosecond lasers, the variability of the surgery