Determining Corneal Power

According to the Gullstrand eye model, total corneal power is 43.00D, which is the sum of the anterior corneal surface (49.00D) and the posterior corneal surface (-6.00D). Corneal power measurements using conventional manual keratometers are estimates based on the curvature of the anterior corneal surface in the paracentral 3mm and a conversion to a total corneal power using the keratometric index of refraction, which assumes Gullstrand proportions between the anterior and posterior corneal curvatures. However, changes in corneal shape after refractive surgery alter these relationships and render incorrect the assumptions necessary for the accurate use of conventional keratometers. Furthermore, normal corneal curvature is prolate, not spherical, and the validity of using the paracentral cornea to determine the effective anterior corneal curvature once the aspheric shape has been altered by refractive surgery has not been confirmed.

Wang et al3 evaluated five methods of determining corneal power after myopic LASIK in 11 eyes of eight patients with visually significant cataracts. The investigators also compared the IOL power predicted by each method to the postoperative refractive outcome. They assessed four IOL formulas (SRK/T, Hoffer Q, and Holladay 1 and 2). The five methods are described with results for the SRK/T formula.

Clinical history. This method requires corneal power measurements prior to laser refractive surgery (Kpre) and a record of the change in manifest refraction at the corneal plane induced by the surgery (∆ MRcp). The corneal power after the refractive procedure (Kpost) is estimated by the formula: Kpost = Kpre - ∆ MRcp. Using this method, the mean spherical error was 0.87 ±1.27D (range, -1.12 to 3.07D).3

Feiz-Mannis method. The post-LASIK corneal curvature is not required. The IOL power is calculated with the pre-LASIK corneal power and axial length measurements, and it is adjusted by the change in spherical equivalent postoperative refraction at the spectacle plane multiplied by 0.7 (to vertex-correct for the IOL's position).1 The 11 eyes studied by Wang et al3 found that, with this method of IOL calculation, the mean spherical error was -0.78 ±1.40D (range, -3.44 to 2.00D). Feiz et al1 proposed a nomogram to adjust the IOL power values when a preoperative change in refractive error is known but when preoperative keratometry values are not available.3

Adjusted effective refractive power (EffRPadj). This method, described by Wang et al,3 requires a knowledge of a change in refractive error (∆ MR) induced by LASIK and a measurement of postoperative corneal power using the Eyesys Corneal Analysis System (Eyesys Technologies, Inc., Houston, TX), which provides an EffRP in the Holladay Diagnostic Summary. The EffRPadj = EffRP - (0.15 X ∆ MR). Using the EffRPadj as the corneal power value with the SRK/T formula for their series of 11 eyes, the investigators found that the mean spherical error was 0.88 ±0.81D (range, -0.13 to 2.12D).

Contact lens overrefraction (CLOR). In eyes with a visual acuity of 20/70 or better (eg, can be accurately refracted), the plano hard contact lens of a known base curve that induces no change in refraction has the same dioptric power as the anterior corneal surface.5 Using this technique to estimate corneal power, Wang et al3 found in their series that the mean spherical error was 4.26 ±3.13D (range, 0.09 to 8.54D). Haigis5 has demonstrated that this relatively high error is due to alterations from the Gullstrand relationship between the anterior and posterior corneal curvatures after refractive surgery. Haigis proposed formulas based on theoretical models to correct for these changes using the CLOR method, but the investigators' formulas have yet to be tested in a clinical series.

Maloney method (personal communication, Robert K. Maloney, MD, and Ling Wang, MD, October 2002). Central corneal power is determined with the Humphrey Atlas Topography System (Carl Zeiss Meditec Inc., Dublin, CA) and modified using the formula: corneal power = (central topographic power X [376/337.5]) - 4.9. Using this method, Wang et al3 found in their series a mean spherical error of 1.94 ±1.00D (range, 0.64 to 3.40D).

Sonego-Krone et al2 evaluated a direct method to measure corneal power after myopic LASIK using the Orbscan II topographer (Bausch & Lomb, Rochester, NY). The investigators noted the limitation of other topography and manual keratometry systems that measure only the anterior corneal curvature and considered the advantages of the combined slit-scanning and Placido-disk corneal videokeratographer (Orbscan II), which is capable of taking into account corneal thickness and posterior corneal curvature. The investigators examined corneal power before and after myopic LASIK in 26 eyes using the Orbscan II and the clinical history method. They found the best agreement with clinical history to be from the 2-mm total mean power calculation (difference from refractive ∆ = 0.07 ± 0.62D) and the 4-mm total optical power calculation (difference from refractive ∆ = -0.08 ±0.53). This method has not been used in a clinical study to test its validity as an estimate of corneal power.

ELP

ELP is related to anterior chamber depth and has a relationship with corneal shape (eg, hyperopic eyes tend to have shallower anterior chambers with a more anterior ELP. Myopic eyes tend to have deeper anterior chambers and a more posterior ELP). Numerous IOL formulas, including SRK/T, Hoffer Q, and Holladay 1 and 2, use corneal power in their calculation of ELP, thereby compounding the effect of changes in corneal shape after refractive surgery on IOL power calculation.3,4

Aramberri4 studied a series of nine eyes with a history of laser refractive surgery that underwent cataract surgery and compared the refractive outcome predicted by the standard SRK/T formula to a “double-K” modified SRK/T.4 The modified formula used the prerefractive surgery keratometric value to determine the ELP and the postrefractive surgery corneal power (determined by the clinical history method) to calculate IOL power by the vergence formula. Using the standard formula, the mean spherical error was 1.82 ±0.73D (range, 0.96 to 3.19D). Using the double-K method, the mean spherical error was reduced to 0.13 ±0.62D (range, -0.56 to 1.47D).4

Wang et al3 applied the double-K method to each formula tested in their series of 11 eyes. They used the Feiz-Mannis approach that included only pre-LASIK keratometric measurements in the IOL calculation, thereby avoiding the problem with the ELP. The results for the three other methods of determining corneal power using the double-K SRK/T formula are:

• a mean spherical error of -1.02 ±1.13D (range, -3.25 to 0.97D) with the clinical history method;
• a mean spherical error of -0.98 ±0.72D (range -2.09 to 0.01D) with the EffRPadj method; and
• a mean spherical error 0.45 ±0.51D (range, -0.45 to 1.13D) with the Maloney method. Wang et al3 proposed a modified Maloney equation to reduce the risk of a hyperopic outcome such that K = (central topographic power X [376/337.5]) - 6.1.

Although the Aramberri double-K method improves accuracy, many IOL calculation formulas do not readily allow for the entry of two different K values. Ladas and Stark6 described a method to utilize standard IOL formulas to determine an outcome that is very similar to the clinical history double-K method or the Feiz-Mannis method. Using a given IOL formula, the surgeon inserts the prerefractive surgery corneal power. He then inserts the change in manifest refraction at the spectacle plane as the target refraction for emmetropia. Using the SRK/T formula, investigators found that this method produced essentially the same results as those presented by Aramberri.6

BOTTOM LINE

As data have emerged to evaluate methods of IOL power calculation after refractive surgery, studies have shown that the assumptions underlying conventional methods based on the Gullstrand eye produce errors both in corneal power and in the estimation of the ELP. The best method for a given patient depends upon the available clinical history and instrumentation. A perfect method would require no clinical history and would accurately estimate both the corneal power and ELP based solely on measurements taken after refractive surgery. For now, the studies presented herein provide a useful framework for approaching the challenge presented by each patient with a cataract after refractive surgery.

Reviewer
Gregory J. McCormick, MD, states that he holds no financial interest in any company or product mentioned herein. He may be reached at (585) 256-2569; mccormick_greg@hotmail.com.
Panel Members Y. Ralph Chu, MD, is Medical Director, Chu Vision Institute in Edina, Minnesota. He states that he holds no financial interest in any product or company mentioned herein. Dr. Chu may be reached at (952) 835-1235; yrchu@chuvision.com.
Nina Goyal, MD, is a resident in ophthalmology at the Rush University Medical Center in Chicago. She states that she holds no financial interest in any product or company mentioned herein. Dr. Goyal may be reached at (312) 942-5315; ninagoyal@yahoo.com.
Wei Jiang, MD, is a resident in ophthalmology at the Jules Stein Eye Institute in Los Angeles. She states that she holds no financial interest in any product or company mentioned herein. Dr. Jiang may be reached at (310) 825-5000; wjiang70@yahoo.com.
Baseer Khan, MD, is a senior resident in ophthalmology in the Department of Ophthalmology at the University of Toronto. He states that he holds no financial interest in any product or company mentioned herein. Dr. Khan may be reached at (415) 258-8211; bob.khan@utoronto.ca.
Jay S. Pepose, MD, PhD, is Professor of Clinical Ophthalmology & Visual Sciences, Washington University School of Medicine, St. Louis. He states that he holds no financial interest in any product or company mentioned herein. Dr. Pepose may be reached at (636) 728-0111; jpepose@peposevision.com.
Paul Sanghera, MD, is a resident in ophthalmology in the Department of Ophthalmology and Vision Sciences at the University of Toronto. He states that he holds no financial interest in any product or company mentioned herein. He may be reached at (416) 666-7115; sanghera@rogers.com.
Tracy Swartz, OD, MS, states that she holds no financial interest in any product or company mentioned herein. She may be reached at (615) 321-8881;
drswartz@wangvisioninstitute.com.
Ming Wang, MD, PhD, states that he holds no financial interest in any product or company mentioned herein. He may be reached at (615) 321-8881;
drwang@wangvisioninstitute.com.