Refraction after IOL Exchange

Jin et al1Jin G.J. Crandall A.S. Jones J.J. Intraocular lens exchange due to incorrect lens power.Ophthalmology. 2007; 114: 417-424Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar developed empiric relations for predicting the refraction after intraocular lens (IOL) exchange from a sample of 22 eyes. These relations have some value, but also some disadvantages. Proportions of variance explained by the regression were stated as r2 = 0.76 for hyperopic eyes and r2 = 0.95 for myopic eyes. In fact, the data from cases 3 and 4 were omitted from their Figure 1. With inclusion of these cases, the coefficient of determination for hyperopic eyes drops to r2 = 0.53. Because the predictive equations did not have a zero intercept, replacing the lens with a lens of identical power was predicted to change the spherical equivalent (SE) refraction by +0.87 diopters (D) in hyperopes and by −0.41 D in myopes. The regression equations also do not apply to exchange for an IOL with a different manufacturer’s A-constant or a different effective lens position.2Holladay J.T. Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations.J Cataract Refract Surg. 1997; 23: 1356-1370Abstract Full Text PDF PubMed Scopus (212) Google Scholar A theoretical approach offers several advantages. We noted that double application of the Holladay refractive vergence formula might apply (equation 12 of Ref. 2). This formula predicts the refraction after addition of refractive power to the eye and was offered as a means to calculate the refraction after secondary or piggyback lens placement.2Holladay J.T. Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations.J Cataract Refract Surg. 1997; 23: 1356-1370Abstract Full Text PDF PubMed Scopus (212) Google Scholar We used the formula once to determine the refraction after IOL removal. This predicted aphakic refraction and the new IOL power were input into the formula to determine the final refraction after IOL exchange. A similar approach was taken by Hideyuki et al in 3 patients.3Hideyuki T. Yoshiaki N. Yoshiaki H. et al.A simple and accurate method to calculate emmetropic intraocular lens (IOL) power for IOL exchange.Folia Ophthalmol Japonica. 2005; 56: 765-767Google Scholar We applied this method to the Jin et al data and plotted the actual change in refraction against the predicted change in refraction. The proportion of variance explained by the method was 0.93 (Fig 1 [available at http://aaojournal.org]). The regression equation was change in observed SE = 1.09 * (change in predicted SE) − 0.026. When the 3 cases of Hideyuki et al were added to the analysis, the r2 value was still 0.93. This method has several advantages. First, it provides a theoretical underpinning to the problem. The regression coefficient close to 1 and the intercept close to 0 mean that the theory predicts the observations without scaling coefficients or fudge factors. Second, the same equation applies to both myopes and hyperopes. Third, the method theoretically may apply to changes in the IOL A-constant or in the effective lens position (e.g., anterior chamber, sulcus fixation, capsular fixation), although this dataset does not provide empirical validation of this possiblity. Finally, the low intercept value (0.026 D) correctly indicates that an IOL exchange with an identical lens will not change the refraction. This method also complements traditional biometry formulas, based on axial length and keratometry. First, the presence of an unexpected refraction may indicate that traditional formulas are inaccurate in a particular patient. This refractive method does not use axial length, which may be difficult to measure accurately with a staphyloma, intraocular silicone oil, etc. Second, although keratometry enters the equation, the double-application method is very insensitive to changes in keratometry. Therefore, the method may be particularly useful when effective keratometry values are uncertain, such as after refractive surgery or with irregular astigmatism. Refractive vergence and traditional biometry equations can both be used before IOL exchange to ensure that all information is considered. Author replyOphthalmologyVol. 115Issue 4PreviewWe greatly appreciate the comments of Dr Leffler et al on our work and acknowledge their recommendations regarding the intraocular lens (IOL) power prediction for IOL exchange. Full-Text PDF