3 Postrefractive Intraocular Lens Power Calculation: Choosing the Right Nomogram Abstract Since the advent of refractive procedures in the late 20th century, a growing number of patients have undergone these procedures to enhance their vision and social outlook. As these patients grow older and develop cataracts, they require a surgical procedure to remove the opacified lens and implant an intraocular lens (IOL). Given the vastly satisfactory outcomes of their refractive surgery, these patients understandably maintain high expectations from their subsequent cataract surgery. Although the process of choosing an appropriate IOL remains a challenging task in virgin eyes, the task is even more complex in postrefractive eyes. Ironically, the therapeutic effects of refractive surgery make the outcome of IOL calculations for an eventual cataract surgery relatively suboptimal. Most modern IOL formulae and instruments were devised prior to the popularity of refractive surgery. Thus, they provide a high level of accuracy for eyes with physiologic corneas, but not so for ones that have undergone corneal flattening or steepening effects of myopic or hyperopic refractive surgery, respectively. These corneal changes result in inaccurate measurement and calculation of two important components of modern IOL formulae: corneal power and estimated lens position. Consequently, experts in the field have developed a multitude of various methods, formulae, and devices to address these challenges. Further, newer and more sophisticated methodologies and devices are forthcoming, which may result in more precise calculations and measurements. Therefore, it becomes important to understand and consider these methods carefully in combination with clinical judgment to achieve improved refractive outcomes. Keywords: cataract, cataract surgery, postrefractive cataract surgery, intraocular lens, intraocular lens calculations, intraocular lens formula, laser-assisted in situ keratomileusis, photorefractive keratectomy, LASIK, PRK Every year, approximately a million patients in the United States choose to undergo some form of corneal refractive surgery to correct their vision.1 This growing patient population will subsequently develop a cataract with age and have high expectations from their cataract surgery refractive outcomes like those of their previous refractive procedures. Advances in technology, instrumentation, and surgical technique have led to significant improvement in cataract procedures. However, compared to the results of refractive surgery, cataract surgery outcomes in these patients are relatively suboptimal.2 A major reason for this disparity is due to the inadequate accuracy of intraocular lens (IOL) power calculation formulae in postrefractive eyes. Given the rising number of patients with a history of refractive surgery and their high expectations, calculating accurate IOL power for postrefractive eyes is of considerable importance. Many different forms of refractive procedures exist that are based on a variation of approach, instrumentation, and anatomy. Procedures involving myopic correction with excimer laser treatment (i.e., photorefractive keratectomy [PRK], laser in situ keratomileusis [LASIK]) are of most interest as a growing number of patients have undergone this procedure in the previous few decades. After the advent and Food and Drug Administration (FDA) approval of myopic LASIK in the late 1990s, it was found that accurate IOL power calculation prior to cataract surgery is more challenging in eyes that underwent refractive surgery than those that did not.3,4 Cataract surgeries in patients with a history of these refractive procedures may lead to an undesirable hyperopic refractive result. Conversely, cataract surgery in patients with a history of hyperopic LASIK may lead to an undesirable myopic refractive shift. These risks are due to the changes to the corneal surface that take place as part of refractive surgery, which affect the process of accurate IOL calculations. Experts in the field have described a variety of methodologies and approaches to minimize the errors in postrefractive IOL calculations.5,6,7 Despite the continued improvement in outcomes of these patients, there is lack of a single, perfect solution to this problem, as demonstrated by the existence of a plethora of formulae, devices, and potential solutions.8 Modern IOL formulae have evolved over the years and have become extremely sophisticated and accurate in their ability to calculate lens power. These theoretical and regression formulae were mostly developed prior to the advancement of refractive surgery. Thus, these formulae are accurate for physiologic corneas but not so for nonphysiologic corneas such as those altered by refractive surgery. Third-generation IOL formulae such as Hoffer Q, Holladay I, and Sanders–Retzlaff–Kraff (SRK)/T rely on two main variables: axial length and corneal power.6 These two variables are crucial components in the calculation of vergence and estimation of the postoperative lens position (i.e., estimated lens position [ELP] or anterior chamber depth [ACD]). As subsequent generations of IOL formulae have been developed, more variables have been introduced in formulae such as Holladay II, Barrett, and Haigis to further refine the lens power calculation. However, axial length and corneal power remain the two primary variables in the modern IOL formulae. Modern IOL formulae and instruments are based on certain assumptions about the optical and anatomical characteristics of a virgin cornea. However, following most corneal refractive procedures, these assumptions are invalidated. Thus, when standard IOL formulae and biometric instruments are used in these eyes, a significant “refractive surprise” may be encountered after cataract surgery. There are two main sources of error that a surgeon will encounter when attempting IOL calculations for postrefractive eyes: (1) the imprecise measurement of the true corneal power after refractive surgery9,10 and (2) the inaccurate predictability of the ELP by the third- and fourth-generation IOL formulae.11 The refractive power of the cornea is one of the two most important variables in IOL calculations.5 It is the most obvious source of error in performing IOL calculations in postrefractive eyes. This is due to the changes to the cornea that occur as a result of refractive surgery. These changes can vary from being a dramatic resurfacing to an alteration of refractive index of the cornea.12,13 Standard manual keratometry, automated keratometry, and topography instruments for virgin eyes are not suitable for the measurement of postrefractive eyes4,14 due to the surgical manipulation of the anterior cornea. As only the anterior surface of the cornea is impacted, this affects the refractive relationship between the anterior and posterior surface of the cornea. Not only are standard keratometers unable to account for the changes to the anterior surface of the cornea, but also they are unable to directly measure the posterior corneal power. Traditional keratometry instruments measure the corneal power at four points of a paracentral 2.5- to 3.2-mm ring. These devices work on the assumption that a cornea is a spherocylindrical surface. Indeed, in a virgin eye, that holds true, as the central 2- to 3-mm area of the cornea is approximately spherical. In the postrefractive eye, however, the spherical nature of the central cornea is affected due to surgical manipulation ( Fig. 3.1 Anterior flattening of the cornea due to myopic refractive surgery. Cross-sectional graphical renderings are shown in the (a) axial plane of a physiologic cornea and (b) anteriorly flattened central cornea due to myopic refractive surgery. It is important to note that the posterior cornea remains unaffected. A standard keratometer or topographer measures the corneal power (P) value by deriving it from the measured radius of curvature of the anterior corneal surface (r) and the “effective” index of refraction (n) according to the following formula: P = (n − 1)/r. However, these keratometers and topographers are unable to directly measure the radius of curvature of the posterior cornea (i.e., power of the posterior cornea). For virgin eyes, most keratometers and topographers compensate for the negative power of the posterior cornea by assuming that the radius of curvature of the posterior surface of the cornea is 1.2 mm less than the anterior curvature.18 Based on Gullstrand’s model eye,19 a modified index of refraction of 1.3375 is used as opposed to 1.3760 of the cornea itself. However, refractive surgery, by causing a decrease or increase of anterior corneal curvature, alters and invalidates the anterior-to-posterior cornea relationship ( The ELP is a calculated prediction of the postoperative distance between the optical center of the implanted IOL and the anterior surface of the cornea. Thus, it cannot be physically measured preoperatively. The incorrect calculation of the ELP in third- and fourth-generation IOL formulae is further responsible for the inaccuracy of IOL power calculation in postrefractive eyes.6 Newer generations of IOL formulae have sought to improve the ELP predictability by introducing additional measurable variables such as preoperative refraction, horizontal white-to-white distance, ACD, lens thickness, and patient age.4 The anterior cornea radius of curvature (i.e., corneal power) is closely related to the ELP predictability. After myopic refractive surgery, the cornea is flatter than before. This affects the calculation of the ELP using traditional IOL formulae as a more forward lens position is predicted. This overestimation of ELP results in calculation of an underpowered IOL. Implantation of an underpowered IOL will then result in more hyperopic refractive outcomes.6 Conversely, after hyperopic refractive surgery, the cornea is steeper than before. In these eyes, an underestimation of the ELP occurs, which then results in calculation of an overpowered IOL. Implantation of such an IOL will result in more myopic refractive outcomes.12,22 The result of using standard methods of keratometry in a patient with a history of myopic LASIK or PRK refractive surgery would be an overestimation of the true corneal power.23 This overestimation, thus, leads to more hyperopic results than intended after cataract surgery as a lower powered lens is calculated by IOL formulae. Conversely, hyperopic refractive surgery such as LASIK and conductive keratoplasty (CK) will cause the keratometric process to result in an underestimation of the corneal power. As the central cornea is now steeper, standard methods of keratometry are unable to appropriately measure the now steeper cornea.22 The effects of this underestimation yield an IOL power higher than intended and thus results in more myopic refractive outcomes after cataract surgery.11,24,25 Unlike the effects of LASIK and PRK, radial keratotomy affects both the anterior and posterior corneal surfaces. This may more closely preserve the anterior–posterior corneal relationship.16 It could also cause a more severe overestimation as the cornea undergoes greater central-than-paracentral flattening.26,27 Due to the variability of refractive change in the anterior and posterior cornea, refractive outcomes after cataract surgery in patients with a history of radial keratotomy are less predictable.15 There is a plethora of solutions available to address the problems that exist with IOL calculations in patients with a history of refractive surgery ( The current “gold standard” for IOL calculations in postrefractive eyes is to rely on the biometric and refractive data prior to the refractive surgery. These data include the prerefractive corneal power (Kpre
3.1 Introduction
3.2 Origins of Error
3.2.1 Keratometric Error
Measuring the Anterior Cornea
Fig. 3.1).15,16 Thus, these devices ignore flatter or steeper more central regions after myopic or hyperopic refractive surgery, respectively.10 Such measurement technique will not provide an accurate measurement of the central flattening or steepening of the anterior cornea that occurs with myopic or hyperopic laser refractive surgery, respectively.17
Measuring the Posterior Cornea
Fig. 3.1). Thus, the index of refraction (1.3375) used by standard keratometers would be invalid and result in measurement of unreliable corneal power values.12,20,21
3.2.2 Estimated Lens Position Error
3.2.3 Surgery-Specific Error
3.3 Solutions to Postrefractive IOL Calculations
Table 3.1). The proposed methods can be organized by those that require prerefractive “historical” data or not, and further by those that require topographic measurements or not.
3.3.1 Historical Data Methods
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