Cataract Surgery After Refractive Surgery

Ravi H. Patel, MD; Carol L. Karp, MD; Sonia H. Yoo, MD; Guillermo Amescua, MD; Anat Galor, MD, MSPH

Disclosures

Int Ophthalmol Clin. 2016;56(2):171-182. 

In This Article

Sources of Error

Corneal Power

The main cause for refractive error in IOL prediction lies in the difficulty to accurately measure corneal power after refractive surgery. The general principle in keratometry is that the cornea acts as a convex mirror reflecting the light source off its surface producing a virtual image. The position and size of this image is measured and, as the light source size and distance to the cornea is known, the radius of the cornea can be calculated. Both manual and automated keratometers measure the intermediate areas around the central cornea and the central corneal power is then calculated. In manual keratometry, the reflected mires are approximately 3mm and in automated keratometry the reflections are at a 2.5mm optical zone. Thus, while corneal measurements are taken, the corneal power given is derived from calculations with assumptions of the corneal index and relationship of the anterior and posterior cornea. To measure true corneal power both the anterior and posterior surface must be considered.

The above methods of corneal power estimation are less accurate in an eye with refractive surgery for several reasons. First, corneal measurements are taken at a 2.5mm or a 3.0mm optical zone. However, after myopic (or hyperopic) ablation the power of the central cornea is flatter (or steeper) than the measured central power, respectively.[2,3] In addition, the assumption of a spherocylindrical cornea leads to an overestimation of corneal power by 15% to 25% leading to a hyperopic outcome after myopic ablation and a myopic outcome after hyperopic ablation.[3–5] And finally, by ablating the corneal surface the relationship between the anterior and posterior corneal curvature is changed after refractive surgery, thus the assumption of a refractive index of 1.3375 is no longer accurate.

Formula Error

One of the most common regression formulas used for IOL power estimation is the SRK formula, which is:

where, P is the dioptric power of the IOL, L is the axial length of the eye (mm), K is the average corneal power (K), and A represents a constant specified for the type of lens. Newer generation formulas (Holladay I, SRK/T, and Hoffer Q) also utilize the relationship between the steepness of the cornea and the anterior chamber depth to estimate the effective lens position (ELP). However, after a myopic ablation the formulas predict a falsely shallower anterior chamber depth and thus a more anterior ELP. This ultimately results in an underestimation of IOL power partially contributing to the hyperopic surprise. Conversely, after hyperopic ablation, these formulas predict a falsely deeper ELP resulting in an overestimated lens power with subsequent myopic surprise.

Newer Techniques/Formulas

Given these challenges, newer techniques and formulas have been applied to postrefractive eyes in attempts to improve outcomes after cataract surgery.

The Historical Method

The historical method of keratometry was first described by Holla-day[6] to determine corneal power in patients with a history of radial keratotomy. This method relies on the patients' prerefractive surgery corneal power and spherical equivalent in combination with their postrefractive surgery spherical equivalent.[6,7]

It remains important to note that this method provides only an estimation of corneal power which can then be used in regression formulas for IOL power prediction. Argento et al[8] compared the predictability of various methods of IOL power calculation in 7 cases [6 post-laser-assisted in situ keratomileusis (LASIK) eyes and 1 postradial keratotomy eye] using the Holladay 2, Hoffer Q, and SRK/T formulas and found that the clinical history method with the Hoffer Q formula provided the best results with a mean dioptric error of – 0.98±0.87. Although this may be more effective than standard methods of IOL prediction in patients with a history of refractive surgery, subsequent studies have shown that large variations of IOL power prediction and large refractive errors still occur.[6–9]

Double-K Method

Aramberri[10] described the double-K method to account for the IOL formulas use of corneal power to determine ELP. Similar to the clinical historical method, the double-K method utilizes prerefractive surgery data to obtain the postrefractive surgery corneal power. However, the double-K method also relies on prerefractive surgery corneal power data to determine the ELP. The Holladay 2 formula allows direct entry of 2 corneal power values for the double-K calculation. Subsequent studies have shown greater accuracy of IOL power prediction in third-generation and fourth-generation formulas when utilizing the double- K method.[11]

ASCRS Online

A commonly used tool from the Web site of the American Society of Cataract and Refractive surgery is the IOL power calculating tool. From this site, the type of refractive surgery is selected and then all known preoperative and current data are entered. The IOL power is calculated by a variety of formulas, both those requiring historical data and those that do not. All predicted IOL powers are displayed when data required for them are entered, as well as an average of all available formulas. This remains a useful tool in estimation of IOL power calculation and can be utilized at http://iolcalc.org.

Tomography

With the advent of devices that can measure the tomography of the cornea with Scheimpflug images or Fourier-domain optical coherence tomography (OCT), the anterior and posterior surface powers can be more accurately calculated using Gaussian optics. Thus, a benefit of this methodology includes the possible elimination of regression formulas.[12,13] Fourier-domain OCTcarries the advantage of higher resolution and speed of image acquisition to traditional Scheimpflug images, thereby decreasing potential motion artifact. However, no difference in predictive accuracy of Scheimpflug and OCT-based IOL calculations has been demonstrated.[14]

Intraoperative Aberrometry

Intraoperative aberrometry is a relatively new technique for IOL power determination that has shown promise in patients with a history of refractive surgery.[15] This technique uses Talbot-Moire wavefront aberrometry to measure the refraction of the entire optical system in an aphakic eye during surgery. The independence of historical or even preoperative data is an advantage to previously discussed methodologies. Recent studies found this to be statistically better at IOL power prediction to several techniques and can reach target refraction in similar percentages to virgin eyes undergoing cataract surgery.[16–18] Even this technique has its limitations, however, as aphakic measurements must be taken in the operating room, which can be altered depending on intraocular pressure, patient fixation, and external pressure from the speculum. In addition, determining the ELP still remains a source of error.

Preoperative Planning

Given the imprecise nature of IOL prediction in eyes with a history of refractive surgery, several methods are typically used. It is recommended to review the IOL prediction using several techniques and eliminate any outlier values. Even with intraoperative aberrometry it would be advisable to obtain multiple methods for IOL selection to minimize refractive surprise. Some surgeons will aim for slight myopia to avoid a hyperopic outcome in patients with a history of myopic procedures; however, it is important to note that in patients with previous high myopic ablations it may be easier to perform a hyperopic ''touch up'' ablation in a centrally thinned cornea.

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