IOL Power Calculations After Keratorefractive Surgery

 

IOL Power Calculations After Keratorefractive Surgery

How the Pentacam simplifies an increasingly important task.

BY WARREN E. HILL, MD

Performing IOL power calculations for eyes with prior RK and LASIK is becoming more and more of a problem for ophthalmologists; in fact, it is something of a tsunami that will wash over many practices. In the US alone, millions of patients have undergone RK and LASIK since these procedures were first introduced, and a significant portion of these individuals are nearing the age at which cataract surgery is becoming commonplace. More than ever, ophthalmologists now need a workable strategy for easily and successfully handling IOL power calculations following all forms of keratorefractive surgery. 

A TWO-PART PROCESS 

Calculating the IOL power for the postkeratorefractive surgery patient is a two-part process. First, we must estimate the central corneal power as best as we can. The problem with using standard equipment is that keratometers are completely blind to the central cornea, measuring instead an intermediate zone and extrapolating a central value. Standard topographers, which generate simulated K readings, suffer from a similar problem in that they do not see the very center of the cornea and are, in essence, a topographer trying to be a keratometer. For patients who have undergone prior myopic keratorefractive procedures, this problem may lead to a significant overestimation of the central corneal power. This is especially true for eyes that have undergone prior RK. For example, if the device measures a power of 34.00 D, but the very center of the cornea is 28.00 D, we may get an unpleasant hyperopic surprise postoperatively. Although some topographers try to average the power of the central cornea, they are still blind to this area, which happens to be the flattest point in RK eyes as well as for eyes that have undergone myopic LASIK. The benefit of using a Scheimpflug camera is that it rotates around the center of the cornea, successfully capturing a measurement of that area so it is included in the power measurement. This is why the Pentacam Comprehensive Eye Scanner (Oculus, Inc., Lynnwood, WA) is such an important advance in corneal mapping technology. Second, the artifact of a very flat or very steep cornea’scentral power can lead standard third-generation, twovariable formulas to incorrectly estimate the effective lens position when carrying out the IOL power calculation. This is because several popular two-variable formulas, such as SRK/T, in part tie the effective lens position to the central corneal power. If a central corneal power of 34.00 D is entered into one of these formulas, it may incorrectly assume that the anterior segment is very shallow and call for less IOL power than is actually required. This is a second and often overlooked cause of unanticipated postoperative hyperopia following myopic keratorefractive surgery. The problem can be overcome by performing a “double-K” method of calculation, as is done by the Holladay 2 formula (Holladay Consulting, Inc., Bellaire, TX), or by performing a double-K method correction to two-variable formulas as outlined in the literature.1,2 IOL calculations after refractive surgery must take both of these aspects into consideration: (1) the correct estimation of central corneal power and (2) have some methodology for removing the artifact of a very flat or very steep central corneal power from the effective lens position part of the IOL power calculation. 

READING THE DATA Eyes With No History or Previous RK Another notable feature of the Pentacam is its ability to give basic information about eyes for which there are no available records. Most surgeons at some time have encountered a patient who underwent previous refractive surgery but for various reasons cannot provide his records. The Pentacam allows us to have a very good idea as to whether the patient had myopic or hyperopic LASIK or RK, simply by noting some important differences in the numbers. The ratio between the posterior and anterior corneal radii of a normal, unoperated eye is approximately 82.5%. Eyes that have undergone RK typically have a very flat central cornea, and the ratio between the posterior and anterior portions of the cornea is very high, typically above 90% (Figure 1). This is the signature of an eye with prior RK. For measuring eyes with prior RK, the Pentacam uses a 5-mm equivalent K reading. However, one caveat is that RK corneas are often highly multifocal, so even the Pentacam’s measurement can be imprecise. The multifocal nature of these corneas can lead to wide variability in the estimation of central corneal power. On a typical front-surface–power map, an RK cornea may show an 8.00 D range of power across a 2-mm area. 

In 2007, Jack Holladay, MD, and I conducted a study in which we measured post-RK eyes using the Pentacam. This research showed a fairly good correlation between the equivalent K reading for a 5-mm zone and the backcalculated central corneal power using the Holladay 2 formula. As would be expected, the correlation was not as tight as we saw for the post-LASIK eye, due to the highly multifocal nature of these post-RK eyes. In performing IOL power calculations for eyes that have previously undergone RK, it is helpful to keep in mind that very often, RK is the gift that keeps on giving. By this statement, we mean that some degree of hyperopic drift may continue through a patient’s lifetime. It is a very common story that many patients who had a good initial refractive outcome have since shifted toward significant hyperopia. Because this effect may continue throughout their lives, it is generally wise to err on the myopic side when doing a post-RK calculation. 

This strategy is a good idea for two reasons: (1) hyperopic errors are more common than myopic errors and (2) as the years pass, these patients are more likely to have their refraction shift toward more favorable vision, rather than away from what they want. Instead of plano, I prefer to target -0.50 to -1.00 D for the post-RK patient, so that 5 years following their surgery they are not hyperopic. Rule of Twos When performing cataract surgery on patients who have had prior RK, expect some degree of hyperopia during the first postoperative week. In fact, I like to see approximately +1.00 D of hyperopia for an eye with eight incisions in the immediate postoperative period, especially if there is a small optical zone. If an unstable refractive outcome worries you, use what we call the rule of twos—two refractions on two consecutive visits, approximately 2 months after surgery—before making plans for a lens exchange or secondary piggyback implantation. This gives the cornea enough time to shift back to its presurgical configuration.

 Myopic LASIK In contrast to the RK eye, eyes that have undergone prior myopic LASIK or PRK are somewhat steeper in the central cornea and have a lower-than-normal ratio between the posterior and anterior cornea, typically less than 70% (Figure 2). So, if the Pentacam shows a flat central cornea and a low ratio, then the eye has undergone myopic LASIK or PRK. In estimating the central corneal power for the eye that has undergone prior myopic LASIK or PRK, the equivalent K reading for the 4.5-mm zone will often give a good estimation of the central corneal power. However, depending on the type of ablation, sometimes a smaller equivalent K reading may work better. Hyperopic LASIK The hallmark of hyperopic LASIK is, of course, a steep central corneal power as well as an increased ratio between the posterior and anterior corneal radii (Figure 3). It does not increase as much as with RK, but it increases nonetheless. This effect would seem counterintuitive, because one procedure is myopic incisional and the other is hyperopic ablative, but they both have the same effect on the measured ratios. So, if you are unsure whether an eye has undergone hyperopic or myopic LASIK, look at the central corneal power and the ratio between the posterior and anterior corneal radii as a place from which to begin. 

CALCULATIONS 

The next critical step is performing the IOL power calculation. In our practice, we use the Holladay IOL Consultant Software and Holladay 2 formula (both by Holladay Consulting, Inc.), which was the first formula to incorporate a double-K method. Check the box on the preoperative data screen that says “previous RK, ALK, LASIK” to remove the artifact of the iatrogenically altered central corneal power. Then, input the Surgeon-Entered Value for the central corneal power. Some surgeons have found it helpful to compare several IOL power calculation methodologies side by side. Doing this work by hand used to be quite cumbersome and time consuming and was frequently prone to errors. But, thanks to the generosity of luminaries in the field such as Jack Holladay, MD; Wolfgang Haigis, PhD; Douglas Koch, MD; and Li Wang, MD, PhD, a free online postkeratorefractive calculator is now available that allows for the input of data from several different instruments, including the Pentacam. This tool is very helpful and saves a tremendous amount of time. You may view the calculator online at http://www.iol.ascrs.org. 

SUMMARY

 Remember that IOL power calculations following all forms of keratorefractive surgery are a two-part process. First, we must estimate the central corneal power. Depending on the degree of multifocality of the cornea, this estimation can be easy or very challenging. Next, the IOL power must be calculated employing some form of a “double-K” method to remove the artifact of an iatrogenically changed central corneal power. The Holladay 2 formula is becoming the standard for this type of calculation, but other methods may also be employed using more common third-generation, two-variable formulas. In the Pentacam, we now have another useful tool in performing IOL power calculations for the postkeratorefractive eye. 

■ Warren E. Hill, MD, is Medical Director of East Valley Ophthalmology in Mesa, Arizona. He is a consultant for Oculus, Inc., but acknowledged no direct financial interest in the company or its products. Dr. Hill may be reached at (480) 981-6130; hill@doctor-hill.com. 1. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: Double K method. J Cataract Refract Surg. 2003;29:2063–2068. 2. Koch D, Wang I. Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg. 2003 29:11:2039-2042.