EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/76185
18 EW NEWS & OPINION July 2012 Complicated cataract cases IOL power calculations in patients with prior corneal laser refractive surgery by Andrea L. Butler, M.D., and Bonnie An Henderson, M.D. A A challenge that cataract surgeons face with increasing frequency is the patient with a keratorefractive surgical history. These patients usually have high expectations for good unaided vision. Addi- tionally, their surgical past makes it difficult to calculate intraocular lens power. There have been a number of keratorefractive procedures over the years, and each alters corneal shape in a unique way. A limited list includes radial keratotomy, hexagonal keratotomy, laser thermal keratoplasty, conductive keratoplasty, epikeratophakia, keratomileusis, automated lamellar keratoplasty, Intacs ring segment (Addition Technology, Des Plaines, Ill.) implan- tation, PRK, LASIK, and LASEK. Far and away the most common today are the excimer procedures, PRK and LASIK. Excluding cases of keratoectasia, excimer laser procedures alter anterior corneal shape without changing posterior corneal shape. In doing so, they alter the ratio of the curves of the two refracting surfaces. This change makes it difficult to calculate total corneal refracting power by traditional methods that measure only anterior radius of curvature. This change also confounds the prediction of effective lens position (ELP), which is the plane inside the eye where the intraocular lens will sit after cataract surgery. While ELP calculations were never extremely precise with any formula, ELP is more likely to be predicted by the pre-op anterior corneal curve than the anterior corneal curve follow- ing LASIK or PRK. Changes in corneal asphericity and treatment-induced high order aberrations can also confound the outcomes of cataract surgery. The question and challenge for the cataract surgeon is how to compensate for the changes caused by corneal laser refrac- tive surgery when calculating intraocular lens power. It is widely understood by most in the industry that we have not arrived at a com- pletely satisfactory solution, especially for patients who lack surgical records, but we are considerably further along than we were 10 years ago. Most patients can expect to achieve post-op refractive results that are within a diopter of intended. Patients with rel- atively minor LASIK and PRK corrections can expect to achieve even better results. In their EyeWorld article, Drs. Butler and Henderson offer their approach to managing patients with a history of PRK or LASIK. Kevin Miller, M.D., Complicated cataract cases editor seminal paper published in the American Journal of Ophthalmology in 1983 in- troduced the concept of refractive corneal surgery. New York City ophthalmologist Dr. S. Trokel, in conjunction with a photochemist working at IBM, Dr. R. Srinivasan, and researcher B. Braren, described the technique of "ablative photodecomposition" using the excimer laser to remove corneal tissue in enucleated cow eyes. Since approval in 1995 by the FDA, pho- torefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK) have become common surgical procedures. An estimated 5,000,000 cases are performed worldwide each year. While the development of re- fractive surgery has created wonder- ful benefits for patients by providing less dependence on spectacles, it has created new obstacles for ophthal- mologists when these same patients require cataract surgery later in life. IOL power calculation becomes a challenge, as the true corneal refrac- tive power after refractive surgery becomes more elusive. Measuring keratometry Standard manual keratometry meas- ures the anterior corneal curvature of four data points in two perpendi- cular meridians within the central 3- mm zone. These four values are then applied to a formula, which is based on spherical geometry, to derive the corneal power. A source of error re- sults from standard keratometry as- suming that the cornea is a single refracting surface by only taking an- terior corneal curvature into account and simply assuming that the poste- rior curvature is a percentage of the anterior curvature. The keratometry formula attempts to correct for this in a normal prolate cornea by under- estimating its refractive index. Be- cause of the contribution from the posterior surface and the cornea's true index of refraction, the true net power of the cornea is lower than the value reported by standard ker- atometry even in normal eyes. This discrepancy between true corneal power and that reported by keratom- etry is more pronounced still in post-refractive eyes; LASIK and PRK change the ratio between the ante- rior and posterior curvature of the Figure 1 Source: Andrea L. Butler, M.D., and Bonnie An Henderson, M.D. cornea, which is the basis for deriv- ing the index of refraction. This often results in post-refractive patients having a hyperopic refrac- tive error after cataract surgery. Errors also occur because refrac- tive surgery alters the corneal as- phericity, thereby increasing the range of central corneal powers. This is a source of error in using tradi- tional keratometry to calculate IOL power in post-refractive eyes. Advancements have been made in tools available for pre-op corneal assessment. Current generation topographers sample more data points than traditional keratometers and do so over a larger area of the cornea than simply the central 3- mm zone. Even so, many machines still have a central "blind spot" that results in difficulty calculating the actual posterior corneal curvature in post-refractive corneas. Assumption of ELP The FDA suggested the term "effec- tive lens position" (ELP) in 1995. While a number of variables are used to aid in predicting the ELP pre-op, corneal architecture is an important one. In simple terms, if the cornea is flat, as in a hyperopic eye, a shallower anterior chamber is assumed, thereby assuming that the IOL will be closer to the cornea. However, if the cornea is steep, as in a myopic eye, a deeper anterior chamber is assumed, thereby assum- ing that the IOL will be farther from the cornea. Because the corneal architecture of post-refractive eyes has been al- tered, this approach to estimating the ELP is inaccurate. The predicted anterior ELP after myopic LASIK/ PRK, for example, underestimates the IOL power with all formulas, with the exception of Haigis. Approaches to overcome this inaccu- racy include a number of methods (Figure 1). The most practical adjunct tool in IOL calculation for these patients is the ASCRS Post- Keratorefractive Intraocular Lens Power Calculator, created by Warren Hill, M.D., Li Wang, M.D., and Douglas Koch, M.D. (iol.ascrs.org). By employing pre-LASIK/PRK K-values and change in manifest re- fraction, this online IOL calculation program can estimate corneal power and effective lens position using the clinical history, Feiz-Mannis, and corneal bypass methods. By employ- ing a change in manifest refraction only, the online program can estimate corneal power using the Adjusted EffRP, Adjusted Atlas 0-3, Masket Formula, Modified-Masket, and Adjusted ACCP methods. And when no prior data are available, by employing current corneal measure- ments only, the Wang-Koch- Maloney, Shammas, Haigis-L, and Galilei methods can estimate corneal power. The program then uses the resultant values to calculate IOL powers in an appropriate fashion (Figure 1). Prediction errors of these various methods have been assessed and validated. For example, these errors are noted to range from 0.57+0.51 (average power) to 1.31+1.11 (Feiz-Mannis method). Corneal aberration In considering IOL selection for post-refractive patients, it is impor- tant to remember that refractive sur-