Eyeworld

EW CATARACT 30 January 2011 by Warren Hill, M.D. Avoiding pitfalls in IOL calculations O ne of the most important ways surgeons can avoid pitfalls when making IOL calculations is by develop- ing and using validation criteria. Doctors should not blindly accept whatever information their calculating machines give them without a lot of understanding as to what the numbers mean. For example, if the axial length of two eyes are being measured, and the right eye is 23.4 mm while the left eye is 25.6 mm, some physicians may simply use these numbers with- out question. However, in reality, eyes are usually symmetrical unless there are specific reasons for them not to be. In this case, there is a huge disparity in the measurements between the two eyes and unless the physician switches on his or her brain when the machine is turned on, this discrepancy will not be picked up. If the patient has no dif- ference in the refraction between his or her eyes and yet the machine gives those numbers that are very different, lights should go on, and the physician needs to find out what it is that accounts for such a large difference between the two eyes. Using validation criteria, which can be applied to axial length, ante- rior chamber depth, white-to-white measurements, and the lens power itself, physicians can look at the numbers and see if they actually make sense in the context of each particular patient. Another equally important way surgeons can avoid IOL calculation errors is by taking into account the artifacts created by refractive sur- gery. Physicians are commonly met with eyes that have undergone prior refractive surgery, LASIK, PRK, and radial keratotomy, and these eyes cannot be approached with normal calculations. The central corneal power that is estimated by the ma- chine is going to be wrong and the older formulas are going to be thrown off by artifacts created by re- fractive surgery. Physicians, there- fore, need to develop an approach to doing calculations for these eyes. The ASCRS website provides a calcu- lator for eyes with prior radial kera- totomy, myopic LASIK, and hyper- opic LASIK. Finally, surgeons need to aban- don older formulas—which take into account fewer variables—for newer, more advanced ones. It is mind-boggling that physicians spend so much time obsessing about what type of microkeratome or what phaco settings to use and yet neglect to upgrade the part of the procedure by which they're judged by their pa- tients and their peers. Some eyes may have anterior segments that are very small or very large, and the most common third generation formulas—the Holladay I, Hoffer Q, and SRKT—would be blind to such anatomical differ- ences. These 20-year-old formulas are completely unable to make the distinction between certain parts of the anatomy. Surgeons cannot ex- pect older formulas to perform at a very high level when they use so lit- tle information to come to the right conclusion. Instead, physicians should think about using modern tools like the Olsen formula, the Holladay II, or the Haigis formula because they are much better at coming up with the right answer for eyes that might be a little bit un- usual. It is helpful to keep in mind that even the newer formulas are theoretical formulas, which means that they make assumptions based on prior experience. The more so- D espite the advances in oph- thalmic education, biometric in- strumentation, and IOL calculation formulas, I am still amazed to hear on occasion the stories of fellow surgeons using automated keratometric measurements for their post-RK IOL calculations or outdated IOL formulas for their routine cases. Warren Hill, M.D., is one of the world's leading au- thorities on IOL power calculations, es- pecially in regard to difficult and challenging cases. In this month's col- umn, he touches on some of the pitfalls encountered during lens calculations and gives us a brief view of where the future of this field may be heading. It is impossible to thoroughly cover this complex aspect of cataract and re- fractive surgery in this short column, but I would encourage surgeons to visit Dr. Hill's website (www.doctor-hill.com). This site is one of the most complete and valuable resources for understand- ing the nuances of proper IOL power calculations and keratometric measure- ments, especially in post-refractive sur- gery patients. A half day completely reviewing the information on his site will go a long way to improving the re- fractive results in your cataract patients. Richard Hoffman, M.D. Column Editor continued on page 33 A master in IOL calculations, Warren Hill, M.D., Mesa, Ariz., discusses how to avoid potential errors in one of the most crucial elements of refractive cataract surgery

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