Eyeworld

EW REFRACTIVE 38 May 2018 Refractive editor's corner of the world by Michelle Stephenson EyeWorld Contributing Writer better job of predicting the effective lens position. "Until we get to that point, we're still going to be bat- ting, at the very best, 90% within ±0.5 D. Errors in corneal power measurement are the other major cause of refractive misses. Being off by more than 0.5 D often compro- mises quality of vision in patients with presbyopia-correcting IOLs and is something we as surgeons must be prepared to acknowledge and address," he said. Daniel Chang, MD, Bakersfield, California, said there are several additional factors that prevent sur- geons from hitting the target every time. "Variability in keratometry can be a factor. If there is dry eye or fluc- tuation in the tear film, a one-time measurement may give you good numbers, but it may not represent the typical refractive state of the eye. Additionally, keratometry is only an approximation of the true corneal curvature. Keratometry provides three numbers that approximate the three-dimensional shape of the cornea, and describing the refractive state of that cornea with a keratom- etry model is a simplification. Most of our current formulas are based on vergence formulas created from that model, and sometimes the models are off. Ideally, if we can ray trace the entire cornea and measure stabil- ity over time, we can better calculate what patients need," Dr. Chang said. When to consider an enhancement According to Dr. Chang, patients dictate when an enhancement is necessary. "I let patient satisfaction drive things. If patients are happy with their refractive state after sur- gery, I call it good. Even with all of the technology we have for preop- erative and intraoperative measure- ments, there are still variables that can prevent us from hitting our re- fractive target, but patient happiness is always my primary goal," he said. Dr. Chang doesn't have a nu- merical threshold for enhancements; it is mostly determined by patient satisfaction and their appreciation of the potential improvement with a refractive correction. "If a patient's uncorrected vision is 20/40 and I do a refraction to 20/20, patients typically notice the difference. If it's a smaller improvement, I'll some- times give them a prescription for glasses to wear temporarily to see if they like that improvement. I occa- sionally even do a contact lens fit to demonstrate whether a refractive correction will make a difference. I use this approach in subtler cases where I'm not sure the refractive error is the entire problem because there may be other issues involved. Most commonly, I choose laser vi- sion correction as my enhancement technique," he said. Dr. Chang said his enhance- ment rate is in the single digits. "I have not needed to perform many enhancements. If you counsel pa- tients thoroughly, if you set expecta- tions correctly, if you are meticulous about your preoperative measure- ments, and if you do good surgery, most patients do well and won't need an enhancement. However, everyone has a different threshold Premium implant cataract surgery P atients who undergo cataract surgery fall into two basic groups: those who want to do a lot with glasses after lens replacement surgery and those who want to do a lot without glasses after surgery. We know that the number one reason patients can end up unhappy is if their uncorrect- ed vision goals are not met. Thus it is not unusual for the premium cataract surgery patient to need a refractive fine tune, which is typically refractive corneal surgery in the form of AK, PRK, or LASIK. I like to tell my patients considering their cataract surgery implant/vision options that in general, all their options involve a fine tune after surgery. I go on to explain that in traditional cataract surgery the fine tuner is glasses (or contacts), and in premium cataract surgery the fine tuner is laser vision correction. Someday soon I will add another exciting category to my preoperative options discussion: adjusting the power of the implant in the eye to truly custom- ize it to patients in the form of light adjustability. The point is enhancing the final refractive/op- tical result is an important part of cataract surgery, and our ophthalmic experts in this column tell us why. Thank you to Steven Schallhorn, MD, Douglas Koch, MD, and Daniel Chang, MD, for teaching us the reasons that we do not hit plano or our precise (I emphasize "precise") refrac- tive endpoint goal are because of issues such as: • the final effective lens position is still something we estimate; • keratometry values still involve assumptions in corneal curvature measuring technology; • it is harder for us to accurately measure the posterior cornea than the front (and both matter!); • biometry variables; and • the incision healing increases or decreases the final astigmatism amount. Even though lens calculations formulas have gotten so amazing, it still comes down to limitations in math and measuring technology; preoperative assumptions have to be used and they help, but they can't see the future so they are not perfect. When we include the fact that individuals have their own unique and variable healing responses, it is not difficult for the well-educated patient to understand why all cataract surgery is a two step process for the very best vision: lens replacement plus glasses or lens replacement plus laser fine tune. I hope you enjoy this column as much as I did. Thank you again to our world class experts. Vance Thompson, MD, Refractive editor High patient expectations necessitate enhancement plans P remium IOLs have greatly improved patient out- comes after cataract sur- gery. With these improved outcomes have come higher patient expectations, so it is important for surgeons to have an enhancement plan for every patient. "The ability to predict the intra- ocular lens power has improved over the years; we and our patients appre- ciate the value of hitting the refrac- tive target," said Steven Schallhorn, MD, chief medical officer, Carl Zeiss Meditec. "Years ago, being within 1 D of the postoperative target was acceptable. Today, there is a grow- ing need to understand how we can get closer to the target. In some patients, being even 0.5 D off is not acceptable." Dr. Schallhorn noted that care- ful preoperative planning can mini- mize the number of patients requir- ing an enhancement. "The majority of today's patients are within 0.5 D of predicted," he said. "Accurate biometry remains the single most important measurement for calcu- lating the correct lens power. This is followed by the power calculation that utilizes the biometry data. However, there are other variables that we do not currently take into account, such as the variability in the posterior corneal curvature and a more accurate method to predict the position of the IOL in the eye (effective lens position)." Douglas Koch, MD, Houston, agreed. He thinks that minimizing the number of enhancements relies primarily on being able to do a Even with the careful preoperative planning and meticulous intraoperative technique, it is important to have a good strategy for possible postoperative enhancement. Source: Daniel Chang, MD

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