Questions and Answers

Questions and Answers

Paolo Vinciguerra, MD: We are measuring the wavefronts of the patients in a steady way. We froze one moment, but the accommodation of the eyes would, of course, change something. The tears phase – that changes during the day, like the intraocular pressure. There are many factors that are in continuous mode change. So, how we can choose which is the best compromise between what you suggest recommending, for example, to take many measurements for intraocular pressure during the day, and then for the future, think of something that compensates for the average. And the second question – if you are compensating by ablation, biomechanics is not only the important point, but also the epithelium, because the epithelium changes a lot, which is not accepted in terms of dramatically changing the curvature of the cornea. What is your opinion?

Raymond A. Applegate, OD, PhD: I will tell you how I interpret your questions, and just shake your head if I have it wrong. The first question is that the aberrations of the eye are dynamic. There are lots of factors, and we can go down a long list and show that the aberration structure is not one fixed value, as David and I might have been implying. I think everybody is aware of this fact. To me, the solution to this fact is down the road a little bit. The first step is demonstrate that we are not inducing new aberrations. Because the variations you are talking about are much smaller than the aberrations we are currently inducing. So, to me, the first step is, let’s get down there. But now, let’s assume that we are down there. Let’s assume that we are not inducing new aberrations but we are actually going in like we have done, and there are some eyes that actually have had their aberrations reduced, some normal eyes that have had their aberrations reduced. Certainly, in highly aberrated eyes, we are doing that all the time now. But, let’s take a normal eye which we are trying to reduce. Paul’s question is, let’s say we have a 25-year-old eye. The eye can still really accommodate, and it’s all over the board. There is no simple answer to that because the aberration structure, regardless of when you measure it, interacts with the object. You almost have to tell me what you want the patient to look at, because the spatial information in that object is going to interact with the aberration structure. So, if you told me that you would really like the person to read at near, that that was the highest priority for the patient, then I would want to look at aberration structures with the eye somewhat accommodated, and correct in that way. It is almost like what we do now when we ask a person if they want a pair of computer glasses. You are fixing it for some distance. Right now, we don’t have the option to fix it. In the future, further down the road, I think we are going to have materials that are implantable, that are constantly changeable. So, as the eye ages and the aberration structure changes, you can go in and alter the aberration structure of the device you have placed in the eye. I

Wavefront and Emerging Refractive Technologies, pp. 21–22

Proceedings of the 51st Annual Symposium of the New Orleans Academy of Ophthalmology, New Orleans, LA, USA, February 22-24, 2002

edited by Jill B. Koury

© 2003 Kugler Publications, The Hague, The Netherlands

don’t remember the second question, Paolo. Oh, the epithelium. Do you have a comment on that?

David R. Williams, PhD: Heidi Hoffer at the University of Rochester built a realtime wavefront sensor to look at some of the temporal variations on the wave aberration. While we can measure these, we can’t always identify exactly where they are coming from. But we don’t think the epithelial changes are enormous on the scale of things we worry about in correcting refractive surgery. We can easily see microfluctuations of accommodation, which, of course, are well known. You can take repeated measures if you want to eliminate those, but, again, those are small, and over a longer time scale than the time scale of microfluctuation of accommodation, the wave aberration is relatively stable. If you are aware of diurnal variations, for example, and I am not aware of these, I’d like to see data on them, but generally, it is a fairly stable thing. I think Ray’s point about the aberration structure varying with vergence is a very important one, and one that may lead us to customizing our correction based on the lifestyle of a particular patient, and if they are doing a lot of near work, we might choose to correct the aberration structure for near work as opposed to far work, and so on. It depends on the patient, perhaps.

Questions and Answers on ”Customized prolate eximer ablation”

(Jack Holladay, MD)

Raymond A. Applegate, OD, PhD: I had a comment about something you mentioned in passing at the end, a study on pilots with coma and so on. For a long time, we have assumed that having no aberrations in the eye will give you the best subjective appearance of, for example, a letter target, and that deserves careful tests. We have actually done that now with our adaptive optic system, where we give people the option of dialing in various amounts of aberrations in order to find out what actually does give the best visual impression of the target. It is when the aberrations are zero. So we do know where the bull’s eye is.

Jack Holladay, MD: What was the target?

Dr Applegate: The target was an E.

Dr Holladay: An E was the target you used. The only thing I would like to point out there is the studies that show basically that the vertical astigmatism with the rule is better in the Arabic world because we have more vertical strokes in the letters. And, of course, in Eastern cultures, that doesn’t work. And it needs to be against the rule because it changes. So the only thing I would point out to you is, again, I don’t think you can know that the bull’s eye is at zero because the fact is that, in the Western world, if there are more vertical strokes in the things we look at, it is better to have vertical coma, and a little with the rule because it clears up that image if you are not absolutely perfect.

Dr Applegate: Just one comment on that. I think it is important to distinguish between what gives the best image quality and how targets can affect how effective aberrations are. So you are absolutely right. Different aberrations will be differently potent, depending on the target you are looking at. Still, the best image quality occurs when you zero out the aberrations.

Ronald R. Krueger, MD, MSE: To comment on the prolate shape, it is true that we induce a lot of spherical aberration with our current laser algorithms and we need to improve upon that, but that is actually what wavefront does. Wavefront takes into consideration what is a unique preoperative spherical aberration, and comes up with an ablation profile to correct that, taking into consideration some of the other peripheral effects to really minimize spherical aberration. So, we could come

Wavefront and Emerging Refractive Technologies, pp. 23–24

Proceedings of the 51st Annual Symposium of the New Orleans Academy of Ophthalmology, New Orleans, LA, USA, February 22-24, 2002

edited by Jill B. Koury

© 2003 Kugler Publications, The Hague, The Netherlands

up with a whole prolate algorithm and eliminate wavefront, but not everybody starts out with the same spherical aberration. There are some people who are negatively aspheric instead of positively aspheric. So, really to cover the bases for everybody you are doing, we will need to move ahead with wavefront.

Dr Holladay: Ron, I’m glad you made that point because we have a little difference of opinion here. Let me see if I can make myself clear. My point is, you’re right, there is one in about 500 people who have negative spherical aberration over the age of 40 years, so it is important that we do wavefront on everyone in order to pick out the one in 500 patients who is negative. But the fact is, every laser is putting insufficient energy in the periphery – and it doesn’t have anything to do with the algorithm. What happens is this. When you do your wavefront and you come up and find out that that patient has positive spherical aberration and then you generate the ablation profile you should generate, that theoretical ablation profile was done on plastic originally and doesn’t compensate for the peripheral energy, so every single patient you have treated who had positive spherical aberration was basically increased in the amount because of the laser’s calibration in the periphery. And, that is why you can’t come up with one single patient you have done who has an asphericity from topography that is negative Q value. All I am saying is, if you correct the laser function, then you can implement the wavefront you are doing right now, but if your laser is putting insufficient energy in the periphery, that’s the reason why the patients are getting more spherical aberration than they had before the surgery. Do you have one patient with a negative Q value?

Dr Applegate: I think Jack is making an important point. Certainly, you would need to know what the laser is doing, but you shouldn’t be confused by adding energy in the periphery to make sure you get the ablation you want, with the necessity of having a prolate cornea. Almost all corneas will be prolate. I totally agree with Jack. What is important is to make it the right amount of prolateness. As to the 20/ 20 acuity, having a bunch of pilots with 20/10 acuity and others with 20/20, I would just draw the audience’s attention back to the slide I tried to show. There are a lot of factors that play into the percept. We all know that if you take a 20/ 20 patient who sees 20/20 and now you reduce their aberrations and they don’t see better, there may be other factors involved in why they see 20/20 and not the aberration. If you take an eye that has a neural component and the capability of seeing better, it will see down to the sampling limit of the retina, which is about, depending on how the spacing is exactly, around 20/8 to 20/10.