Questions and Answers on “Supervision with wavefront”

(Jack Holladay, MD)

Question from a panel member: In terms of topographers, it seems as if topographers are going to be more important today, and I know your Astra is a new thing. What is available and what is your favorite topographer today, and why?

Jack Holladay, MD: That’s a loaded question, isn’t it? Let me just rank what I think they are. In terms of repeatability, I think that the Humphrey Atlas is an excellent unit. It gives you good points. The ISIS 2000 is a good unit, and ISIS is back in business, as you probably know. Joe Wakiel bought it back, so they have units. You can repair your old ISIS. The Tomey unit that Steve worked on for so many years is very repeatable and also does a very good job. The Technomed made by Cscan in Germany is also a very good unit. The only one with which I have a little problem with the Placido aspect is the Orbscan. The height data are very good from the slit, but I think the Placido data from the Orbscan is poor, and I think they would agree with that. You take two Orbscans in a row and the Placido picture is never the same, but the slit data are very good. So, those are the four that I would say were pretty close. I have a question for the panel, those that presented the results for today. The studies I have seen show that the tolerance on our predictability with the laser today is about plus or minus, at best, 0.4 diopters. My question is, what do you think the order of magnitude of these higher order aberrations is, and do you think it is greater than or less than this 0.4 diopter that we can do with the laser today?

Ronald R. Krueger, MD, MSE: I think it was Ray or Dave who said something this morning about higher order aberrations being about one-third of a diopter or thereabouts, right? So we are looking at a very, very small amount. If we would try to equate the higher-order aberrations and put it into sphere, it would be somewhere around one-third of a diopter. On average, for a normal patient. That’s not for abnormal patients, for patients who are post-surgical.

Dr Holladay: So that’s my question. Does anyone on the panel think that with the tolerances on the laser today, which in every study I showed today showed that the standard deviation or the mean absolute error was between 0.4 and 0.5 diopters, we can correct higher-order aberrations that are smaller than a half diopter?

Dr Krueger: I would just like to say in response to that we are looking at our current laser vision correction algorithms with conventional treatments and then

Wavefront and Emerging Refractive Technologies, pp. 47–52

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

custom, and Karl was showing some good stuff with his laser, down to one-quarter of a diopter standard deviations. When we look at the whole platform, we are trying to move this whole field into better and better predictability of results. There are so many different factors involved that we are going to start answering these questions one step along the way. Last week at the wavefront congress in Interlaken, Switzerland, we asked a panel of nine experts what they thought was the greatest problem with wavefront-guided treatments. And we had something like nine or ten different choices, whether it was biomechanical effects, wound healing, profile issues of the laser, delivery of the laser, wavefront aberration accuracy from the wavefront device, all kinds of different things, and it turned out that, although there were some that seemed to be slightly more popular – biomechanical effects was rather popular – they were all about average. Everyone had sort of their own special area that they thought was a major problem in the field. So, these are things that we are all going to have to tackle individually, one step at a time, in order to really get this thing as perfect as possible. We will never get rid of all the aberrations. But if we can get close, that would be nice.

Dr Holladay: Was that a ‘No’? The question was, can you correct higher order aberrations that are less than half a diopter with a device that basically has the precision of a half diopter.

Dr Krueger: In the future?

Dr Holladay: Yes.

Dr Krueger: And today?

Dr Holladay: No. We’re not there yet.

Paolo Vinciguerra, MD: With regard to spherical aberration. Sometimes we have a ten-diopter spherical aberration. This can be dramatically improved. And if you see a patient who has been treated decentered and is seen to recenter, the calculation of custom ablation can be extremely useful. For cases where the astigmatism is not symmetrical in the two halves of the cornea, that can be managed much better now.

Ioannis G. Pallikaris, MD: I think that Jack is right on this point, but again, we are not looking to actually correct the aberration today. We are looking to eliminate the induced aberrations, which is this age right now. We are not so far that we can correct with this accuracy you are talking about right now with the present technology. But it seems that, from the clinical results, we can eliminate this, which is also important.

Dr Krueger: The results of what we have with laser vision correction, although very good, and patients are happy and everything, the fact that we have induced aberrations and that we have some patients out there with bad outcomes and symptoms, etc., that has fueled the whole creation of this wavefront field, to try to figure out ways of measuring these problems and ways of hopefully solving and eliminating them in the future.

Dr Holladay: Let me ask this question. I am trying to play devil’s advocate, because the point is, we have to spend money to buy these devices. If they are going to help us in the future, maybe we should wait until the future to buy them. That’s the only point I’m making. But here’s the thing. If an epithelial cell is 5 µm thick, and Dan Rheinstein says that the epithelial is growing in, and Ming Wang wrote that little article he did on Planck’s constant, which just says there is a biological limit to everything we do. If we say that 5 µm is the limit of an epithelial cell, do you feel that’s probably the biological limit of how good we can get, and that is down in the range of about a quarter to 0.3 diopters?

Dr Holladay: That’s what I want to hear. Ioannis, how do we do half an epithelial cell?

Dr Pallikaris: I do not think this single epithelium you get is related to the final optical surface, and what we get on the surface is epithelium plus tear film, etc. So, I think it is not right to correlate in this way.

Dr Krueger: You never see a series of epithelial cells with one just sticking up there by itself, or a whole new row starting without some smoothing effect that makes it a more gradual surface along the edge.

Dr Holladay: So one layer would be about the limit?

Dr Krueger: One layer, yes, but then that’s talking about spherical rather than the aberrations themselves. Hopefully, by making a very smooth surface, you can eliminate most of those, and maybe you’d have a little bit of sphere one way or the other, and if it’s one whole layer, maybe you haven’t changed the sphere that much.

Question from the audience: Let me get a little more practical than one cell. Let’s say that we had this technology available, the wavefront technology, and say that we had a patient for some reason that the first LASIK needed to be touched up. Is the panel now saying that it would be better to do a surface ablation in this touch-up, or to go ahead and open up the flap?

Dr Holladay: No, I wasn’t saying that. I am saying a surface ablation on top of a LASIK is dangerous, because you already have compromised tissue, and so to come back again and ablate that surface when I already know some of those nutrients are coming from below, and that has all been messed up, that every single custom, or any ablation I have seen on a flap that was past Bowman’s, ended up with terrible haze and a terrible result. I was only talking about virgin corneas and virgin PRKs. I have to go under a LASIK flap, but my results aren’t going to be as good.

Question from the audience: So the answer is, with the LASIK you still lift up the flap.

Dr Holladay: Absolutely.

Stephen G. Slade, MD, FACS: The other part of this is that you really should never ablate on top of a flap. There have even been a couple of studies now. But we always do that. But the other thing is that, in my opinion, you should never do a custom ablation surface-wise on those patients who probably need it the most, such as a patient who has had a graft, a patient who has had an RK, a patient who has had T-cuts, a patient who has had a therapeutic lamellar, a homoplastic lamellar, or a LASIK. I think any time you have compromised cornea, you are going to have a radically different type of wound healing from what you suspected before. Unfortunately, in those patients on whom you can lift up the flap, you are often going to be at the limits of what you can do because of the bed thickness. The patients who need this the most are those who have had previous surgery and are disabled. They are the patients we can fix most easily, because you are not worried about such fine adjustments. Their aberrations are ten times what the typical person’s aberrations might be, but there are going to be special challenges in addressing these people, simply because you can’t do it on top and you might not have enough room to do it on the bottom.

Dr Holladay: Steve, the only thing I would say is that, from my perspective, the most common patient that walks into my clinic with irregular astigmatism and on whom I would want to do a custom ablation is a 20-year contact lens wearer who has an asymmetric bowtie with irregularity below that is about 20/25, 20/30. There are a whole lot of those out there, and I think they are the majority of virgin corneas with irregular astigmatism that are going to want refractive surgery, or long-term hard contact lens wearers who have an induced irregular astigmatism that doesn’t go away after one month, and I believe we can help all those patients today with PRK.

Dr Slade: I would still put that second on my priority list behind those people whom we have already messed up before.

Dr Krueger: That’s good for the whole field. Actually, even for Steve too maybe, it’s just this whole concept in terms of future. Perhaps in some of these patients who have aberrations that are very small and the refractive error is close to plano, we may in the future use a femtosecond laser to do intrastromal finetuning of aberrations, if we get to that stage?

Dr Slade: It’s the same sort of thing. You know the article about Planck’s constant, and that thin is the size. We have done intrastromal ablations and I will present that data tomorrow with the femtosecond laser. You could do that theoretically with a slit lamp, like a no-touch technique. You remove somewhat less than a micron, but we don’t know how much. The problem with that is you can remove very precisely wherever, how much you want, with a femtosecond laser without really touching the cornea. It’s like a YAG, a misfired YAG. But now the problem is, once you have created this cavity, will the front surface, the corneal first surface, predictably collapse into that cavity to give you what you wanted to do? I’ll show you some things tomorrow.

Dr Pallikaris: May I make a very practical comment on your question, because the question was actually related to when we are looking at a secondary LASIK or

secondary treatment on the previously LASIK-ed eye, what we have to choose, and I am telling you that, based on my experience, the last thing we want to choose is another LASIK. I am saying this because of having had experience with other combined approaches, which are ICR or, let’s say, conductive keratopathy or thermokeratoplasty procedures. I find it is much easier to see with this approach and the point here is that, if this patient is undercorrected then you make an ICR, if they are overcorrected you think about the thermokerotoplasty procedure, with conductive keratoplasty it works very nicely. Because my impression is that, using these peripheral approaches, you have another advantage, stretching out the center of the cornea. And, by the way, all LASIK patients have microstriae, but in a lot of them there are so many that they can affect the center high quality of vision. So, I would find it very interesting to use those approaches in the future. You have an additional stretching effect to the center which doesn’t occur with a higher order wavefront approach.

Raymond A. Applegate, OD, PhD: There are other blips on the radar screen that people should know about, and that I think are particularly exciting. Let’s forget about the cornea for a minute and go to man-made materials that we understand all the contents of, because we made them. We can imagine developing an intraocular lens that is tunable over time. We can imagine, even now, doing contact lenses that move minimally, highly gas permeable, where you are carving on the contact lens instead of on the cornea. All these materials are out there and available and I can tell you that a lot of contact lens companies are looking into it, and I know that a lot of IOL companies are looking into it. So, my prediction for the long term is that we may even see a movement away from operating on the cornea, because you might be able to manipulate these materials in much more predictable ways than corneal material.

Question from the audience: I have a question about wavefront analyzers. I have had some experience with them, and one of the things I noted on some of our patients is that when we take that wavefront analysis in the predilated state, we sometimes see a lot of higher-order aberrations and then, after dilation, all those aberrations just melt away when you repeat the scan. I was wondering how you account for this, because you would expect that, with the larger pupil, you would see more of those higher-order aberrations. Is this due to a diffraction effect with the smaller pupils when doing the wave scan? My second question was, how much of the cornea are we actually scanning with these different devices? Are we looking at just a couple of millimeters when we project the Hartmann-Shack grid, or how many millimeters of the cornea are we actually scanning or looking at with these studies?

Dr Holladay: You see the grids that they have out for the Hartmann-Shack are usually 10 x 10 or 15 x 15, so you can have between 100 and 250 sample points on most of those Hartmann-Shack grids. If you know that’s 6 mm and you know it’s 10 across, then you know the resolution there is about 0.6 mm between points, or 600 µm, so you have a hole between them. With topography, you end up with somewhere between 8000 points, and the resolution there gets down to about 100 µm, but you are always going to have holes because you don’t cover the whole thing. Your point about the pupil getting smaller, there are lots of reasons why

that can happen. If they have the sliding scale, where it goes up and down, then, as the pupil gets smaller, you do get a diffraction effect, so that could be showing up. But, if it is on an absolute scale where it’s showing the same and you don’t get under about a 3.5 mm pupil, then there is absolutely no reason in a normal person why you should end up with higher aberrations centrally than you would in a 6-mm aperture, that just doesn’t make any sense. I would think that the machine has more of a problem than it’s real.

David R. Williams, PhD: I would concur with that view. It sounds like a very atypical, unusual observation, and it would not be due to diffraction, which does not appear in any of the Zernike coefficients. The diffraction is not confused with the Zernike coefficients that show up on a typical aberrometer.

Question from the audience: We thought it was very unusual and something that should be mentioned because you wouldn’t want to be treating those higher-order aberrations in the undilated state, although that is what most people are walking around in. They are not walking around dilated. At least, for us, it’s a cause for concern in developing this technology.

Karl G. Stonecipher, MD: Steve, can you make a comment with the VISX when you show them the PreVue, though I know you are only doing patients who actually improve with the PreVue lens. Was that hard to find? Did it commonly occur? Where were you with that, and do you think that it is going to help us in the future?

Steven Schallhorn, MD: Firstly, I wasn’t an investigator in that study, so I don’t know how many patients were excluded because of the PreVue lens. I don’t know if Greg can comment on that, if he’s in the audience. I have tried the PreVue lens and used it, played around with it a bit. It is very difficult. I guess the registration is what you are actually doing when you sight through the PreVue lens. It is difficult. It is very dependent on lining it up. XYZ rotation, it’s very difficult to use. The way I look at it, it is sort of a reassurance factor, both for the surgeon and for the patient, that these higher-order aberrations, or what the wavefront is measuring, can improve the vision of the patient, can help the patient out. I don’t know how many patients were excluded because of the PreVue lens requirement.