Are all aberrations equal?

Are all aberrations equal?

Raymond A. Applegate

University of Houston, Houston, TX, USA

It is well known that even a simple myopic eye has aberrations. Recently, we began to study methods of quantifying these aberrations and of examining whether each individual component has an equal effect on visual acuity. Our study showed that, for an equal amount of root-mean square (RMS) error, not all coefficients of the Zernike polynomial resulted in equivalent losses of visual function, as measured by high and low contrast logMAR acuity. Our clinical conclusion was that visual acuity does not do a good job of reflecting optical quality – which is becoming more and more important as researchers attempt to improve the optics of the eye. Each subject’s eyes were measured for wavefront error, i.e., the physical distance between actual wavefront and ideal wavefront. These errors were broken down into individual components, including defocus or spherical error, astigmatic error, and the higher-order aberrations, such as trifoil, tetrafoil, coma.

Using the CT View program, we examined patients with normal vision without surgery and those who had undergone LASIK. For example, we created simulations to compare a normal eye at 3 mm with a 3-mm post-LASIK patient who has 20/15 Snellen acuity. Although the LASIK patient was satisfied with the postLASIK results, this patient did have a typical, slight increase in aberration and resulting loss of contrast and edge sharpness. The purpose of this particular study was to determine, for a fixed RMS error of the wavefront, how each term of a normal eye’s Zernicke polynomial affects high and low contrast visual acuity. In other words, whether visual acuity is equally affected by a fixed level of RMS error, regardless of the source of aberration. The subjects of the study were three healthy volunteers aged between 25 and 51 years, with 20/16 or better visual acuity. In order to select the level of RMS wavefront error to be introduced, we increased the error attributable to defocus, which is term 4, until between one and two lines of visual acuity were lost. We noted that it took about 0.25 µm of RMS defocus over a 6-mm pupil to decrease acuity by eight Snellen letters, or 1-3/5 line on a logMAR acuity chart. We performed the experiment in object space; we did not change the optics of the eye. We then loaded 0.25 µm of RMS error into each term, one at a time, from term 3, which is astigmatism, to term 14, which is quadrafoil. Each subject was dilated and viewed the aberrated charts through a 3-

Address for correspondence: Raymond A. Applegate, OD, PhD, University of Houston, 4800 Calhoun Road, Houston, TX 77204, USA.

Wavefront and Emerging Refractive Technologies, pp. 189–190

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

mm artificial pupil aligned to the eye’s foveal achromatic axis, using an optimized spherocylindrical correction for the 10-foot distance. Questions could be asked about the eye’s aberrations. We found that, consequently, the effect of the eye’s aberrations was minimal because we used a 3-mm pupil.

We considered the diffraction limit for a 7.3-mm pupil and a typical eye. The point is that, as you make the pupil smaller, you approach the diffraction limit of the eye. If you go too small, it becomes a spatial filter. The ideal pupil size is approximately 3 mm for experiments like this. Head and eye position was fixed using a bitebar. Each chart was created using a random selection of letters from an equally identifiable letter set, and printed on a high-resolution printer. Each subject read each chart until five letters were missed.

What would be expected? If the combined sphere and cylinder (the typical ophthalmic prescription) were combined with every term up to the 14th order, and each of these aberrations affected vision equally, patients would lose about eight letters for a 0.25-RMS wavefront error. We examined the data for letters lost for high-contrast letters. We noted that, as measured by visual acuity, different aberrations affected visual performance quite differently. The same holds true for lowcontrast letters. The errors are larger, but the same relative position occurs.

The largest effect occurred between quadrafoil and secondary astigmatism. We checked the acuity chart and found the quadrafoil to be the least affected. The letters had little fuzzy edges, but you can read everything down to the bottom of the chart. Compare that to secondary astigmatism, which was the most affected. You can clearly see that it is harder to read for the same aberration level. But the most surprising thing was that less than eight letters were lost. This shows that a

patient can interpret blur quite well.

In conclusion, not all coefficients of the Zernike polynomial induced equivalent visual acuity losses. It was striking that the large change in appearance was not reflected in equally large decreases in visual performance; the subjects could correctly identify highly aberrated letters. A direct corollary is that our brains are the world’s best image processors.

Clinically, although visual acuity is a good clinical tool, it will not show how subtle improvements in aberration structure improve performance. The wavefront error, or that RMS error, affects visual acuity differently, depending on which Zernike mode is loaded with the error. This will guide us as to which terms we should pay careful attention to as we try to improve optics in the human eye.

Acknowledgments

These studies began as a project by Vick Cansara, a resident at the Department of Ophthalmology, which was presented at the Alamo Day Conference in San Antonio in March, 2001. We have taken them further, and this work is supported by the National Institutes of Health, a San Antonio area foundation, and unrestricted grants from Research to Prevent Blindness. Two of my laboratory technicians, Charles Valentine and Tom Aguilar, helped organize the data.

Questions and Answers on “LADAR LASEK”

(Daniel S. Durrie, MD)

Moderator: Dr Durrie, one of the ophthalmologists in the audience would like you to tell us about the prices and the difference in price between the LTK and hyperopic LASIK you referred to.

Daniel S. Durrie, MD: As far as LTK is concerned, one of the problems is that the machine was so expensive that people were forced to charge fairly high prices. I always thought that this was because it was the procedure that wasn’t as good as hyperopic LASIK or as permanent as hyperopic LASIK, but that it was safer. I charged half as much. In our case, our LASIK price is around $2000.00, so we charged $995.00. Actually, what I would like to charge is somewhere in the $500.00 to $600.00 range, or $400.00 to $500.00 range, if it wasn’t so expensive to have the machine. And I think this is what we are going to see with other procedures that are coming out. Because I think patients will understand that it is not the same as hyperopic LASIK, that the price is a lot different, and that their expectations should be more realistic. I think of it more like a Botox injection, that it’s something you’re going to pay for and know you’re going to have to do again. I think we need to learn a lot more from our aesthetic doctors about this. Also, this is kind of the long answer; hyperopic patients and procedures are very different. You are going to enhance 100% of your hyperopes. Just keep that in mind. If your business plan now has free enhancements, you are going to enhance 100% of those patients, because as they get older, they get more hyperopic and more presbyopic. So we have a totally different situation. Not every hyperope gets free enhancements. With myopes, the eyeball doesn’t grow longer. It’s basically a totally different ball game, and a lot of people haven’t thought about that. Then they wonder why they have these huge enhancement rates on their hyperopes. I tell them up front, “I can’t keep you from getting older, and you are going to be paying me again.” That’s the process if they want to continue without reading glasses. So bear that in mind.

Moderator: For Dr Soloway: In the literature, there are reports of up to a ten-point IOP lowering effect after scleral expansion bands have been implanted. One of the questions is, “Do you have any data or comments on the possible effect of scleral expansion bands in aqueous fluid dynamics? Does this cause hypotony or could it?” And in follow-up, in cataractagenesis or in axial length changes or anterior chamber depth changes.

Wavefront and Emerging Refractive Technologies, pp. 191–195

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

Barry Soloway, MD: In terms of axial length or anterior chamber depth changes, we haven’t seen any and we have got good measurements in patients whom we have done in the USA with it. This was part of the data which we needed to record for the protocol. In terms of pressure lowering in open-angle glaucoma, it is felt to be an increase in pore size. Dr Schacker looked at both presbyopia and chronic openangle glaucomas, two things that occur when people get to their mid-forties and start to get worse and worse as they age, and felt that lens crowding was decreasing pore size for filtration. What they have seen in the Canadian studies was an improvement in the lowering of intraocular pressure in patients who have had higher pressures. I haven’t seen any lowering of pressure in the patients I have, all of whom started out with normal pressure in their teens. So, we haven’t seen any lowering. But, again, according to the theory proposed by Dr Schacker, this should be the case, that if the pressure is already normal, we are not going to see an improvement, but it is really going to show itself as an improvement once the pressure starts to get higher and higher and pore size and filtration become problematic.

Moderator: The last part of that question, is there any link to causing further cataract progression?

Dr Soloway: We haven’t seen any in the data, certainly in my office, or in the data that I have seen presented by other people doing the surgery, of loss of distance vision or changes in distance vision. There hasn’t been anything specifically reported in terms of cataract, but no-one has reported any changes in distance acuity over time, which you would think at least would show that lens clarity is staying reasonably good, certainly in the patients I have performed on, we haven’t seen any changes in lens density.

Moderator: One of the questions is, “How do you enhance LASEK?”

Dr Durrie: You just do it again. There isn’t really any difference, but as I was showing, you have to have a laser that is relatively smooth or you would have a tough time making a flap. But there really isn’t any difference. If you smooth it out the first time, you can do an enhancement quite easily.

Question from the audience: I have one comment on the previous point in regard to the cataracts. Anecdotally, I have spoken with a couple of surgeons who put in a number of scleral expansion bands, and they have actually seen some lens vacuoles decrease when the accommodation is theoretically improved, the movement, or something, is perhaps massaging the lens or something, but at least anecdotally, they have actually reported some decrease in lens opacities.

Moderator: We have another question for Dr Durrie. What percentage of your patients get LASIK versus LASEK?

Dr Durrie: Right now, and in my practice we just looked at this, 80% have standard LASIK, 20% have surface ablation of one kind or the other, and right now, not to confuse the patients, I use this for people who have dry eyes, people who are very

conservative, people who come in with a long list of Internet questions, and everything else and are looking for the safest procedures. Those are the patients I head in this direction. People who play contact sports, whom we have talked about before, or people who need to have epithelium replaced, then I will do PRK. That is still an 80% standard procedure, but a year ago, I was doing one percent. So if you look at a growth area, and I think this has been an additional growth area in my practice, it has not been just the LASIK patients. These are patients on whom I either didn’t want to do surgery or they didn’t want to have surgery before, so I think it does grow in your practice.

Moderator: As a sort of follow-up, another question is, “How are your LASEK results with regard to postoperative pain and dry eye, and how do they compare to LASIK patients?”

Dr Durrie: They hurt a lot more than LASIK patients. I think what you want to do is to exceed their expectations. So I tell them that they are not going to be happy for a week. They are going to wish they had had the other procedure for a week. By a month, they are going to be happy they had this procedure, and beyond that, I think we all know the visual acuity is a little bit better. I want to get them used to the fact that they are going to have a lot of trouble for the first week. But I do have patients who experience no pain, see well, and are 20/25, 20/30 with the contact lens in. You are trying to protect yourself from the patient whose contact lens comes out, they now are a PRK, they are uncomfortable, and that happens. You don’t know which patient is going to do it, so get their expectations there. As far as dry eyes are concerned, we know because of the PRK data that has been out there for years, that the corneal nerves return to normal within a matter of weeks, as opposed to a matter of months to years in LASIK. So one of our feelings about dry-eye patients is that this disruption of the neural feedback area may be less in these patients. But we don’t have enough data yet. I think that would be a good question for Dr Vinciguerra who has been doing this for three years, but anecdotally, I haven’t heard of many dry-eye problems with surface ablation, while we certainly see a lot of them with LASIK.

Moderator: One last question, Dr Durrie. Someone has asked, “Which company manufactures the LASIK hook that you use?”

Dr Durrie: Actually, ASICO, Katena and Storz all make these instruments. I have a set of each one of them, and they are all fairly interchangeable. I think that the hand-held instrument companies have done a good job of coming up with sets of instruments. So they are readily available and not very expensive.

Moderator: I see that there are a few other questions. Does anyone have any other comments about other instrumentation for LASIK? Someone asked, “Does chronic contact lens wear affect the result of LASEK?”

Dr Durrie: I’d like to turn that over to Ray. Have you studied, or has anyone talked about the higher order aberration permanently induced by contact lens wear, either soft or hard? I don’t remember hearing that last week. Has anybody looked at that?

Raymond A. Applegate, OD, PhD: Not to my knowledge, but surely the chronic contact lens wearer has higher order aberrations, probably as bad as refractive surgery. I don’t have data to support that statement, though.

Dr Durrie: I am turning the question around because I think it is quite interesting, the statement that Ray just made. I think it is important that we look at the 35- year-old contact lens wearer. We talk about corneal warpage. You put them in front of your aberrometer. These people have some really bizarre shaped warpage that appears to be permanent. Some of these patients haven’t worn their lenses for five to ten years. So I think we are finding a lot of things that we thought were pretty benign that can really cause a lot of distortion in vision.

Questions from the audience: I have a question for Dr Durrie. I know that, with my LASEK practice, I recommend, and I would say that probably around 75% of the patients are going along with this, doing one eye at a time. I was wondering whether you are doing anything like that with LASEK versus LASIK?

Dr Durrie: I give patients the option. Sometimes it’s like having one wisdom tooth out versus two. So, if they really have someone that can take them back and forth and they are not bothered that they are going to have trouble driving for several days. And always remember that this patient you are talking to could be the one whose contact lenses come out. Because they do. People will knock them out or they will blink them out. So when you are counselling someone, always take the time to explain these things to them.

Barry Soloway, MD: Basically, it’s being able to see up close without actually changing the power in the eye, due to pupil size changes where we are getting more into a diffraction pattern, as Dr Applegate was talking about. Certainly, when you see these patients trying to accommodate, and pupil size coming down as you are watching them on a wavefront machine, or what have you, you realize that you are getting to a point where you have a much smaller pupil, so your circles of confusion and depth of field are going to improve as well.

Moderator: A last couple of questions for Dr Durrie before we resume the talks. Dr Durrie, someone asked, “Do you charge or bill the insurance companies for punctal plugs after LASIK?”

Dr Durrie: I guess I don’t know the answer to that question, because I think we are very haphazard about it. The punctal plugs are reimbursed from insurance if you have documentation of a Schirmer test or a tear lysozyme, such as a touch scientific machine with which you can actually document that the tear is low, and then you can bill the insurance company. Dr Vance Thompson, who is in Sioux Falls, South Dakota, has a very good program, I think. He does a tear lysozyme and a Schirmer test on everyone preoperatively, has certain criteria that are kind of go, no go for surgery, but then he treats the patients, and they have to come up to a certain level before he moves ahead. This kind of protects you a little if you have treated your dry-eye patients and had them respond ahead of time. Then, if they get drier afterwards, you have kind of told them ahead of time. I like that approach, and I think it is probably what we will aim to do: a lot more pretreat-

ment with punctal plugs on those patients who need it, and wait for their surgery. That is usually reimbursed by insurance. Once you have operated on them, people think everything you do is free from then on. One of our problems is that, if their eyes are dry, they don’t even want you to charge their insurance. They want you to provide it, because you caused the problem.

Moderator: Do you do LASEK on both eyes on the same day, and do you use different nomograms for LASEK versus LASIK?

Dr Soloway: As I said, in about 75% of the patients on whom I am doing LASEK, we do one eye at a time. We don’t use any shields on the patient, although what I have been finding, and definitely on the one eye at a time patients, I have been taping their eye closed just with some of that perforated Dermatape or what-have- you, overnight, so that they literally cannot open their eye. They use their other unoperated eye or previously operated eye to get around, and we have definitely been seeing an improvement in terms of how they look the next day. We are potentially a little concerned about things like infection, or what-have-you. I use Ocuflox liberally during the whole procedure, hopefully to prevent this, and we haven’t yet seen this as being a problem. By doing one eye at a time, we are able to tape the eye closed, and we have found that patients are slightly more comfortable and seem to heal a little better, but we really don’t have any firm data on that occurring.

Dr Durrie: How about a nomogram?

Dr Soloway: Actually, I use the LADAR System laser pretty uniformly on patients. I take off a fair amount on my LASIK patients. I take off more than any of the other users at the center when I’m doing LASIK. I work on a very dry technique. What I use with LASEK is not doing my typical LASIK nomogram change, although I make some changes that I would normally make on larger astigmatism and on larger optical zones, so the portion that I am kind of taking off on my lower myopes, so to speak, I’m not taking off on my LASEK patients, but the portion that I take off as I go in LASIK from a smaller zone to a larger zone or up to a higher astigmatism, I do still reduce. I have done about 85 or so eyes with LASEK. Some patients are one eye LASEK, one eye LASIK. I mainly do LASEK on patients with pachymetry concerns, and I would say that about 80% of the people on whom I do LASEK are people on whom I wouldn’t do LASIK, because of thickness issues. I have not yet had a single person in whom I needed to go back and enhance.

Dr Durrie: I do not have a nomogram adjustment, probably just because I don’t have enough data at this point in time, and I have had excellent results, as you can see, without adjusting, so I am just using the same thing I used for LASEK. The one question I have is that, other than Dr Barron, not a lot of people have reported LASEK Visx results, which is one of the things... I can’t quote that, and most people are using Visx lasers, so you are going to have to ask a Visx user rather than Barry or myself, because I think there is going to be a difference in those lasers, depending on what laser you use.