Preliminary LADARWave measurement of flap-induced aberrations

Preliminary LADARWave measurement of flap-induced aberrations

Ronald R. Krueger

Cole Eye Institute, Cleveland Clinic, Cleveland, OH, USA

Preliminary results with the LADARWave CustomCornea Wavefront System from Alcon are encouraging. However, it is important to characterize the kind of aberrations that are created with this device in order to achieve increasingly better outcomes, with fewer induced aberrations. Overall, three groups of patients can be evaluated: non-surgical eyes, surgical eyes to detect how many laser aberrations are created, and flap-only patients. For laser treatment only, we are probably increasing the spherical aberration by over 100%, and the trefoil and coma by 50%. Aberrations induced solely by the laser still represent the major amount of aberrations created by us. But there are subtle aberrations that we are also creating with flaps.

A patient who underwent LASIK for myopia achieved 20/15 three months after her surgery. Despite the spherical aberration expected after myopic LASIK, she is doing well. The patient has reasonably good visual acuity because her sphere and cylinder have been eliminated, and the aberrations are relatively small. On the other hand, she has a heightened amount of coma compared to a normal patient. Likewise, there is greater positive asphericity, plus the patient has a little bit of trefoil.

For hyperopic photorefractive keratectomy (PRK), a patient was slightly undercorrected (about +0.5 D) at one month postoperatively. This hyperopic pattern usually consists of a mountain top wavefront pattern, However, the higher order aberrations have a slight bowl-shaped area in the center, indicating that a previous hyperopic procedure was performed. The spherical aberration indicates more of an inverted sombrero or donut shape, which is typical of hyperopia correction.

Only six eyes in our practice have so far been evaluated for flap-induced aberrations, using two different microkeratomes (the Moria M2 and the SKBM). Flaps are created in both eyes of the patient, then checked at one day, one week, and one month with the LADARWave and also with optical coherence tomography (OCT) to actually measure the thickness of the flap and the profile of that thickness. There is some flap swelling in terms of thickness during the first day, with variable

Address for correspondence: Ronald R. Krueger, MD, MSE, Cole Eye Institute, Cleveland Clinic, 9500 Euclid Avenue, i32, Cleveland, OH 44195, USA.

Wavefront and Emerging Refractive Technologies, pp. 159–160

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

160 R.R. Krueger

changes in the sphere. There is also a slight shift toward hyperopia over time, but this is very small.

Moreover, there are some coma changes that are different between the two microkeratomes, although these observations are still preliminary. With OCT and/ or ultrasound measurements we can calculate a certain flap thickness. Using ultrasound, flap thicknesses are slightly more, and there is additional swelling on the first postoperative day.

When using the Moria M2, a nasal hinge in one eye and a superior hinge in the other can be performed because the device is flexible with hinge orientation. Initially, we observed a slight shift toward myopia (swelling), but this was followed by a shift toward hyperopia. Overall, the sphere term may be slightly less myopic than before flap creation. Astigmatism is somewhat variable. Likewise, coma is slightly less or slightly more, but it definitely changes orientation according to the flap.

One patient happened to be negatively aspheric, and even flipped over to positive spherical aberration after the flap. This patient had a nasal hinge in the right eye and a superior hinge in the left. In the right eye, coma orientation tended to shift toward the nasal area, then later returned toward the superior because that is where the preoperative coma was found. Another patient who underwent surgery (nasal hinge right eye, superior hinge left eye) was a much higher myope. Again, there was a slight shift toward hyperopia in the left eye. There was also more superior coma which remained superior, but in the right eye with the nasal flap, it shifted toward a more oblique pattern (new horizontal and preoperative vertical coma).

Another patient had both eyes nasally hinged with the SKBM. Again, the sphere seemed to be slightly less myopic (more of a hyperopic shift). In addition, in terms of orientation, the induced coma was actually negative coma. It shifted to where the myopic component of the coma was oriented with the hinge.

When making the flap nasally with the SKBM, the myopic component rather than the hyperopic component of coma becomes apparent nasally, and this is different than what was experienced with the Moria M2, where the hyperopic component of the coma orients toward the hinge. This suggests that different microkeratomes give us different levels of coma. Why this is so, has yet to be determined. To be honest, though, these are early results. It may not even turn out to be much of a trend.

Overall, the LADARWave wavefront system is valuable in helping to assess the aberration pattern. Despite worsening of spherical aberration and coma with the laser part of treatment, patients are happy. The kind of flap-induced aberrations depend on the particular microkeratome used, with a tendency for either myopic or hyperopic coma toward the hinge. In the future, preoperative wavefront maps may become commonplace. Such a resource may improve outcomes. For example, a patient with slight myopic coma that is noted superiorly, could become a candidate for a superior hinge. The coma might be negated simply by orientation of the hinge and the microkeratome used.

Questions and Answers on “Results of the Wavelight Allegretto Laser for the treatment of myopia and myopic astigmatism”

(Charles R. Moore, MD)

Moderator: We will start with this to the general panel. These are patients with various conditions and you are going to correct them by any means possible. I just need brief answers, and if you agree or if you disagree. The first patient is an overcorrected, eight cut RK, who comes in basically with a +2.00. I want to know first, what is your goal? Are you going to try to overcorrect them? The second question is, “Are there any reports that this is a stable operation, or do you anticipate they will shift more hyperopically again?”

Daniel S. Durrie, MD: We did a 300 eye FDA study on hyperopic LASIK over RK just because everyone was worried about safety, whether it would really be done, so this was back with the Summit laser, and we were very impressed, not just by the lack of complications...we had three patients with epithelial ingrowth that didn’t need to be removed and really didn’t have any progressive weakness. We are now three years postop in these patients and we are going to continue to follow them. But it doesn’t seem as if we stopped the progression of hyperopia, but we certainly didn’t make it worse, and if anything, we might have made it a little better.

Moderator: Is your end point then more minus?

Dr Durrie: I was going to say that when you take a patient who has been overcorrected, consecutive hyperope, if you want to call it that, whether it is with a laser with an RK, they get about 40% more correction than what you dial into the laser. So if you put in two diopters, you are going to get 2.8. I put in the correction expecting them to get over-corrected by 40% and get them over the myopic side. I haven’t had a lot of surprises doing that because I would like to have them myopic and let them drift back. I just let the fact that they get over-corrected take care of my target.

Moderator: Any other additions?

Dr Moore: I think the other thing to be aware of with these consecutive hyperopes is to look at the base keratometry because if they are really super flat and very unstable corneas, they are the ones that really over-respond a lot of times to the

Wavefront and Emerging Refractive Technologies, pp. 161–163

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

treatment, but they are also the ones that, by creating the prolate cornea, again you can stabilize them and make them much happier.

Moderator: How about an over-corrected myopic LASIK with an 8-mm or less flap?

Dr Durrie: I still lift the flap and just use whatever they have. As long as it is a good flap. A few pulses hitting the epithelium out in the areas, you have to protect the back of the flap, but I think it is much more dangerous to re-cut a flap to try to get just a millimeter larger.

Moderator: Any other comments?

Karl G. Stonecipher, MD: I agree with that.

Moderator: How about a postcorneal transplant for keratoconus? Obviously a phakic patient, the other eye is ametropic or thereabouts, and the refraction is a -10.00 +6.00 x 180.

Dr Durrie: The situation with doing LASIK over a corneal transplant.

Moderator: Incidentally, this does not have to be LASIK.

Dr Durrie: I’m just saying that, with LASIK over a corneal transplant, some people have reported that you get a shift when you cut across the graft margin, especially someone who is high cylinder, and do two-stage procedures. Actually, what I usually do if I do LASIK over a graft is that I don’t do a two-stage procedure. We do a flap only and do it later. But I am conservative on the amount of correction I put in. So, I want to help them now and maybe lift the flap later. Recently, I have been doing surface ablation with the Alcon Autonomous laser and I have not gotten any haze on these patients. So, I am thinking that maybe we should be rethinking surface ablation with the smoother lasers on our graft patients so that we don’t have to worry about weakening the cornea again and cutting a flap. Right now, I am doing surface ablation.

Moderator: Does 6 D of astigmatism bother you?

Dr Durrie: In that patient, I would try to correct 6; with the Alcon, I can put in 6 and usually get 5. I would attempt that as long as they have good best-corrected visual and are not irregular astigmatism.

Steven Schallhorn, MD: I think the other issue with the LASIK is just the depth too. You have to be very careful about the depth of the ablation. I think that the results of PRK are kind of unusually good. That has been my experience.

Moderator: The last patient was a congenital astigmatism, Plano +6.00 x 180. How would you correct him or her?

Charles Moore, MD: That is really a +6.00 –6.00 when you flip those cylinders and so you have to correct it that way. I assume that is a manifest refraction and not a cycloplegic.

Moderator: Either way you want.

Dr Moore: Let’s just assume they are the same, which they probably aren’t. Assuming it is a +6.00 –6.00, you are going to try to correct as much of it as you can, but you are probably not going to correct it 100% of either of those two.

Dr Schallhorn: This may have to be combined with another procedure. You are not going to get the full correction.

Moderator: Would anyone use incisional keratotomy in this type...?

Dr Durrie: Actually, it is interesting now that we are getting into not only the incisional keratotomy, but also the conductive keratoplasty area, because Dr Mendez and I have been down there and seen his postop patients. He has corrected patients just like this with a +6.00 –6.00 with conductive keratoplasty only, because of the mixed astigmatism and the coupling you get out of it, and just treating one side and getting the cup link. So I think, as we get some of these new tools, we may add them together. But I don’t think you are going to get this all with a laser.