Introduction of Issues for Excimer Laser Guidance




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НазваниеIntroduction of Issues for Excimer Laser Guidance
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Report of Walter J. Stark, M.D.

DR. STARK: Thank you. Could you turn the slides on for me, please?

I enjoyed reviewing this issue, and as a start off for the review, I was given the article by Jones et al. from Emory contributed by  my secretary wrote Stulting et al., but it's Jones at all  and contributed by Edelhauser and Thompson, and Doyle Stulting, all coauthors.

The purpose of this study was to assess the effect of the laser in situ keratomileusis on corneal endothelium, and it is probably difficult for some of you to read that, but it is in a handout that was provided at the desk.

In effect, they studied the corneal endothelium in 98 eyes of 65 patients, with correction from 2.75 D up to 14.5 D of myopia, 2 weeks and 12 weeks after LASIK. Of importance, probably, is that 91 percent of the patients or 59 patients had a history of contact lens use.

With the analysis of their data, just as a quick summary, there was no statistically significant change in the mean endothelial cell density or coefficient of variation at 12 weeks and at the times postoperatively comparing to preoperative values, but the percent hexagonal to cells will decrease by 1 percent at the 12 week visits.

So their conclusions were that corneal endothelial cell density and morphology were unchanged 2 and 12 weeks after LASIK, and so there was no clinically significant effect on corneal endothelial cell density by cuts down as deep to the point that they left in their higher myopic cases a residual corneal bed of 208 microns.

The variation in corneal bed depth or thickness at the end of treatment would have gone down to that 208 microns.

One of the problems with this report and most of the other reports on this field is the calculation and thickness of the residual stromal bed. There have been several studies that have shown  they used the Chiron corneal shaper  but there have been several studies that have shown the variability in flap thicknesses within +10, +20 microns, but there have also been a number of studies that have shown that there are tremendous variabilities in how thick these corneal shapers or corneal cutters actually cut. And I told a number of people and spoke with others, and Eric Donnenfeld has a paper that is submitted  it was presented to ASCRS  and they used the Chiron automated corneal shaper with a 160 micron plate.

In this study, they measured the corneal thickness by pachymetry and calculated the corneal thickness of the flap by subtracting that, and the thickness of the flap using a 160 micron plate to cut a 160 micron depth ranged between 81.4 microns and 170 microns. That means that this corneal cutter in their hands was off by up to 60 microns.

The only way you can tell this is by measuring the residual bed by ultrasonic pachymetry after you have made your corneal flap. And there are several problems with that  it is a little bit rough, and it doesn't measure quite as easily. Surgeons are reluctant to hydrate the bed to give them a better surface to measure the actual thickness, because that might affect the rate of ablation.

But I have also found from other surgeons that this has been a problem not only with the automated corneal shaper but the new hansatome [ph.] Several surgeons in our area have had unexpected thin flaps and buttonholes and have sent their units back on several occasions. I have just visited with Lindstrom and Hartan in Minneapolis. They did a study and calculated that using a 180 micron blade with the hansatome, they were initially getting cuts of 110 microns, and they sent it back on a couple of occasions, and now the 180 micron blade seems to be cutting 150 microns.

So that's a major problem with any of the studies that have not measured directly the thickness of the bed or tried to measure the thickness of the flap. There may be some ways to get around that. It has been suggested that one might be able to take high frequency ultrasound and measure the actual thickness of the anterior flap by a little reflectant that one can see there, and that would be a good study for some refractive surgeons.

So there are two questions that were asked of me: Is there short  and long term damage to the endothelium as evidenced by specular microscopy, and is there a loss of integrity of the cornea that can lead to ectasia. I think these are very important questions because, not shown well there, but on page 3 of the handout, there is depth versus the amount of cut. If we are going to try to correct and leave 250 microns of tissue in the residual corneal bed, with a 6 mm diameter cut, you can correct about 10 D with the LASIK procedure. If you want to leave 200 microns, you can go up to about 14 D. If you make a smaller optical zone, you can correct more, but one of the problems of LASIK and high myopia, especially the smaller diameters, is night time glare, so that would increase that.

If you use the multizone treatment  Figure 2, for those who have the handout, and I passed out to the panel the updated one  you can correct up to about 13 D and still leave 250 microns of tissue in the residual cornea  but that's assuming your cut is 160 microns, and it is also assuming an average corneal thickness of about 0.52 microns.

The potential mechanisms for laser damage to the endothelium would include thermal, mechanical damage from the shock wave, and damage from the ultraviolet light. Also, we have to consider that the different lasers have different levels of fluence and repetition rates. But experimental studies on the rabbit and non human primates demonstrate that superficial ablation probably doesn't cause any damage from the laser. But if you get down to cuts that are within 40 microns of Descemet membrane, there is damage to the corneal endothelium, as shown by Koch et al. A recent presentation by the Emory group, Lim, Jeon and Edelhauser showed that if you get to 175 microns from endothelium in the rabbit, you are going to cause some damage to the endothelial barrier function at 175 microns and also, another aspect of their study, to 150 microns.

The clinical studies in general don't show a loss of corneal endothelium after excimer laser, except Pallikaris  and I e mailed him and faxed him, but I have not gotten a response  reported in 10 eyes a 5.7 percent loss of endothelial cells at 6 months and a 10.6 percent loss of endothelial cells at one year, and treatment of  8 to  17 D.

An interesting article by Proeme and associates  and I put some of the references in here  and also by Perez Santonja  shows that contact lens use apparently decreases the central endothelial cell count by about 7 percent. And if you take those contact lenses off, people will get an apparent 7 percent increase in corneal endothelial cell loss. So I can't remember exactly in the Emory article, but the patients may have actually had a loss of 7 percent endothelial cells, but gained 7 percent because the contact lenses were taken off, so it's a little difficult to tell if they are not standardized. So in effect, to show a 10 percent endothelial cell loss, you would have to lose 17 percent endothelial cells.

We did show that excimer laser probably does do something to the endothelium. This was the histology of a macular corneal dystrophy patient that we reported after a failed PTK, and you see this electron dense collagen deposition in Descemet membrane, and that was shown in non human primates in experimental studies, and this was shown in a clinical case. So the excimer laser tickles the endothelium  does something  and I asked Dick Green if he had any more thoughts on it, and he said he didn't, that the collagen that's migrating up through Descemet membrane is produced by the endothelium, and he didn't know why, except that it showed some minor trauma to the endothelium  and that was a cut of about 300 microns; we were 300 microns away from the endothelium. So that's just food for thought.

The Jones et al. article did not address the issue of ectasia, and ectasia has not been reported to be a great problem after excimer laser, except by Seiler and Associates in the Journal of Refractive Surgery in 1998, this year. He reported on three cases of ectasia after use of the excimer laser, and these were in corrections from  10 to  13 D.

I calculated two of them that I could best calculate  and I have included this article; I am sure some of you will want to do the addition again  but one of the cuts left about 215 microns of tissue, and the other, 175 microns of tissue, again, assuming that the keratome did what it did, and that's a big assumption.

So that would suggest that as you get close to 215 microns, you may increase the risk of ectasia.

I talked to Richard Abbott, and he said that he has now got a case of ectasia in a patient who was referred to him after three repeat lasers.

Lindstrom said that he uses 200 microns as the minimum amount of residual corneal tissue and has found no ectasia, and he points out that hyperopic ALK gives no effect if 200 microns of tissue are left. So you don't get the bowing forward effect, again, assuming that the machine is cutting what it is supposed to cut.

While Slade states that he thinks 200 microns of the tissue residual is okay, he plans for 250, to be conservative, and this would also allow for some retreatment. He points out that in MKM, myopic keratomileusis, which is no longer done, that you can go down to the 200 microns, and that's a full cut across at 200 microns rather than 200 microns residual thickness only being right in the center, which you would expect with LASIK, and he said that they did not get any cases of ectasia.

So in summary, I think there appears to be little evidence from the literature, and in particular, the Jones paper, even considering that there are some deficiencies in the paper, about what is the actual corneal thickness left. There seems to be little evidence of serious loss of endothelial cell function at 12 weeks, but the limitation of that study as with others is that it is only 12 weeks' follow up, and I think one would have to look at at least a year follow up to see if there is any effect from the endothelium or other long term problems.

Ectasia has not been reported to be a problem, except that by Seiler. Gary mentioned that  I think he indicated in his presentation that they have not had any problems with ectasia, and there is a large number of studies that they are following. But with Seiler, that one case did have ectasia with 250 microns of tissue. And I polled several people, and they are listed here, who are I guess what you could consider respected refractive surgeons who have had a moderate experience, and most of them, except for Lindstrom and Slade, strongly recommended adhering to the 250 micron limit.

So in summary, I think we don't have adequate data to tell us what the minimal safe residual thickness of the corneal bed after LASIK would be without the risk of ectasia or corneal endothelial damage. Several surgeons polled have stressed that we need to have guidelines not only on the residual corneal bed thickness, but also on the optical zone profiles and the diameter and thickness of the flap. And I would add to that that I think the FDA should look at some way to document good manufacturing practice of the makers of the keratome  that, even though they are 510k, if people are getting these keratomes, especially people who might not be corneal experts or experts in refractive surgery, and go in and set the machine up right and have a 160 or 180 micron plate, they should be getting 160 or 180 micron cut, plus or minus 10 microns, rather than plus or minus 50 or 60 microns.

So I think we ought to stick to 250 at this point, especially with other modalities of refraction becoming available, maybe the intracorneal or the intraocular lenses, but I really think we should do something about standardizing these corneal cutters.

DR. McCULLEY: That was outstanding. I agree with a lot that Walter said. I could restate, but I don't know how much restating we need to do of what he said. I think he covered it very thoroughly and brought up at one point or another throughout his discussion all the major points that I have thought of.

One flaw in these LASIK studies that some people seem to continue to fall into is not to measure the corneal thickness preop and then make assumptions; it's easy to do.

Yes?

DR. GRIMMETT; Michael Grimmett.

I thought that was a beautiful presentation by Walter. I have one comment. I am not sure the data that underlines Dick Lindstrom's comment regarding hyperopic ALK doesn't give ectasia of 200 micrometers of tissue are left posterior to the cut. Perhaps there is some additional data that I don't know of. In Archives of Ophthalmology in April of this year, Lyle et al. published a series of hyperopic ALK in which approximately one in five developed progressive ectasia, and they analyzed to see whether the depth of the cut was related to those who developed ectasia, and they did not find a statistically significant result.

Perhaps Lyle's study just points out the fact that you have discussed, that these keratomes don't cut accurately, so maybe they were all over the map, and that's why they couldn't find it. But unless Dick has published otherwise  I tried to look that up recently and was unable to find that data.

DR. McCULLEY: Dr. Wang?

DR. WANG: Ming Wang.

I understand the 250 guideline actually came from mid air, with the intuitive assumption that the biological system tends to have a full degeneracy, without any experimental data. Is that true, to the knowledge of panelists?

DR. McCULLEY: Where did the 250 come from? I think what we have seen  and Walter alluded to it with the CRS study  they had no ectasia, but they limited it to 250. So I think there is a good deal of data to support that 250 undisturbed is safe for endothelium and structural/anatomical integrity. So whether it came from the air or not, it seems to be a good number, and the question is whether we can go deeper or not, and I don't think we have that information.

Something else that Walter brought up that is a really big point and that Dr. Grimmett also hit on is that we need to know  and I am surprised  there is variability on the depth of the cut depending on the intraocular pressure, so it may vary with the same keratome cutting well, if the pressure in the eye varies. But there seems to be more to it than that, that the keratomes don't necessarily always cut with everything being standardized within a tolerance limit that we are keratorefractive surgeons would like to see. And I would agree with the +/  10 microns. If you start getting more than that, until we know whether we can go below 250 or not  right now, at 250, if those things cut 20 to 30 microns deeper, we may be approaching 200, and that may not be safe.

DR. WAXLER: Just to comment on Dr. Wang's comment, the 250 didn't come totally out of thin air. Basically, it came out of an analysis of what was reported in the literature with regard to animal work at the time and also adding some 50 microns. The data that was available at that time, several years ago, was that somewhere in the neighborhood of 200 was probably a threshold area. And then, we knew that the microkeratomes were sloppy, but no one really knew how sloppy, so we just added 50 microns. So it was kind of out of air, but not completely. Every now and then, we revisit that to see if we have any more information.

I appreciate your analysis; it was excellent.

DR. McCULLEY: Dr. Bullimore?

DR. BULLIMORE: Just a question in terms of the ectasia. Can you only actually tell it is ectasia when it is progressive, or may there be some ectasia happening in a large majority of patients, but you just don't know what is ectasia and what is the laser not working properly?
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