Overcorrection with wavefront

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Overcorrection with wavefront

Postby RobyRob » Thu Aug 17, 2006 5:44 pm

Is is possible to do intentional spherical overcorrections with Allegretto Wave Eye-Q when ablation is wavefront guided?
I've heard nomogram adjustments are possible only when ablation is standard (not custom).
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Postby LasikExpert » Thu Aug 17, 2006 7:49 pm

My knowledge of the Wavefront Allegretto Eye-Q wavefront-guided excimer laser is limited. The Eye-Q is not approved by the US Food and Drug Administration for use in America. The Wavefront Allegretto in the US is wavefront optimized, not wavefront guided. I do have greater knowledge of the custom wavefront systems of the Alcon LADARVision and Bausch & Lomb Technolas 217z Zyoptix and some basic understanding of wavefront analysis. I’ll try to respond based upon a limited, but general, understanding.

I also want to be sure I understand your question clearly. “Spherical correction” means correction of myopia (nearsighted, shortsighted) or hyperopia (farsighted, longsighted) vision. Its counterpart would be cylinder correction, which is the correction of astigmatism.

It would be technically possible to create a custom wavefront ablation map that would be designed to overcorrect sphere, however the effect would be limited and quite possibly counterproductive. Any excimer laser surgery will affect Higher Order Aberrations (HOA). Sphere and cylinder are Lower Order Aberrations (LOA). Current wavefront guided systems do well at reducing the amount of increased HOA during surgery, but they do not reliably and predictably reduce HOA or even keep HOA at the same level. Surgery to overcorrect sphere would undoubtedly increase HOA even more than surgery to correct sphere to plano (no refractive error). Overcorrection with today’s systems may provide the desired “quantity” of overcorrection of sphere, but would very likely decrease the “quality” of vision by increasing HOA.

It is currently possible to “offset” the LADARVision and Zyoptix to slightly undercorrect, but undercorrection runs into the same limitation as would be expected of an attempted overcorrection.

Today’s methods and means of wavefront guided excimer laser surgery are to target full correction of sphere and cylinder to plano, and HOA to levels within the population norms with a minimum of increase. Changing the spherical target in the world of wavefront is rather complicated. Possible, but complicated and I do not believe any of the laser manufacturers have accomplished this...yet.

If you desire overcorrection of sphere, a topography or refraction guided ablation would probably provide the most accurate outcome.
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Postby RobyRob » Sun Aug 20, 2006 9:17 pm

Thank you for answer Glenn.

I read that Eye-Q is FDA approved since July 2006 (http://biz.yahoo.com/bw/060717/20060717005367.html). Is this only for wavefront optimized surgeries?

Anyway, I will discuss this issue with my surgeon.
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Postby LasikExpert » Mon Aug 21, 2006 3:17 am

The US Allegretto is wavefront optimized, not custom wavefront-guided. That may or may not make a difference in your case.
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Postby RobyRob » Mon Aug 28, 2006 10:29 pm

Glenn, since I am only weeks away from having my surgery and I have my final examination this Thursday I would like to ask you a few more questions.

1.) Does surgeon have to enter any numbers (diopters, angles,...) when surgery is wavefront guided or he leaves it all to aberrometer?

2.) There is this great surgeon, but for some reason she does only LASEK, even though she does also have epikeratome. Why do you think she does that?

3.) Should I demand from her epi-LASIK? She says that both methods have equal results.

4.) What is the correct use of IR pupilometer? Do you have to put it on the eye or just hold it near the eye? Must the other eye be closed? Why do some surgeons measure only in dim light and not in total darkness?

5.) Should I have wavefront guided surgery?
Two doctors told me NO because:
- new technology,
- measurements not reproducible,
- causes more damage in case of decentered ablation
One doctor said YES:
- it gives better vision
My data is: 8mm pupil, -3.00 SPH, -1.00 CYL, 20/10 vision with glasses, 26 years old, 550 microns. Here are screenshots from Pentacam:
left
right

6.) Why does wavefront guided surgery take more tissue? Some say it takes less because it takes only what is neccessary.
It takes more compared to what? Wavefront optimized or standard? Does "more tissue" mean the tissue in peripheral area (to reduce spherical aberration)?
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Postby LasikExpert » Mon Aug 28, 2006 10:57 pm

RObyRob wrote:1.) Does surgeon have to enter any numbers (diopters, angles,...) when surgery is wavefront guided or he leaves it all to aberrometer?


Even in wavefront-guided mode an "offset" is generally possible, which is often used to accommodate specific patient or environmental needs, however the primary ablation pattern is derived from the wavefront analysis.

RObyRob wrote:2.) There is this great surgeon, but for some reason she does only LASEK, even though she does also have epikeratome. Why do you think she does that?


Because in her experience her patients get better results.

RObyRob wrote:3.) Should I demand from her epi-LASIK? She says that both methods have equal results.


Epi-Lasik requires the application of a microkeratome affixed to the eye with a suction ring. This is an additional process that rarely causes any sort of problem, but every additional process is a process that can have a problem. Keep in mind that if Epi-Lasik were to have some sort of problem, the doctor would likely just revert to PRK and remove the epithelium.

RObyRob wrote:4.) What is the correct use of IR pupilometer? Do you have to put it on the eye or just hold it near the eye? Must the other eye be closed? Why do some surgeons measure only in dim light and not in total darkness?


People don't see in total darkness, we see in low light. A measurement in total darkness is a valid reference point, but it is also valuable to know just how much the pupils constrict with a little bit of light. Low light pupil measurement is important if you have large pupils, but is probably not terribly important if your pupils are small.

RObyRob wrote:5.) Should I have wavefront guided surgery?
Two doctors told me NO because:
- new technology,


"New" is relative. The technology has been around several years and over a million eyes. It has proven itself time and again.

RObyRob wrote:- measurements not reproducible,


I personally believe that this is a disengenous statement. Wavefront measurements are reproducible within a certain range, but the evaluations are so detailed that normal biological fluctuations will provide changes. It is like saying that measuring dust on an atomic scale is not reproducable, but measuring a brick on an airport scale is.

RObyRob wrote:- causes more damage in case of decentered ablation


The same tracking system is used whether the ablation is wavefront-guided or conventional. Decentration is a neutral issue between the two.

RObyRob wrote:One doctor said YES:
- it gives better vision


In study after study of patients who are appropriate candidates for both have shown that wavefront-guided ablations provide better outcomes than conventional ablation.

Because I am not a doctor, scientist, or technician trained to evaluate Pentacam scans, I will refrain from comment on your scans.
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Postby RobyRob » Tue Aug 29, 2006 9:27 pm

Thank you Glenn for these detailed answers.

I would like to add some comments.

1.) Can I ask the surgeon how many diopters of myopia and astigmatism will wavefront guided laser remove? Or he doesn't have this information because it's wavefront guided?

2.) So it's actually possible that some surgeon has better results with LASEK than epi-LASIK? That doesn't make sense since epi-LASIK is improvement of LASEK. Could there be any financial reasons in background (less expenses with LASEK)? She charges the same for both methods.

3.) So you think she considers suction ring more dangerous than trephine blade and alcohol? I have more fear from the second.

6.) You didn't answer question 6. Forgot?
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Postby LasikExpert » Tue Aug 29, 2006 10:12 pm

RObyRob wrote:1.) Can I ask the surgeon how many diopters of myopia and astigmatism will wavefront guided laser remove? Or he doesn't have this information because it's wavefront guided?


I think what you meant to say was how many microns of tissue will be removed. The laser systems are able to generate a reasonably accurate estimation of how much tissue will be removed and where it will be removed. The information is not totally precise because we are talking about biological tissue and the laser manufacturers are not going to reveal all the secrets of their algorithms. I don't see why the doctor would object to sharing that information.

RObyRob wrote:2.) So it's actually possible that some surgeon has better results with LASEK than epi-LASIK? That doesn't make sense since epi-LASIK is improvement of LASEK. Could there be any financial reasons in background (less expenses with LASEK)? She charges the same for both methods.


"Improvement" is always relative and interpretative. As an example, Lasik is an "improvement" over PRK, but many doctors are going back to PRK for some patients because the results are statistically better with PRK for that particular group of patients. It is accurate to say that Epi-Lasik is different that Lasik, but there are those who will argue Epi-Lasik is better than Lasik, some who will argue it is worse than Lasik, and some who will say Epi-Lasik is so much expensive luggage. All can point to established facts to support their claims.

The difference in material costs between LASEK and Epi-Lasik is negligible and the doctor would just pass on any additional cost to the customer/patient in the form of higher prices. Also, no doctor in his or her right mind is going to provide a sub-optimal result to save what is probably less than the price of lunch. If you want to take a totally cynical view, the doctor will make much more with a happy patient referring friends. It is in the doctor's best ethical, medical, and financial interest to do what will provide you the best outcome.

RObyRob wrote:3.) So you think she considers suction ring more dangerous than trephine blade and alcohol? I have more fear from the second.


I could only assume this is accurate, but you could ask the source to know for sure.

RObyRob wrote:6.) You didn't answer question 6. Forgot?


Actually, yes I did forget. To repeat it here:

RObyRob wrote:6.) Why does wavefront guided surgery take more tissue? Some say it takes less because it takes only what is neccessary.
It takes more compared to what? Wavefront optimized or standard? Does "more tissue" mean the tissue in peripheral area (to reduce spherical aberration)?


All comparisons are to conventional ablation.

Wavefront often requires more tissue because it is correcting more than just sphere and cylinder. The wavefront-guided ablation attempts to neutralize increases in Higher Order Aberrations (HOA) that are normally associated with laser assisted refractive surgery. That is accomplished with changes in the ablation depth. This may be less in one spot and it may be more in another, but overall more tissue is generally removed.

There is both positive spherical aberration and negative spherical aberration, requiring less tissue removal and more tissue removal, respectively. Your unique wavefront will determine what is appropriate.

Wavefront-optimized lasers (Allegretto in the US) used wavefront diagnostics to determine the best overall static ablation pattern. It is only correcting sphere and cylinder, but does so with an optimized ablation pattern.
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Postby RobyRob » Mon Sep 18, 2006 8:45 pm

Glenn, my surgery is in couple of days and I would like to ask you a few more things.

1.) Can I get a copy of informed consent signed by the surgeon? Just in case something goes terribly wrong that is not mentioned in there so I could sue him then... Do surgeons usually give this to patients?

2.) They said they use a LASEK trephine blade with 8mm diameter but that the ablation zone is 9mm! Do you have any idea how this is possible?

3.) Informed consent says that night problems are only temporary and go away in few weeks. How is this possible? Can surgery induced HOA (like spherical aberration) go away after time?

Thank you again for all your help so far.
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Postby LasikExpert » Mon Sep 18, 2006 9:59 pm

RObyRob wrote:1.) Can I get a copy of informed consent signed by the surgeon? Just in case something goes terribly wrong that is not mentioned in there so I could sue him then... Do surgeons usually give this to patients?


Your doctor will undoubtedly provide you a copy of your informed consent. In fact you could probably ask for a copy now.

Remember that an informed consent is not a contract that assures or guarantees a paticular result. It is a medical-legal process to inform the patient of potential side effects, complications, and undesired results. You are signing that you understand these issues and accept them as real possibilities. The doctor keeps a copy to affirm that s/he has provided you with the information.

RObyRob wrote:2.) They said they use a LASEK trephine blade with 8mm diameter but that the ablation zone is 9mm! Do you have any idea how this is possible?


You may want to verify this information. The excimer laser can ablate through the epithelium, but there would probably be a rather defined edge where the epithelium ends and Bowman's/stroma begins.

RObyRob wrote:3.) Informed consent says that night problems are only temporary and go away in few weeks. How is this possible? Can surgery induced HOA (like spherical aberration) go away after time?


Take another look at that informed consent. Night vision problems may be temporary and usually are temporary, but can also be permanent.

Many people who have night vision or other quality of vision problems have them resolve during the normal six month healing period. The vast majority of patients who have problems immediately after refractive surgery have them resolve with healing or additional treatment.
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Postby RobyRob » Thu Sep 21, 2006 7:43 pm

I have one more question.

The doctor said that my optical zone will be 6.5mm. I know that my pupil is often larger, sometimes even 8mm.

Tho doctor said that I shouldn't worry because the entire treatment zone will be 9mm.

I've read that only optical zone has optical effect.

Could it be that tranzitional zones are different in custom ablations than in conventional ones? Because I will have wavefront guided LASEK.
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Postby LasikExpert » Thu Sep 21, 2006 11:58 pm

The optical zone is targeted to fully correct. All transition zone is less than full correction. You should read about Lasik pupil size issues.
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