Systematic underablation in laser in situ keratomileusis: ablation pattern identified by advanced topographical analysis.

J Cataract Refract Surg. 2003 Aug;29(8):1621-5
Brown SM, Campbell CE.

Excerpt: The corneal loci responsible for the aberrations were difficult or impossible to identify on axial topographies but were readily identified with curvature topography. The patient's ablations appeared to be miniature versions of the intended ablation profiles, with small areas of emmetropic central cornea surrounded by annuli of rapidly increasing keratometric power; that is, systematic underablation. This may explain why some patients have visual aberrations with pupil diameters smaller than the programmed optical zones.

PMID: 12954316

Click here for link to full text of abstract.

More information.

Well, that hits the nail right on the head.

The laser was the Alcon LADARvision laser.

http://www.ascrs.org/publications/jcrs/editmar01.html

"For a variety of reasons that are not entirely understood, planned ablation zones are not fully realized in practice."

the issue of underablation and optical zones is really starting to be understood more.

one problem with most excimers is surgeons dont understand all the physics they need to know.

the laser beam on most laser is non collimated which means there is actually a specific focus point for the beam to have a specific diameter and energy density.

if the cornea is above or below this specific point you can see over and under ablations with smaller or larger optical zones.

there are a bunch of other factors but this one focusing issue along with corneal curvature that decreases in the periphery of the cornea result in alot of optical zones being smaller than intended

Sam Omar MD
Orlando, FL

quote:

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Originally posted by Sam Omar, MD:
the issue of underablation and optical zones is really starting to be understood more.

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That's odd. We patients on Surgical Eyes have understood this for a very long time.

Broken, the issue of optical zones and laser fluence is a very different area than the standard issues regarding pupil size and optical/treatment zones. Further compounding this issue are the cosine effect of decreased ablation in the mid/periphery limiting the effective optical zone and optical zone centration. Most folks who discuss this are really simplifying the actual optics however i am not a post refractive surgery patient and must respect all patients observations.

Sam Omar MD
Orlando, FL

My goodness, this is what I have been saying for ages.

As well as the topography being shown up with axial plots, lasers shooting from the optical axis are also at fault.

I certainly agree with Sam that the physics of the process is not properly understood by most doctors. With a better understanding of what you are actually doing (not just burning flesh) the potential for better outcomes is increased dramatically.

This is a great article and should startmuch discussion within the industry.

grant
grantm@connexus.net.au

quote:

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Originally posted by Sam Omar, MD:
Further compounding this issue are the cosine effect of decreased ablation in the mid/periphery limiting the effective optical zone and optical zone centration.

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I think until the laser industry has figured this out, maybe you should stop doing this surgery. What do you think?

P.S. And don't call me "Broken". "Broken Eyes" or "BE" for short. I am never broken.

I guess this explains why I only have one millimeter of perfect vision in the middle of my left (good) eye. Don't understand how I could have -3.5 vision (my original prescription) within the central 6 mm of an 8 mm ablation zone. Wish someone could explain THAT to me.

I'll explain it to you. Lasering a cornea is like trying to precisely sculpt jello.

(Thanks for that one, Sandy).

http://www.revophth.com/index.asp?page=1_54.htm

"These values also indicate that 17 percent of the patients have pupils larger than 7 mm and approximately 2.5 percent have greater than an 8-mm pupil.

Surgeons should also consider that the “effective optical zone” size is always smaller than the “intended” optical zone size stated by the manufacturer. Studies have demonstrated that the effective optical zone for almost every commercial laser is 10 percent smaller than intended for a – 5 D treatment, 23 percent smaller for a – 10 D treatment, 37 percent smaller for a – 15 D treatment and 52 percent smaller for a – 20 D treatment.

For an intended 6.5 mm treatment, the actual effective optical zone size with existing lasers is 5.9, 5.0, 4.1 and 3.1 mm for the – 5, –10, –15 and – 20 D treatment, respectively. To avoid any glare from pupil and effective optical zone disparity, the effective optical zone size should be the same size as the scotopic pupil size and perfectly centered with the pupil. An additional 1- or 2-mm blend zone beyond the effective optical zone is needed to avoid a sharp transition from the treated to untreated area that can cause unwanted images in the peripheral vision."

http://www.la-sight.com/LS_lsk_advancedOptics.asp

"Another important limitation inherent in every corneal laser system currently available has become known as the "cosine correction problem." Ophthalmic lasers are all calibrated by measuring beam dynamics on a flat plastic test surface. When excimer laser energy strikes the test plastic at a right angle, 100% of the energy is absorbed. The real cornea is curved, however, not flat. In treating a real cornea, the energy delivered everywhere but the very top of the corneal dome will strike on a down-sloping angle, away from the perpendicular. The energy absorbed is proportional to the angle of the downslope, hence the "cosine" reference. Until laser systems can measure, anticipate and correct for this appropriately, some error will invariably be introduced, and this error cannot be corrected even by fancy "wavefront" systems."

This sounds a lot like what Dr. Holladay talks about at the podium, prolate to oblate.

The way I see it, all of these factors are influenced by pupil size. Regardless of how you look at it, if the pupil is small, as in bright light, the oblate shape or cosine effect does not affect the vision. The larger the pupil dilates, the more the cosine effect/oblate shape becomes problematic for vision.

You can multifactorial it all day long and it's still pupil size dependent. The larger the pupil, the worse it gets.

Of course I realize that other factors cause night vision problems, like decentrations just to name one. But when you think about it, unless it is so grossly decentered that it's way off on the sclera somewhere, most patients with a decentered ablation are not bothered in bright light. But when the lights go out, the ghosts appear.

My eye was finally picked up with the new Customvue wave.
OS +5.41 DS -4.85 DC x 77 @12.5 mm (4.0 Rx Calc)
Pupil (mm) 4.00 High Order 21.5%
Eff. Bur (D) 5.11 Rms Err 2.95

What does this mean as far as Abberrations or irregular astigmatism?
Thanks

Alexis,

If I am reading this correctly, at the time of the measurement, your pupil was 4mm. Was the room dark when the measurement was taken? The point I'm getting at is that if the room was not dark, then the measurement doesn't tell you the whole picture.

I'm not the best at reading these. Hopefully someone else will have more insight. But one thing seems obvious. If your refractive error is +5.41 and your Higher Order is 21.5%, you have some serious aberrations, even at 4mm. I *think* that the RMS Err represents the total higher order stuff, and 2.95 is a lot at 4mm.

Irregular astigmatism is all the higher order aberrations, everything but your sphere and cylinder.

You got some serious problems, girl. From http://www.cquest.com/russo/OPD_maps.html, "Basic Understanding of Nidek OPD Map Displays": "The Total WF RMS error as displayed as the bottom of the Higher Order Map is also a good measure of irregular astigmatism. Values greater than 1.0um appear to be clinically significant." It says that individual Zernike RMS values of greater than 0.5um are clinically significant. I have 0.355um of coma in my right eye and it is very annoying. I can only imagine what you're seeing.

Alexis, do you have the individual values? Apparently they tell more than the total rms error. I had posted about my frustration with my vision and one of the dr.s said that he suspected that my coma value was high (greater than 0.3um). Some higher order aberrations have more of an impact on vision than others. For instance, spherical aberration may not affect your day vision, but it'll cause starbursts and glare. Coma can cause a lot of blur and possibly double images even during the daytime. Trefoil is apparently not as big of deal as coma or spherical aberration. Also, higher order aberrations will have a bigger impact on emmetropes (people with no prescription) than on people who have decent amounts of astigmatism and/or defocus. I have a picture of the moon - one pic shows how a perfect eye sees it, one shows how an abe emmetrope with 0.16um of spherical aberration sees it, one shows how it looks to a 1D myope, and another shows how it looks to a 1D myope with 0.16um of spherical aberration. Pretty interesting stuff. Unfortunately, I don't have the web address where I found it. It was about visual significance of variability.

I think your "High order 21.5%" means that 21.5% of your total aberration is of the higher order (ie.coma, spherical, trefoil, etc.). The rest (78.5%) is defocus and astigmatism. Maybe someone else has better or more information.

Thank you for response and your interesting information. My left eye sees 8 moons and I do have multiple images during the day and at night. I also have cornea pain when my eye is dry especially in the middle of the night which I think is because of more severe dryness. I think the pain is from damaged nerves.

My eye was finally picked up with the new Customvue wave.
OS +5.41 DS -4.85 DC x 77 @12.5 mm (4.0 Rx Calc)
Pupil (mm) 4.00 High Order 21.5%
Eff. Bur (D) 5.11 Rms Err 2.95

What does this mean as far as Abberrations or irregular astigmatism?

if I remember correctly the room was dark
Thanks[/QUOTE][/QUOTE]

Diameters of Topographic Optical Zone and Programmed Ablation Zone for Laser in situ Keratomileusis for Myopia

Journal of Refractive Surgery Vol. 19 No. 5 September/October 2003



Andreea E. Partal, MD; Edward E. Manche, MD

PURPOSE

To compare topographic optical zones with programmed ablation zone settings of eyes treated with laser in situ keratomileusis (LASIK) for myopia using the VISX S2 excimer laser.
METHODS

Two-hundred three eyes treated with LASIK using the VISX S2 excimer laser were retrospectively evaluated to determine the size of the topographic optical zone. Three to six months after LASIK, the topographic optical zone was measured at the zone of highest curvature on topography and subtraction topography. Eyes were divided into four groups (A, B, C, D) in order of increasing myopia.
RESULTS

When the topographic optical zone was compared with the programmed ablation zone, an optical zone reduction of 0.5 ± 0.1 mm and 0.4 ± 0.1 mm was found for the longest and shortest diameters, respectively. For eyes with spherical ablation zones, this reduction was 0.6 ± 0.1 mm and 0.4 ± 0.1 mm shorter than the programmed horizontal and vertical dimensions. Groups A, B, C, and D, in order of increasing myopia, all showed reductions of approximately 0.5 ± 0.1 mm for the longest and 0.3 to 0.4 ± 0.1 mm for the shortest diameters of the optical zone.
CONCLUSIONS

The topographic optical zone was reduced from the programmed ablation zone. This reduction was statistically significant for both elliptical and spherical ablations, and seemed to be independent of the amount of myopia. [J Refract Surg 2003;19:528-533]