250 Cornea Thickness Rule

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250 Cornea Thickness Rule

Postby DryEye » Sat May 12, 2007 4:16 pm

Does the 250 Number mean total Cornea Thickness? If one is told they have a 476 Cornea thickness post lasik is that plenty to work with or does the 250 number entail a certain part of the Cornea only?
Thanks
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Re: 250 Cornea Thickness Rule

Postby JPD » Sat May 12, 2007 5:36 pm

DryEye wrote:Does the 250 Number mean total Cornea Thickness? If one is told they have a 476 Cornea thickness post lasik is that plenty to work with or does the 250 number entail a certain part of the Cornea only?
Thanks


I'm not for sure, but I think in the case of Lasik, the the amount of cornea you're told you have left to work with would be the amount under the flap. Before you have a flap cut, it would of course be total cornea thinkness.
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Postby LasikExpert » Sat May 12, 2007 5:47 pm

It is necessary for at least 250 microns of a healthy cornea to remain untouched to maintain stability. This does not commonly mean 250 micron total cornea, but 250 microns under the flap and under the ablation that is untoched.

More is always better.
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Postby DryEye » Sat May 12, 2007 8:37 pm

Is there a way to interpret the numbers looking at ones Orbscans or PentaCam's?
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Postby LasikExpert » Sat May 12, 2007 8:50 pm

The Orbscan and Pentacam can provide total corneal thickness, but not thickness of just the flap or just the untouched cornea. That would require a Visante OCT or Artemis, which are relatively rare.
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Postby DryEye » Sat May 12, 2007 8:52 pm

So, how do these doctors know if someone is a good candidate risk wise when doing enhancements?
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Postby LasikExpert » Sat May 12, 2007 11:01 pm

Subtraction technique. The total thickness is measured and calculations of intended flap thickness and expected tissue ablation are subtracted.

Many doctors do a thickness test of the underlying stromal bed after creating and moving back the flap but before applying laser energy. This provides the actual flap thickness and residual corneal thickness, from which the expected tissue ablation is subtracted.
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Postby DryEye » Mon May 14, 2007 9:07 pm

So, if my right eye was 541 prior to lasik and it is now 476 is that enough information in order to figure out if it is enough?
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Postby LasikExpert » Tue May 15, 2007 2:42 am

Not with much precision. It would be important to know the thickness of the flap.

Flaps range from 100 to about 180 microns thick, which would mean a guestimate would be your untouched cornea is between 376 and 296 microns.
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Postby DryEye » Tue May 15, 2007 2:49 am

I have all my paperwork - would it be stated somewhere on there?
It says estimated flap 160? Flap Thickness 160 Superior Flap?
It was CustomVue Surgical Parameters:
Flap Diameter (mm) 9.00
Flap Thickness : 160
Residual Bed Depth : 313

Anything else I should be looking at on the sheet/printout?
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Postby LasikExpert » Sat May 19, 2007 1:10 am

It looks like all the numbers are there.
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Postby DryEye » Sat May 19, 2007 3:41 am

What does all that mean?
Thanks
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Postby JPD » Sat May 19, 2007 4:20 am

DryEye wrote:What does all that mean?
Thanks


You've got 473 total corneal thickness (including the flap). You have 313 microns under the flap. So for future work in order to maintain the 250 minimum you have 63 microns of tissue which can be removed. So if you need an enhancement which would require the removal of more then 63 microns you're only option would be PRK on your flap.
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Postby DryEye » Sat May 19, 2007 5:45 am

PRK would remove top portion only vs lasik doing it internally with 63 microns of tissue for safety? Can a surgeon get it down (lasik enhancement) to a certain amount of tissue to remove safely when doing an enhancement - say 30 or 40 microns in this case?
With PRK they would have more to work with for removal if need be over a period of one's lifetime?
Do you know what the average (cornea left) is for most people after lasik?
How do the two differ in terms of corneal stability over long run?
With PRK one can do more than 63 microns if need be?
Also, I read here that once one does PRK over a flap that is the way it has to be done for additional enhancements down the road.
Would I have had more cornea left if I had chosen Intralase instead of microkeratome (VISX CustomVue)?
I think I would prefer lasik if I decide on an enhancement unless I am told that it isn't a wise choice.
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Postby JPD » Sat May 19, 2007 3:17 pm

DryEye wrote:Would I have had more cornea left if I had chosen Intralase instead of microkeratome (VISX CustomVue)?


Yes, I believe Intralase cuts the flap somewhere between 100-120 microns. I'm not knowledgeable enough to answer any of your other questions.
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