Re: K values and PRK/Lasik?

The K value tells the curvature of the cornea. A high K value
indicates a more prolate cornea (shaped like the point of a football)
whereas a low K value indicates a more oblate cornea (shaped like the
top of a hamburger bun).

Birds and animals that are predators tend to have forward pointing
eyes with more prolate corneas. A prolate cornea provides excellent
central forward vision. A good example is an eagle. An eagle has very
prolate coronas with a very dense retina that allows him to see very,
very clearly at a great distance. This is important if you are trying
to grasp a mouse in a field when you are flying 40 miles per hour.

A frog, on the other hand, is more prey than predator. His eyes are
set back on his head and are less forward looking than an eagle. His
corneas are very flat when compared to the eagle. The frog's flap
oblate cornea gives him good peripheral vision. This way the frog can
see things coming at him from above and behind.

Human corneas are more like the eagle than the frog. They are prolate
and pointed forward. We have good central forward vision and not so
very good peripheral vision. This serves us well as predators and
occasional prey.

Refractive surgery for myopia (nearsighted, shortsighted) vision
effects the change by flattening the central portion of the cornea,
thus making the cornea more oblate. If this flattening is not severe
and there is enough of a prolate shape after surgery, we won't turn
into frogs. Well, frog-type vision, anyway.

If you want to read all the technical details on this, go to
http://www.pubmed.com and search on "prolate", "excimer", and
"Holladay" (yes the spelling is correct).

You surgeon is being wisely cautious about making your already
somewhat flat corneas too flat. If your cornea becomes too flat, your
central forward vision will probably decrease in quality. Sure, you
will be able to see a bit more clearly in the periphery, but unless
you are a frog or are commonly prey, this is not a good thing.

I recommend that you visit a doctor who uses the Wave Light Allegretto
excimer laser (http://www.allegrettowave.com). This laser has an
ablation pattern that is designed to maintain a more prolate cornea
and may (emphasis on "may") be able to provide the correction you need
and not cause a loss in central forward vision.

Also, congratulations on selecting a doctor who is knowledgeable on
these issues and is advising you correctly. There have been too many
patients who ended up with poor vision quality because of a cornea too
flat after refractive surgery. Additionally, although the recovery
time is longer with PRK than LASIK, studies have shown PRK has a
better outcome long-term.

Glenn Hagele
Executive Director
USAEyes.org

"Consider and Choose With Confidence"

Email to glenn dot hagele at usaeyes dot org

http://www.USAEyes.org
http://www.ComplicatedEyes.org

I am not a doctor.

Glenn - USAEyes.org wrote:
> The K value tells the curvature of the cornea. A high K value
> indicates a more prolate cornea (shaped like the point of a football)
> whereas a low K value indicates a more oblate cornea (shaped like the
> top of a hamburger bun).
>
> Birds and animals that are predators tend to have forward pointing
> eyes with more prolate corneas. A prolate cornea provides excellent
> central forward vision. A good example is an eagle. An eagle has very
> prolate coronas with a very dense retina that allows him to see very,
> very clearly at a great distance. This is important if you are trying
> to grasp a mouse in a field when you are flying 40 miles per hour.
>
> A frog, on the other hand, is more prey than predator. His eyes are
> set back on his head and are less forward looking than an eagle. His
> corneas are very flat when compared to the eagle. The frog's flap
> oblate cornea gives him good peripheral vision. This way the frog can
> see things coming at him from above and behind.
>
> Human corneas are more like the eagle than the frog. They are prolate
> and pointed forward. We have good central forward vision and not so
> very good peripheral vision. This serves us well as predators and
> occasional prey.
>
> Refractive surgery for myopia (nearsighted, shortsighted) vision
> effects the change by flattening the central portion of the cornea,
> thus making the cornea more oblate. If this flattening is not severe
> and there is enough of a prolate shape after surgery, we won't turn
> into frogs. Well, frog-type vision, anyway.
>
> If you want to read all the technical details on this, go to
> http://www.pubmed.com and search on "prolate", "excimer", and
> "Holladay" (yes the spelling is correct).
>
> You surgeon is being wisely cautious about making your already
> somewhat flat corneas too flat. If your cornea becomes too flat, your
> central forward vision will probably decrease in quality. Sure, you
> will be able to see a bit more clearly in the periphery, but unless
> you are a frog or are commonly prey, this is not a good thing.
>
> I recommend that you visit a doctor who uses the Wave Light Allegretto
> excimer laser (http://www.allegrettowave.com). This laser has an
> ablation pattern that is designed to maintain a more prolate cornea
> and may (emphasis on "may") be able to provide the correction you need
> and not cause a loss in central forward vision.
>
> Also, congratulations on selecting a doctor who is knowledgeable on
> these issues and is advising you correctly. There have been too many
> patients who ended up with poor vision quality because of a cornea too
> flat after refractive surgery. Additionally, although the recovery
> time is longer with PRK than LASIK, studies have shown PRK has a
> better outcome long-term.
>
> Glenn Hagele
> Executive Director
> USAEyes.org
>
> "Consider and Choose With Confidence"
>
> Email to glenn dot hagele at usaeyes dot org
>
> http://www.USAEyes.org
> http://www.ComplicatedEyes.org
>
> I am not a doctor.

There is some virtue in brevity, or so I thought.

DrG

LOL

Well, you had given the short answer, but I find that some people
won't settle for the clear, concise, and quick advice of a
knowledgeable doctor. When you give them all the details, they may
understand that advice of a professional is worth a thousand words.

And I'm sure you know me well enough that when you ask me what time it
is, I'll explain why time was invented.

Glenn Hagele
Executive Director
USAEyes.org

"Consider and Choose With Confidence"

Email to glenn dot hagele at usaeyes dot org

http://www.USAEyes.org
http://www.ComplicatedEyes.org

I am not a doctor.

Yes, now it's time for the explanation.

How exactly is the central forward vision better in prolate (vs. oblate) -
is it the central image sharper or is it a change in perspective perhaps?
Can it be said that it is a choice between sharp central vision (prolate)
vs. sharp peripheral vision (oblate)? (Obviously neither do have real
/problem/ with sharpness both centrally or peripherally.)

The optic part of the answer is that the prolate cornea will focus
more light energy at the fovea. The density of the receptor cells in a
human makes the best possible vision with absolutely no aberrations at
about 20/6, however due to aberrations the most that can be achieved
is about 20/8, and that is very rare.

Glenn Hagele
Executive Director
USAEyes.org

"Consider and Choose With Confidence"

Email to glenn dot hagele at usaeyes dot org

http://www.USAEyes.org
http://www.ComplicatedEyes.org

I am not a doctor.

Glenn - USAEyes.org wrote:

>The optic part of the answer is that the prolate cornea will focus
>more light energy at the fovea.
>
I don't think that it's the quantity of light energy that makes the
difference, it's more the quality of the image, which is indeed affected
by the optical quality (presence or absence of refractive error defocus
and/or aberrations) and pupil size.

> The density of the receptor cells in a
>human makes the best possible vision with absolutely no aberrations at
>about 20/6, however due to aberrations the most that can be achieved
>is about 20/8, and that is very rare.
>
>
In healthy humans, 20/10 is pretty common, so I'd guess there are quite
a few 20/8s out there, it's just that the standard charts don't
generally get that fine, so they *rarely* get recorded.

w.stacy, o.d.

Have you checked your room dimensions?

DrG

Not enough info to know to whom or what you are posting. Please snip at
least a little from the post you're addressing...

w.stacy, o.d.

Dr. Leukoma wrote:
> Have you checked your room dimensions?
>
> DrG
>

> I don't think that it's the quantity of light energy that makes the
> difference, it's more the quality of the image, which is indeed
> affected by the optical quality (presence or absence of refractive
> error defocus and/or aberrations) and pupil size.

Well, it's a bit confusing. I don't mean to be rude, but isn't it trivial
that refractive error and aberrations will reduce the image quality.

I didn't even mention the quantity of light -possibility, because I didn't
believe it would have essential effect, except possibly in dark (but only if
there is near to zero spherical aberration). I would guess that it doesn't
make any difference whatsoever in daylight or possibly even in room light if
the central image is brighter since the eye senses brightness
logarithmically. And, even a slight aberration would probably far exceed the
benefit of brighter central image (= the case of prolate, apparently).

If it is indeed central or peripheral brightness that differs in
prolate/oblate, I would imagine that it doesn't have nearly any significance
in practice, except if you're the lucky 1% of patients or so, prolate might
be good when there is nearly zero aberration present.

I don't know if prolate has other significant benefit like better contact
lens compatibility or better tear film properties.

Please comment if you have any more information about the subject.

Glenn - USAEyes.org wrote:
> The optic part of the answer is that the prolate cornea will focus
> more light energy at the fovea. The density of the receptor cells in a
> human makes the best possible vision with absolutely no aberrations at
> about 20/6, however due to aberrations the most that can be achieved
> is about 20/8, and that is very rare.
>
> Glenn Hagele
> Executive Director
> USAEyes.org
>
> "Consider and Choose With Confidence"
>
> Email to glenn dot hagele at usaeyes dot org
>
> http://www.USAEyes.org
> http://www.ComplicatedEyes.org
>
> I am not a doctor.


Glenn is correct. The oblate cornea results in increased spherical
aberration, which results in an annulus of defocus.

DrG