Lasik Eyes Surgery - K values and PRK/Lasik?

Author

K values and PRK/Lasik?

Darrell Criswell

2005-07-24, 1:00 am

I had a preliminary evaluation for PRK or Lasik. I have about a -5.0
diopter correction. After corneal topography I was told I have a
relatively flat cornea, my K value was about 42 and to do PRK they
would have to remove about 5 so I would end up with a K value of 37 (I
was told the lowest they would take the K value to was 36). They said
the corneal surgeons might not think I was a good candidate for
refractive surgery. They said the low K value might not give good
results because of the optics of visual system. I haven't talked to
the surgeon yet so I don't know what he will say.

Can anyone tell me what problems the low K value has for PRK and why
low K values could give poor results for PRK or LASIK?

I have read if you have LASIK the low K value can cause problems with
the creation of the flap.

Any explanations or advice would be appreciatated.

Glenn - USAEyes.org

2005-07-24, 6:52 pm

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.

Sandra

2005-07-26, 11:09 pm

A low K value represents a flat cornea. A normal eye is smooth and round
like an eagle's eye, to
gather light well. Frogs have flat eyes. They don't see so well. If you
want to
see like an eagle, you want a round cornea that gathers light well and
focuses light on the retina.
Don't let some refractive surgeon flatten your cornea.

Wearing corrective lenses helps you focus light better on your retina, but
once
your cornea has been flattened by refractive surgery it is flat for life.
You may
have serious problems getting contacts to fit that flattened deformed
post-surgical
cornea. Every post-LASIK cornea has wrinkles in it called Bowman's crinkles.
That's
100%. That doesn't help much with optics. This may be the source of those
sparklebursts
that almost every LASIK patient (perhaps every LASIK patient) sees.

To make matters even worse, LASER eye surgery damages the corneal nerves,
causing dry eye. Recent studies show this damage is significant and
permanent.

Loss of contrast sensitivity is also reported to be UNIVERSAL after LASIK
and permanent. Who wants to lose part of their ability to see normally? For
vanity?

Be nice to your eyes and skip the eye surgery altogether. It's not good for
you.
Tell a friend.

"Darrell Criswell" <dcriswel@satx.rr.com> wrote in message
news:b626e1ddh713hemv45ptaaa5jpv7rr80ad@4ax.com...
>I had a preliminary evaluation for PRK or Lasik. I have about a -5.0
> diopter correction. After corneal topography I was told I have a
> relatively flat cornea, my K value was about 42 and to do PRK they
> would have to remove about 5 so I would end up with a K value of 37 (I
> was told the lowest they would take the K value to was 36). They said
> the corneal surgeons might not think I was a good candidate for
> refractive surgery. They said the low K value might not give good
> results because of the optics of visual system. I haven't talked to
> the surgeon yet so I don't know what he will say.
>
> Can anyone tell me what problems the low K value has for PRK and why
> low K values could give poor results for PRK or LASIK?
>
> I have read if you have LASIK the low K value can cause problems with
> the creation of the flap.
>
> Any explanations or advice would be appreciatated.

serebel

2005-07-26, 11:09 pm

Sandra wrote:
Every post-LASIK cornea has wrinkles in it called Bowman's crinkles.
> That's
> 100%. That doesn't help much with optics. This may be the source of those
> sparklebursts
> that almost every LASIK patient (perhaps every LASIK patient) sees.
>

You've gotta be kidding. Bowmans crinkles? Sounds like a new brand of
potato chip.

> To make matters even worse, LASER eye surgery damages the corneal nerves,
> causing dry eye. Recent studies show this damage is significant and
> permanent.
>

Also not true. What recent studies?

> Loss of contrast sensitivity is also reported to be UNIVERSAL after LASIK
> and permanent. Who wants to lose part of their ability to see normally? For
> vanity?
>
Not true again. A lot of people have much improved contrast
sensitivity. Most of what you call this loss is miniscule.

> Be nice to your eyes and skip the eye surgery altogether. It's not good for
> you.
> Tell a friend.
>
> Tell a friend that "Sandra" here is probably just another "Sandy" type of alias.

SErebel

Glenn - USAEyes.org

2005-07-27, 9:01 am

> Bowmans crinkles?

I have not heard that one in a while. Bowman's crinkles was a term
briefly used to describe micro-striae in the LASIK flap. I suppose
there is somebody out there still using the term, but I have not heard
it at any of the conventions in a long time.

For details on striae and resolution techniques, see
http://www.complicatedeyes.org/striae.htm

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.

serebel

2005-07-27, 11:01 pm

Glenn - USAEyes.org wrote:
>
> I have not heard that one in a while. Bowman's crinkles was a term
> briefly used to describe micro-striae in the LASIK flap. I suppose
> there is somebody out there still using the term, but I have not heard
> it at any of the conventions in a long time.
>

I prefer my "crinkles" with salt and a bottle of Bud.

SErebel