Lasik Eyes Surgery - Prolate in Zeiss MEL-80

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Prolate in Zeiss MEL-80

Pauli Soininen

2005-04-19, 10:51 am

Zeiss MEL-80 lasers the eye so, that the shape of the eye will be prolate as
illustrated in this brochure for example
http://www.grane.cz/pdf/MEL80%20brochure.pdf . My understanding is, that
spherical aberration correction is based on the curvature of the lens. How
is it possible, that prolate (MEL-80) and oblate (others like Zyoptix etc.)
will produce the same refractive correction?

Are there any research on the benefits (or disadvantages) of prolate? Or
what are the commonly stated benefits of prolate?

Can an eye lasered with MEL-80 be retreated with other manufacturer's
system?

Glenn - USAEyes.org

2005-04-19, 10:51 am

I just had a long and (always) interesting conversation with Dan
Reinstein, MD, who is a researcher for Zeiss. Simply put, prolate
ablation patterns such as those found on the Wave Light Allegretto and
the Zeiss Meditec lasers do produce the same refraction correction,
but in very different ways and in ways that may make a big difference
in the quality of vision for the patient. For some (many actually)
the difference is negligible, but for those who need the advantages of
a more prolate cornea, the difference is great.

BTW, one of the most significant advances of the wavefront-guided
ablation is that they tend to leave the cornea more prolate.

As for research, Dr. Reinstein will be presenting several studies here
at the ASCRS meeting in Washington DC about these differences and
their apparent advantages. OF course, Dan is a paid consultant for
Zeiss, so one must consider that possible bias.

An eye treated with any laser can be retreated with any other laser.
Whether or not an eye SHOULD be retreated is another matter.

Glenn Hagele
Executive Director
Council for Refractive Surgery Quality Assurance

Email to glenn dot hagele at usaeyes dot org

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

I am not a doctor.

Pauli Soininen

2005-04-19, 10:51 am

> Simply put, prolate ablation patterns such as those found on
> the Wave Light Allegretto and the Zeiss Meditec lasers do
> produce the same refraction correction, but in very different
> ways and in ways that may make a big difference
> in the quality of vision for the patient.

Thank you. That sounds familiar; there is a big difference. I would really
like to know what that difference is (and if it's the curvature, how can the
refractive power be the same)?

> As for research, Dr. Reinstein will be presenting several studies here
> at the ASCRS meeting in Washington DC about these differences and
> their apparent advantages.

Would be great to hear about these!

Does prolate require more, less or same amount of ablation than oblate?

Ragnar

2005-04-19, 10:51 am

That's a good question Pauli. The MEL80 is not available to U.S.
patients for some reason.

On Sat, 16 Apr 2005 23:40:03 GMT, "Pauli Soininen" <no@spam.com>
wrote:

>Zeiss MEL-80 lasers the eye so, that the shape of the eye will be prolate as
>illustrated in this brochure for example
>http://www.grane.cz/pdf/MEL80%20brochure.pdf . My understanding is, that
>spherical aberration correction is based on the curvature of the lens. How
>is it possible, that prolate (MEL-80) and oblate (others like Zyoptix etc.)
>will produce the same refractive correction?
>
>Are there any research on the benefits (or disadvantages) of prolate? Or
>what are the commonly stated benefits of prolate?
>
>Can an eye lasered with MEL-80 be retreated with other manufacturer's
>system?
>

Glenn - USAEyes.org

2005-04-19, 10:51 am

Feel free to give me a call if you would like to discuss all the gory
technical details, or better yet I could possibly put you in touch
with one of the doctors who helped develop those profiles. The doctor
who has probably done more and for longer about prolate corneas is
Jack Holladay (that is the spelling) of Houston Texas. A trip down
PubMed lane with a search for "prolate" and "Holladay" will give you
more detail than the average lay person (and most surgeons) can
comprehend. Dr. Holladay is also an engineer.

To put things simply (LOL), we tend to think of light entering the eye
and being bent by the cornea and lens to focus on the retina as a
single line. This is what most of the diagrams you see show; one
line. The problem is that this is not what happens. There are
innumerable "rays" of light passing at the same time through different
points of the cornea, lens, and they do not all focus on the same
point. If they did, we would have no peripheral vision. The more
rays that focus directly on the fovea, the better the forward vision.
The more rays that focus off the fovea, the better the peripheral
vision, but at the cost of reduced vision quality (not necessarily
quantity) of forward vision.

Dr. Holladay uses the frog and eagle as an example. An eagle has his
eyes pointed forward (as do about all predators) and has highly
prolate corneas that focus the majority of light on the fovea to give
maximum quality of forward vision, but the eagle has lousy peripheral
vision. That really doesn't make much difference because on the chain
the eagle is up there pretty high in the food chain and does not need
to worry much about something sneaking up from the side. He does need
good forward vision because if you swoop down on a mouse at 45 mph and
miss by as much of an inch, you are going to last be known as a
blazing ball of feathers and a long grease spot.

A frog, on the other hand, is not much of a predator (I'm sure the
pro-mosquito zealots will argue with me about that) but really needs
to watch his back-side. His eyes are placed more flat and up, and he
has a very oblate cornea. This gives him much better peripheral
vision, but his forward vision is not so great. Lucky for him he has
a long wide sticky tongue to whack those mosquitoes.

People are in between the frog and the eagle. We have forward set
eyes, reasonably prolate corneas, but not too prolate so we have some
peripheral vision to keep the predators off our back. When you
flatten the cornea, by design you are making the prolate cornea more
oblate. Some technologies really make it oblate, some much less.
This is why the more prolate ablation patterns in the Wave Light
(Holladay was instrumental in this design) give better quality of
vision, but realistically there is also some increase in peripheral
vision and decrease in forward vision quality. We are seeing this in
all the outcome reports (contrast sensitivity, etc.). Whether this
decrease in forward vision is enough to be clinically significant is
another issue.

Wavefront attempts to compensate for this prolate to oblate change by
over accommodating the higher order aberration changes caused by the
ablation and the change in overall shape. That is why wavefront
guided ablations are able to make a more oblate cornea see good
quality. It is kind of like lowering the water because you can't
raise the bridge.

Wavefront requires more ablation than conventional, but I have not
seen where there is much difference between more oblate and more
prolate tissue loss overall. Most certainly there will be a
difference between an oblate peripheral tissue loss and prolate
peripheral tissue loss.

Virtually everything presented here at ASCRS will be available online
within a few months. Well, everything of value, anyway. If you are
really a glutton for punishment, you can purchase all courses on audio
CD.

Glenn Hagele
Executive Director
Council for Refractive Surgery Quality Assurance

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

2005-04-19, 10:51 am

Because Zeiss has not yet decided to spend millions of dollars to
enter the US laser competition, that's why. It is a conscious
decision by the company, although this is changing.

Glenn Hagele
Executive Director
Council for Refractive Surgery Quality Assurance

Email to glenn dot hagele at usaeyes dot org

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

I am not a doctor.

Pauli Soininen

2005-04-19, 10:51 am

Thank you very much, this was one of the best posts I've read here.

> The more rays that focus directly on the fovea, the
> better the forward vision.

Usually it is said, that good acuity is based on densely distributed cells
(we are assuming that refraction is ok). The amount of rays usually equals
to brighter picture.

What I understand that you are saying is that prolate will gather rays more
densely on the fovea and less densely to the peripheral area (the density of
cells on retina being a different story). I assume that the picture formed
on retina has essentially the same proportions on a prolate and oblate
lasered eye in terms of dimensions.

I'm still wondernig how this is possible. I could explain it maybe if there
was a counter-shape in the crystalline lens to compensate or if the
refrative index would be somehow different on the "extra"-curved part of
cornea. But these are probably bad guesses (especially the first one) and
the correct answer is something else.

> An eagle has his eyes pointed forward (as do about all
> predators) and has highly prolate corneas that focus the
> majority of light on the fovea to give maximum quality
> of forward vision

In addition, an eagle has two foveas per eye. From what I found out, it
seems that the extra fovea is used in underwater dives.

> This is why the more prolate ablation patterns in the Wave Light
> (Holladay was instrumental in this design) give better quality of
> vision, but realistically there is also some increase in peripheral
> vision and decrease in forward vision quality.

I found this part confusing, wasn't it before that prolate means good
forward vision quality?

> We are seeing this in all the outcome reports
> (contrast sensitivity, etc.).

I personally find the term contrast sensitivity not so good because in my
opinion, LASIK does not affect the cells' ability to detect the amount of
light. For example, bright objects (such as light coming from a window) will
spread over dark objects and then it will be impossible to see the dark
object properly (because the retina has adjusted to the bright picture and
the bright object is covering the dark object). And text will always blur
(lighter areas will spread over darker areas) more or less, which can be
interpreted as contrast sensitivity loss but in fact is just caused by
spreading (and/or retinal adaptation and/or large pupil size exposing the
transition zone).

Pauli Soininen

2005-04-19, 10:51 am

Glenn - USAEyes.org

2005-04-19, 10:51 am

My description of the advantage of a prolate cornea is an
oversimplification. It seems that you want to know all those gritty
details. I think you will find Dr. Holladay's articles very
interesting.

The image size is not the same with a more oblate cornea. What occurs
with a grossly flattened cornea is too may rays are focused off the
point of the object. That is manifested as blur. Burr is simply part
of an image being expanded, but not all the image.

The cornea accounts for about 1/3rd of the total refractive power of
the eye, the rest is in the crystalline lens. Theoretically, the
fluids in the eye are refractive neutral, but of course they
contribute also. You are correct that the lens "reverse compensates"
for the prolate shape of the cornea, but remember that human corneas
are not all that prolate. There is not so much compensation going on.

One of the reasons the quality of our vision changes as we get older
is that the shape of the lens changes with age. Even before
cataracts, the lens makes a significant change that almost reverses
the shape from childhood. This reduces the forward vision quality,
but increases peripheral It is opined that this was nature's way of
keeping old folks around longer. The young agile predators becomes a
slow old prey. That is of course speculation, but it is consistent
with the nature of vision in predators and prey.

The density of the light sensitive cells at the fovea determines the
best possible vision acuity for the animal. A human with a perfectly
compacted highest natural density of cells and a perfect
wavefront-neutral optical system could see 20/5. Due to imperfections
in the optics, the best one could reasonably expect is 20/8. The fact
that people achieve 20/12 and even 20/10 vision after wavefront-guided
surgery is really pushing the biological envelope.

I didn't know about the above water and under water dual fovea in the
eagle. Very interesting.

The reason there is some decrease in forward vision quality and
increase in peripheral vision quality of an Wave Light Allegretto
ablation is because while it is a MORE prolate profile, it is still
changing he shape of the cornea toward prolate. It is necessary to
flatten the center of the cornea to give a myopic correction.

The "sensitivity" part of contrast sensitivity is not based upon the
individual cells' ability to perceive differences in light and dark,
but is considering the entire neuro-optical system's ability to
decipher the difference between light and dark. Yes, the cell's
ability stays the same with refractive surgery, but the quality of the
light, and even the wavelength of that light, can be changed in
refractive surgery enough to cause reduction in perception of change.

You are correct that "contrast sensitivity" is an inaccurate use of
the English language to define an event, but then try to explain why
"pretty well" does not mean an attractive hole in the ground. English
is rather inexact.

BTW, I've seen at the ASCRS medical conference more presentations
showing an INCREASE in contrast sensitivity after LASIK and PRK than I
have ever seen before. Doctors are reporting routine outcomes that
would have been impossible to even predict four years ago.

Glenn Hagele
Executive Director
Council for Refractive Surgery Quality Assurance

Email to glenn dot hagele at usaeyes dot org

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

I am not a doctor.

Pauli Soininen

2005-04-19, 10:51 am

> The image size is not the same with a more oblate cornea. What occurs
> with a grossly flattened cornea is too may rays are focused off the
> point of the object. That is manifested as blur. Burr is simply part
> of an image being expanded, but not all the image.

What I understand from this is that more oblate cornea is the same thing as
partial refractive error which sounds similar to the situation of refractive
error free central cornea and outer cornea with refractive error.

> This reduces the forward vision quality, but increases
> peripheral It is opined that this was nature's way of
> keeping old folks around longer.

I can believe this is true. But I have to say I don't understand the
mechanism. At least the image I'm seeing seems perfectly consistent in all
parts. The focus, amount of superimposed spreaded picture, the brightness,
everything is consistent. Of course perihperal area isn't as perceivable as
the forward vision (due to cell density and above that, subconscious routing
of visual information to the consciousness, I think). But when the balance
of vision quality between forward and peripheral changes, do you mean the
brightness changes between the areas or something else?

> The reason there is some decrease in forward vision quality and
> increase in peripheral vision quality of an Wave Light Allegretto
> ablation is because while it is a MORE prolate profile, it is still
> changing he shape of the cornea toward prolate. It is necessary to
> flatten the center of the cornea to give a myopic correction.

I still don't understand this part. I feel like "toward prolate" should read
"toward oblate".

> Yes, the cell's ability stays the same with refractive
> surgery, but the quality of the light, and even the
> wavelength of that light, can be changed in refractive
> surgery enough to cause reduction in perception of change.

I would guess, that after the light rays have passed through everything and
are now hitting the cell, there are only two meaningful variables:
wavelength and the superpositional amplitude of many rays (or in case of
just one photon, there exists only one amplitude).

> You are correct that "contrast sensitivity" is an inaccurate use

I would suggest something like the opacity of superimposed blurred picture.
Or even blurriness is better in my opinion.

It is not that the similar shades are difficult to tell apart from each
other but the countour is blurred, that is why the acuity suffers (and also
dark areas are not visible because of forced adaptation and obstruction from
spreaded light). And basically this is exactly the same problem as in simple
refractive error, only not as severe as here is also some of the good, sharp
picture to help.

> BTW, I've seen at the ASCRS medical conference more presentations
> showing an INCREASE in contrast sensitivity after LASIK and PRK than I
> have ever seen before.

Yes, some patients (somewhat rare cases?) are saying, that they see better
for example in dark than before with glasses/contacts. Logically the
explanation would be that there are less aberrations. How on earth is this
possible, if there is a transition zone with refractive error and the pupil
is large enough to uncover the transition zone?

Glenn - USAEyes.org

2005-04-19, 10:59 pm

>I can believe this is true. But I have to say I don't understand the
>mechanism. At least the image I'm seeing seems perfectly consistent in all
>parts. The focus, amount of superimposed spreaded picture, the brightness,
>everything is consistent. Of course perihperal area isn't as perceivable as
>the forward vision (due to cell density and above that, subconscious routing
>of visual information to the consciousness, I think). But when the balance
>of vision quality between forward and peripheral changes, do you mean the
>brightness changes between the areas or something else?

Not the brightness per se, such as lumens. But the concentration of
the light changes. The same amount of light enters the eye and
reaches the retina, but the concentration (focal point) of that light
is changed.

>
>
>I still don't understand this part. I feel like "toward prolate" should read
>"toward oblate".

A more prolate shape when compared with conventional ablation
profiles.

>
>It is not that the similar shades are difficult to tell apart from each
>other but the countour is blurred, that is why the acuity suffers (and also
>dark areas are not visible because of forced adaptation and obstruction from
>spreaded light). And basically this is exactly the same problem as in simple
>refractive error, only not as severe as here is also some of the good, sharp
>picture to help.

This blur causes a mix between light and dark images. The location of
the edge of the black letter on the Snellen chart is not
distinguishable because of the blur. Contrast sensitivity measures
the ability to define these edges at different sensitivity levels.

>Yes, some patients (somewhat rare cases?) are saying, that they see better
>for example in dark than before with glasses/contacts. Logically the
>explanation would be that there are less aberrations. How on earth is this
>possible, if there is a transition zone with refractive error and the pupil
>is large enough to uncover the transition zone?
>

It happens not just because there are less aberrations. Some
aberrations increase vision quality. It happens because the overall
change of the entire system, including aberrations, changes to allow
the neural system to better decipher what is being sent. It is not
just optics, it has a lot to do with what the brain does with the
information. There are many people with moderate optics but
exceptional vision. Now were are getting into neural adaptation, etc.

Glenn Hagele
Executive Director
Council for Refractive Surgery Quality Assurance

Email to glenn dot hagele at usaeyes dot org

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

I am not a doctor.