The HUGE problem with attempting to incorporate wavefront correcting
Higher Order Aberration (HOA) correction into lenses is registration.
The slightest misalignment of optics to correction causes a cascading
effect and degradation of vision.

I am relatively knowledgeable of optic wavefront analysis in humans
and the use of wavefront-guided corneal ablation during Lasik and
similar refractive surgery with the goal of correcting Lower Order
Aberration (LOA), which is sphere and cylinder, and not inducing HOA,
which is commonly represented in Zernike polynomials.

I have just returned from a medical convention where iZon had a booth.
I have always been suspicious of iZon's "wavefront glasses" because I
understand the role of registration in HOA correction. Let me give a
(believe it or not) simple example.

The Zernike polynomial Spherical Aberration (SA) can be described as
the periphery of the optic having a different spherical refractive
error as the center. A person can be plano in the middle of the optic
and a have a 2.00 myopic ring around the edges.

Make an "O" with your right thumb and finger, put it in front of you,
look through the center, and imagine that this represents the SA.
Center is plano, your fingers represent a ring that is -2.00.

Now make a circle with your left thumb and finger. This circle is a
lens created to correct the SA. The outer ring has 2.00 diopters of
correction and the center is plano. Align the two rings together. This
is the theoretical correction of the HOA known as Spherical Aberration
with the use of a lens. What is -2.00 in the optic of the right hand
is corrected with a perfectly aligned 2.00 lens.

Now move your left hand to the left and take the two O's out of
alignment. What have you done?

Where the lens (left hand) is over the plano portion of the optic
(right hand), you have created 2.00 diopters of hyperopia, but only
where these two planes intersect. Where the lens is over the myopic
portion of the optic you have plano. Where there is no lens over the
plano center of the optic you have plano. What you have created is the
HOA known as Horizontal Coma, which is on the next level down the
Zernike pyramid. All this by simply changing the alignment of the lens
to the optics.

And that is only with SA, which is probably the least exotic of the
Zernike polynomials of HOA.

Each time you misalign optic to correction at a Zernike, you induce
HOA for each lower level of the Zernike pyramid, cascading down
through an infinite number of mathematical possibilities.

That is a long way to say that you cannot correct HOA if you cannot
align the correction precisely to the aberrated optic. When you get
down to the seventh level of Zernike, which is what iZon says their
aberrometer measures, you are talking about alignment with a tolerance
of less than a portion of microns and less than a percentage of a
single degree.

Glasses cannot maintain this kind of alignment. Even contact lenses
are unable to maintain this kind of alignment. Think of your toric
lens fittings. Try to achieve that to within .0005 degree. And forget
about what the tear film does.

I had talked with iZon's representatives before and never received an
adequate answer to my probing questions about their "wavefront
glasses". At this convention I spoke at length with their
representatives and this time I received comprehensive responses.
Well, at least more than I had received before.

The iZon aberrometer purportedly measures LOA (sphere and cylinder)
and HOA down to the seventh Zernike level. That really is far enough
as much farther and the aberrations are of proportionately less
clinical significance. Anyway, they take this "wavefront" and
translate it into a spherocylindrical correction which is then ground
onto high quality material lenses within 1/8th of a diopter tolerance.
The representative finally acknowledged that they don't really correct
HOA with their glasses, but that they use a wavefront analysis to
create a spherocylindrical lens.

Okay, now that makes sense. That is also why if you check a set of
iZon glasses, you will have both spherical and cylinder correction at
all points on the lens. Sorry iZon, but that is not correcting HOA.
That is using an autorefractor (albeit an autorefractor on steroids
that is probably better than anything else on the market) and very
good materials with a higher than average tolerance.

Don't get me wrong. I'm not suggesting that iZon glasses are inferior.
Not at all. In fact we have received reports from patients with
refractive surgery induced vision problems report that they were
greatly impressed with the quality of their vision with iZon
spectacles and I often suggest that a patient consider iZon and
discuss them with there eye doctor. But for all that iZon glasses are,
they are not correcting HOA.

I'm sure that an iZon mucky-muck would say that they have never
actually said that the correct the HOA wavefront, but you have all
heard the sales pitch so you draw your own conclusions.

Not surprisingly my questioning raised another question that the rep
could not answer. The iZon representative said that their aberrometer
does not use Hartmann-Shack or Tscherning technology for their
aberrometer...but could not say what they do use. That is like saying
that their car does not use an internal combustion engine or an
electric motor, they don't know what is under the hood, but it goes
faster than everyone else's car.

I guess you can describe me as a cynic who desperately wants to
believe.

Glenn Hagele
Executive Director
USAEyes (R)
Patient Advocacy Surgeon Certification

"Consider and Choose With Confidence" (TM)

Email to glenn dot hagele at usaeyes dot org

http://www.USAEyes.org

Lasik Bulletin Board
http://www.USAEyes.org/Ask-Lasik-Expert/

I am not a doctor.

Copyright 2007
All Rights Reserved

What Glenn said... :)

It boils down to this - wavefront correction only works if it moves
precisely with the eye.

-MT