A Message From Dr. Maller
The 2004 kickoff! I would like
to take this opportunity after a rather long newsletter hiatus to welcome all
the new subscribers to the No Blur! EyeMail as well as thank all the former
subscribers for still being there. After much reprogramming of the
website, the new format I believe is much cleaner and neater, more
comprehensive, and hopefully easier for you to find exactly for what you
are looking. This newsletter has also received a facelift. As
seems appropriate to start this new format was my recent trip into the local
broadcast news. Many people who have had refractive surgery have been left
with less than desirable outcomes. My work to restore functional vision to
these very irregular eyes caught the attention of WSVN. This along with
Orthokeratology, was the "focus" of the news broadcast. I hope that you
enjoy the new EyeMail format and continue to find it useful.
Please share the newsletter with your family and friends. Sight is so
important and my hopes with this newsletter is to keep the public better
informed about issues relating to this most precious gift that we call
sight.
Dr. Maller
What is Spherical Aberration?
Recently it has been brought to my
attention that the term "spherical aberration" is being discussed very heavily
in the general public yet the optical problems of spherical aberration are not
accurately understood by these people. That is not really a surprise since
not many people in the general public have a theoretical optics
background. The reason it has become a relatively "hot topic" is because
it is one of the complications after refractive surgery, degrading the quality
of the image and causing visual disturbances that are difficult to function
through. I will give a fairly accurate but hopefully easy to understand
explanation of spherical aberration here.
When light travels from one medium to
another (such as water to air), the path of the light changes - it is
"bent." Anyone who has stood in a pool looking down at their own legs is
familiar with this effect since their legs appear to bend at the surface of the
water. This bending of light is referred to as refraction. When
light passes from air through a curved surface such as a lens used in
making glasses, the bending is more pronounced. The more curved the
surface, the more the light is bent. If the refracting surface is
spherical (this would be like the surface of a baseball), the light hits all
parts of the surface and bends appropriately, but not equally.
The light rays that enter along
the "B" path bend less than the light rays along the "A" path. What
this does is it creates a "distortion" in the processed light since what had
been a single point of light at the object turns into multiple points when it is
refracted into the image.
So, you might be asking yourself what
does this have to do with eyes and why does this become a problem after
refractive surgery? First, spherical aberration only becomes a problem as
more peripheral light is let into the system. For example, if we placed a
"filter" that blocked out all of the light along the "A" path, the image formed
after the refraction would be less distorted since those rays of light never
even get through the spherical surface. The eyes normally do something
very similar to this filtration by using the pupil to limit the peripheral light
rays from entering the eye. Second, spherical aberration only becomes a
problem at higher refracting powers. For example, if our lens in the above
example were of very low refracting power, the points "A", "B", and "C" would
almost be on top of each other but as the refracting power gets larger those
points spread out causing more distortion.
Our corneas in their pristine
"virginal" state are not in fact spherical, but are aspheric. As you move
along the surface of the cornea from the center outward, the amount of curvature
decreases very gradually. This progressive flattening of the cornea from
the center outward minimizes the optical problem of spherical aberration because
with this system the light traveling along path "A" focuses further back at
point "C" and not forward like in our example above. This is very
important since the refracting power of the cornea is enormous and as you
remember, when the refracting power is large, spherical aberration becomes an
important factor. After refractive surgery is done on a cornea (i.e.
LASIK) the aspherical shape is lost and so too is lost the ability of the cornea
to neutralize the spherical aberration. This is the reason that spherical
aberration increases after refractive surgery. The effect is even more
problematic when the individual has large pupils since that will allow all of
the peripheral rays to enter the eye thus degrading the quality of the
image.
I know this is somewhat technical but
I have tried to be accurate while also making it understandable. I hope
that I have succeeded!
Contact Lenses for Keratoconus
A recent study was released
concerning Keratoconus and contact lens wear. Since contact lenses are the
primary form of correction for this condition the study has considerable
significance. The effect of contact lens wear particularly with contact
lenses that do not transmit much oxygen through to the cornea, always brings up
concern with the health of the corneal endothelium. This tissue is
important in keeping the cornea functioning normally. Continued oxygen
deprivation to the cells of the corneal endothelium cause their
destruction. Since these cells do not grow back, there is certainly a
concern if they die off and are not going to be replaced. The results of
the study showed that gas permeable aspheric design lenses and soft toric lenses
compared favorably but the Softperm (a hybrid soft and hard lens) causes
considerable long term damage. Unfortunately, many Keratoconic patients
are wearing this Softperm lens because no other lens could address their
problems. Hopefully, with truly custom designed, topography guided contact
lenses, these people can now be refit to healthier options.
Message from Dr. Maller
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