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Home > Archive > Lasik Eyes Surgery > November 2004 > Ectasia? Night Vision Problems? Structural Issues With Ablating Too Much Tissue...
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Ectasia? Night Vision Problems? Structural Issues With Ablating Too Much Tissue...
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| Wizkid 2004-11-03, 2:08 am |
| Structural issues with oblating too much tissue:
John Marshall, PhD, delivered the Charles D. Kelman, MD, Innovator's
Lecture on the topic of "Wavefronts and Biomechanics" here at the
American Society of Cataract and Refractive Surgery meeting.
The cornea is made up of collagen filaments arranged in grids not
unlike the steel skeleton of a building or car tire, according to Dr.
Marshall. When tissue is removed in PRK, he said, the surgeon cuts
through 5 million of those fibers. In LASIK, by comparison, 232
MILLION FIBERS ARE DISRUPTED, he said.
He challenged future innovators to make sure that "wavefront
technology has [not] gotten ahead of the biology."
Though an eye implosion seems unlikely, it sounds like the eye's
structural integrity will sag with Wavefront in the higher diopteric
ranges causing poor vision much like presbiopia.
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| Glenn - USAEyes.org 2004-11-03, 7:12 pm |
| I don't think anyone disputes that if too much tissue is removed, the
cornea will become unstable.
What you don't provide here is why LASIK would disrupt 232 million
collagen fibrils and PRK would disrupt only 5 million. The reason is
the flap.
When the LASIK flap is created, it is created about 90-180 microns
below the surface of the cornea. Creating the flap must cut through
the collagen fibrils to get down that far.
Because the fibrils are like many long strings stacked one upon the
other and are stretched from edge of the cornea to edge, creating the
flap releases the internal energy and strength provided by those
fibrils.
Not only does cutting the fibrils release this internal tension, but
the lack of tension allows material to expand between the foundation
fibrils. This causes a "swelling" outside the flap and induces a
small amount of myopic correction. Cynthia Roberts, PhD has published
a very detailed article on these biomechanics.
If the purpose of refractive surgery is to disrupt as few collagen
fibrils as possible, then PRK would win over LASIK every time, and
lens-based surgery would win over PRK, but refractive surgery is not a
one-issue event. Late onset corneal haze too often develops when high
correction is made with PRK. Lens-based refractive surgery is
significantly more invasive than either PRK or LASIK.
Collagen fibril disruption is just one of the myriad of factors that
surgeons, manufacturers, and researchers must consider when developing
and offering a refractive surgery procedure. It is certainly an
important one, but it is not the only one and other issues may vastly
overshadow this one.
Yet again, the importance of selecting a knowledgeable surgeon is
raised.
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.
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