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Tom Lucas wrote:
> The chisel is bigger than the individual hairs it carves. 500um diameter
> spot does not mean that is exactly the area ablated - for one thing the
> beam is not perfectly coherent in any laser so the edges of the beam are
> slightly less energetic than the centre. There are other factors too -
> flying spot means that the laser doesn't just move to one place, zap,
> and then move to another creating a mosaic - it means that the beam is
> smoothly passed over several points like spraying paint on a car. This
> gives a much greater resolution that 500um.
>
> As you are probably aware then the orders of the aberrations are derived
> from the polynomial equation used to mathematically describe the shape
> of the cornea. As you rightly say, one aberration on its own doesn't
> affect much however when a lot of them are grouped together they
> collectively affect the rate of change of that area of the cornea - i.e.
> they act together. Mathematically, rate of change is differentiation and
> when you differentiate a polynomial all the orders go down by one. This
> means that the higher orders are promoted to lower order status and will
> actually affect vision. Myopia is nothing more than a collection of HOAs
> working together which is why it can be modelled with a lower order
> equation.
>
> However, the Laser can confidently deal with lower order aberrations and
> will fix them using the traditonal ablation method - meaning that the
> interraction between the previous HOAs is lost. Perhaps a few may remain
> but the density of them is greatly reduced and they can now be taken as
> individuals and classed as HOAs. It is like a few splinters sticking up
> from an otherwise smoothly sanded floor which had previously been rough
> everywhere.
>
> If there are enough of these HOAs left to affect vision then it is
> because the Lower orders were not completely fixed. Perhaps a term like
> medium order aberration would be useful in these cases? Anyway my point
> is this - lasers treat lower (and, with wavefront, medium) order
> aberrations. There is no need to treat higher order aberrations unless
> they are dense enough to be visual which is exactly when they cease to
> be higher order and are thus treatable.
>
> This post is a bit complex and mathematical so please shout if anything
> is not clear but I think it somes up what I've been trying to say about
> HOAs for a while. Anyway the gist of it is this - any anomaly that
> affects vision is of a low order regardless of whether it is formed of a
> collection of higher orders (which is the definition of low order).
Your attempt at explaining is good, but unfortuately its not correct.
Let me show you proof:
One study reported by the university of Rochester and Ohio State
University demonstrated that the flap cut in LASIK also may interfere
with custom LASIK, which involves precise measurements of the eye's
optical system to detect vision flaws that in the past went
uncorrected. Replacing the flap after precise laser vision correction
may interfere with optimal outcomes because the flap itself was not
reshaped with custom measurements.
Ace's comments: Aberrations were measured before the formation of the
flap. You know this and know why.
Additional LASIK complications can include dry eye, inflammation,
infection, irritation, redness, and visual distortions (see details in
the chart). An individual also might experience vision defects such as
seeing glare and halos surrounding brightly lit objects at night. Night
vision problems could be due to a LASIK treatment zone too small to
accommodate the pupil's larger size in low lighting.
Total aberrations increased on average by a factor of 1.92 and corneal
aberrations by a factor of 3.72. For the low preoperative myopia group
(-2.5 to -6.5 D) the average increase was 1.53 (total) and 1.97
(corneal), whereas for the high preoperative myopia group (-6.8 to
-13.1 D) the average increase was 2.29 (total) and 4.37 (corneal). In
terms of RMS differences (before minus after surgery), total RMS
difference changed from -0.05 to 0.80 =B5m, reaching statistical
significance in 11 of the 14 eyes, and corneal RMS changed from -0.16
to 2.04 =B5m, statistically significant in 13 of the 14 eyes. Part of
this increase is accounted for by an increase in the third-order
aberrations (increasing by a factor of 1.98 for total and 2.73 for
corneal) and by an increase of the fourth-order aberrations (increasing
by a factor of 2.54 for total and 3.93 for corneal).
Lasik can correct myopia, hyperopia and astigmatism, something glasses
and contacts can do. The problem at hand is lasik induces more high
order aberrations that can_not_be corrected except with RGP contacts
which most people cant tolerate after lasik anyway due to dry eyes.
Tom, if you get your color topographies before and after lasik, you
will find that the RMS is higher after lasik. You see well from near
thanks to the undercorrection and good accomodation.
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