| Tom Lucas 2006-09-20, 8:26 am |
| "Ace" <acemanvx@yahoo.com> wrote in message
news:1158740618.694516.199730@i3g2000cwc.googlegroups.com...
<snip mathematics>
[vbcol=seagreen]
> Your attempt at explaining is good, but unfortuately its not correct.
> Let me show you proof:
What is incorrect? Calculus is not open to opinion - the rules are
rigid.
<snip study>
> Ace's comments: Aberrations were measured before the formation of the
> flap. You know this and know why.
This doesn't affect anything I've said in my post. I have said that
there are aberrations but I have not said where they come from - that is
not important to my post and would introduce subjectvity.
<snip obligitary Lasik alarm>
<snip>
> total RMS difference changed from -0.05 to 0.80 µm, reaching
> statistical
> significance in 11 of the 14 eyes, and corneal RMS changed from -0.16
> to 2.04 µm, statistically significant in 13 of the 14 eyes.
They seem to have an odd method of using RMS because, by definition, it
cannot produce a negative value, unless used in imaginary number
mathematics which I don't think is being used here.
Putting that aside then we are talking about aberrations which are
roughly a micron - when averaged by RMS! My point still stands that they
cannot affect vision indivudually, even if they are "statistically
significant" which could mean anything. If they are sufficient in
density to affect vision then their order reduces because they become a
collective.
> 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).
How big a part? That is an important question. The higher the order then
the smaller the aberration and the more complicated an equation you need
to describe it. Fourth order aberrations are, by definition, four orders
of magnitude smaller than a first order aberration like myopia.
Lets use my floor sanding analogy again - the 1st order is getting the
floor flat and level. The second order is getting the splinters off from
around the screw holes and the third order is splinters on the
splinters. By the time we are at fourth order then we are talking dust
on the splinters on the splinters. Sure, those particles could be 3.93
times bigger but there needs to be a hell of a lot of them to contribute
to the shape of the floor. If there is enough to affect the flatness of
the floor then that then becomes a first order problem and the fact that
the lump consists of fourth order problems is not relevant.
This is basically what I said in the first post and there is nothing in
it which is incorrect as far as I can see.
> Lasik can correct myopia, hyperopia and astigmatism, something glasses
> and contacts can do.
Lasik will fix those three whereas glasses/contacts can only offer you a
work around.
> 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.
I am not surprised that some HOAs are increased because they are such
small things and all processes have a tolerance - glass might look
smooth but you can still strike a match on it. The question is whether
they affect the quality of vision and what I am saying is that they can
only do that if they are sufficiently dense to form a lower order
aberration.
Lasik reduces first and second order aberrations until they are small
and irregular enough to be classed as higher orders so of course the
number will increase. Whether they still actually affect vision is
something else entirely.
> 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.
All my lower orders were reduced (although I could use a bit more on the
first order) and the, of the higher orders, spherical stayed about the
same. Another went down a bit (coma) and another went up (trefoil). At
my third scan a few weeks back the coma and trefoil had both gone down a
bit although the spherical was still maxed out. I expect this because
healing is going to have the greatest effect on the smallest lumps. I
can't tell if speherical has reduced because it has always been of fthe
chart.
|