Could the laser shots be cause of common anomolies?

[mike r]

I recall seeing a diagram online showing how the laser shots maybe leave microscopic divots (for example sake not exact obviously - ^^^^^^^^^^) across the surface plain of the cornea during the sculpting process and wonder if these mInute indentations because they are not as smooth as the intitial cornea surface could induce these common refractive anomolies...

May sound bizarre or ridiculous but just a thought anyway?

[mike r]

based on the same theory of an unsmooth laser surface which is essentially occuring even if at nano levels compared to a virgin surface. This could mimic taking mini prisms for that matter creating the same effect that is placed on the lenses of headlights and other various lighting to create greater illumination. This prism effect maybe the residue of the laser imbedded inside the cornea. Thus when light passes through who knows what anomoly may occur. Remember the flap never completely heals so these divots/ prisms may not either. Thus the long term effect of halos bright lights double vision.

Im sure this has been discounted as a factor thru studies eeehh?

[LasikExpert]

In fact what you describe is exactly what occurs during laser assisted refractive surgery. The creation of the Lasik flap and the ablation of tissue create slight imperfections at the surface. This is why as a general rule all laser assisted refractive surgery techniques increase higher order aberrations (HOA). This increase is not always problematic because human vision is limited. The increase in HOA may not cause vision quality problems. It depends upon the level of HOA the patient has naturally and the amount of HOA that are induced.

It is possible for HOA to decrease with Lasik and similar surgery, but that is not reliably predictable.

The tendency to increase HOA is one of the reasons a wavefront diagnostic analysis before surgery is so important. The surgeon needs to know what HOA exists before surgery to know if the patient is on the threshold of poor vision quality and that Lasik would push the patient across that threshold into poor vision.

[mike r]

And I thought you were just going to let this one ride. It was to far out there...HA!

Sometimes I feel like Im sitting in the circle on the 70s show in these forums of but of course Ive been clean for 19yrs but have a habit of really putting things under a microscope from prior endeavors...

Anyway thanks for the sonopsis...Good enough for a layman I guess. Its nice to no how hard it is to get the procedure right even with all the technology.

<mike r>

[LasikExpert]

Since you seem to like the errata; the smallest epithelial cell is about 5 microns across. The epithelium is the outermost layer of the cornea.

According to Munnerlyn's formula, the removal of 12 microns of corneal tissue will effect a 1.00 diopter refractive change. With 12 microns equaling 1.00 diopter and an epithelial cell being 5 microns wide, that means that if the surgery misses one cell it can be off by about 0.40 diopters. That can be the difference between 20/20 and 20/30ish.

This is a gross oversimplification, but it gives you an idea of the level of sophistication required to provide a good outcome and how remarkable it is that the vast majority of Lasik patients have good outcomes.

[DryEye]

Is there anything out there (proceudre) than can smoothen ones surface permanently similar to how sees after inserting tears?

[LasikExpert]

I don't now of anything that creates a surface as smooth as the tear film. That is not so much a limitation of lasers, as it is because the tear film is so very smooth.

The epithelium does a very good job of resolving minor irregularities. The combination of a healthy epithelium and tear film can resolve a lot of problems, whether they be natural or induced by surgery.

[Kean]

mike r,
Regarding smoothing, this additional process is done only by a few specialists/experts. It has clinical study data, this procedure is pretty new in Lasik/PRK, accordingly the smoothing technique is borrowed from PTK (I didn't read too much into PTK, so Glenn can shed more light) which existed for a long time but never considered for .

I've read chapter 8, "Smoothing in Excimer Refractive Surgery" in the 2006 book "Refractive Surface Ablation: PRK, LASEK, Epi-LASIK, Custom, PTK and Retreatment" which discusses this procedure and why it is critical to be a part of the procedure.

In general, the paper describes ablated surface irregularities associated with flyspot (based on your description) and broadbeam laser (more like a step) and how this additional step can improve visual acuities.

The smoothing pretty much equates to further ablation (eg. your example ~~~~~ surface to -------- means more cells gone and flatter)

It all sounds very promising and makes so much sense, Clinical studies show quicker recovery, better visual acuity to name a few. However, it is very new which means very-very-very-very few surgeons have the experience and not that much of this specific experience incorporated in Lasik/PRK, plus not many have adopted it. The article also listed out quite a bit of prohibitive limitations including dehydration, induced hyperopia (more cells gone due to smoothing) etc to name a few if not handled properly.

This is at its baby stage, of course, it will get better and it may be adopted widely in the future to provide better visual outcome. It sounds very promising, I sure wish the technology has matured today, it will benefit future patients when the risks and drawbacks are addressed/minimized so that it can be applied globally.

[mike r]

I really apreciate that follow up Kean WOW!

I guess I could have researched that book you speak of at the library????
Maybe not. Or saw to much to follow thru with the procedure.
I had earlier deducted the Allegretto is flying spot maybe ladar to I forgot. The flying spot beam I surmized is lets say more like a fine ball point pen instead of the broad beam(a magic marker) which lets say takes out flat spots but the flat spots would nt detail as well do to the wider swath.
But I see the theory of the smoother surfacing thus creating better acuity.
Very interesting. And I overlooked that. hhmm. Maybe I could of had 20/15. But I also believe I saw that the visx custom procedure will eat away more cornea and cause more haloeing ghosting and starbursting.
Also the allegretto works with the curvature of the cornea whereas the visx does not. At least to some degree.
These latest procedures you mention arising coupled with our life expectancys and wanting to just reap some benefit now of better vision, I guess boils down to dealing with the side effects and just excepting whats currently available and hoping for the best.
The other side of the coin.


This type of subject matter could influence a whole new topic discussion board on just technicals and theory. I think it could be quite beneficial in the decision making process and in general better educating the public.

Hows that for eloquence Rgonyer could I be a contender?

BUT I will leave this wonderful idea to the judgement of the moderator.

<Mike R>

[LasikExpert]

One of the drawbacks to a broadbeam is the ablation plume. When the cells are released by laser energy they eject away from the cornea at high speed. Even though there is nearly no heat involved, it looks like the mushroom cloud of an atomic bomb.

The problem with the plume is that it gets in the way of the next laser pulse. Laser is only light, and the laser light can be blocked by the evacuating plume. This blocking of subsequent laser pulses causes shadows on the treatment area and can cause central islands - undertreated areas that are raised like islands and can cause significant visual disturbance.

There are vacuum systems to pull away the plume, but one of the major advantages of the flying spot and adjusting slit technology is that the following pulse is probably going to be somewhere else on the cornea and will not need to worry about the plume being in the way.