astigmatism question

[teller4]

Thanks to the posters of this community and the physicians who spend time with us.  
I am going to ask about cataract surgery soon with an eye MD, amongst other things.
Regarding astigmatism, will measures be taken with machines that are more accurate than the differing measures I've gotten from optometry and ophthalmology refractive exams where they judge it by my reading and judging /comparing the eye charts?
Thanks.....

[JohnHaganMD]

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[JohnHaganMD]

Best of luck

[SoftwareDeveloper]

In the clip above CA stood for chromatic aberration.

Yup, Tecnis is a product family of IOLs from  AMO=Abbott Medical Optics.  I don't know offhand if it was merely a brand name they came up with internally, or a perhaps a company AMO bought.

At least when I had my surgery in Europe they did the preop testing and then ordered the lens and had it shipped, but I was getting a premium lens and was highly myopic so the power wouldn't have been common. I don't know if high volume centers bother keeping any stock of common monofocal powers  or if they always order, but I'd suspect they'd order.

[teller4]

I appreciate all the answers, Dr. Hagan and Software Developer.

[JohnHaganMD]

Most surgery centers and surgical venues do not let you order from a full menu of IOLs available in the country. It would be prohibitively expensive to carry a full inventory of powers and sizes.

[JohnHaganMD]

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[teller4]

I wonder if places order an iol or they only carry certain brands?
I had never heard of AMO before which is Tecnis, right?  

[SoftwareDeveloper]

The astigmatism that optometrists measure for a glasses/contacts prescription includes a portion of astigmatism which comes from the natural lens, in addition to astigmatism from the cornea. When the natural lens is removed during cataract surgery, you won't have the astigmatism from that source anymore so the prescription measurement of astigmatism isn't what they need to consider when planning for cataract surgery. They only need to consider the astigmatism from the cornea so they discover that by having a scanner look at the shape of the cornea.

Until a few years ago they focused on eye scans that measure the front of the cornea, anterior corneal astigmatism,  and so many scanners only analyze that. Then they discovered that although astigmatism from the back of the cornea, posterior corneal astigmatism,  is small in many people, in others it can be large enough to have an impact. Here are a couple of articles mentioning that:

http://eyeworld.org/article.php?sid=6609
http://www.aao.org/eyenet/article/toric-iol-calculations-consider-posterior-cornea


Older scanners that did measure posterior astigmatism   apparently didn't always do a good job. There are newer ones that do better at measuring total corneal astigmatism including posterior astigmatism, but I don't know if all surgeons are up to date so its likely worth raising the issue with the doctor's office of what equipment they use.

Since you are talking about the issue of aspheric vs. spheric lenses, it sounds like you've done some research and have decided on a monofocal rather than a premium lens?

In terms of aspheric lenses, I think that is the default these days since that is best for most people, although in theory a surgeon can check how spherical your eye is to decide what lens is best for your particular case.  If you are getting a monofocal lens, usually the doctor decides which one is best, though they aren't all created equal  and some surgeons don't look closely enough at optics since they are more concerned with surgery&medicine e.g. they have different levels of chromatic aberration,  as an article in this discusses:


http://eyeworld.org/supplements/EW-December-supplement-2014.pdf
" Cataract surgery with an IOL with an Abbe number greater than that of the natural lens (47) can improve CA, so that our cataract patients could actually experience better vision quality than they did as young adults. "

The Tecnis lenses tend to have a higher Abbe number, while the Alcon lenses have a lower one. There are also some monofocal lenses that claim to have a slightly larger depth of focus than standard monofocals (i.e. a little bit better near) while still being monofocal, like the Softec HD, but its not clear is much of a difference (I don't have a study link offhand)  and sometimes they charge extra for those I'd heard, and I don't know how they compare in other ways (like their abbe number).  I know there is a Hoya monofocal, the iSert Gemetric  which I had seen study results indicating has  bit better depth of focus than other monofocals, but last I'd heard they oddly hadn't commercialized it despite getting it approved by the FDA. I don't know if that has changed.

[teller4]

CA stands for corneal  astigmatism or aberration?

[JohnHaganMD]

Most monofocal and toric IOLs are aspheric now and the advantages of aspheric designed over spheric well recognized.

[teller4]

I mean, aspheric IOL's.

[teller4]

Thanks, Doctor H.  Should be interesting to learn what the measurements are.
I just went to a site that mentions advantages of aspheric lenses over spherical ones.  I suppose that if I look hard enough, there must be information that lists more pros and cons, but so far, this one seems to indicate that aspheric tends to give a better result ( it was the company manufacturing the aspheric type).

[JohnHaganMD]

Yes they use instruments that measure the radius of curviture of your cornea and an "A-Scan" that measures the length of the eye and location of certain key areas like depth of anterior chamber and then use specialized formula to determine IOL power. Some allow constant updating from constants (K) derived from the surgeons previous surgery. For most people with normal eyes the error rate from expects is plus/minus 0.50 diopters and for many plus/minus 0.25 diopters.