How much does uneven tear film affect IOL calculations?

[OhMyEye]

A last minute problem with my insurance company required rescheduling my cataract surgery.  It may've been divine intervention and leads me to ask:

How critical to getting precise IOL calculations is an even tear film?  I had IOL calculations done by two technicians.  The second one I saw told me "Blink, so we get an even tear film".  Being a former  aerospace machinist who dealt with tolerances in the millionths, I know how important precise measurements are to the final outcome.  I was floored when the tech told me to blink for an even tear film.  I knew immediately she wanted the most precise measurements possible.  She was an absolute professional and exuded supreme confidence.  She appeared to be in her 50's, so she was no new kid on the job.  She knew the Zeiss machine like the back of her hand.  She exuded pride of workmanship.  I was very impressed.

The first IOL technician I saw appeared mid to late 20's.  Well dressed.  Pleasant.  Used IOLMaster 500.  But she didn't ask me to blink.  Never told me to open my eye a bit more.  Didn't exude the same cool confidence of the first tech.  And most of all, this tech had rebuked me for something I said when I entered her office.  She'd misinterpreted what I said and politicized it.  She caught herself rebuking me and said "I'm sorry, that's none of my business".  Then she commenced testing me.  I was stunned at the rebuke and mostly mute during the testing.

30 miles from home I thought "What if she purposely messed up my testing?"  I called the surgeon when I got home.  He was horrified at what I told him.  He said his staff was too politically correct and they had no business rebuking any patient based on their (staff's) political views.  He said even so, the tech would "never" intentionally mess up my measurements.  He said the measurements are verified several times before and during surgery, and he'd make a note on my file to recheck the measurements before surgery.  The surgeon's compassion and understanding impressed me.  He's very well known.  For these reasons I decided to go with him.  Even so, I still had doubts about his IOL tech's measurements.  Then the insurance issue caused surgery rescheduling.

I have the IOL calculations and measurements from both eye techs.  These measurements are different between the two technicians.  Are the differences a big deal?

AL difference is .06
K1 difference is .17
K2 difference is .18

Both surgeons recommended Alcon SN60WF.  The first recommended 16.5/-0.39 and the surgeon I chose recommended 16.0/0.14.

What are the differences between each and how will each IOL affect my vision?

[OhMyEye]

SoftwareDeveloper;

Thank you very much for a great explanation.  I especially appreciate the math formula to convert the IOL #'s into cm/inches.

The 16.5/-0.39 figure is from Surgeon #1.  His tech examined my eye with IOLMaster 700 and no other machines/devices.  My copy of the IOLMaster 'workup sheet' shows the figures 16/5/-0.39 in bold font; apparently the suggested IOL per IOLMaster 500.  The surgeon's notes on this sheet state "Target PL +0 -1.0".   I see that a post op refraction of -0.39 falls in the middle of the target.

The 16.0/0.14 figure is from Surgeon #2.  His tech examined my eye with IOLMaster 500 and Lenstar 900/Haag-Streit machines. My copy of a Haag-Streit Diagnostics 'workup sheet' shows the figures 16.0/0.14 in bold font; apparently the suggested IOL per Lenstar 900/Haag-Streit.  The surgeon's notes state "Target -0.50".  ** Do I see a problem here, or am I calculating wrongly? **

Using your formula, a target of -0.50 has focal point of 200cm/78.75 inches.

Still using your formula, IOL 16.0/0.14 has focal point of 714cm/23.5 feet.  That's no where near the surgeon's target of -0.50, is it?  Should I mention this to the surgeon?

Ironically, I was scheduled to have the 16.0/0.14 IOL implanted two days ago.  An insurance mix-up by the surgeon's staff caused me to cancel the procedure pending insurance resolution.  Maybe it happened so you could reply to my post and save me from more eye troubles than I already have.

I have some questions about mini-monovision.  Might I PM you with them?  Seems I may be testing the limits of patience and posting about my cataract/ERM/macula pucker saga.

Thank you.

[JohnHaganMD]

Yes testing an eye and doing these calculations is not like dealing with machines, machining and tolerances to 10 decimal points.

One other thing yesterday in a medical journal they reported that eyes with ERM/pucker more prone to post op cystoic macular edema (CME)  you may want to ask your surgeon if he/she wants to keep you on steroid drops and NSAIDS longer than usual and do post op macular OCT

[SoftwareDeveloper]

The lens powers aren't very different. Unfortunately studies show  eye measurements can vary for a person slightly throughout the day even when measured  by the same equipment and person.  Lenses don't come in exact powers, typically they might come at multiples of 0.5 so the two lens powers they are suggesting are likely one notch apart and your actual ideal lens power would theoretically be between the two. Since   lens formulas aren't exact, there is no way of knowing which of the two is closest to your actual ideal lens power.

You don't explicitly note what the numbers are, but the recommendation of "16.5/-0.39"  appears to be a typical IOL Master type result which is suggesting a lens power of 16.5D which it estimates would leave you very slightly myopic at -0.39D which is a best focal point of (100 / -0.39) = 256.4 cm = about 100.9 inches. That is a small enough amount of myopia you'd likely still have good distance vision with that eye (depending on whether you have any residual astigmatism, or other eye health issues interfering with best corrected vision).

Unfortunately lens formulas aren't exact however so that won't be the actual final result, they are based on statistical analysis of past  patient results, and there is no way to know in which direction the error might be.The potential error tends to be small for those who had low prescriptions before cataracts, and higher for those with higher prescriptions.

There is some chance the error might be in the farsighted direction and leave you less myopic, closer  to best focus at 0D, i.e. infinite distance, or even slightly farsighted (with still good distance),  .. or the error could be in the other direction and you are left a bit more myopic than that, e.g. perhaps with a focal point closer to a meter or two.

The other suggested power is 16D which their formulas estimate  would leave you very slightly farsighted, 0.14D, which would lead to a slight reduction in near vision with that eye (by 0.14D). Again, unfortunately the lens power calculations aren't exact so if it errs in one direction it might leave you even more farsighted and reduce near vision.. or it might leave you very slightly myopic. This option might have trivially higher odds of having better distance vision than the first option, but with less near vision, but that isn't guaranteed since there is no way to know what lens power would be ideal for you.

[JohnHaganMD]

Nicely said

[OhMyEye]

Very nicely said.

[OhMyEye]

Your response has encouraged me to talk openly about the varying eye measurements with my surgeon.  My call has been placed.

Are you able to tell me if the disparity between AL, K1, K2 measurements are anything to be concerned about?

Are you able to tell me the end result vision differences are between the two recommended IOL's?

Thank you again for your good advice.

[JohnHaganMD]

At this point I'm going to say that you need to discuss this with your surgeon and their staff. You have spent a huge amount of time doing research and finding what you believe to be the best surgeon. If that is the case that surgeon and their staff, if they are in fact that good, should be more than happy to discuss your concerns.  Best of luck.