From: Emil Chynn July 4, 2015 at 2:22 AM dear keranet pals i have recently been graciously invited (by dick and billy) to submit a talk to speak at hawaiin eye and the ASA congress in utah in 2016, and wanted to ask if some of you out there have good experience treating extreme myopes (eg -10 to -20) with ASA or PRKi believe some of you have said yes, several in south america where one would think the risk of scarring (the major complication of extreme ablations besides KC) would be higher bc of high UV exposurei am taking billy's sage advice that pooled results are always more convincing that that of 1 surgeon, as it shows reproducibility at different centers by different surgeonsplease reply to me directly, or if you prefer, through knet, if you have similar good results on extreme ablations, and whether or not you would be interested in pooling data for presentation and/or submission as a paperattached are 2 presentations that 2 of my fellows did recently about this topicthanks!yours-----------------------------------------------------------------------------------------------*Emil William Chynn, MD, FACS, MBA*

From: Ronal July 4, 2015 at 3:38 PMEmilHow much is the lowest final pachimetry you go. On my case with Lasek I choose myself would not go below 400 micrasRonal PerinoBrazilFrom: Emil Chynn July 4, 2015 at 4:41 PM RonalI used to have a lower limit of 350Then studies showed a LASIK flap contributes 0 to the structural integrity of the cornea bc it's cut 300 degrees and is only stuck on loosely by proteoglycans or whatever. Like a hat sitting on your headIn LASIK almost everyone uses 250 right?So wouldn't it be OK to go down to 250 on an ASA??The logic seems reasonable. And I know of some doctors who now follow this logicHowever I've not been so brave yet. But about 4 years ago I went down from 350 to 300 with 0 cases of ectasia yet??I also have an upper minimum of 475 because the 1 pt I gave KC to 15 yrs ago would up w 350 after LASIK (was still cutting flaps back then) but in retrospect had an initial patchy of 470. We really didn't identify initial pachy as an independent risk factor parameter back thenOf course this assumes all orbscan and/or Pentacam patterns and other indices are normal, including Stulting/Randleman and the patient manifests to 20/20, etcInterested to see if others find this logic also reasonableContact me privately w your extreme cases. Another MD already has. We would each be authors, and each contribute cases and help write up the paper??Thanks and too bad the U.S. men's soccer team isn't as good as our women's (or the Brazilian women's team isn't as good as your men's, right?);)---From: William Trattler July 4, 2015 at 4:51 PM Emil,One of the challenges on your 470 microkeratome case - is that microkeratomes often went deeper than what they were labelled - and most doctors did not measure the stromal bed prior to the initiation of the excimer laser on the bed. So - with a preop thin cornea and a metal microkeratome - the risk may be more likely to be related to a deeper than expected flap rather than the cornea itself being thin. I am aware of some cases where two patients underwent LASIK on the same day by the same surgeon with the same microkeratome - and despite both have a preop corneal thickness above 500 microns - both patients developed ectasia. Do you still have the topos of your patient who experienced ectasia? Did you measure the stromal bed prior to performing the excimer ablation? Which microkeratome were you using back then?best regardsBillFrom: William Trattler EmilCorneal thickness and corneal strength are not directly relatedWhen you perform corneal crosslinking on a patient - the cornea becomes stronger. Does it become thicker or thinner?When a patient ages - the cornea becomes stronger. Does the cornea become thicker or thinner with ageIf you can answer these questions - it becomes obvious that looking at a patients corneal thickness as a pseudo-measurement of an eyes corneal strength does not hold water.What matters is the strength of the cornea preopThe only method we have to accurately determine a patient's preop strength is corneal mapping (topography and tomography). Preop corneal thickness is not an independent factor if the corneal shape is normal in both eyesSo yes - my answer is: a patient with a 440 corneal thickness and bilateral normal topography is not at increased risk for ectasia with PRK.BillPS: In your case of ectasia in a thin cornea - would you be able to share the preop topography. Most of the time, in retrospect, there are some risk factors visible on the topography. As well, you are speaking of averages. Dan Reinstein has two papers showing that the standard deviation of flap thickness with various microkeratomes is quite wide, regardless of which eye. So yes - one of the eyes could have had a deeper than expected flap.

From: Emil Chynn July 4, 2015 at 7:32 PMBillyI did consider those factorsI doubt it was a deeper accidental cut as initial pachy was lower in the affected eye. Plus the affected eye was the second left eye not the first right eyeAs you're probably aware many studies including the pioneering one by Kerry Solomon demonstrated thicker flaps in first right eyes bc the blade is new and sharper than in second eyes (brief article appended below)So it's unlikely that the ectasia was caused by a thicker flap being cutAre you actually suggesting that you don't agree patients with an abnormally thin cornea have an abnormal cornea?For example:A patient has initial pachys of 440. Topos look normal in every way and all other indices are normal. Rx is -3 so you calculate you'd be left with 400 microns of RSB after ablationYou'd actually PRK this person??

Abstract PURPOSE: To determine the flap thickness accuracy of 6 microkeratome models and determine factors that might affect flap thickness.SETTING: Magill Research Center for Vision Correction, Storm Eye Institute, Medical University of South Carolina, Charleston, South Carolina, USA.METHODS: This multicenter prospective study involved 18 surgeons. Six microkeratomes were evaluated: AMO Amadeus, Bausch & Lomb Hansatome, Moria Carriazo-Barraquer, Moria M2, Nidek MK2000, and Alcon Summit Krumeich-Barraquer. Eyes of 1061 consecutive patients who had laser in situ keratomileusis were included. Age, sex, surgical order (first or second cut), keratometry (flattest, steepest, and mean), white-to-white measurement, laser used, plate thickness, head serial number, blade lot number, and occurrence of epithelial defects were recorded. Intraoperative pachymetry was obtained just before the microkeratome was placed on the eye. Residual bed pachymetry was measured after the microkeratome cut had been created and the flap lifted. The estimated flap thickness was determined by subtraction (ie, mean preoperative pachymetry measurement minus mean residual bed pachymetry).RESULTS: A total of 1634 eyes were reviewed. Sex distribution was 54.3% women and 45.7% men, and the mean age was 39.4 years +/- 10.6 (SD). In addition, 54.5% of the procedures were in first eyes and 45.5%, in second eyes. The mean preoperative pachymetry measurement was 547 +/- 34 microm. The mean keratometry was 43.6 +/- 1.6 diopters (D) in the flattest axis and 44.6 +/-1.5 D in the steepest axis. The mean white-to-white measurement was 11.7 +/- 0.4 mm. The mean flap thickness created by the devices varied between head designs, and microkeratome heads had significant differences (P