Comparison of LASIK Flap Thicknesses with the IntraLase Compared to Microkeratomes in 10,000 Eyes

1. Comparison of LASIK Flap Thicknesses with the IntraLase Compared to Microkeratomes in 10,000 Eyes David Kent Christchurch, New Zealand The author has no financial interest in any of the products in this presentation

2. Methods • All LASIK flaps measured by subtraction ultrasound pachymetry since July 2000 • New microkeratome blade or IntraLase disposable patient interface used for every eye • Same surgeon did all measurements with Sonogage 50 MHz pachymeter • Thinnest central pachymetry pre-operatively • thinnest of three readings • Intra-operative in-the-bed pachymetry • thinnest of three readings • Data recorded in MS Access database • analyzed in MS Excel and SyStat SigmaStat

3. 10,584 LASIK Flaps 2000-2009

4. Four IntraLase Settings One ACS head Red ovals around the most extreme values Ten Hansatome Trays (27 heads) One Zyoptix XP Tray (2 heads)

5. Thinnest Flap Creators Zyoptix XP Hansatome Z16 head IntraLase Hansatome & ACS IntraLase

6. Lowest Flap Thickness Standard Deviation N>100

7. Flap Creator Means and Standard Deviations • One Way ANOVA, P=0.002 P = 0.002, One way ANOVA N=8 N=2 N=4 N=20

8. Risk of Making a Too-Thick Flap • Excessively thick flap defined as central thickness of 190microns or more • Risk of a too-thick flap greater with microkeratomes • P>0.001, z-test of proportions

9. Sources of Measurement Error • Variability due to not being central on the cornea affecting both: • Pre-operative pachymetry • In-the-bed pachymetry • Pachymeter or probe inaccuracies • Variable hydration of the corneal bed • If the bed is wet the stroma swells and thickens

10. Corneal Bed Hydration • Corneal bed is dry with the IntraLase • In-the-bed measurement affected by having to press more firmly with the pachymeter probe to achieve a reading • Therefore more likely to have overestimate flap thickness • Corneal bed is wet with a microkeratome • Cornea is more hydrated giving a thicker in-the-bed reading and hence thinner flap measurement • Not as much pressure required to get a reading in-the-bed • More likely to underestimate flap thickness

11. Microkeratome Summary • Microkeratome heads vary widely with regards to average flap thickness and standard deviation of flap thickness • Some microkeratome heads: • have excessively high flap thickness variability • make flaps that are too thick • Zero Compression Hansatome heads had smaller standard deviations than the Standard Hansatome • Microkeratomes have a greater risk of making an excessively thick flap than with the IntraLase, P<0.001

12. IntraLase Flap Summary • Flap thickness standard deviation is lower with the IntraLase than with microkeratomes, P=0.002 • The IntraLase 90 micron setting makes consistent thin flaps with a low standard deviation and a high level of safety • IntraLase 100 and 110 micron settings also have a low standard deviations • My IntraLase FS60 makes flaps that measure about 20-25 microns thicker on average than the nominal thickness