Study of Flap Thickness Precision with a Femtosecond Laser

1. Study of Flap Thickness Precision with a Femtosecond Laser Shinagawa LASIK Center Tatsuya Yonekawa,MSc ; Minoru Tomita,MD.PhD ; Youhei Iida,CE ; Yuko Inada,CE ; Sachiko Imada,BSc ; Mika Morizumi,BSc Masashi Shimizu,BSc; Hiroyuki Nomura,CE ; Naoyuki Kurokawa, MD Yutaka Makisima, MD ; Nobuo Nakamura, MD. PhD ; Tadayuki Nakamura, MD Masaru Aoyama, MD ; Tadahiko Tsuru, MD.PhD

2. Introduction • Recently, many authors have reported advantages of corneal flap preparation using Femtosecond laser(FS 60™;AMO). • FS 60™ requires Patient Interface (PI)for connection with the patient. • FS 60™ can make corneal flaps with precise thickness (Planar-Flap)by the applanation of the cornea with PI. applanation Planar-Flap

3. Purpose During operation , we often encounter occasions that flap thickness differs from the input value. May it have an individual error in PI? Isn't it a possibility of the effect of PI that the flap thickness is different from the input value? Therefore, we measured PI by the measuring device. Error value corrected in correlation to PI regulation. Corrected value is inputted (manually correction) into FS60 ™. (Shinagawamethod) We studied whether we could make a flap without errors.

4. Subjects and methods • The subjects were 240 eyes of 120 patients (mean age 34.65 years old, mean cornea thickness 538.49±23.91um) who underwent LASIK using measured PI with FS 60™ at our clinic from July 2007 to September 2007. • The thickness of the flaps was calculated by the following equation: Subtraction pachymetry method (pre-operative cornea thickness - post lifting bed thickness). The corneal thickness was determined by a contact ultrasonic pachymater (SP3000™;Tomey). • The subjects were divided into two groups by flap thickness: 90 um group (98 eyes) and 100 um group (152 eyes). • IntraLase FS60™ Raster flap mode(Power 0.80uJ) 9.0 mm – 8.8 mm diamater Hinge angle was 60 degree (superior type) Side-cut angle was 70 degree(Power 0.80uJ)

5. Method of PI measurement • Sterilized clinical engineer picked PI (n=531) randomly under the clean room and examined for measurement right before the operation. • Using an precise measuring device (IDH0530™; Mitutoyo), we measured five spots without damaging the PI lens surface. • We limited the measurement range to be ±10μm, within the range of official PI error by the manufacturer. • Utilized the PI with lens tilt of ±5um or less. • PI error was determined by the difference between the value from the lens center and the PI regulation by the manufacturer.

6. Result of PI measurement n=153

7. Input for Correction • PI does have few errors. PI thickness can differ a little and it effects the flap thickness. • We want to make a flap according to the corrected value, thus less errors. • Therefore, we measured PI thickness before surgery to make corneal flaps with more correct thickness. Our method utilizes a measured PI Input for correction (Shinagawamethod) is necessary! • A flap thickness of 100um is made at the time of 100um input in PI of ±0um • As for PI of a + error, a flap is made thinner • As for PI of a - error, a flap is made thicker • We defined PI correction value to be the difference between the PI standard value of ±0um and the value from the lens center, which was reflected onto the system of FS60 ™ laser. PI regulations → Thin !! Thick !!

8. Flap Results①

9. Flap Results② measured PI90μm Measured PI100μm p Value p Value

10. Summary • The mean flap thickness was 92.18±4.11um(Range:82-99um) in 90 um group, and 102.34±4.93um (Range:91-112um) in 100 um group. • In the flap, a right and left correlation was not seen.

11. Conclusions • We could reduce the errors for flap thickness by measuring PI value and making correction manually. • Creating more reliable and better flaps are possible by implementing this method, measured PI value, for all operations. • When making thin flap is needed, by controlling input values for less errors, foreseeable accidents and troubles can be avoided. • The flap with good accuracy was able to be achieved in reproducibility good by applying this measured PI to all the operations. • The present study shows that the use of measured PI enables to make flaps with more precise thickness than the method using unmeasured PI as reported by Binder et al: J Cataract Refract Surg 2004; 30:1-9. • We will classify the combination with anterior segment OCT into a field in the future.

12. References • Flap dimensions created with the Intralase pulsion laser . Binder et al : J Cataract Refract Surg 2004 ; 30:1-9. • Anterior Segment OCT Analysis of Thin IntraLase Femtosecond Flaps . Jason E.Stahl et al : Journal of Refract Surgery 2007 ; Vol23:555-558 • Flap thickness reproducibility in laser situ keratomileusis with a femtosecond laser : Optical coherence tomography measurement. Jin-Hyoung Kim et al : J Cataract Refract Surg 2008 ; 34:132-136.