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 60AMO).
• 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 .
• (Shinagawa method)
• 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
  538.4923.91um) 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 (SP3000Tomey).
• 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 (n531)
  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

• n153

Mean(µm) SD(µm) Min(µm) Max(µm)
Error 3.40 1.61 None 9.0
Tilt 3.33 1.00 None 8.0
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 (Shinagawa method) 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 Results?
Flap Thickness(µm) Number of Eyes Mean Flap Thickness (µm) SD (µm) Min (µm) Max (µm)
90 98 92.18 4.11 82 99
100 152 102.34 4.93 91 112
Mean(µm) SD(µm) Min(µm) Max(µm)
90µm Right 92.22 4.02 84 99
90µm Left 92.33 4.30 82 99
100µm Right 101.81 4.60 91 109
100µm Left 101.97 4.63 92 112
9
Flap Results?

• measured PI 90µm

• Measured PI 100µm

p Value
p Value
10
Summary

• The mean flap thickness was 92.184.11um
  (Range82-99um) in 90 um group, and 102.344.93um
  (Range91-112um) in 100 um group.
• In the flap, a right and left correlation was not
  seen.

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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 301-9.
• 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 301-9.
• Anterior Segment OCT Analysis of Thin IntraLase
  Femtosecond Flaps . Jason E.Stahl et al Journal
  of Refract Surgery 2007 Vol23555-558
• 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 34132-136.