Assessment of VisuMax Femtosecond Laser Accuracy and Precision of Flap Thickness and Centration

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Assessment of VisuMax Femtosecond Laser Accuracy
and Precision of Flap Thickness and Centration

• Dan Z Reinstein MD MA(Cantab) FRCSC1,2,3,4Timothy
  J Archer, MA(Oxon) DipCompSci(Cantab) 1Marine
  Gobbe, MST(Optom) PhD 1
• 1. London Vision Clinic, London, UK2. St.
  Thomas Hospital - Kings College, London, UK3.
  Weill Medical College of Cornell University, New
  York4. Centre Hospitalier National
  dOphtalmologie, (Pr. Laroche), Paris, France

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Financial Disclosure
The author acknowledges a financial interest in
the Artemis
Many aspects of the Artemis technology are
patented. Patents are administered by the Cornell
Research Foundation. This intellectual property
has been licensed by Cornell to ArcScan Inc, a
company in which the author has a financial
interest.
The author is a consultant for Carl Zeiss Meditec
(Jena, Germany)
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PURPOSE

• To measure Accuracy
• Reproducibility
• Flap Centration (corneal vertex centration
  intended)

of VisuMax central flap thickness
Carl Zeiss Meditec Jena, Germany
Accuracy refers to the closeness of the
measurement to the actual value Precision refers
to the distribution of values obtained when
making multiple measurements of the same object
under specified conditions Reproducibility refers
to the distribution of measurements made of
multiple objects intended to have a single
measurement value
In determining the reproducibility of flap
thicknesses produced by a femtosecond laser, the
precision of the measuring tool MUST be 2 SD
smaller than the reproducibility of flap
thickness provided by the femtosecond laser. For
example, lets assume that Instrument 1 has a
precision (i.e SD) of 1 µm and Instrument 2 has a
precision of 10 µm. Also assume that Flap 1 was
105 µm thick and Flap 2 was 115 µm thick.
Instrument 1
Instrument 1 would measure Flap 1 in the range
104-106 µm and so can distinguish between flaps
which differ in thickness by 3 µm. The 1 µm
precision adds an error of 1 µm to the process
of measuring flap thickness precision.
1 µm
1 µm
95
104 105 106
114 115 116
125
Instrument 2 would measure Flap 1 in the range
95-115 µm and Flap 2 in the range 105-125 µm. The
overlap of these ranges demonstrates that
Instrument 2 may not be able to distinguish
between the two flaps.
Instrument 2
10 µm
10 µm
Instrument 2 does not have sufficient precision
to measure a population with a reproducibility
lt10 µm
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METHODS VisuMax Femtosecond Laser

• Tissue disruption is reduced to submicron volume
  by tighter focusing and lower energy. The lower
  energy reduces the risk of an inflammatory
  response compared to other femtosecond lasers.
• Suction is applied to the cornea rather than the
  sclera, which means that the suction required is
  low and the increase in IOP is low. This also
  means that there is no risk of corneal shift
  during suction.
• The contact glass has a curved surface, so that
  the eye does not need to be applanated to a flat
  surface. The contact glass is available in 3
  sizes according to the limbus diameter.
• This results in no vision loss during suction and
  the patient is able to fixate which helps to
  achieve optimal flap centration.
• There is minimal to no opaque bubble layer (OBL)
  remaining after bilateral sequential flap
  creation eye tracking can be effected without
  delay to begin excimer laser ablation immediately
  after flap creation.
• The manifest refraction is entered to focus the
  fixation target for each eye individually.
• The keratometry is also entered to calculate the
  laser focus depth in the peripheral cornea.

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Artemis very high-frequency digital ultrasound
arc-scanner

• The Artemis uses a 50 MHz VHF ultrasound
  transducer
• Immersion scanning means the tear-film is not
  incorporated into measurements and there is no
  contact of the transducer with the eye
• Arc-scan mechanism enables maximum
  perpendicularity of the transducer to the corneal
  surface to minimize refractive errors to the
  ultrasonic signal in the peripheral cornea
• Digital signal processing used to significantly
  reduce noise and enhance signal-to-noise ratio
  has been shown to double resolution and increase
  measurement precision by a factor of 3 compared
  with analog processing1
• Patient fixation beam is coaxial with the
  infra-red camera, the corneal vertex and the
  centre of rotation of the scanning system so that
  each scan plane can be centered on the corneal
  vertex
• Meridional B-scans of the cornea enable
  localisation of the epithelium, Bowmans, the
  flap interface and the back surface
• Thickness measurements made by computer-analysis
  of peaks on the I-scan trace each peak provides
  a surface localization of 0.87 µm2
• Axial resolution of 21 µm enables measurement of
  layers thicker than 21 µm, ie sufficient to
  distinguish epithelium, flap, stroma and cornea
• 3D layered pachymetry calculated by interpolation
  between multiple meridional scans with a
  precision lt 1.0 µm2,3

ArcScan Inc Evergreen, Colorado
Surface localization 0.87 µm
1. Reinstein DZ, Silverman RH, Rondeau MJ,
Coleman DJ. Epithelial and corneal thickness
measurements by high-frequency ultrasound digital
signal processing. Ophthalmology
1994101(1)140-6. 2. Reinstein DZ, Silverman
RH, Raevsky T, et al. Arc-scanning very
high-frequency digital ultrasound for 3D
pachymetric mapping of the corneal epithelium and
stroma in laser in situ keratomileusis. J Refract
Surg 2000414-30. 3. Reinstein DZ, Silverman RH,
Trokel SL and Coleman DJ. Corneal pachymetric
topography. Ophthalmology 1994432-8.
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METHODS Population
Artemis B-Scan (above) of VisuMax Flap 6 months
post LASIK. Edge detection by I-scan digital
signal processing (red outline, below) based on
raw scan data

• Spherical equivalent
• mean -3.60 1.61 D
• range -1.00 to -6.38 D
• Cylinder
• mean -0.80 0.55 D
• range 0.00 to -2.00 D

• VisuMax Flap Settings
• Intended thickness 110 µm
• Flap diameter 8.5 mm
• Hinge 5.0 mm
• Sidecut 110?

• 24 eyes of 12 patients
• Age
• median 30 years
• mean 31.6 7.5 years
• range 24 to 52 years
• BSCVA
• 100 20/20
• 63 20/16

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METHODS Flap Thickness Measurement

• Flap Thickness Measurement
• Epithelial changes are known to occur after
  LASIK,2 therefore postop flap thickness
  measurements are not valid. Addition of the
  preoperative epithelium to the stromal component
  of the flap provides a closer representation of
  the original flap at the time of creation.
• Artemis I VHF digital ultrasound scans are
  performed before and 3 months after treatment to
  ensure no remaining edema in the stromal
  component of the flap.

Epithelial thickness
Original flap thickness
Pre-op

Stromal component of the flap
Post-op 3 months
1. Reinstein DZ, Sutton HF, Srivannaboon S,
Silverman RH, Archer TJ, Coleman DJ. Evaluating
microkeratome efficacy by 3D corneal lamellar
flap thickness accuracy and reproducibility using
Artemis VHF digital ultrasound arc-scanning. J
Refract Surg. 200622431-440. 2. Reinstein DZ,
Srivannaboon S, Silverman RH, Coleman DJ. The
accuracy of routine LASIK isolation of
biomechanical and epithelial factors. Invest
Ophthalmol Vis Sci. 200041(Suppl)S318.
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Comparison of Methods of Flap Thickness
Measurement

• Artemis Reinstein Flap Thickness
• Method 1 µm precision localization of flap
  interface, flap thickness calculated as stromal
  component of the flap plus preoperative
  epithelium
• Sources of error
• Instrument flap thickness measurement precision
  of 1.4 µm1
• Potential post-operative stromal thickness
  changes
• Intraoperative Handheld Ultrasound
• Method subtract intraoperative residual stromal
  bed thickness measurement from corneal thickness
  measurement
• Sources of error
• Instrument corneal thickness measurement
  precision of about 6 µm2
• Instrument residual stromal bed thickness
  measurement precision (not published, but likely
  gt6 µm)
• Stromal hydration during surgery
• Misalignment of probe location for corneal and
  residual bed measurements
• Optical Coherence Tomography
• Method 1 direct measurement of flap thickness by
  automated computer algorithm
• Method 2 direct measurement of flap thickness by
  manual placement of measuring tool on OCT B-scan
  image
• Sources of error
• (Method 1) Instrument central flap thickness
  measurement precision of 6.5 µm3
• (Method 2) Instrument flap thickness measurement
  precision of 6.5 µm added to manual measurement
  precision flap tool only allows flap
  measurement to the nearest 6 µm (12 µm
  increments)
• (Method 2) Intra-observer error of flap interface
  location

Most accurate method of determining original flap
thickness produced by flap creating device
1. Reinstein DZ, Silverman RH, Raevsky T, et al.
Arc-scanning very high-frequency digital
ultrasound for 3D pachymetric mapping of the
corneal epithelium and stroma in laser in situ
keratomileusis. J Refract Surg 2000414-30. 2.
Yaylali V, Kaufman SC and Thompson HW. Corneal
thickness measurements with the Orbscan
Topography System and ultrasonic pachymetry. J
Cataract Refract Surg 19971345-50. 3. Li Y,
Netto MV, Shekhar R, et al. A longitudinal study
of LASIK flap and stromal thickness with High
Speed Optical Coherence Tomography. Ophthalmology
2007114(6)1124-32
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METHODS Flap Centration Measurement
Flaps were intended to be centered on the corneal
vertex. The patient aligns their eye naturally to
the corneal vertex by focusing on an internal
fixation target.
C
Distance Corneal Vertex Flap Centre Horizontal
Offset 0.5 x (VN VT) Vertical Offset VI
(0.5 x FD
I
V
Total offset Centration of the Flap relative
to the Corneal Vertex (CV) ? (Horizontal
Offset 2 Vertical Offset 2)
T
N
FD Flap Diameter
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RESULTS Central Flap Thickness

• Intended flap thickness 110.00 µm
• Average flap thickness 112.31 µm
• Accuracy 2.31 µm
• Reproducibility (SD) 7.89 µm
• Minimum flap thickness 102.61 µm
• Maximum flap thickness 132.94 µm
• Range 30.34 µm

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RESULTS Flap Centration
Distance Corneal Vertex - Flap Centre

• Average 0.32 mm
• Standard Deviation 0.17 mm
• Minimum 0.13 mm
• Maximum 0.83 mm

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DISCUSSION

• The VisuMax femtosecond laser system was found to
  produce very accurate and highly reproducible
  flaps, well centered to the corneal vertex
• Accuracy 2.31 µm
• Reproducibility 7.89 µm
• Flap centration 0.32 mm

The Artemis VHF digital ultrasound arc-scanner
has a flap thickness measurement precision of 1.4
µm and therefore provided sufficient precision to
determine flap thickness reproducibility as high
as 2.8 µm ( 2 x 1.4). Paradoxically,
pachymetry by devices with lower measurement
precision may give falsely high flap thickness
reproducibility results. If the measurement
precision of the instrument being used is unable
to discriminate between flaps of similar
thickness, the random error associated with each
flap thickness measurement could incorrectly
cluster the measured values resulting in a
falsely high flap thickness reproducibility.