LASIK-Basics and Microkeratome Theory

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LASIK-Basics and Microkeratome Theory

• Dr. Rupal Shah
• LASIK India, LASIK Information India, Blade Free
  LASIK India
• New Vision Laser Centers

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LASIK

• Laser In Situ Keratameleusis
• Followed from the procedure known as ALK or MLK
• Basic theory is decades old

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Automated Lamellar Keratomeleusis(ALK)

• Consists of two incisions
• First, a slice of cornea 160 microns thick and a
  diameter of 8 mm is removed
• Second, a thin slice of cornea corresponding to
  the refractive error is removed
• The first slice is replaced back

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Problems with ALK

• There are limits to the accuracy of a mechanical
  instrument
• The second slice could never be accurate or
  precise enough to compete with other forms of
  refractive surgery

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Excimer Laser

• Can ablate tissue with great accuracy
• Since the first cut is not critical, that is done
  with the microkeratome
• The refractive lenticle is removed with the
  excimer laser
• Refractive change can occur with the excimer
  laser without disturbing the epithelium

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LASIK-Technique

• The microkeratome makes a horizontal cut on the
  cornea
• Slice is not excised completely
• A tongue like flap is removed to one side
• The laser is applied in the usual manner
• The flap is replaced and sticks in place

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Horizontal-First Cut
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Horizontal-First Cut
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Horizontal-First Cut
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Flap is lifted to a side
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Laser is applied under the flap
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Flap is replaced
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Microkeratomes-History

• Basic Theory was evolved by Jose Barraquer
  decades ago
• First Used for ALK/MLK
• In the 90s, modified for use in the procedure
  that has come to be known as LASIK

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Microkeratomes

• Used for making thin lamellar slices of the cornea

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Microkeratomes Used in LASIK

• Capable of creating thin lamellar slices of the
  cornea of fixed or adjustable depth
• The Slice interface should be smooth, and free of
  spherical aberrations
• The slice should have an appropriate diameter
  along with an appropriate hinge size.

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Principle of Microkeratomes

• Work on the principle of a carpenters plane or
  randho
• The blade is at a fixed distance from an
  applanation plate, which determines the thickness
  of the slice

Blade
Plane
Blade to Plate Gap
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FLAP CREATION
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Problem of applanation

• The cornea is a spherical object, and unlike
  wood, will not be in contact with the applanation
  plate at all or any points along the blade motion
• Therefore, very high suction is applied all
  around the cornea, to ensure high IOP, and
  thereby pressure of the cornea against the
  applanation plate

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SUCTION and IOP

• The suction ring will induce a rise in
  intraocular pressure.
• An adequate vacuum will induce pressure greater
  than 65mm Hg, which is the recommended minimum
  requirement.
• Insufficient vacuum will not provide the optimum
  positioning of the eye within the suction ring.
  If this occurs, an irregular flap may be produced.

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First Component of a Microkeratome

• A Suction ring and a vacuum pump, to ensure
  adequate applanation of the cornea by the
  applanation plate
• If the cornea is not applanated perfectly, we
  would get thin flaps, no flap or a small free
  lamella of the cornea

Plate
Blade
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RING SELECTION

• 8.5 mm ring
• Steep corneas (K gt 45), thinner flaps
• Small diameter corneas (prevent dissection of
  blood vessels)
• 8.8 mm ring
• 9.0 mm ring
• Standard myopic ring
• 9.5 mm ring
• 10.0 mm ring
• All hyperopes and flat corneas (K lt 40)
• Extremely steep corneas (K gt 47)

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RING SELECTION

• Ring selection closely corresponds to the desired
  flap diameter
• Slightly larger with steeper Ks
• Slightly smaller with flatter Ks

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Second Component

• A means to arrive at a conclusion whether there
  is sufficient applanation or not
• Indirect Way Through an applanation tonometer,
  measuring IOP
• Direct Way Through a transparent applanation
  plate

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APPLANATOR

• The applanators are used to verify the cut
  diameter prior to flap creation.
• The applanator does not replace, or function as a
  tonometer.
• Diameter check vs. pressure check
• IOP measurement is recommended for every eye
  prior to flap creation

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Applanation of the Cornea
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Flap Interface Should be Smooth

• An ordinary knife would lead to lot of scarring
  on the cornea
• A special blade is used which oscillates at a
  high speed to and fro in the direction orthogonal
  to the direction of forward motion
• Higher the oscillation speed, the smoother is the
  cut

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Oscillation of the blade

• A rotating shaft with an eccentric tip is used.
  The shaft is rotated by a turbine motor, either
  gas driven (faster oscillation) or electrically
  driven

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Third Component

• A means of forward translation of the blade,
  along with oscillation in an orthogonal direction
• Forward Motion should be smooth, uniform and
  independent of load
• Can be done by hand (manual machines), a cable
  drive or gears

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FLAP FACTORS

• BLADE SPEED (12,000 rpm)
• BLADE ANGLE AND SHARPNESS (25 degrees)
• SPEED OF TRANSITION ACROSS THE CORNEA (4.0
  mm/sec)
• DISTANCE BETWEEN THE BLADE AND THE PLATE
• IOP (gt65mmHg)
• PRESSURE DURING TRANSITION
• NASAL DECENTERING (0.5mm)

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FLAP FACTORS

• Pressure exerted by the surgeons hands on the
  instrument during the surgery could effect the
  outcome of the procedure.
• Too much downward pressure will create a thicker
  flap.
• Not enough downward pressure, a thin flap or loss
  of suction may occur.
• The weight of the keratome has been adapted to
  the speed of the keratome head across the cornea.
  

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Cable Drives for rotational and axial motion
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Fourth Component

• The thickness of the flap is determined by the
  blade to plate gap
• This gap can be varied by physically increasing
  the gap, or by using different thickness
  applanation plates

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HEADS

• Stainless Steel Construction
• Available in multiple depths
• 130 µm
• Thin corneas (500u 530u), high myopes
• 160 µm
• Moderate corneas (530u 560u)
• 180 µm
• All thick corneas (gt560 µm)

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Four Essential Components

• Suction ring and Vacuum pump
• Applanation plate with means of checking
  applanation
• A means of oscillating the blade at high speed
  and a mechanism for forward translation of the
  blade
• An adjustable plate to blade gap

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HANDPIECE
Fully assembled, no on eye assembly required
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CONSOLE
Blade Change
Vacuum Level
Test
Vacuum Adjust
Battery Indicator
On/Off
Pedal Connect
Handpiece Connect
Vacuum Port
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Other Microkeratomes

• Suction Ring is first applied on the eye.
• The Handpiece is then placed later

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FLAP FACTORS

• Your success is dependent on close attention to
  detailed
• assembly
• operation
• maintenance
• The device is a precisely manufactured instrument
  designed to cut precise corneal lenticules.
  Damage to any part of the instrument may lead to
  undesired results

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CLEANING

• Always follow the recommended cleaning regimen
• Failure to use the proper cleaning technique or
  cleaning agents may
• Damage the components
• Lead to undesired clinical outcomes

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Laser Microkeratomes

• Intralase, Femtec 20/20
• All laser procedure
• Uses Photodisruption

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Thank You
Rotational Cable
App.Plate
Axial Cable
Hinge Stop
Suction Ring
Blade