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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 New Vision Laser Centers – PowerPoint PPT presentation

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Title: LASIK-Basics and Microkeratome Theory


1
LASIK-Basics and Microkeratome Theory
  • Dr. Rupal Shah
  • LASIK India, LASIK Information India, Blade Free
    LASIK India
  • New Vision Laser Centers

2
LASIK
  • Laser In Situ Keratameleusis
  • Followed from the procedure known as ALK or MLK
  • Basic theory is decades old

3
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

4
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

5
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

6
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

7
Horizontal-First Cut
8
Horizontal-First Cut
9
Horizontal-First Cut
10
Flap is lifted to a side
11
Laser is applied under the flap
12
Flap is replaced
13
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

14
Microkeratomes
  • Used for making thin lamellar slices of the cornea

15
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.

16
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
17
FLAP CREATION
18
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

19
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.

20
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
21
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)

22
RING SELECTION
  • Ring selection closely corresponds to the desired
    flap diameter
  • Slightly larger with steeper Ks
  • Slightly smaller with flatter Ks

23
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

24
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

25
Applanation of the Cornea
26
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

27
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

28
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

29
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)

30
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.

31
Cable Drives for rotational and axial motion
32
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

33
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)

34
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

35
HANDPIECE
Fully assembled, no on eye assembly required
36
CONSOLE
Blade Change
Vacuum Level
Test
Vacuum Adjust
Battery Indicator
On/Off
Pedal Connect
Handpiece Connect
Vacuum Port
37
Other Microkeratomes
  • Suction Ring is first applied on the eye.
  • The Handpiece is then placed later

38
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39
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

40
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

41
Laser Microkeratomes
  • Intralase, Femtec 20/20
  • All laser procedure
  • Uses Photodisruption

42
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43
Thank You
Rotational Cable
App.Plate
Axial Cable
Hinge Stop
Suction Ring
Blade
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