LASIK Flap Architecture

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        Medicals International




        LASIK Flap Architecture
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        Technical Communiqué

        Ophthalmic Department

        Issue # 1/2005
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            Technical Communiqué (Ophthalmic)
           Flap construction for LASIK surgery.
 With the increasing number of refractive LASIK procedures done and the
advancement of the Excimer laser machines it is worth to stop a while and think about
an important factor influencing the LASIK clinical outcome that is the Flap
construction and architecture.

A LASIK flap differs from other surgical ophthalmic wounds in several significant
ways. In flap making we do not enter the eye. The flap is made with little control from
the surgeon (compared to other wounds where the surgeon’s hands control the wound
parameters). The above renders the instrument performing the flap of primary
importance. Being the largest dissection done onto the eye compared to other
ophthalmic surgeries, understanding the flap making process becomes imperative to
achieving best clinical outcomes. In the following, we will present and discuss all the
elements that contribute to adequate flap making.

The Elements of the Flap Architecture

There are several essential elements that affect the flap architecture:

   1- Flap diameter: with the introduction of the flying spot lasers it is now possible
      and essential to have larger ablation and transition zones especially for the
      hyperopic treatment where a treatment zone of more than 8.5mm is needed
      and to have larger ablation/optic zones to prevent night vision problems. To be
      able to make an ablation of 8.5mm diameter we need a flap diameter of 9.5
      mm at least preventing that the ablation hits the hinge. The effective part or
      tissue available for ablation under a flap of 9.5mm is effectively between 8.5-
      9.00mm viewing the hinge size which is at least 0.5-0.7mm. On the other hand
      the large flap diameter size will give in addition a better stability of the flap
      when repositioned back and enhances postoperative vision.

   2- Flap thickness: the flap thickness must be a compromise between: thick
      enough to avoid buttonholes and to allow manipulation, and thin enough to
      leave enough tissue or stroma available for ablation.

   3- Stromal Bed: should be completely homogenous and smooth with No signs of
      chatter marks or rough edges along any part of the circumference. A well
      defined bevel of the edge is vital to help hold the flap back in place.

   4- Hinge position: it may be desirable to position the hinge on different places
      (e.g. nasal or posterior). Yet, a large number of refractive surgeons use
      exclusively the Nasal hinge position. The reason is that cutting temporally and
      creating a nasal hinge preserves the nasal innervations, which carries signals
      indicating dryness from the ocular surface to the brain.




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The flap construction is very important to the outcome of the surgery and may affect
the whole surgery and vision quality in case not achieved properly. The instrument
responsible for doing this flap is the Micro-keratome and we will try below to focus
on the specifications that one shall look for when choosing a Micro-keratome.

Main specifications for Selecting Micro-keratome:

A good micro-keratome must be able to control all the above mentioned parameters as
follows;

•   It should preserve the cornea's biomechanical structure.
•   I should not induce any buttonholes, and the risk of inducing a free cap should be
    negligible.
•   It should make a regular cut and leave a smooth stromal bed surface. Flaps of
    consistent thickness and high stromal beds quality provide better visual acuities.
•   It should cover both myopic and hyperopic patients. For hyperopic patients, it
    should produce a small hinge and a large flap with a diameter that can range from
    8.5 to 10.0 mm.
•   It should prevent epithelial in-growth: the key to prevent epithelial in-growth is a
    good, tight fitting flap. That is both keratome and surgeon related. A micro-
    keratome must cut a flap with steep, clearly defined edges.
•   The suction rings should be highly effective for the device to create excellent
    quality flaps.
•   Surgeons appreciate the device’s ability to customize the procedure for a variety
    of corneas, refractive errors, optical zone sizes, pupil diameters, and K values.
•   In addition the user must be able to visualise the flap position before doing the cut
    and be able to modify as needed.
•   It must have proper speed of the oscillation and of translation.
•   It should avoid the use of chemicals.
•   It is preferable if it is multifunctional (LASIK & EPI-LASIK with one single
    device).
•   It must be easy to use, upgradeable, and it must provide automatic operation.
•   It should be flexible to help the surgeon to individualize the hinge and flap size for
    each patient.
•   It should have predictability of the parameters selected by the surgeon.

Highlight on the Lasitome (Krumeich-Barraquer) from Gebauer/ Visijet
The Lasitome system (Gebauer Medizintechnik GmbH, Neuhausen, Germany) is a
new micro-keratome available on the European market since February 2003 (Figure
1), and it shows considerable promise. Since its introduction, the device’s efficacy
and quality in performing its minimally invasive, alcohol free Epithelial flap have
been documented.



                                    Figure 1. The Lasitome microkeratome.




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The Epi-LASIK procedure using the Epi-Lasitome solves many problems associated
with refractive corneal surgery where a lamellar flap (LASIK) is used. Many surgeons
prefer Epi-LASIK as it better saves stromal tissue, eliminates all possible epithelial
defects and eliminates all the long term problems experienced at the level of the flap
interface.

DESIGN


Applanation Window: Visibility of Blade Path

The Lasitome features one main modification that renders its performance superb.
The success rate of flaps is highly related to the ability to clearly view the pass of the
blade through the meniscus once the head is properly engaged on the suction ring. In
addition to this feature, the Lasitome allows to clearly view the border of the soon-to-
be created flap by following the two lines drawn on the applanation window. The
Lasitome incorporates the Preview-Window which takes the presumption off the
procedure. You know exactly how large the flap is going to be before you begin the
procedure. The applanation window also gives you full intra-operative view of the
cornea. You have total control throughout the entire operation.




                              Figure 2. The applanation window of the device’s head




Its applanation window is laterally secured to the head by two independent fixtures,
micropins and setscrews, in order to more safely hold the window in the accurate
position. Moreover, the applanation window provides a view of the entire cut of the
cornea and corneal meniscus (Figures 2 and 3).




        Figure 3. The Gebauer Epilift uses an applanator bar to help maintain an even epithelial separation.




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Suction Rings

A variety of Suction Rings enables you to adapt the system to your patients’
individual needs. You may choose from three sizes and two depths and set up your
system to obtain an ideal eye and corneal profile fit. The Suction Rings are designed
to avoid aspiration of conjunctiva and pseudo-suction as well as to provide a safe but
moderate elevation of the IOP (no black out). Besides, the suction rings have many
holes along the side that adhere to the cornea. The holes are protected by diagonal
bands and help the suction ring to attach itself more strongly to the cornea (Figure 4).




                                   Figure 4: Suction Rings


High Precision Cutting Head

The Lasitome Cutting Head is created using state of the art precision generation 4
CAD/CAM technology. It meets the most severe mechanical specifications to ensure
consistent flap thickness, cut after cut. Its new improved design incorporates a
mechanical stop to hold the Applanation Window securely in its position.

The micro-keratome has two heads, one regular and one hyperopic, which can create
various types of flaps. The standard-sized head cuts at 160 µm for applanations of up
to 9.5 mm, and the larger head cuts at 130 µm for applanations exceeding 9.5 mm.
The latter is useful in hyperopic cases in which the ablation zone of the excimer laser
is usually wider in diameter. Both types of heads can also create thinner flaps (130
µm).

Smooth Stromal Bed: Speed and Dual- Motor Function

The Lasitome micro-keratome system is designed to produce excellent flaps with
defined thickness and smooth stromal beds, procedure after procedure. The computer-
controlled micro-motors are independent of each other and may be set to the
parameters you prefer. The oscillation motor shuts off before the back drive to the
starting point which protects the fine stromal bed during retraction.The
microkeratome’s transverse speed may be set from 0.4 to 3.0 mm/second.
The Lasitome has two motors, one for oscillation and the other for advancement. As
the head returns to the starting position after making the cut, the blade’s motor stops.
With only one motor functioning, the blade does not move. Therefore, the risk of
damaging the flap is minimal during this reverse movement of the head.

High Quality Blades

Unlike other micro-keratome blades, Lasitome Blades are not “upgraded”
razorblades. It is manufactured to the highest Quality Standards; they are unique in


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dimensional precision and cutting edge quality. Lasitome Blades (Figure 5) are
Gamma-sterile, packaged in a special cartridge to ensure accurate insertion.




                                  Figure 5: Lasitome Blades


Alcohol-Free Treatment

The EPI-Head and EPI-Blade combination creates epithelial flaps without the
necessity of a preceding treatment with an alcohol solution. The Epithelium is
mechanically dislocated and separated from Bowman's Layer.

The EPI-Head / EPI-Blade (Figure 6) are fully compatible with the Lasitome. The
clinical results for all patients including LASEK re-treatments are excellent. Making
the Epi-flap in this group poses no problems and the outcomes were no different than
any regular stromal flap.




Figure 6: EPI-Head/EPI-Blade


Customization: Easy and Multi-functional

Surgeons would be extremely pleased with the ease with which the Lasitome creates
an epithelial flap. The Lasitome’s suction to the cornea can be easily controlled by the
foot pedal. However, some surgeons prefer that the technician or circulating nurse
manage everything manually from the console so that he may concentrate on the
position of the ring and the cut. Lasitome offers both choices (figure 7).

You can input the applanated flap diameter, and select hinge width, cutting speed and
translation speed. The system will meet these parameters precisely. It continually
monitors all these settings throughout the procedure, including the vacuum level. In
case of a commercial power failure it will provide enough backup power to complete


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the initiated procedure safely. LASIK and EPI-LASIK are done with one single
device.




                           Figure 7: The console of the Lasitome




Epi-LASIK/ LASEK

All LASIK patients suffer the loss of one or two Chart lines 2 to 3 years down the
road after surgery. The main reason is related to the flap stromal interface and the to
the long-term changes to the stromal fibers above the flap demarcation line. Kindly
examine figure 8 below.




              Figure 8: Microscopic picture of a 1 year old LASIK treated eye.

The long term stability of the PRK visual outcomes, the advancements in the laser
beam profiles, as well as the desire to performing pain free surgery lead to the
increasing popularity of the LASEK procedure.

However, LASEK seems not to become the procedure of choice for so many
refractive surgeons. The main reasons are:

       Use of Alcohol
       Dead epithelial tissue
       Difficulty in handling the epithelial flap
       The relatively slower healing process
       The relatively higher pain then LASIK
       The Integrity of the created flap

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The Epi-LASIK is the                procedure that
allows all the benefits             of the LASEK
while not having to use             Alcohol,      the
benefits from a stronger       La   more alive flap,
and the benefits from a        mi   faster and lesser
painful healing process.            Figure 9 below
represents a diagram of        na   the     epithelial
tissue.                        De
                               ns
                               a




   Figure 9: Diagram                representation of
    the Epithelium.

Basal cells play a major            role in epithelial
re-growth           and                  metabolism.
They are very important             for        future
adhesion of epithelium              to Bowman’s.
Lamina Lucida is the                layer           of
adhesion structures. On             the other hand,
Lamina            Densa                    (Collagen
lamella) is a cell- free            structure.

The Lasitome cleaves                between Lamina
Densa and Bowman’s.                 The Epithelium
needs to be fully intact            with      both
Lamina Layers.

Anchoring Fibrils -                         “Anchor
Plaque”               or
Hemidesmosome                             Complex-
anchor the Epithelial               layer to the
Bowman’s Membrane.                  If these are
separated at Bowmans,               they can “re-
attach” the Epithelial              flap once the
flap is sited back down.




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The body natural response is to “kill” all cells above any destroyed Lamina Lucida as
the Lamina Densa layer has to be built first by the limbal basal cells. This layer then
builds the layer above, then the one above it, etc... This is what happens with LASEK.
The proposed theory is that If the Anchor Plaque fibres are cut between the Lamina
Densa and the Lamina Lucida, they cannot “re-anchor” the epithelial flap to the
bowman’s capsule. The whole flap is then ultimately non viable and almost non
functional.

Why Epi-LASIK?

Let us examine the evolution of the Laser vision correction refractive surgery and that
evolved and led to our better current visual outcomes:

        1983 – 1986: RK
        1983 – 1990: ALK
        1986 – Present: PRK
        1995 – Present: LASIK
        1999 – Present: LASEK
        2004: EPI-LASIK

The imperfection of each of the above procedures led to the birth of its successor.
Today Epi-LASIK is the procedure that carries a load of advantages that are worth
considering. Those include:

       Fast Visual recovery
       Little pain experience
       Long term superior and stable visual outcomes
       Safer procedure (tissue saving)
       Aberration free flap allows better surgical outcomes when custom treatments
       are applied

The Epitome / Lasitome system from Visijet is one of the most tested, most reliable
and most researched devices allowing the performance of all modern refractive
procedures.

References:
   1- Refractive EyeCare for Ophthalmologists: June 2001 and September 2001. Techniques in
      refractive surgery

   2- Un-complicating LASIK tools and techniques: supplement to refractive EyeCare for
      ophthalmologists volume 5, number 11 November 2001

   3- Ophthalmology times special Section : microKeratomes February 2002

   4- Journal of Ophthalmology 2002; D. S. Durrie, MD et al; “Epithelial Defects: A Technology-
      related Complication”.

   5- Journal of Ophthalmology 2003; B. D. Soloway, MD et al; “Flap Architecture: Quality and
      Consistency Are the Keys to Fewer Complications and Better Outcomes”.

   6- Extracts from Chris Lohmann, MD, ESCRS Paris Sep 2004.


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