Understanding LASIK Technology

8/3/2019 Understanding LASIK Technology

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Understanding Vision Correction Technology- Your Guide to Better Vision

Understanding LASIK Technology

Your Guide to Better Vision1st Edition

byDr. Dean Dornic, MD

www.VisionAuthorities.com

Copyright Laser Eye Center of Carolina. You may not copy this book.


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Contents

INTRODUCTION AND HISTORY of LASIK..........................................................2

FEMTOSECOND FLAP MAKERS...........................................................................................5 Different Types of Femtosecond Lasers..............................................................................6 Features of Different Flap-Making Lasers.........................................................................8

Summary of Safety Advantages of Femtosecond lasers....................................................9

WAVEFRONT VISION CORRECTION TECHNOLOGY.......................................................10

Higher order Aberrations....................................................................................................10 Wavefront-Optimized LASIK.............................................................................................12

Wavefront-Guided LASIK...................................................................................................13

CONCLUSIONS AND GLOSSARY.........................................................................................16 Description of LASIK Technologies....................................................................................16

Comparison Chart of LASIK Technologies.......................................................................17

Copyright Laser Eye Center of Carolina. You may not copy this book. 1


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Introduction

What is LASIK Eye Surgery?

LASIK eye surgery refers to a vision correction procedure that utilizes a laser to reshape the

cornea. An intrical part of LASIK is the creation of a LASIK flap. Unlike PRK, where the laser

is applied to the corneal surface, LASIK first involves lifting the top layers of the cornea (the

flap) and applying the laser to reshape the cornea under the flap. Performing the reshaping stepunder a flap and then replacing it, leads to faster visual recovery and a shorter period of

discomfort. Some advantages of LASIK over other vision correction procedures is the quickness

of the procedure itself: generally less than 15 minutes, the relative lack of pain and the quickreturn of visual function: usually 6-8 hours.

Illustration 1: What makes LASIK "LASIK", isthe creation of a flap, followed by reshaping

of the cornea under the flap with a laser.

History of LASIK Eye Surgery

LASIK was first described in 1991. LASIK is really just the evolution and combination of olderprocedures that have their roots in the Automated Lamellar Keratoplasty (ALK) procedure that

dates back to the 1950's. In ALK, flaps were made with an instrument which had an oscillatingblade called a microkeratome. This same instrumented was used in the initial, flap making stage,of the LASIK procedure. The primary difference between ALK and LASIK is that in ALK, the

cornea was reshaped with a lathe. In LASIK, the cornea is reshaped with a laser.



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Illustration 2: When LASIK was first

performed, it made use of the same

instrument used to make a flap inALK: the microkeratome.

Right from the beginning, LASIK eye surgery enjoyed tremendous success with high patient

satisfaction and growing acceptance, but, as with any medical procedure, it was plagued by asmall number of patients suffering from troubling complications and side effects.

Chief among these complications were:

Flap complications

Quality of vision issues such as night halos

Dry Eyes

Illustration 3: Some patients who had LASIK were troubled

by disabling halos and other night time driving difficulties.



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Over the years, newer technology and techniques were introduced to reduce the number and

severity of these adverse outcomes.

The two major innovations that led to improves in LASIK eye surgery were:

1. The femtosecond laser to replace flap creation with a blade

2. Wavefront technology to improve quality of vision and reduce the incidence of night visiondifficulties.

Let's take a look at these two innovations in a little more detail.



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The Femtosecond Laser

The use of a blade to create a LASIK flap has several limitations. Even with the best qualitycontrols, flaws on the blade edge produces defective flaps. Variability in flap thickness (a result

of variability in exposed blade) are sometimes severe enough to produce excessively thick or thin

(buttonholed) flaps. Machine malfunctions can result in partial, decentered or free flaps (flapswithout a hinge).

A more precise way to make the LASIK flap is a femtosecond laser. The femtosecond laser

creates a flap by producing photo disruption bubbles a set distance below the corneal surface.


Illustration 5: A femtosecond laser

creates a bubble beneath the corneal

surface.

Illustration 4: Variability in the exposed

blade leads to variability in flap thickness.

Illustration 6: The corneal flap is

created by a sheet of closely spaced

bubbles.


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Different Types of Femtosecond Lasers

All femtosecond lasers produce flaps of more predictable thickness. The initial femtosecond

lasers used in LASIK were much slower and were not as sophisticated as some of today's latest

models. These early models of lasers had their own potential side-effects and risks. Some of the

initial lasers required more energy and were more prone to such complications as: torn flaps,incomplete flaps, diffuse lamellar keratopathy (excessive, sometimes vision damaging,

inflammation) and transient light sensitivity syndrome. Some models of femtosecond lasers stilloperate on the same principles as microkeratomes: simply replacing the blade with a laser and

thus are prone to some of the same complications, primarily: irregular, decentered and free flaps.

As is true in most things in medicine, improvements in technology resulted in improvements inpatient care. The lasers became faster and were able to produce flaps using less energy. They

also became more sophisticated with some femtosecond lasers allowing for true customization of

size and shape. Some models of femtosecond laser allowed the surgeon to visually monitor the

flap creation process and allowing intervention, if necessary, to correct a problem before flapcreation was complete. Some of the latest models of lasers allow for the production of inverted,

beveled flap edges causing the flap to fit back in position more precisely and more securely. Allof these newer features make the LASIK procedure safer with improved outcomes. Some of theimprovements in femtosecond flap creating technology and the specific lasers that have these

improvements are listed in table 1.



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Illustration 6: IntraLase lasersallow for beveled edges (top picture)

which make the flap more resistant

to trauma vs a straight cut (bottompicture) created by microkeratomes

and the Ziemer laser.

Illustration 5: Some models of femtosecondlasers have a surgeon dashboard allowing for

true customization of the flap.


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Illustration 8: Some femtosecond lasers permit the surgeon

to visualize the flap as it is being created. This may allowthe surgeon to stop the procedure and correct a problem

before flap creation is complete (second chance capability).


Illustration 7: Various studies confirm that aninverted bevel edged flap, as can be created

by the iFS model laser is the type most

resistant to displacement.


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Table 1. Features of Femtosecond flap-creating lasers used in LASIK.

LASER MODEL FEATURES SIGNIFICANCE

Ziemer Emulates microkeratome.Limited flap diameters and

thicknesses. Straight cut.

Surgeon cannot see the flapbeing created.

Lower risk of cornealabrasions than microkeratome

but still has the potential to

produce all the complicationsseen with a microkeratome.

Intralase FS Dashboard allows for greatercustomization of flap with

unlimited flap diameters and

thicknesses. Flap creation

visible to surgeon. Secondchance feature.

Lower risk of flapcomplications than Ziemer.

IntraLase iFS All the features of the FSmodel PLUS capability tocreate oval flaps and inverted

bevel flap edges.

Lower risk of flap creation

complications (as the FSabove) PLUS more stable

cornea and lower risk of

complications after flap

creation. Safest of the three.

Summary of Safety Advantages of Femtosecond Lasers

Ziemer (Z-LASIK): reduces risk of corneal abrasions.

IntraLase FS: reduces or eliminates the risk of corneal abrasions, button-hole flaps,decentered flaps, irregular flaps free-flaps, epithelial ingrowth.

IntraLase iFS: reduces or eliminates the risk of corneal abrasions, button-hole flaps,decentered flaps, irregular flaps free-flaps, epithelial ingrowth. In addition reduces therisk of slipped and wrinkled flaps.

The Ziemer laser creates flaps more precisely than a bladed microkeratome. TheIntraLase FS laser is safer than a Ziemer because it reduces the risk of flapcomplications during the procedure. The Intralase iFS laser takes safety a step furtherby reducing complications and side-effects after the procedure.



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Wavefront Vision Correction Technology

The Problem of Higher Order Aberrations

Early on after LASIK eye surgery began to be performed it soon became apparent that somepatients, even when corrected to 20/20 levels of vision, were unhappy with the quality of their

vision. Some patients reported that their vision was smudged, ghosty, not clear or even

doubled. Many of these patients complaints of quality of vision can be explained by thepresence of higher order aberrations. Most patients are familiar with the terms associated with

lower order aberrations: farsightedness, nearsightedness and astigmatism. These lower order

aberrations are problems of defocus ie problems that can be corrected with eyeglasses.

Higher order aberrations refer to blur created by distortion not corrected by standard eyeglasses.An extreme example of higher order aberration is the distorted vision created by a fun house

mirror.


Illustration 9: The distorted image ofa fun house mirror is an extreme

example of a higher order aberration:

ie poor image not correctable witheyeglasses.


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During a LASIK procedure, the cornea is reshaped. In the case of nearsightedness, the central

cornea is flattened. The result is an abnormal shape termed 'oblate' (cornea flatter in the middle

than in the periphery. Oblate corneas produce a type of aberration termed spherical aberration.There are many other forms of aberration, all of which can contribute to poor image quality and

other, unwanted, visual side effects.


Illustration 10: Corneal topography of patient before

(above) and after (below) LASIK. The result is anoblate cornea. A difference map is on the right.

Illustration 11: Spherical aberration results in a degradation of image

quality after LASIK. Because the 20/20 size letter can be read in bothexamples, both patients would be classified as "20/20", but obviously

the patient above has better quality vision.


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In everyday life, higher order aberrations cause the most problems when the pupil is large. Thepupil is largest at night and patients with higher order aberrations have the most difficulty with

night driving. Severe aberrations can result in severe haloing or star bursting with night

driving.

Wavefront Optimized LASIK

Wavefront optimized LASIK uses knowledge of optics to limit the severity of sphericalaberration. Patients who have their LASIK performed with a laser that is wavefront optimized,

in general, show better vision quality than conventional LASIK. The problem of night halos andstar bursts is reduced.


Illustration 12: The presence of higher order aberrations can result in problems with night

driving.


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Wavefront Guided LASIK

A step above wavefront optimized LASIK is a wavefront-guidedLASIK. Wavefront optimized

LASIK attempts to limit the degree of new aberrations during the LASIK procedure. Awavefront-guided LASIK procedure attempts to correct pre-existing aberrations. Obviously, if a

patients pre-existing aberrations are to be corrected, they must first be measured. Measurement

of both lower-order and higher order-aberrations takes place through the use of a wavefrontanalyzer.

The concept of correcting higher order aberrations actually came about through the efforts ofNASA. They developed a technology to correct the affect of higher-order aberrations in images

captured by the Hubble telescope. The result of NASA's wavefront correcting technology was

the ability to see distant stars and galaxies in detail never before possible. (see Illustration 14).

This same technology was applied to the correction of wavefront measured vision abnormalitiesby VISX (now Abbott Medical) in their proprietary CustomVue technology. The result was

vision correction with LASIK at an unprecedented success level with many patients achieving a

level of vision (including night vision) surpassing that of glasses or contact lenses.


Illustration 13: A wavefront analyzer

measures both lower order (e.g.

myopia) and higher order (e.g.spherical) aberrations.


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Patient satisfaction with night vision before and after a wavefront-guided LASIKprocedure.


Illustration 14: Wavefront correcting technology developed by NASA improved the

resolution of astronomical telescopes. This same technology improves the quality ofhuman vision in wavefront-guided vision correction procedures.

Illustration 15: Patient satisfaction with night vision actually

improved after wave-front guided LASIK as compared to theireyeglass or contact lens corrected vision pre-op.


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Key Point: A wavefront-optimized procedure attempts to limit the introduction of new

aberrations. A wavefront-guided procedure measures pre-existing aberrations and

attempts to correct these along with the patient's refractive error.


Illustration 16: A phoropter is used to

plan a wavefront-optimized LASIKprocedure. Which is better, one or

two?

Illustration 17: A wavefront-analyzer is usedto plan a wavefront-guided procedure. This

technology measures and corrects higher-

order aberrations.


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Conclusions and LASIK Glossary

LASIK is not one procedure but a group of procedures all having incommon the presence of a corneal flap.

Since it's introduction 20 years ago, a number of innovations and improvements in technology

have resulted in a safer and more sophisticated procedure. One should not assume that allLASIK providers offer the most up-to-date technology. While there has been a gradual shift to

all-laser technology, almost half of LASIK procedures performed are still performed with a blade

(microkeratome). Newer, better technology such as wavefront-guided LASIK are more costlyand more time consuming and may not be offered by high volume, discount LASIK centers.

LASIK is not a commodity but includes a professional service component. The consumer is

wise to educate himself in the differences in technology in order to insure that they understandexactly what they think they are receiving. What follows is a description of some commonlyoffered LASIK procedures.

Z-LASIK

Z-LASIK refers to LASIK performed with a Ziemer laser to create the LASIK flap. TheZiemer laser emulates a microkeratome (bladed flap maker). All complications possible with a

bladed microkeratome can occur with Z-LASIK. In Z-LASIK, the Ziemer laser to create a flap

is often paired with a wavefront optimized laser to perform the vision correction.

iLASIK

iLASIK refers to LASIK performed using an IntraLase laser to create the flap along with a

wavefront-guided procedure to reduce the risk of night halos and starbursting. iLASIK has a

lower risk of flap complication compared to Z-LASIK. It also tends to produce better visionquality.

OmniLase

OmniLase refers to an all-laser LASIK procedure in which both the flap-making step and thevision correction step is fully customized. It offers the highest likelihood of excellent vision

quality and may result in improved night vision. It has the lowest risk of flap complication of

any of the three all-laser LASIK procedures and tends to offer the least amount of post-operativedry eye problem.

The following chart compares and contrasts the commonly offered LASIK eyesurgery options.



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Table 2. Comparison of LASIK Options and Technologies

Copyright Laser Eye Center of Carolina You may not copy this book 17

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Understanding LASIK Technology