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Customized Corneal Ablation

Customized LASIK & PRK will dominate innext few yearsSpeedy recovery, good quality of visionsatisfactory outcomeDisadvantage of conventional refractive

surgery in some patients:* Increase in HOA* Reduction in visual quality

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Wavefront Customized Visual Correction

Ocular wavefront sensing:Will be increasingly employedWill become routine in visionassessment

Wavefront customization is employed tooptimize any refractive surgeryprocedure

Wavefront Customized Visual Correction(cont)

Future wavefront customized refractiveprocedures

Implantation of optimized IOL’s e.gTechnis aspheric lensCustomized IOL’s preinsersion,customized phakic IOL’s

Types of customizationTwo main methods are available in refractive surgery:

1. Topography guided ablation2. Wavefront guided ablation

According to recent statistics, approximately 55% of North American refractive surgeons have wavefront analyzers in their practice and routinely perform wavefront-guided ablations.

Definition of Wavefront

Wavefront-Guided (Customized) Excimer Laser Refractive Surgery

Definition: Wavefront-guided custom ablation is used to correct higher-order aberrations, in addition to spherocylindrical correction.

Incidence in General population

10-15% have significant higher-order aberration

Hard contact lenses

Corrects higher order aberrations resulting from the cornea.

How to measure the higher order optical aberration?

By using a Wavefront analysis system called aberrometry

Pupil and Wavefront

Larger diameter leads to a larger Wavefront error

Evaluation of vision qualityCompromised more at dim light during night and represented by:

a) Double visionb) Ghostingc) Glared) Halose) Starburstsd) Reduced contrast sensitivity

Quantification of magnitude of the aberration

1)Usually by RMS wavefront error

2)It gives a rough estimate

Types of aberrationsLow Order Aberration (LOA) : 1st & 2nd

order aberration

High Order Aberration (HOA) : 3rd to 6th or 10th

How to evaluate the quantity of optical system

• Root Mean Square ( RMS ) The RMS is SD of height (depth) of wavefront

relative to the reference at all the point in the wavefront. RMS is very useful measure of optical quality.

• Peak-To-Valley (PV) Distance from the highest to lowest point on the

deformed wavefront relative to the reference wavefront.

• RMS1 is qualitative presentation of 1st order aberration, RMS2 is due to 2nd order and etc.

• RMSh represents the total RMS of HOA (including 3rd to 6th OA)

• RMSg represents the total RMS of HOA and LOA• Total RMS increases with aging• LASIK increases RMS especially in subjects with

larger pupils• RMSh > 0.3 and > 20% of total RMS is clinically

significant and indicative for customized ablation

Wavefront measuring devices

Hartmann-shack style devices are the most common used

Hartmann-shack style

1)A narrow beam of light is projected on to the retina and the light reflected from the fovea passes through the lens and the cornea and exists the eye

Corneal topography guided ablation has been attempted on patients with regular and irregular astigmatism, decentered ablations, and central islands. The irregular astigmatism group is more challenging and may benefit most from corneal topography guided ablation as the systems become more refined.

Customization can be basedon corneal topography or wavefront

measurements.

Hartmann-shack aberrometerA low intensity laser beam is directed to retinal

surface

Light rays from this laser spot are reflected back to the front of the eye

Laser spot on retinal surface

light rays are reflected back to the front of the eye

Lens arrey focuses these

light rays

Photodetector (charged coupled device,CCD)

Dots of light

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Real eye with Aberrations

Considering refractive lens at corneal surface

Excimer etching by customized ablation

Converting wavefront to flat wavefront

Perfect focus on fovea

Ideal eye

10/14/2007 ٧

Zernike’s Polynomials

Zero order (no order)=axial symmetry, flatwavefrontFirst order = linear aberration, tilting around ahorizontal (x) or vertical axis (y)Second order = focus shift,

spherical defocus or astigmatismThird order = corresponding tocoma, triangular astigmatismFourth order = sphericalaberration, complex patternsFifth-10th order = Irregular aberrations,important when pupil is wide dilated.

XVth Congress of Iranian Society of OphthalmologyOctober 2005

Wavefront guided customization Goals

a) Spherocylinder correction

b) To correct the aberration that are induced by conventional laser vision correction and pre-existing aberration.

Requirment for wavefront

1) Efficient eye tracking

2) Good registeration system for eye

3) Small size laser spot

4) Sufficient corneal bed thickness

Challenges to wavefront

Tear film abnormalities

Miotic pupil : If severe may be difficult to measure and provide information beyond the 3mm optical zone and therefore require pharmacological dilation.

Optical effects of tear film disruption

Wavefront map with cycloplegic or mydriatic

1) Has significant difference in pre-op refractive error compared to subjective refraction

(Ablation profile algorithm)

1) After good aberration maps, Ablation profile algorithm is needed

2) Should be designed to make the post-op higher order aberrations as minimal as possible

3) Not only correcting the pre-op higher order aberration but also preventing the induction of new higher order aberrations.

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Conversion to Ablation Profile:Ablation profile is fundamentally inverse of wavefronterror mapGoal: Correction of refractive error and higher-orderaberrationsPupil diameter at least 0.5mm

larger than scotopicLimbal marking for cyclotorsion

detectionWavefront measurement + corneal curvature +biomechanics ablation profile complex

Emphasis on Registration

Registration refers to the method by which a customized ablation pattern based on preoperative wavefront analysis from the aberrometer is superimposed in the same position on the eye during the laser treatment.

Registration

Centration is established by the surgeon when the patient fixates on the target light. The instrument captures a video image of the eye, and the surgeon defines the pupillary ring (red) and limbus ring (yellow). These will be used by the LADARVision tracker to follow the eye during application of the customized ablation pattern.

LADARWave and LADARVision systems:

Torsional alignment refers to the need to compensate for the cyclotorsion of the eye that occurs when the patient lies down.

The eye usually rotates less than 5 degrees, but in some patients, postural cyclotorsion can be as much as 10 to 12 degrees.

Torsional alignment

Misalignment1) A small misalignment in in the axis can have

significant impact on the results of procedure

2) A 5-7 degrees of cyclotorsion when changing from sitting position to supine position. 50% of the visual benefit of HOA is lost.

Bausch and lomb zyolptix system has

1) Diagnostic2) Treatment components for wavefront

customized corneal ablation

Diagnostic part of zioptix systems (2172 laser)

In zy wave aberometer and ORBscan both machine are integrated in one workstation.

It has Hartmann-shack sensor that measures up to 5th order zernike term.

It measures sphere from +6.0 D to -12. D and 0 to 5 D of cylender with pupil diameter from 2.5 to 8.6 mm.

Pupil during exam

1) Orbscan must be performed with an undilatd pupil

2) Wavefront meaurements are taken with a dilated pupil.

Types of beam

Has a truncated gaussion 3 beam to achieve the benefits of both beam types.

1 2 3

Laser beam sizes in Tech217z

1) The 2mm beam treat the majority of the refractive errors.

2) The 1mm beam used for more specific ablation pattern on the transition zone

Eye tracker

The system has 120 Hz active eye tracker with a passive automatic shut-off system corrects for intraoperative eye movement in every dimention including cyclotortion and pupil shift

FDA investigation for outcomes for wavefront guided lasik in myopia for 3 separate laser platforms

1) LADAR vision2) Technolus 217 z for low to moderate

myopia3) visx

Conventional Wave front guided

72% ………….20/20 89% ……..20/20

Environmental ConditionsControl of environmental conditions during treatments is

important. In previous U.S. FDA Multi-Center Clinical Trials, the room conditions were:Temperature ranged from 68ºF to 72ºF (20ºC to 22.2ºC)Relative humidity ranged from 40% to 45%Treatments performed at >24ºC were associated with

less accurate outcomesStability of temperature and humidity is important

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Future of Customized CornealAblation

LASIK vs surface ablation

Each microkeratome induces specific “flaponly” aberration(flap size, thickness)Considering “flap aberrations” in totaltreatment calculations

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Future of Customized Corneal Ablation… cont.

Surface Ablations:Show promising results with use ofimmunomodulating agentsBetter control of cellular and biochemicalreactions

Introduction of new drugs to better regulatewound healing and refractive outcomeGene therapy for better control of post laserkeratocyte activation and wound healing

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Multifocal AblationPresbyopia: Customized multifocal ablationAberrations may be induced when creatingmultifocalityPotential loss of contrast sensitivity and qualityof visual functionFuture results will be improved:* Wavefront mapping, sophisticated eye

trackers and corneal registration* Preoperative simulation of postop condition

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Laser Delivery Refinements

Correction of HOA needssmaller spot delivery

>5th order requires 0.6-0.8mm spot sizeSmaller spot size needs faster and better eyetrackers

Smaller ablation depth per pulse providesideal correction profile for higher orders

Courtesy of IBM

The Human Hair (125 microns)Etched with an excimer laser

Thermal Non-thermal

ExcimerCO2YAG

Effects of Laser Ablation

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Conclusion

Wavefront measurement devices andconsequently wavefront correctionprocedures are still in process ofevolution

Achievement of “supervision”: withadvancement in current procedureswill not be a dream in near future

Thanks for your attention