The Evolution of the Optical Zonein Corneal Refractive Surgery.

Bruce Drum, Ph.D.FDA, Division of Ophthalmic and ENT

Devices, Rockville, MD

Disclaimer

This presentation represents theprofessional opinion of the author and isnot an official document, guidance orpolicy of the U.S. Government, theDepartment of Health and HumanServices, or the Food and DrugAdministration, nor should any officialendorsement be inferred.

Outline

• Theoretical and Operational Definitions• PRK with Early Large-Beam Lasers• Small-Beam Scanning Lasers• PRK vs. LASIK• Wavefront-Guided Treatments• Optimizing Residual Aberrations• Future Developments

What is the Optical Zone?• Theoretical definition: the part of the

corneal ablation area that receives the fullintended refractive correction.

• Operational definition: the part of thecorneal ablation area that receives the treatmentthat is designed to produce the full intendedrefractive correction.

• Effective Optical Zone: the part of thecorneal ablation area that actually conforms tothe theoretical definition.

Early PRK AlgorithmsPRK Ablation for Myopia Munnerlyn Formula

for myopia correction

• Assumed sphericalcorneal surface

• Assumed uniform etchrate

• Ablation target wasspherical surface oflower curvature

• Optical zone assumedequal to ablation zoneEndothelium

Optical Zone

Epithelium

Bowman’sMembrane

TargetSurface

UV Laser Beam

Stroma

Problems with the SimpleSpherical Model

• Epithelial Remodeling• Central Islands• Beam Inhomogeneity• Radial Ablation Efficiency Function• Biomechanical Effects• Induced Spherical Aberration

Epithelial RemodelingEpithelial Regrowth afterPRK for Myopia

• Curvature discontinuity atablation edge promotesepithelial remodeling.

• Remodeling extends intointended optical zone.

• Remodeling induces sphericalaberration, reduces effectiveoptical zone size.

• Attempted solution:Ablate an annular “transition”or “blend” zone at the edge ofthe optical zone to minimizecurvature discontinuity.

• Transition zones reduce butdo not eliminate remodeling.

NominalOptical Zone

RemodeledEpithelium

EffectiveOpticalZone

Central Island Profile

NominalOptical Zone

Central Islands• Causes:

– Fluid pools on centralsurface, interferes withablation.

– Debris plume interferes withcentral ablation.

• Solutions:– Extra central pulses.– Pause treatment, sop up

fluid with sponge.– Use small-beam scanning

laser.– Remove debris plume

before each pulse.

Central Island

TransitionZone

EffectiveOptical Zone

EpithelialRemodeling

Beam Inhomogeneity• Early large-beam lasers used complicated

optical schemes to make the laser beamuniformly intense over its entire area. Theseschemes were not always successful.

• Beam inhomogeneities, particularly dimmingtoward the beam perimeter, contribute tounderablation near the edge of the optical zone.

• Solution: Switching to small-beam scanningalgorithms effectively solves the problem.

Radial Ablation Efficiency Loss Effects of oblique

beam incidence• Larger beam area,

lower fluence.

• Higher reflectance,lower fluence.

• Dim edges of beamfall below ablationthreshold, effectivebeam size shrinks.

• Ablation calibrationsdone on flat plasticdon’t show theseeffects.

NominalOptical Zone

_

F0 F_

F_ = F0 cos(_)

Biomechanical Effects• Ablation of stromal tissue releases tension

on the cut stromal fibers, which then retractfrom the corneal center and thicken thestromal layer toward the edge of the opticalzone.

• These changes flatten the center of thenominal optical zone and steepen the edges,contributing to an oblate contour, increasingspherical aberration and shrinking theeffective optical zone.

Induced Spherical AberrationEffect of True Spherical Ablation • The Munnerlyn formula for spherical

myopic ablation should make thecornea more prolate and reducepositive spherical aberration.

• The actual result is a more oblatecornea with increased sphericalaberration.

• Conclusion: the theoreticalreduction in spherical aberrationdue to the Munnerlyn equation isrelatively minor, overwhelmed byopposing effects of healing,biomechanical changes andablation efficiency losses.

AfterAblation

BeforeAblation

The Effective Optical ZoneMyopia Treatments

• Epithelial remodeling, radial ablation efficiencylosses and biomechanical effects all reduce theeffective ablation in the outer portion of the nominaloptical zone.

• These effects shrink the actual zone of fullrefractive correction, i.e., the effective optical zone.

• They also distort attempted cylindrical ablations byflattening the cornea along the astigmatic axis,introducing an unintended spherical correctioncomponent and reducing the cylindrical correction.

The Effective Optical ZoneHyperopia Treatments

Undercorrections caused bymultiple mechanisms:• Epithelial remodeling fills in

annular ablation, inducesnegative spherical aberration.

• Cornea bulges out at diameterof maximum ablation depth.

• Cut fibers retract toward thecenter, thickening the stromaand flattening the central zone.

• Low efficiency reducesmaximum ablation depth.

• All changes shrink the effectiveoptical zone.

NominalOptical Zone

StromalThickening

EpithelialRemodeling

Bulging

EffectiveOpticalZone

Small Beam Scanning Lasers• Scanning lasers have largely replaced large

beam lasers that relied on variable apertures toshape the ablation.

• Scanning beams provide smoother, moreprecise ablations, and allow elimination ofcentral islands.

• Scanned ablations are also more flexible,making it easy to compensate for peripheralunderablation by adding extra pulses to theaffected regions.

LASIK vs. PRK

The use of LASIK instead of PRK has littleeffect on most of the factors that reduceeffective peripheral ablation depth:• Epithelial remodeling is reduced but not

eliminated.• Biomechanical effects tend to be worse

because the flap results in additional stromalweakening.

Wavefront-Guided Treatments• Recent developments in aberrometry allow us to

incorporate the measurement of higher orderaberrations into corneal refractive surgery.

• Small beam scanning lasers are able, in principle,to correct higher order aberrations.

• Successful wavefront-guided treatments requirealgorithms that compensate for ablation artifactsand anticipate healing and biomechanical factors.

• Operational definitions of wavefront-guided opticalzones should include explicit control of higherorder aberrations within the zone.

Optimizing Residual Aberrations• Eliminating all higher-order aberrations is

probably not the best strategy to optimize visualfunction. For example, some controlledaberrations can improve depth of focus withminimal degradation of image quality.

• The brain can adapt to long-term aberrationpatterns. In some cases, removing aberrationscan therefore impair visual function.

• An operational definition of the optimal opticalzone can specify aberrations to be preserved aswell as those to be removed.

Future Developments• We have come a long way since the early days of PRK toward

understanding the ablation artifacts, healing factors andbiomechanical effects that need to be taken into account to makethe effective optical zone agree with the theoretical one.

• Remaining challenges:– Develop accurate calibration techniques to assure that the

achieved corneal ablation depth map equals the one intended;– predict optimal aberration patterns and correction parameters in

individual eyes; and– characterize refractive variability in individual eyes to establish

fundamental limitations in defining the optical zone.• In the past, operational definitions of the optical zone typically have

been validated by test ablations on flat plastic. In the future, thegoal should be to validate them by direct wavefront measurementsof the cornea.

Conclusions• We have made a lot of progress toward making

corneal refractive surgery safe and effective, butthere is still more room for improvement.

• The rapid development of corneal refractivesurgery is an excellent example of how thecollaborative efforts of manufacturers, scientists,clinicians, and FDA reviewers can lead to majorimprovements in the safety and effectiveness ofmedical devices.

Corneal Refractive Surgery• PRK

– Remove Epithelium– Ablate with Excimer Laser– Epithelium Regrows

• LASIK– Cut 130-180 micron Flap– Ablate with Excimer Laser– Replace Flap

PRK

LASIK

FDA Perspective

• FDA’s mission is to regulate medicalproducts for safety and effectiveness.

• The goal of efforts to improve ablationalgorithms is to maximize the safety andeffectiveness of corneal refractive surgeryto the patient.