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Does Size Matter?Does Size Matter?
Arthur Bradley, PhD,
Pete Kollbaum, OD, Larry Thibos, Ph.D., Xu Cheng,Ph.D.
Indiana University School of Optometry
Susana Marcos, Ph.D.,Instituto de Optica, Madrid, Spain
The impact of ablation zone and pupil size
Typical Soft CL: Lens diameter 14 mm
Typical Soft CL Optical Zone 9-11 mm
Spectacle Lens isalways much
larger than pupil
Most Spectacle and Contact Lens corrections employ opticalcorrections larger than the pupil, thus ensuring full correctionof the entire wavefront that creates the retinal image.
Newer refractive optical corrections, however, employ opticalzones that may be smaller than the pupil, and thus onlycorrect part of the wavefront creating the retinal image.
Phakic IOLs
Artisan anteriorchamber IOL5 or 6 mm OZ
NuVita anteriorchamber IOL
5mm OZ
PhotoablativeRefractive Surgery
Visian ICLposterior chamber4.65-5.5 OZ
Current AZ sizes rangefrom 5.5 - 7 mm
What are the consequences of pupils that arelarger than the optical correction?
Does size really matter? Surely it must!
Should a patient with a > 8 mmdiameter pupil choose RS?
Concern about pupil size and refractive surgery: an unresolved issue
1990: Applegate and Gansel, “The importance of pupil size in optical qualitymeasurements following RK”1996: Pop “The complicated laser -Large pupils cause a devilish problem:halos” (LASIK) Claimed that patients with >8mm pupils are 100% dissatisfied.1998: Nixon (patient letter to JCRS), 9mm pupils, 5.5 mm LASIK suffers lossof contrast, halos, star bursts that make it impossible to drive at night.1998: Davidorf “Measurement of pupil size may be the most frequentlyneglected facet of the refractive surgery evaluation…Clearly, a patient withpupils that dilate to 8 or 9 mm in low light is likely to suffer from visualaberrations following refractive surgery”2003: Freedman et al, identifies and “empirical rule” that “It is generallyaccepted that the transition zone between ablated and unablated cornea must lieoutside the measured pupil size by 0.5 to 1.0 mm to minimize…..halos, glare,ghosting and desaturation”.2004: Pop and Payette, “Pupil size at any month postoperatively was notstatistically predictive of postoperative NVC”2004: Klyce, in editorial following Pop’s article “The pupil proclaimsinnocense”
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Example of pre- and post-LASIK wavefronts(Marcos IOVS 2001), 6 mm AZ, 9mm transition.
Myope
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3LASIK
50m mrange
6m mrange
HO“corr”
HO&LO“uncorr”
PRE POST
RMS0.8 mm
RMS2.7 mm
6.5 mm pupil
PRE POST
Sample Eye
RMS12.5 mm
RMS5.7 mm
Increased HO aberrations in post-LASIK eye are exaggerated with increasing pupil size.Unknown for very large pupils.
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Treatment Zone
Measured Aberrations
Note that this anexample of poorpost-op correctedoptics. Largestlevel of HOaberrations inMarcos study
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Post
HO RMS analysis of same eye as a function of pupil sizeR
MS
mm
Size of large pupils
?
Deq=1.7D
How large are pupils at low light levels?
=low mesopic
Winn et al, 1994, who tookgreat care to remove othercues which might result inpupil constriction.
Periman et al,2003 usingColvardpupillometerreplicate Winnet al.
Mean=7 mm inyoung adults,significantproportionhave >8 mmpupils
FOZ<Ablation Zone
AZ=6mm
Partal and Manche, 2003
AZ=6.5, Effective TZ=5.5
Pre-op Rx -5.25DTZ5.5
AZ6.5
Pupil8 mm
In this example, <50% of areaof night-time pupil is treated
Boxer et al 2002
Successful treatment zone (TZ) generallyappears smaller than planned ablationzone (AZ)
How large is the ablation zone?
Increased aberrations in post-RS central pupil definitelyassociated with decreased visual function.
But what of highly aberrated periphery?
Marcos, 2001, 3 mm pupil
Post RS Aberrations and VisionP
ost
/pre
Oshika et al, 2002, post-LASIK eyes who lost > 2lines of BSCVA had moreinduced SA and coma incentral 3 mm than eyesthat did not lose BSCVA.
Examples
E.g. Atchison et al 1979: Effect of pupil size on VA in normal eyes
MAR
Pupil Diameter (mm)1 2 3 4 5 6 7 8
1
2
2-3 mm pupil is bestAberrationeffect
Diffractioneffect
Does increased pupil size, which should universally lead toincreased aberrations, also lead to decreased vision?
Approach #1: vary pupil size and measure impact on vision
Holladay et al, 1999, post-LASIK loss in BSCVA
Montes-Mico and Charman, 2002
Emmetropic control
Post PRK
12 c/deg
Decrease light level
Conclusion #1: Increased pupil sizesassociated with low light levels reducedvisual function in post RS eyespresumably to the recruitment of thehighly aberrated marginal optics.
Surrogate test of pupil size effect is examine the impact of light level.
Approach #2: Look for correlations between patient pupilsize and and post-RS vision problems.Note that this approach lacks an internal control, requires adequaterange of pupil sizes, clincial relevance requires inclusion of eyeswith large pupils, need to have measurement technique thatsimulates pupil sizes experienced during night driving.
2001 Haw and Manche, pre-op pupil size did correlate with anypost-PRK visual symptoms or measured visual performance.
2003 Lee et al, Surveyed LASIK patients and reportedsymptoms of glare and halos did not significantly correlatedwith pupil size.
2004 Pop and Payette find that pupil size is not a risk factor forNVC after LASIK.
Conclusion #2: Increased pupil sizes associated with low lightlevels did not reduced visual function in post RS eyes.
Without data, resort to modeling refractive correctionsthat are smaller than the pupil.
Develop a theoretical model of eye that is emmetropic in thecentral region of the pupil, but still myopic in the outeruncorrected region of the pupil (a bifocal eye). Analogous toCharman and Walsh’s (1986) bifocal model, originally developedto examine retinal images in eyes with bifocal SVCL.
Data Summary:Mesopic/Scotopic pupils measured up to 9 mm, with a mean ofabout 7 mm for most young adult eyes.Can find no published aberration data for pupils larger than 7 mm.We know little about the peripheral optics and vision of post RSpatients with larger than normal pupils.Large pupil are/are not associated with vision problems.Patients with large pupils should/should not have RS.
Charman and Walsh (1986) Model
B=TZ x Rx=70 arc min
B=P x RxE.g. 110 min
plano
myopic
Simulated PSF For a -4 D Myope with 8 mmpupil, 5 mm TZ
Halo?
Lackner et al 2003, using same argument as Charmanand Walsh proposed that light outside of TZ causesHalos experienced by LASIK patients.
Charman et al, 2002 a, b argue that small TZ in RS cause problemsfor peripheral vision due to increases likelihood of rays entering thepupil that do not pass through the TZ.
Dramatic increase in in mid-periphery RMS, and accompanyingreduction in visual sensitivity
TZ
Simulated PSF
TZ=6mm
pre
post
Experimental Evidence to support/reject this bifocal model of post-RS eye
1. Fan Paul et al 2002:Patient description of PSF
Note that no abruptappearance of Halos for P>TZ
“”Halos may be attributed topupil a diameter that is greaterthan the optical zone diameter”BUT observed halos not thesame as bifocal modelpredictions!
Hal
o s
core
2. Schallhorn et al, 2003, 1 month after LASIK
TZ
3. Lackner et al, 2003:Glare and halo phenomena afterLASIK. “only patients whose measured pupil diameter wassmaller than the ablation zone were included.”
Hal
o s
ize
in s
qu
are
deg
rees
Although the model is compelling, the experimental data do notsupport a bifocal model as a cause of NVC following RS.
WF from -3D Myope, 8.4 mm Same eye with 2ndorder fully corrected
Myopic peripheral optics Emmetropiccentral optics
+ =
Simulated bifocalpost-RS eye
Wavefront Version of Bifocal Model of post-RS eye
Wavefront Version of Bifocal Model of post-RS eye
Can we represent this bifocal model withZernike Polynomials (10th order)
Cross-sections through Myopicand Emmetropic wavefronts
Myope
Pupil diameter = 8.4 mm-5051015202530354045
Pupil diameter = 8.4 mm
Zernike Polynomials do not adequately fit bifocal model
Challenge #1
Emmetrope
Vis
ual s
ensi
tivity
Challenge #2: Due to waveguide optics of photoreceptors, humanpupil behaves as though it contains an apodizing function
cone
Axial ray
Marginalray
Apodization filter
h x( ) = hmax ¥10-r x-xmax( )2
Impact of Apodization on bifocal model PSF
PS
F i
nte
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w/o SCE
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For example, the visualintensity of the halogenerated by the rayspassing through theedge of an 8 mm myopicpupil will be attenuatedby approximately 20%
Apodization will reduce thevisual impact of highlyaberrated rays passingthrough the uncorrectedmargins of a dilated pupil.
BUT: as emphasized byAtchison, pupil apodization isabsent under scotopicconditions.
Challenge #3 Can we predict NVC from aberrometry data?
Cheng, et al, 2004:Experimentally introducecontrolled levels of aberrationsinto the retinal image andmeasure the impact on VA.
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PF
RMS which weights each part ofthe pupil equally was one of theworst predictors of VA loss due toelevated aberrations
Marechal
PF = HiQ areaPupil area
Image Quality Metrics that predictvisual impact of HO aberrations aregenerally unaffected by the highlyaberrated rays passing through themargins of a large pupil.
VisuallyweightedStrehlRatio
The highly aberrated marginal rays are notsignificant contributors to the PF or SR evenbefore RS, and therefore increasing theiraberrations is unlikely to have much impact on thecentral spike in the PSF.
Summary:
Understanding the visual impact of marginal rays entering thepost-RS eyes with pupils larger than the TZ is largely unknown.We lack the empirical data and an accurate model.
We need to examine the aberrations post-RS eyes over ( mmpupils. We need to systematically examine the visual impact ofpupil size.
Such data will lead to improved recommendations to patients withlarger pupils, and will also lead to blend strategies that arefunctionally optimized.
Such data will also help resolve the intriguing question of whyNVC seem to disappear in many post-RS patients (e.g Schallhornet al, 2003, Lackner et al 2003). Is it neural adaptation (Webster etal, 2002; Artal et al, 2003), pupil adaptation (Woodhouse, 1975) orstructural/optical adaptation (Klyce, 2004)?
Sample data fromliterature showingeffect of pupil size inpost RS eyes.
Failed to find data beyond 7 mm,and most for smaller pupils.
Miller et al, 2002
Seiler et al, 2000
6+mm AZ
Pre
PostPRK
LASIK