Aberrometry and the Tear Film— Understanding new methods —

Thomas O. Salmon, OD, PhDNortheastern State University, Oklahoma, USA

Northeastern State University

Today's lecture - overview

A. Dry eye introduction

B. Aberrometry basics

C. Aberrometry and dry eye research

A. Introduction - Dry eye (DE)

• Prevalence ~15-30%• Quality of life (QOL)• More common among– Women– Elderly– Contact lenses wearers– Refractive surgery patients– Computer users– With some medications

Diagnosis of DE

• Traditional clinical tests– Schirmer test– Tear break-up time– Corneal staining– Slit lamp evaluation

• Subjective surveys– Patient complaints– Questionnaires

Diagnostic dilemma

• DE test don’t agree• Objective signs ≠ subjective

symptoms• Need for better tests• New technologies– Corneal topography– Tear osmolarity– Aberrometry

OSDI

TearLab

B. Aberrometry basics

1. What are aberrations?

2. What do aberrometers measure?

3. How do we interpret aberrometer data?

1. What are aberrations?

• Aberrations = refractive errors• Lower order aberrations– Sphere (myopia, hyperopia)– Astigmatism

• Other refractive errors, they are the …• Higher order (HO) aberrations– Coma, trefoil, spherical aberration, …

These aberrations are …

not … but rather …

chromatic aberrations monochromatic aberrations

Seidel aberrations• coma• spherical aberration• oblique astigmatism• field curvature• Petzval

Zernike aberrations• coma• trefoil• spherical aberration• Z(4,-4), Z(4,-2) …•Others

Summary 1

B. Aberrometry basics1. What are aberrations?• Aberrations = refractive errors• Lower order = sphere & astigmatism• Higher order = other more complex abs• Monochromatic aberrations• Pupil size = critical parameter!

2. What do aberrometers measure?

• Refractive errors• Similar to autorefractors• Sphere, astigmatism, … &• Higher order aberrations

COAS

Aberrometry ≠ corneal topography

Autorefraction ≠ keratometry

WavefrontWhole eye opticsHD autorefraction

Corneal topographyCorneal surface shapeHD keratometry

What do aberrometers measure?

• Optical wavefronts exciting the eye• Single pass through all the eye's optics• Perfect wavefront = flat

In an aberrated eye …

• refractive errors distort the wavefronts.• Analyze wavefront shape to learn which

refractive errors caused the distortion.

myopia

Video courtesy of Alcon

Color maps, surface plots

Total & higher order maps

Total aberrations Higher order aberrations

A wealth of information

• Metrics of optical quality– MTF– PSF– Strehl ratio …

• Estimate visual performance• Simulate vision• Design optical corrections

MTF

Simulated vision

Simulated retinal images

Emmetropia Myopia + astigmatism

Summary 2

B. Aberrometry basics1. What are aberrations?2. What do aberrometers measure?• Wavefronts that have passed through

the eye's optics.• Distortions caused by refractive errors• Lower and higher order aberrations • Much other information

3. How to interpret the wavefront?

Each refractive error (aberration) causes one particular wavefront shape.

sphere astigmatism trefoil

Zernike analysis

Common higher order aberrations

Oblique trefoil Horizontal trefoilVertical coma Horizontal coma

Spherical aberration

Z(3,-3) Z(3,-1) Z(3,1) Z(3,3)

Z(4,0)

- 0 +

€

RMS = Z12 + ...+ Zn

2( )

Sample COAS printout

COAS

RMS wavefront error

• A useful summary statistic• Magnitude of combined Zernike modes• Can be used for any combined aberrations• Examples– Vertical + horizontal coma = total coma RMS– Third order RMS– Higher order RMS– Total RMS

€

RMS = Z12 + ...+ Zn

2( )

€

RMS = Z12 + ...+ Zn

2( )

Reference normsJournal of Cataract and

Refractive Surgery, December 2006.

Downloadable dataKeyword search: “Zernike

norms”

Summary 3

B. Aberrometry basics1. What are aberrations?2. What do aberrometers measure?3. How do we interpret aberrometer data?• Zernike coefficient for each aberration• ± µm, … specify pupil size!• RMS wavefront error – combined aberrations• Diagnose by comparing data to norms

C. Aberrometry in DE research

• Tear film - many functions• Nutrition, wetting, comfort• The eye's primary refracting surface• DE -> tear film ∆s -> optical effects• Example: DE -> blurred vision• Aberrometry – can be used to evaluate DE

Serial aberrometry

• Tear film - constantly changing over time• Evaporation, blinking, gravity, eye

movement, etc.• Important to measure changes as a

function of time• Koh & Maeda's method: • Serial aberrometry

Tear film aberrometry research

1994 Liang Aberrometry of the human eye JOSAA

1999 Thibos, Hong Tear film break up OVS

2006 Koh, Maeda Normal tears film over time IOVS

2008 Koh, Maeda Dry eye changes over time IOVSCornea

2008 Koh, Maeda Contact lens wetting ECL

2009 Burger, Head Artificial tears CEO

Pioneers in dry-eye aberromery

Dr. Junzhong Liang Dr. Larry Thibos Dr. Shizuka Koh

Blink Blink

1 2 3 4 5 6 7 8 9(sec)

Etafilcon A with PVP

Etafilcon A

Koh S. Effect of Internal Lubricating Agents of Disposable Soft Contact Lenses ... Contact Lenses. Eye Cont Lens 2008;34:100-5.

Contact lens wetting (Koh, et al.)

Serial aberrometry (Koh's method)A. Every 1 sec, for 60 sec, blink every 10 secB. For each eye, ~60 measurements in 1 minute

1. ~60 HO RMS values2. Plot HO RMS as a function of time3. Fluctuation index (FI) & Stability index (SI)

RMS

60 measurements

HO RMS ∆ over time

SI

FI

1 2 3 4 5 6 7 8 9 ->

blink

blink

blink

-> 51 52 53 54 55 56 57 58 59

Change HO RMS with time

Fluctuation Index (FI), Stability index (SI)

Summary 4• Need for better diagnostic tests for dry eye• Dry eye -> tear film -> changes in the eye's optics• Serial aberrometry - measures changes over time• Applications– Normal tear dynamics– Dry eye diagnosis– Contact lens wetting– Artificial tears– Others

ご清聴ありがとうございました