Wave front Aberrometry

Hamid reza Jahadi Hosseini Professor of ophthalmology shiraz university of medical sciences Feb. 2014

Few decades ago , we considered the Eye as a simple optical system. Just learned the myopia , hyperopia

,astigmatism and correct them with sphere and cylinder.

Most of the cases were happy. But still some of the pt could not be

corrected to the 20/20 !!! WHY ??

Few years later we started to do conventional LASIK.

Most of the cases were satisfied post op with UCVA = 20/20

But few of them were Un happy 20/20 cases !!!

Complaining of halo, double vision and night vision disturbance

WHY ???

The Answer is

ABERROPIA

Astronomists had encountered similar problems when looking at the galaxy through their

telescope,the aberrations leads to blur images of the stars

1970 Dr Bille /Heidleberg/germany Used deformable mirrors ( adaptive Optics) In the telescope to neutralize the aberration And produce sharp images.

wikipedia

Aberration is something that deviates from the normal way ( اعوجاج )

Optical aberration is an imperfection in image formation by

the optical system

Optical aberrations can be measured by 2 different methods

1) only analyzing the aberration 2) measuring effect of aberration on

image in 1) we check the Optical quality of the

system , while In 2) we check the image quality

Image quality is defined by Strehl Ratio It is the ratio of Maximum intensity of

actual image/to the maximum intensity of fully diffraction limited image

Strehl Ratio = 1- ( 2 π RMS ) Pi (π)= 3.14

Optical quality of the eye is measured by the

aberrometers. & is reported as RMS(Root Mean Square )

Another way of looking into image quality is by

POINT SPREAD FUNCTION (PSF)

And MODULAION TRANSFER FACTOR ( MTF)

MODULATION TRANSFER FACTOR

is a measure of system ability to transfer contrast from the object plane to the image plane at a specific

Resolution

PSF POINT SPREAD FUNCTION

Is measure of how well a single object point ,makes an image point on the retina after passing through the optical system of the eye

WAVEFRONT جبهه موج

Defocus ( Myopia)

wavefront

physics of Aberrometers 1) Out going wavefront Aberrometry

2) In going wavefront Aberrometry

Out going wavefront Aberrometry Is Hartmann – shack These are the most popular type of Aberrometers used

100 miliscond ingoing diode laser turns on

ZYOPTIX is a Hartmann – shack type

Technisian first measure the WF at dim light un dilated pupil and then several times with dilated pupil at Dark room or dilatedwith non cycloplegic mydriatics.

And select the WF with PPR which is in better agreement with subjective refraction reported by surgeon

With dilated pupil aberrometer can detect more amount of aberrations of globe

Shortcoming of hartmann shack systems

In highly aberrated eyes( HOA>1.5) because

of intererence of multiple beams which are outgoing in diferrent direction, it fails. So other systems like Tracy which uses single Ray seems superior

2) In going wavefront Aberrometry A- Retinal Image Aberrometer Tscherning Aberrometer Retinal Ray tracing (Tracy) B- Double –pass Aberrometer C – Ingoing subjective adjustable

Aberrometry

TSCERNING ABERROMETER Uses ingoing grid rays

WAVELIGHT is a Tscherning Aberrometer

TRACY is an ingoing Aberrometer based on Retinal RAY TRACING

It uses at each time a single ingoing ray and Traces it to retina &

checks how its image formed On the retina .then emits another ingoing ray and repeat

TRACY Can measure WF of the entire eye = Global WF WF of Cornea alone WF of internal optics of the eye So can easily show abnormal WF is

origionating from which part of the eye?

NIDEK OPD SCAN

OPTICAL PATH Difference to Emmetropia

Is a scanning slit autorefractometer using skioscopy technology

Also has placido disc for topography

Actually is a hybrid system

AUTO, TOPO and WF Aberrometer Uses 1440 data points

NIDEK OPD SCAN

Can measure the largest range of Sphere ( -20D to +22D)

and cylinder up to 12D

NIDEK OPD SCAN

Finally these abnormal wave front detected by each of these systems should be calculated.

Diferrent formula can be used Taylor,…..

But the most popular is ZERNIKE Polynomial formula

Which can measure up to 10th order HOA And 35 coefficients

It measures the square root of sum of square of all deviated distance from ideal point

And reports it as RMS Is reveals the TOTAL aberration,including both low-order and higher –order

aberration

Then zernike has the ability to decompose the wavefront into its components.

ie. separareLower order and higher orders

And break the HOA into its different types

Majority of aberromtery machines usually report the GLOBAL HOA ie HOA which arise from the entire globe. Tear film, cornea, lens, even the retina.

Some aberrometers have the ability to measure the Corneal HOA or Lenticular HOA separately too, like TRACY

The computer makes these calculations and converts it into different maps and displays which can be interpreted easily by clinicians.

HOA can be displayed by different maps eg 2D map , 3D maps, normal band

display , Refractive map,………..

HOA can be symmertical like spherical aberration or asymmetric like coma.

different HOA ,s do not have equal effect on image degradation. Some have more effect. Coma is one of the worst types.

In coma , the image of a point is comet shape.

It is more sharp at one edge than the other edge.

It causes symptoms like bluring of vision, monocular diplopia, and glare

Coma occurs in human eye by 2 main mechanism

1) pupil decenteration 2) when optical components are not

coaxial like decentered lens,IOL, decentered ablation, Keratoconus, decentered PK

Spherical Aberration can arise from normal crystalline

lens,when pupil dilate at dark ( night myopia) or from high power conventional myopic ablation.

It causes night glare and halo and myopia

ZERNIKE PYRAMID

NORMAL BAND VIEW

Zero order is an ideal perfect optical system which has no aberration and is called Piston

Lower order aberration include first order and second order aberrations which can be measured by retinoscopy or autorefractometer too,

First order includes Tilt ( Prism)

Second order includes DEFOCUS (Myopia , Hyperopia ) and Astigmatism

3rd order and above are called Higher order aberration 3rd order includes coma , Trefoil 4th order includes spherical Aberration ,

quadrefold 5th order includes pentafold,secondary

coma, secondary trefoil

Z200 Defocus Z220/Z221 stigmatism Z300 coma Z330/Z331 trefoil Z400 Spherical aberration Z420 Z421 secondary astigmatism Z440/Z441 Quadrafoil Z510 , Z511 secondry coma Z 530, Z531 Secondary Trefoil Z550 Z551 Perntafold

can the tear film cause HOA ??

Yes Dry eye , tear debris,..

Wavefront technology has enabled we physicians to detect different types of HOA and quantify them

Correlate them with patients symptom.

Make clinical decision when to treat them by WF guided ablation.

or avoid iathrogenic HOA by doing Aspheric ablations, or using Aspheric

IOL. and Well centered ablation,……