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,……