IN THE NAME OF GOD

Optical Considerations for IOLs

1.Image magnification

2. Power selection

3.Piggy back IOls

4.Power after cornal refractive

surgery

A number of factors must be taken into

consideration when selecting

IOLs,including :

Image Magnification

Theoretically , replacing a clear or cataractous crystalline lense with an artificial lens is optimal form of aphakic correction.The majority of aberrations and distortions produced by aphakic spectacles derive from their placement anterior to pupillary plane.These include: Image magnification,Ring scotoma,Peripheral distortion,jack-in-the box phenomen,decrease useful peripheral field.

Image magnification – as much as 20% -

35% - is the major disadvantage of aphakic

spectacles.Contact lens correction of

aphakia magnifies the image 7%-12% ,IOLs

magnify by 4% or less.Naturally ,a lens

located in the posterior chamber produces

less image magnification than a lens in the

anterior chamber.Unilateral pseudophakic

patients have better stereoacuity and less

aniseikonia than aphakic patients whose

vision is corrected with contact lens or

spectacle.

IOL Power Selection

Nontoric IOLs provide spherical

correction of the refractive error

without any cylindrical component to

this spherical correction should be

calculated for each eye as accurately

as possible.The optimal postoperative

refraction depends on the situation

and visual needs of the patient.

For example, if a unilateral cataract is

present and the fellow eye is more

than 1.5 – 2 D hyperopic , the surgeon

can consider making the operated eye

slightly hyperopic as well.This strategy

avoid inducing aniseikonia and

anisophoria.In most settings , however

it is desirable to produce emmetropia

or slight myopia.

Biometric Assumption In IOL Selection

1.Axial length :

a) A scan

ultrasonography

b) Partial coherence

interferometry

2.Kerotometry or corneal topography

3.Anterior chamber depth or optical

chamber depth(OCD)

Ascan do not actually measure axial

length.They measure the time required

for a sound pulse to travel through the

ocular media , reflect from the retina ,

and return through the media.Sound

moves faster through the crystalline

lens than through the cornea , aqueous

and vitreous.Even with in the lens itself

, the speed of sound can varry

according to the hardness of the

cataract.

Ascan tends to understimate the axial lens

of short eyes

and overstimate long eyes. 1 mm error in

the measurement of axial length result in a

refractive error of approximately 2.5 – 3 D .

The two primary Ascan techniques –

applanation (contact) and immersion – give

different readings.Applanation method may

give a shorter axial length measurement

perhaps due to corneal indentation.

Partial coherence interferometry

measures the time required for infrared

light to travel to the retina . This

technique does not require contact with

the cornea , so corneal compression

artifacts are eliminated . In addition the

patient must fixate a target thus , the

length measured is the path the light

takes to the fovea , the physiological axial

length .

However , the media must be clear

enough to allow fixation and light

transmission , in dense cataract ,

ultrasound axial length measurements

may still be necessary . Partial

coherence interferometry technique is

probably more accurate and

reproducible than ultrasound method ,

although some cases will still require

ultrasound biometry .

Keratometry

Keratometry or corneal topography

does not measure corneal power

directly , keratometry measures

only a small portion of central

cornea , viewing the cornea as a

convex mirror. Both front and back

corneal surfaces contribute to

corneal power and a keratometer

measures only the front surface.

Anterior Chamber Depth

Formulas based on geometrical

optics generally require a third

parameter , anterior chamber

depth or optical chamber depth

(OCD).

Power Prediction Formulas

The first type of IOL formula based on geometrical optics:PIOL = AL - acd

nvit

1 – [ K(acd) / naq ]

K

PIOL = Power of the IOL

K = Dioptric

AL = Axial length

acd = anterior chamber depth

nvit = index of refraction of the vitreaus

naq = index of refraction of the aqueous

In the 1980 , Sanders , Retzlaff and Kraff

took a different approach.formula turned

out to be a simple linear equation , which

was introduced as the SRK formula.

P = A – (2.5 * axial length in mm) – ( 0.9 * average keratometry in diopter )

A constant , which is provided by

manufacturers for their lens ,is specific to

each lens type .

Factors related to A constant: 1.lense

position in the eye

2.haptic

angulation

3.lens

shape

This formula was less accurate for long or short eyes. The power was too low in short eyes and too high in longer eyes . the formula was later modified as the SRKⅡ formula : Axial length (AL) Modified A

constant

20 > AL A = A +

3

21 > AL ≥ 20 A = A +

2

22 > AL ≥ 21 A = A + 1

24.5> AL ≥ 22 A = A (No

change)

AL ≥ 24.5 A = A −

0.5

Second generation formulas added

modifications based on AL for short or

long.

Third and fourth generation formulas

added modifications for other factors

such as corneal curvature , ACD , and

so on .

These formulas are too complex for

convenient hand calculation.

In the normal range of axial length (22

– 24.5 mm) almost all formulas function

does not have discrepancies.

– But in medium long eye (24.5 – 26

mm) ,the Holladay 1 formula is the

most accurate.

– And is very long eyes ( > 26 mm) , the

SRK/T is more accurate.

– In short eyes ( < 22 mm ) the Hoffer Q

formula is more accurate.

– Holladay 2 formula equql the Hoffer Q

in short eyes but is not as accurate as

the Holladay 1 or Hoffer Q in average

and medium long eyes.