Dioptric Distress

DIOPTRIC DISTRESS How to calculate IOL power under

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DIOPTRIC DISTRESS – The definitive Instruction Course to improve your biometry skills


Gathering Good Data


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Basics of Biometry

Dr. Ashima Aggarwal

INSIGHT EYE CLINIC

Rajouri Garden

New Delhi

Dr. Ashima Aggarwal


Financial Disclosure

I have no financial interest in the contents of

this presentation.

Dr. Ashima Aggarwal


Biometry

• The process of measuring various physical

parameters of an eye, and using this data to

determine the ideal intraocular lens power.

Dr. Ashima Aggarwal


Components of biometry

• Essential

Axial Length

Keratometry

• Additional

AC Depth

White to White

Lens thickness

Dr. Ashima Aggarwal


A matter of great responsibility

Biometry should only be performed by the surgeon or a well-trained assistant.

Dr. Ashima Aggarwal


A matter of timing

• Biometry should be done early

Time to calculate IOL Power.

Time to counsel the patient.

Time to arrange specific IOL.

Dr. Ashima Aggarwal


Keratometry

• Estimation of the corneal power.

• Good K measurement is important !

Directly used in IOL Power calculation.

Indirectly used for ELP estimation.

Evaluation of astigmatism.

Dr. Ashima Aggarwal


Keratometry

• Remember, we are measuring the radius of

curvature of the anterior cornea.

• The actual dioptric power is calculated.

• Assumed overall corneal refractive index 1.3375

Dr. Ashima Aggarwal

Power = 1.3375 – 1.0000 Radius


Keratometry

• Manual

• Automated

• Optical

• Topography

• Pentacam

Dr. Ashima Aggarwal


Keratometry – tips and tricks

• Calibrate regularly.

• Good tear film.

• Untouched cornea.

• Accurate centration – occlude the other eye.

• Average of multiple readings.

Dr. Ashima Aggarwal


Keratometry – tips and tricks

• Dark room

• Evident scarring – use the other eye value.

• Recheck

extreme values : < 40 D or > 48 D

asymmetry : > 1.5 D difference

Dr. Ashima Aggarwal


Axial Length

• Contact Ultrasound

• Immersion Ultrasound

• Optical

Dr. Ashima Aggarwal


Axial Length – Contact Ultrasound

• Principle

Ultrasonic waves are reflected off the ocular

structures.

These echoes are deciphered to yield the

distances traversed.

Dr. Ashima Aggarwal


Axial Length – Contact Ultrasound – TIPS

#1 • Calibration.

• Correct velocity settings.

• Gentle touch.

• Look for good spikes.

• Appropriate gain settings.

• Axial alignment using the fixation light.

Dr. Ashima Aggarwal


Axial Length – Contact Ultrasound – TIPS

#2 • Average of multiple readings, with low standard

deviation (<0.06 mm).

• Repeat if difference between two eyes is > 0.3 mm.

• Record before dilating the pupil.

• Avoid using ointments before the measurements.

Dr. Ashima Aggarwal



Dr. Ashima Aggarwal



• PROS

Easy to perform.

Works for the densest of cataracts.

Cost Effective.

Dr. Ashima Aggarwal



• CONS

Corneal compression or fluid bridge.

Risk of abrasion, infection.

Skill dependent.

Misalignment.

Dr. Ashima Aggarwal


Axial Length – Immersion Ultrasound

• The probe is fixed inside a special shell, most

commonly the Prager Shell.

• This shell is placed between the two eyelids,

centred on the cornea.

• A water-bath is created within the shell, such that

the tip of the probe is immersed.

Dr. Ashima Aggarwal



Dr. Ashima Aggarwal


Axial Length – Immersion Ultrasound –

TIPS

• Make sure that the patient is comfortable.

• Align the probe to the mark on the shell properly.

• Fill the chamber adequately, without air bubbles.

• Ask the patient to look at the fixation light.

• Make sure that the machine is in immersion mode.

Dr. Ashima Aggarwal



• PROS

Accurate.

No corneal compression.

Works for the densest of cataracts.

Relatively Cost Effective.

Dr. Ashima Aggarwal



• CONS

Slight learning curve.

Special shell is needed.

Risk of abrasion, infection.

The patient can get wet.

Dr. Ashima Aggarwal


Axial Length – Optical

• Infra-red diode laser (780-830 nm).

• Partial Coherence interferometry is used to

measure the optical path length.

• Since the wavelength of the infrared rays used is

much smaller than that of ultrasonic waves, the

resolution of the measurement is far better.

Dr. Ashima Aggarwal


Axial Length – Optical – TIPS

• Ensure moist ocular surface.

• Ensure alignment.

• Slight mydriasis may be helpful.

• Choose the correct mode.

Dr. Ashima Aggarwal



• PROS

Very high repeatability.

Observer independent.

Fast.

Precise.

Same machine for all measurements.

Dr. Ashima Aggarwal



• PROS

No physical contact.

No anaesthesia.

No corneal compression.

Visual axis measurement.

Dr. Ashima Aggarwal



• CONS

Expensive.

Does not work if there is significant axial

opacity.

Dr. Ashima Aggarwal


After the gathering.....

Dr. Ashima Aggarwal

• After the data has been gathered, the surgeon

must use one or more IOL power calculation

formulae.


Formula Options

• SRK ii – regression formula

• SRK/T

• Holladay 1

• Hoffer Q

• Haigis

Dr. Ashima Aggarwal


Which formula NOT to use !!

• SRK II – obsolete

45.0, 44.0, A-constant 118

Axial Length 22.00 – IOL Power = 22.95

Axial Length 21.99 – IOL Power = 23.98

0.01 mm difference = 1.03 D difference

Dr. Ashima Aggarwal


Which formula to use, and where

Short eyes : Hoffer Q

Average eyes : Holladay 1

Long eyes : SRK/T

Dr. Ashima Aggarwal


Biometry – keep in mind

• Both eyes should be measured.

• Always use the correct mode.

• Look for asymmetry between the keratometry and

axial length of the two eyes. It can indicate

amblyopia, staphyloma, vitreous opacity, covert

corneal scarring etc.

• Repeat on different days, with different machines.

Dr. Ashima Aggarwal



• Use multiple formulae.

• IOL Position - Bag vs Sulcus

»30.0 D bag = 28.5 D sulcus



• Change of IOL model entails recalculation.

Dr. Ashima Aggarwal


• Audit your results scientifically.

• Record, at least

Mean Error (SD)

Percentage of eyes within 0.5 D of predicted

refraction.


Dr. Ashima Aggarwal


Thank you !

Dr. Ashima Aggarwal


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Biometry in Special Situations

Dr. Vinita Gupta

Assistant Professor

All India Institute of Medical Sciences

Rishikesh

Dr. Vinita Gupta




this presentation.

Dr. Vinita Gupta


Special Situations

• High Myopia / Hypermetropia

• Post-vitrectomized eyes

• Paediatric Cataract

• Scarred Cornea

• Aphakia / Pseudophakia

• Piggy back IOL

Dr. Vinita Gupta


High Myopia - issues

Confounding factors

• Lower scleral rigidity

• Posterior staphyloma

Dr. Vinita Gupta


High Myopia - what to do?

• Lower scleral rigidity - Avoid contact biometry. Use either immersion or optical biometry.

• Posterior staphyloma

Optical Biometry is the gold standard.

Perform immersion biometry as well as horizontal axial B scan.

Add ACD and LT from A-scan to vitreous length from B scan to obtain total axial length.

Dr. Vinita Gupta


High Hypermetropia - issues

• Corneal compression.

• Relative ocular dimensions.

• Formula Errors.

Dr. Vinita Gupta


High Hypermetropia - what to do?

• Immersion / Optical biometry

• HofferQ formula

• Optimization

Dr. Vinita Gupta


Post-vitrectomized Eye

• Lower ocular rigidity, so avoid contact biometry.

• No change in axial length measurements

otherwise.

Dr. Vinita Gupta


Silicone Oil filled Eye

• Sound travels slower through Silicone oil.

• The speed depends upon the density of the

silicone oil.

• Presence of retro-silicone space is a further

confounding factor.

• Instrument range can be exceeded, especially in

very long eyes.

Dr. Vinita Gupta


Silicone Oil filled Eye – what to do?

Ultrasonic Measurement

• Measure axial length in the sitting position.

• Apply correcting factors.

Optical Measurement

• Just set the mode to silicone filled eye, and record

the AL.

Dr. Vinita Gupta


Silicon Oil filled Eye - correction factors

• Vitreous Chamber Depth with oil, VCDoil , can be

extrapolated to yield true VCD

VCD TRUE = VCD OIL x 0.64

Then, AL = ACD + LT + VCDTRUE

• Alternatively, total Axial Length can be corrected

Axial Length TRUE = Axial Length OIL x 0.72

Dr. Vinita Gupta


Scarred Cornea

Dr. Vinita Gupta

• Issues

Unknown corneal contribution to refraction.

Faulty ELP estimation.

Optical biometry may not work.


Scarred Cornea – what to do?

• Measure with contact lens overlay.

• Topography.

• Measure other eye.

• Calculate using only AL and refractive error.

Dr. Vinita Gupta


Paediatric Biometry - Issues

Dr. Vinita Gupta

• How cooperative – optical might be better, or

under GA.

• Low ocular rigidity, greater compression.

• Small eye, more impact of compression

3.75 D per mm


Paediatric Biometry - Issues

Dr. Vinita Gupta

• Different Anatomy

standard IOL Power formulae inadequate.

Ultrasound velocity is different

• Growing eye, changing parameters.


Paediatric Biometry – what to do?

Dr. Vinita Gupta

• Always, always measure both eyes.

• Undercorrect

< 2 years, 20% undercorrection

2-8 years, 10% undercorrection

• No one formula has been proven to be superior.

• High IOL Powers - consider piggyback IOLs.


Biometry in aphakic eyes

• Lack of lens spikes. Manual mode may be required.

• Sound velocity is uniform at 1532 m/s.

• It is actually easier to perform biometry in the aphakic

eye.

• Lens power calculations will need to be adjusted

according to the final position of the IOL.

• Refraction based IOL power estimation can be done.

Dr. Vinita Gupta


Biometry in pseudophakic eyes

• IOL exchange.

• Multiple tall spikes, difficult

to freeze the scan.

• Optical measurement is very

effective.

• Standard IOL Power

calculation formulae apply.

Dr. Vinita Gupta


Piggyback IOLs

• Primary piggyback

Very high hyperopia.

• Secondary piggyback

Correction of residual refractive error.

Can be calculated easily based on existing refraction.

• Myopic correction, Piggyback Power = 1.0 x Refractive Error

• Hyperopic Correction, Piggyback Power = 1.5 x Refractive Error

Dr. Vinita Gupta


Biometry in special situations

• Special situations are generally suboptimal eyes

requiring extra diligence.

• Careful consideration to basic principles can make

a big difference to the outcomes.

Dr. Vinita Gupta


Thank you !

Dr. Vinita Gupta


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Optimization Strategies

Dr. Saurabh Sawhney

INSIGHT EYE CLINIC

Rajouri Garden

New Delhi

Dr. Saurabh Sawhney




this presentation.

Dr. Saurabh Sawhney


A FAMILIAR SCENARIO ?

• Excellent surgery

• Pristine Cornea

• Quiet eye

• Unhappy patient

HIGH REFRACTIVE ERROR

Dr. Saurabh Sawhney


The need to hit the target

Dr. Saurabh Sawhney


Sources of error – bad input data

• Corneal compression.

• Calibration errors.

• Inherent errors due to assumptions of speed of

sound, refractive index of cornea etc.

• Erratic errors

Dr. Saurabh Sawhney


Sources of error – IOL Design Feature

• Angulation of haptics.

• Overall size and shape of the implant.

• Rigidity of the haptic optic junction.

• Biological interaction with capsular bag.

• Position of IOL in the bag.

Dr. Saurabh Sawhney


Sources of error – Surgical technique

• Rhexis size, centration

• IOL placement

• Viscoelastic removal

• Suturing technique

Dr. Saurabh Sawhney


IOL Power

= fx (Lens Constant) (AL) (K)

The IOL power formula

Dr. Saurabh Sawhney


IOL Power

= fx (Lens Constant) (AL) (K)

Since the AL and K are fixed values for a given eye, it is the Lens Constant that combines the

influence of all previously mentioned variables.

The IOL power formula

Dr. Saurabh Sawhney


The ‘CONSTANTS’ are variable !!!

Dr. Saurabh Sawhney


• SRK II A-constant

• SRK/T A-constant

• Holladay Surgeon’s Factor

• HofferQ pACD

• Haigis a0, a1, a2

The ‘CONSTANTS’ are variable !!!

Dr. Saurabh Sawhney


Manufacturer’s values

• The A-constant mentioned on the IOL box.

• Empirical value.

• Based on theoretical considerations, limited

clinical data, and experience with similar lens

designs in the past.

• Useful starting point.

Dr. Saurabh Sawhney


Ingredients for optimization

• Preoperative Keratometry

• Preoperative Axial Length

• IOL Power and model

• Postoperative Refractive error

Dr. Saurabh Sawhney


Basics of optimization

• SRK formula

P = A constant – 0.9 K – 2.5 AL

• Rearranging the equation

P + 0.9 K + 2.5 AL = A constant

(where P is the ideal IOL power for emmetropia)

Dr. Saurabh Sawhney


What next ?

• Calculate A-constants for a series of cases.

• Average these values.

• 25 or more cases are needed for meaningful

results.

• Use this new A-constant for your future IOL power

calculations.

Dr. Saurabh Sawhney


Points to ponder

• One surgeon

• One lens model

• One Biometry setup

• In the bag placement of IOL

• Standardized refraction

Dr. Saurabh Sawhney


Limitations of ‘paper and pen’

optimization

• Very complex equations for the modern

theoretical formulae.

• High possibility of unnoticed error.

• Time consuming.

Dr. Saurabh Sawhney


Back calculation for surgeon’s factor

A scary peek into the maths involved.

Dr. Saurabh Sawhney


Back calculation for surgeon’s factor

Surgeon’s Factor = (((-(PostopSphEq*0.001*((ALHolla*12*((4/3)-1))-((CorRadius)*(ALHolla-

(12*(4/3)))))-((((4/3)-1)*ALHolla)+(1.336*(CorRadius)))))-

SQRT(((PostopSphEq*0.001*((ALHolla*12*((4/3)-1))-((CorRadius)*(ALHolla-(12*(4/3)))))-((((4/3)-

1)*ALHolla)+(1.336*CorRadius)))^2)-(4*((((4/3)-1)-(0.001*PostopSphEq*((12*((4/3)-1))-

(CorRadius)))))*(((1.336*ALHolla*CorRadius)-

(0.001*PostopSphEq*ALHolla*12*CorRadius*1.336))-(((1000*1.336)*((1.336*CorRadius)-(((4/3)-

1)*ALHolla)-(0.001*PostopSphEq*((12*((1.336*CorRadius)-((1.3333-

1)*ALHolla)))+(ALHolla*CorRadius)))))/IOLimplanted)))))/(2*(((4/3)-1)-

(0.001*PostopSphEq*((12*((4/3)-1))-(CorRadius))))))-(0.56+RAG-(SQRT(RAG^2-(modAG^2/4)))))

Dr. Saurabh Sawhney


More sensible optimization strategies

• Biometry Machine

• Separate Software

Dr. Saurabh Sawhney


Using the Biometry Machine

• Save the initial biometry.

• Enter the surgical data – surgeon, IOL power,

model etc.

• Enter the postoperative refraction.

• The machine calculates and updates the relevant

constants.

Dr. Saurabh Sawhney


Is there an even simpler way?

Dr. Saurabh Sawhney


Is there an even simpler way?

Independent Software

• Easier to work on the PC rather than the biometry

machine.

• Easier to back up and transport data.

• Faster.

• No memory overload issues.

Dr. Saurabh Sawhney


SOFTWARE

The ‘IOL constants optimizer’

LCO v 2.1

www.Facebook.com/OphthalmicCalculators

www.InsightEyeClinic.in

Dr. Saurabh Sawhney


Dr. Saurabh Sawhney

www.Facebook.com/OphthalmicCalculators www.InsightEyeClinic.in


Happy Optimizing

Dr. Saurabh Sawhney


Thank you !

Dr. Saurabh Sawhney


any circumstances



Converting constants

A constant 118.0 118.4 118.5 118.7 118.8

Surgeon Factor 1.22 1.45 1.51 1.62 1.68

pACD 4.97 5.20 5.26 5.37 5.43

Surgeon Factor = 0.5663 x A-constant - 65.6

pACD = (A constant x 0.5663) - 65.6 + 3.595 0.9704


any circumstances

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