IOL Power Calculation In Special Situations

Dr Ravish Vaishnav Moderator- Dr Vijay

Shetty

Introduction

Precise IOL power calculation is essential for optimal benefits.

IOL Power Calculation Errors can arise from

Keratometry – 1.0D = 0.9D error in IOL power

Axial length – 1mm = 2.5D error in IOL power

IOL Formula

KeratometryUsed to measure the corneal curvature.

Done before axial length measurement.

Types of Keratometer

1. Manual Keratometer

2. Auto Keratometer, Keratometers incorporated in IOL Master

and Lenstar

3. Topography – Placido disc based or Elevation based

topography

Keratometry

Principle

1. Fixed object and variable image size

2. Fixed image and variable object size

Two types

1. Bausch and Lomb

2. Javel Schiotz

Manual Keratometer – Bausch and LombMeasures radius of curvature of anterior corneal surface and

converts into total corneal power (anterior + posterior) by

using standardized refractive index of 1.3375 instead of true

corneal refractive index 1.36.

Measures a larger zone away from the centre - 3.5mm zone

and calibrated to give central corneal power, Works well in

virgin regular corneas.

Limitation – In post RK and post keratorefractive corneas, the

central corneal power measurement is inaccurate.

Automated Keratometer

Principle- Focuses the reflected corneal image on to an

electronic photosensitive device, which instantly records

the size and computes the radius of curvature.

Zone of measurement- Central 3mm zone.

Slightly better than manual K in terms of the zone

measured.

Keratometry of Optical BiometerIOL MASTER –1. No of points tested – 6 points in hexagonal pattern2. Zone of cornea tested – Diameter of 2.3mm LENSTAR –1. No of points tested – 32 points in two circles (16 each)2. Zone of cornea tested – Inner circle diameter – 1.65mm Outer circle diameter – 2.3mm

BETTER IN TERMS OF MEASURING TRUE CENTRAL CORNEAL POWER

TopographySim K – It is determined from the power of placido mires 7,8 and 9 of videokeratoscope for 128 equally spaced meridians within 3mm zone.EKR – EKR is determined by adding 0.7D to total corneal refractive power (TCRP) measured by Pentacam rotating Scheimpflug camera. This conversion factor is applicable after myopic refractive surgery because changes in TCRP are equal to changes in refraction in the 4mm zone.Flattest central K on topo – Standard keratometry tends to overestimate the corneal curvature and we expect a hyperopic shift post-RK. So flattest central K is used for IOL power calculation in post RK. Seo K, Young C et al. New equivalent keratometry reading calculation with a rotating Scheimpflug

camera for intraocular lens power calculation after myopic corneal surgery. JCRS 2014. Vol 40. 1834-42. Savini G, Hoffer K. Pentacam equivalent K-reading. JRS 2010. Vol 26. 388-89.

Topography

Posterior corneal measurement in Oculyzer – Selecting toric

IOL based on anterior corneal measurement can lead to

overcorrection in eyes with WTR astigmatism and

undercorrection in eyes with ATR astigmatism.

Oculus Pentacam AXL – Utilise the anterior + posterior corneal

astigmatism and axial length on the same machine to calculate

toric IOL power.

Koch D, Ali S et al. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2013;39(12):1803-1809.

Axial Length Measurement

1 mm error leads to 2.5D error in postoperative refraction

2 types

1. Optical

2. Ultrasound

A-scan

PRINCIPLE- The ultrasound probe has a piezoelectric crystal

that electrically emits and receive high frequency sound

waves.

Measurement is from anterior corneal surface to internal

limiting membrane.

WAVES- One thin parallel sound beam is emitted from the

probe tip at a frequency of 10MHz, with an echo bouncing into

the probe tip as the sound beam strikes each interface.

A-scan USE OF GAIN IN DIFFICULT SITUATIONS- Gain refers to

electronic amplification of the sound waves received by the

transducer. Increase in gain is required when height of echoes

achieved is inadequate as in dense cataracts. Decrease in gain

is required when artefacts are seen near the retinal echoes as

in silicone filled eyes, pseudophakic eyes.

IMMERSION ADVANTAGE- It is more accurate than contact

method as the corneal compression is avoided.

LIMITATIONS- Does not give accurate readings in silicone filled

eyes, posterior staphyloma.

Optical Biometer

PRINCIPLE OF IOL MASTER – Based on

‘Partial Coherence Interferometry (PCI)’. Diode laser (780nm)

measures echo delay and intensity of infrared light reflected

back from tissue interfaces– Cornea & RPE.

PRINCIPLE OF LENSTAR – Based on ‘Low coherence

optical reflectometry (LCOR)’. Superluminescent

diode laser of 820nm is used.

Optical Biometer

PRINCIPLE OF IOL MASTER 700 – Based on swept source OCT

technology. It provides an image-based measurement,

allowing to view the complete longitudinal section of eyeball.

Akman A, Asena L. Evaluation and comparison of the new swept source OCT-based IOLMaster 700 with the IOLMaster500. Br J Ophthalmol . 2015-307779.

Optical BiometerADVANTAGES OF LENSTAR OVER IOL MASTER V5 –

- Lenstar measures ACD using optical biometry as compared to

IOLMaster which measures ACD using slit imagery.

- Measures ACD from posterior surface of cornea which is

important in short anterior chambers and thicker corneas.

- Takes two sets of readings so covers more central area.

- Pachymetry, Pupillometry, Retinal thickness.

- Lens thickness is measured which is one of the factors in latest

- formulas like Holladay II.Hill W, Angeles R, Otani T. Evaluation of a new IOLMaster algorithm to measure axial length. J Cataract Refract Surg 2008;34:6:920-4.

Optical Biometer

ADVANTAGES OF IOL MASTER 700 OVER LENSTAR-

- In PSC cataracts, opacity are located nearer to the nodal point

of the lens, so more light rays are affected and thus AL

measurement becomes difficult with LENSTAR. IOL MASTER

700 overcomes this problem.

- Unusual eye geometries like tilt or decentration of lens can be

detected.

IOL Formulae

3RD and 4th generation formulae are superior to earlier formulae.

These are a merger of linear regression methods with theoretical

eye models.

They predict better ELP.

Haigis uses multiple A-constant.

Holladay uses multiple parameters including lens thickness

measured by LENSTAR.

IOL formula depending on AL

Circumstance Choice of formulaAL < 20 mm Holladay II/ Hoffer Q

20- 22 mm Hoffer Q

22- 24.5 mm SRK/ T; Holladay

24.5- 26 mm Holladay I

> 26 mm SRK / T ; Holladay I

IOL power in specific situation

Myopic refractive surgery Haigis L

Nanophthalmos Hoffer Q ; Haigis

Post refractive surgery Topography with true net k and flattest k with formula according to the AL

Biometry in specific situations

Aphakic eyes

Aphakic eyes

Two lens spikes are replaced by a single spike - Anterior

vitreous face and Posterior lens capsule.

Immersion technique is the method of choice.

In present Biometers, options are available for aphakic mode

where in the calculation compensate for the change in speed

of the sound waves in cataract lens or aqueous or vitreous.

In ACIOL or Scleral fixated IOL, the appropriate A constant is

used.

Keratoconus eyes

Keratoconus eyes

As cornea with keratoconus is steep, using K reading of such eyes

will yield an overestimated reading due to ELP calculation error.

Formulas which consider only axial length and not Keratometry to

calculate ELP gives better prediction of true IOL power.

K reading has less of this effect in the Hoffer Q formula.

Also, overestimation is not a factor with the Haigis formula as it

does not use the K reading in estimating the lens power.

Eyes with silicone oil

Eyes with silicone filled vitreous/vitrectomized eyesThe refractive index of the oil is much less than that of the

vitreous.

Usage of a standard sound velocity can give an error of upto 8

mm.

Difficulty of measuring the AL can be overcome by increasing

the ‘system gain’.

Eyes with silicone filled vitreous/vitrectomized eyesError in AL measurement occurs as ultrasound travels slower in

silicon oil compared to vitreous and thus taking a longer time to

reach the probe which is interpretated as longer wavelength.

Usually a factor of (0.72) gives a rough estimate of the power.

TAL = 1133/1550 × AAL.

As silicone oil alters the optics of the eye due to its refractive index,

further adjustments in IOL power are required. However it is less

affected in optical biometer

Usually, IOL required is 2 – 3 D stronger than indicated by standard

power calculation.

High myopic eyes

Eyes with high myopiaAccurate axial measurement is critical for IOL power

calculation

Paraxial measurement in ultrasonic measurement due to posterior staphyloma is likely to give a refractive surprise

It is partly overcome by optical biometer using a fixation target

Most of it is taken care in IOL master 700 by directly visualising fovea on the OCT image during measurement

Eyes with high myopia

IOL formula needs to be chosen accordingly

Minus IOL powers have to be chosen carefully by reducing amount of minus power

The surgeon should aim for a -0.50 D to -1.00 D postoperative refraction as most elderly will prefer being near-sighted

Pediatric eyes

Pediatric biometryAs myopia increases rapidly in pediatric age group, goal should

be undercorrection.

Undercorrection of 60-75% is recommended depending upon

the child’s age.

Younger the age, more is the undercorrection.

Pediatric biometrySimple rule of thumb- Target refraction = 7 – age in years

A greater undercorrection can lead to anisometropia and

difficulty in amblyopia correction. At times managing

amblyopia is more difficult than future IOL exchange.

Therefore undercorrection has to be done with care especially

in unilateral cases.

Piggyback IOL

Primary piggybackHaigis or Hoffer Q

Ideally 1 acrylic and 1 silicon IOL to avoid interlenticular opacification

Usually single piece in the bag and 3 piece in the sulcus

Divide the power between the IOL and reduce 1 D for sulcus placed IOL

Secondary piggyback IOL for pseudophakia

Patients with refractive error following the primary IOL

implantation.

Calculated based on refractive error.

Holladay’s refractive formula.

No knowledge of primary implant or the AL is required.

Secondary piggyback IOL for pseudophakiaThe IOL power can also be calculated by one of these formulae:

Myopic correction: P = 1.0 × Error

Hypermetropic correction: P = 1.5 × Error

Rayner sulcoflex IOL power is calculated by giving the residual

refraction while ordering.

Corneal refractive surgery

Common Keratorefractive surgeries which pose a challenge for Cataract surgery

MYOPIC LASIK PRKRK WITH OR WITHOUT AK

HYPEROPIC LASIKPRK

Keratometric error in RKRadial keratotomy (RK) flattens both the

anterior and posterior corneal surfaces, but only in a small

central optical zone.

The effective optical zone diameter can be significantly smaller

than the measurement zone of standard keratometry.

Therefore standard keratometry tends to overestimate the true

corneal power. Furthermore, there can be central flattening after

cataract surgery due to corneal edema. Most of the flattening

effect resolves over several months.

Sources of error in post refractive cases (LASIK/PRK)Radius measurement error/ Keratometer error

Keratometry index error (Altered Gullstrand ratio)

Formula Algorithm error (ELP)

These 3 errors cause hyperopic error in myopic LASIK and

myopic error in hyperopic LASIK.

Methods of estimating true postoperative corneal power

1. Clinical history methodK=K PRE + R PRE – R PO

2. Contact lens methodK= B CL + P CL + R CL – R NO CL

3. Maloney” central K methodK= 1.1141 * TK PO-CTRI

4. Shammas methodK = 1.14 * K MANUAL PO – 6.85. Haigis methodK = -5.1625 * K IOL MASTER + 82.2603 - 0.35

Web Help

http://www.doctor-hill.com/iol-main/keratorefractive-calculator.html

www.ascrs.org, http://iol.ascrs.org/wbfrmCalculator.aspx

Choice of IOLIOL with negative spherical aberration like Technis or Acrysof

IQ would correct spherical aberration induced by myopic LASIK.

Refractive multifocal IOLs further reduce contrast sensitivity and so not recommended.

Diffractive multifocal IOL is not preferred as diffractive ring adds to already multifocal cornea and so not recommended.

Following RK, the overall lack of precision of IOL power calculations, a diurnal fluctuation in refractive power and increase in spherical aberration precludes use of refractive and diffractive multifocal IOL.

Rule of 2s

Inspite of our best efforts, if the final refractive objective

remains elusive, plans for an IOL exchange, or a secondary

piggyback IOL, should not be made until at least 2 months

have passed and two refractions are stable at two consecutive

visits.

Warren Hill Cataract Surgery after keratorefractive surgery

Our study Purpose:

To compare the various methods for IOL calculation post radial

keratotomy (RK).

To predict the most accurate method.

Study Type:

Retrospective analysis

Sample size:

27 eyes post RK included in the analysis

Our study Methods:IOL Power was calculated by 3 methods:

IOL Master: K used were the one derived from IOL masterTopography: K values used were the flattest K on corneal

topographyESCRS calculator: Average value of IOL power to be

implantedResults:

IOL power calculation using Topo-K gave a significantly higher proportion of patients in whom the difference between predicted and actual SE was more than 0.75, compared with ESCRS method. [p=0.04]

Our study Conclusion:

No single reliable method for accurate calculation of IOL

power in post RK patients.

Multiple methods of IOL power calculation using IOL-M K,

flattest central K on topography and ESCRS calculator with final

fine tuning based on surgeon’s experience helps in achieving

near accurate post op refraction.

Thank you!!