A brief history of IOL Formula & the importance of A constant optimization

Subhabrata Bhattacharya

A Brief history of IOL Formula & the importance of Aconstant optimization

Subhabrata Bhattacharya

Biometry is the art of applying mathematics to biology

BRIEF HISTORY OF BIOMETRY

The first IOL was implanted by Sir Harold Ridley on November 29, 1949.

The second implantation by Ridley was performed at Moorfield’s Hospital the following year.

Over the next couple of years Ridley had implanted over a thousand lenses

So how were the power (diopter) of the IOL determined prior to A scan machines ?

1. IOL power was calculated on the basis of clinical history – pre operative refractive error prior to cataract development in eye.

2. Empirical Method – Adherents to the empirical method would implant a 19.0 diopter lens thinking that this would behave like the natural lens

3. The Basic Refractive Error method – One would add or substact 1.25 diopter to every pre operative refractive error in the eye on a 18.0 diopter lens

The first Biometry A scan machines were introduced in 1970s

Hence IOL implantation predates Biometry and A scan machines

Introduction of A scan machines brought in a host of first generation theoretical formulae for the calculation of IOL power

1. Binkhorst (1972)

2. Colenbander ( 1973)

3. Fyododrov ( early 70s)

4. Thijssen (1975)

These were all theoretical formulae or two – lens formulae based on theoretical calculation of IOL power

based on the Cornea and the Lens.

The concept of the A constant was not there

All lenses were assumed to have a pre determined fixed position in the eye

This was the age of Anterior Chamber Lenses

The assumption of a fixed position of the lens was not incorrect given the fact that it was an age of

ICCE or Intra Capsular Cataract Surgery

The mid 1980s saw the introduction and acceptance of Phacoemulsification.

By then the issues faced with AC IOLs ware widely felt.

The concept of IOL implantation in the bag started getting widely recognized.

The stage was set for the second generation IOL calculation formulae and the concept of

A constant

The difficulty of predicting the IOL in the capsular bag due to capsular contraction, vaulting

characteristic, etc, set the stage for a host of pioneering work involving constants.

Second Generation formulae –

Holladay, Prager Chandler et al (1988)

Colliac (1990)

Olsen ( 1987)

Sanders, Retzlaff, and Kraff (1988)

The SRK Formulae

P= A-2.5L-.9K The original SRK formulae through introduced the concept of A constant, was still a theoretical formulae and the A constant was directly derived from the manufacturer’s lens box

Second Generation SRK II Formulae

The second generation theoretical formulae were distinct from first generation formulae in the sense that the value for the position of the IOL was not fixed but varies as a function of at most two variables – axial length and corneal

curvature.

Though the word ELP or Effective Lens Position is comparatively a new term, the concept was recognized now in the form of ACD.

Later the ACD was replaced by ELP as a term which suitably describes the lens position in the eye after cataract surgery.

ELP – Distance from corneal vertex to principal plane of thin IOL

Same as ACD, but avoids confusion with anatomy

SRK II formulae

Notable among the second generation formulae was the development of the original SRK formulae into SRK II formulae .

The SRK II was a regression formulae which attempted to fit a function to the distribution of Axial Length and IOL power through a polynomial regression analysis.

This regression analysis was linear for average axial length eyes , it exhibited non linearity in non average eyes

Adjustments to the Axial Length through A constants by SRK II formulae.

The below table shows the relationship between A constant of SRK and the adjustments done in SRK II

However though the SRK II formulae was a vast improvement and predicted fairly accurately the IOL position in the bag for average eyes, yet there were

unexpected error in longer and shorter eyes.

The reason was that until 1996 it was assumed that a longer eye would have a longer ACD , and a shorter

eye would have proportionately shorter ACD.

The works of Holladay (1996) showed this was often not to be.

Hence a formulae like SRK II that predicts the ELP or the IOL position in the eye as a linear function of axial length only, does not do justice in non average eyes.

The SRK T formulae was a further improvement and combined the linear regression method with the theoretical eye model to predict the IOL position

correctly even in non average eyes.

It is a non linear theoretical optical formula empirically optimized to predict the ELP.

It combines the advantages of theoretical and regression formulas

The SRK T now took into account the corneal curvature in addition to Axial Length to predict IOL position in non average eyes through alteration of A constant over the SRK II

Fourth Generation Formulae like Holladay II takes into account several variables to predict

the IOL position ( ELP ) in the eye.

Axial Length

K readings

White to White

ACD

Manifest Refraction

Lens Thickness

Age

Uses Surgeon Factor as Constant

Fourth generation Olsen formulae – the concept of C constant.

A key feature of the Olsen formulae is the C constant which can be regarded as a

ratio by which the capsular bag will encapsulate the IOL in the bag.

The Olsen formulae takes the c constant as a function of Lens Thickness and ACD.

There are largely three IOL constants in use :

The SRK T as indicated earlier uses A const

The Holladay I formula uses Surgeon Factor or SF

The Holladay II and the Hoffer Q formula uses ACD

The Haigis uses three constants – A0, A1, A2

d ( ELP ) = a0 + (a1*ACD)+(a2*AL)

HAIGIS

The a0 constant behaves like the other constants in SRKT, Holladay I, Holladay II, etc.

The a1 constant is tied to the measured anterior chamber depth

The a2 constant is tied to the measured axial length

The Importance of Constant optimization !

A C O N S T A N T & O P T I M I Z A T I O N

At a glanceIn an average IOL patient, there would be no difference if you use the IOL Master or Lenstar or an immersion ultrasound device, as long as you use the proper constants – Dr. Haigis

CALCULATING IOL POWER ??

Best biometry

Best keratometry

Best formulae

Right A constant

Keratometric Index

White to White

Preoperative Refraction

E V E N D O I N G T H I N G S R I G H T M AY S T I L L N O T B E E N O U G H

H E N C E C O N S TA N T O P T I M I Z AT I O N I S I M P O R TA N T

W H Y O P TI M I ZE ? ?

If you have your a constant optimized then the number of patients falling within .5D of target refraction increases four

fold.

WHY OPTIMIZE ??

WHY OPTIMIZE ?

No two surgeons’s have the same technique.

Rhexis ??

Surgeons NEED to UPDATE biometers;

Lens Constant Database !

UPDATE Lens Constant Database !

AcrySof® IQ Toric ‘A-Constant’

See USER MANUAL for instruction !

IOL DATA OPTIMIZATION

• It is strongly recommended to customize and personalize A-constant, ACD, and Surgeon Factor based on their individual biometry techniques. • Data obtained from at least 30 patients should

be used. • To successfully customize, refractive surprises

should be consistently hyperopic or myopic when compared with predicted values before adjustments are made.

Thank you !!