This document provides a brief history of intraocular lens (IOL) formulae and the importance of optimizing the A constant value. It discusses how early IOL implantation relied on empirical methods before the introduction of biometry and A-scan technology. First generation formulae in the 1970s calculated IOL power theoretically without considering the A constant. Later formulas in the 1980s-1990s introduced the A constant and took factors like axial length and anterior chamber depth into account non-linearly. Current fourth generation formulas like Holladay II consider additional variables but surgeons must optimize constants based on their own surgical techniques and patient data to accurately predict IOL power for individualized patient outcomes.

1. A brief history of IOL Formula & the
importance of A constant optimization
Subhabrata Bhattacharya

2. A Brief history of IOL Formula & the importance of Aconstant
optimization
Subhabrata Bhattacharya

3. Biometry is the art of applying mathematics to
biology

4. 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

5. 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

6. The first Biometry A scan machines were introduced in 1970s
Hence IOL implantation predates Biometry and A scan machines

7. 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)

8. 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

9. 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

10. 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

11. 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.

12. Second Generation formulae –
Holladay, Prager Chandler et al (1988)
Colliac (1990)
Olsen ( 1987)
Sanders, Retzlaff, and Kraff (1988)

13. 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

14. Second Generation SRK II Formulae

15. 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.

16. 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

17. 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

18. 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

19. 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.

20. 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.

21. 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.

22. 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

23. 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

24. 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.

26. 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)

27. 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

28. The Importance of Constant optimization !

29. A CONSTANT & OPTIMIZATION
At a glance
In 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

30. CALCULATING IOL POWER ??
Best biometry
Best keratometry
Best formulae
Right A constant
Keratometric Index
White to White
Preoperative Refraction

31. EVEN DOING THINGS RIGHT MAY
STILL NOT BE ENOUGH
HENCE CONSTANT OPTIMIZATION IS
IMPORTANT

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

33. WHY OPTIMIZE ??

34. WHY OPTIMIZE ?
 No two surgeons’s have the same technique.
 Rhexis ??

35. Surgeons NEED to UPDATE biometers;
Lens Constant Database !

36. UPDATE Lens Constant Database !
AcrySof® IQ Toric
‘A-Constant’
See USER MANUAL for instruction !

37. 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.

38. Thank you !!