- IOL power calculations require considering several factors including axial length, corneal curvature, lens constants, and intended postoperative lens position. - Different IOL power calculation formulas are better suited for different eye characteristics like normal versus long axial lengths. - Accurate biometry measurements from devices like the IOL Master and precise consideration of effective lens position are important for accurate IOL power calculations. - Newer fourth generation formulas like Holladay 2 and Haigis that incorporate additional parameters generally provide the highest accuracy over a wide range of axial lengths.

1. IOL Power calculations
- Dr Aman dalal

2. Father of IOL —
Dr Harold Ridley.
Pioneers in India —
Dr Daljeet Singh.

3. Principles —
factors to consider :
1) Axial length of eyeball
2) corneal K values
3) lens related constants ( Ant chamber/A
constant )
— provided by manufacturer
4) post Op lens position

4. based on situations —
Normal Axial length : SRK 2, SRK t , Hoffer
long Axial length : HOLLADAY 1 / SRK T
post Lasik pt : Haigis formula, ASCRS calculator,
Gaussian optics formula, Barrets
Silicone oil filled eye : Increase calculated IOL
power by 3-3.5 D
Post keratoplasty pt : take K values of other eye

5. GENERATIONS of IOL formula —
1st gen : SRK
Binkhorst
2nd gen : SRK 2.
3rd gen : SRK/T
Hoffer Q
Holladay
4th gen : Holladay 2
Haigis
Olsen
Wolfgang Haigis
4th gen formulas dont depend on assumptions of Ant
chamber depth,and requires real measurement of the
same

6. SRK formula
Sanders, Retzlaff and Kraff
Though now replaced with the newer generation
formulas, it is still useful for understanding the relation of
the variables and A-constant to the IOL Power (P).
P = A – 0.9 K – 2.5 AL
P= Power of the IOL in diopters
A= A constant
K= Average keratometry value in diopters
AL= axial length (mm)
✰ SRK 1 & 2 - obsolete now

7. Barrets - location of principle plane of refraction is
retained as a relevant variable in the formula
recommended for short-long eyes
Parameters - AL, corneal power , ACD, Lens
thickness
HAIGIS - 3 variables used - a0, a1, a2
recommended for Normal eyes ( a0 optimised formula)
long eyes (a0,a1,a2 optimised )
Parameters - AL, ACD
Hoffer - relies on Personalised ACD, AL, Corneal
curvature
parameters - Al, Corneal power

8. Holladay 1 - Uses “postOp stabilised refractive
value”, power of implanted IOL, PreOp corneal and
AL measurements
Parameters used - Corneal power, AL
Holladay 2 - uses 7 parameters
AL, ACD, Corneal power, Lens thickness, Age, white
to white corneal diameter, preOp refraction data
SRK/T - thereotical approach
optimisation method involve PostOp ACD
prediction, Retina thickness correction factor,
Corneal μ index

9. Accurate power calculation depends on —
preOp biometric data ( axial length, ant chamber depth, K
index )
Use of right formula for different axial lengths
IOL power quality control

10. Two eyes with same Axial length and same keratometry
values may have diff IOL powers for emmetropia
why ?
ELP
Geometry of each lens model
ELP/ Effective lens position —
actual distance from corneal ant surface to lens plane ( considering lens is infinitely
thin)
ELP decreases in short eyes and flat corneas
explains position of IOL postoperatively
Difficult to predict :
1) IOL is thinner than Cataract
2) Ant chamber depth increases in pseudophakia
3) variable lens geometry across diff powers

11. IOL master —
Non contact device, with AL along visual axis
measuring range :
5-10 mm for corneal radii
1.5-6.5 mm for ant chamber depth
14-40 mm for axial length
PRINCIPLE : Dual Beam partial coherence
Measure reflected infrared laser light form internal
tissue interfaces
From cornea to retinal pigment epi
( std ultrasound biometry measures AL from corneal
vertex to internal limiting membrane
Takes 5 AL measurements in diff quadrants
Helps to get around less denser opacities in cornea

12. Axial myopia —
different anatomy of posterior pole
↑ incidence of Post staphyloma —> biometry
misalignment between visual axis and
anatomic axis
With IOL master pt looks directly to fixation light — so AL
measurement is to fovea , giving exact refractive error

13. OKULIX & OLSEN -biometric computer program
Utilise ray tracing
Pentacam & Orbscan — used for corneal ectasia and now for
precise corneal power measurements
Pentacam uses a rotating Scheimpflug camera to take optical
cross sections of the anterior segment. The Orbscan uses a
series of horizontal slit images
Bausch and
Lomb Orbscan
IIz Corneal
Analysis
illuminated ring pattern
and a beam of light that
is shined across the
cornea, the image
sensors in the Orbscan
IIz acquire over 9000
points of data in less
than 2 seconds.

14. Real time IOL power calculation -
Intraoperative aberrometry
ORA by Alcon/ Optiwave refractive analysis device
Helps avoid Refractive surprises
With each procedure, it captures the new data in its AnalyzOR™ database
Includes correction for surgically induced astigmatism

15. Which formulas to use when ?
For normal ALs ( 22.5-24.5) - most formulas work well
Recently, in a database study of 8,108 eyes undergoing
cataract surgery, the Hoffer Q formula was found to provide the
best refractive outcomes in eyes shorter than 21.00 mm.
x Holladay 1 and Hoffer Q formulas were equally reliable for
eyes with an AL between 21.00 mm and 21.49 mm.
4th gen formulas( Barrett, Haigis or Holladay 2 ) have the
advantage of including the non-proportional relationship
between the ACD and AL — p rovide the highest accuracy
over the full range of ALs

16. Commonly used lens constants -
SRK/T formula — A constant
Holladay 1. — Surgeon factor
Holladay 2. —. Ant chamber depth
Hoffer q — uses pseudophakic ant chamber depth
Haigis formula — 3 constants - a0, a1, a2
4th gen formulas Haigis, Holladay 2 - give best refractive
outcomes in extremes of axial lengths

17. HAAG streit LENSTAR LS 900 —
◉ measures all intraocular distances, CCT, ACD , lens
thickness
◉ one shot laser
◉ Utilises OLSEN IOL calculation formula
◉ C constant is used - accurate IOL prediction in all
kind of human eyes

18. THANK YOU