1. Accurate IOL power calculations require precise measurements of axial length and corneal power using modern devices like optical biometers. Errors in these measurements can lead to incorrect IOL powers. 2. Newer theoretical formulas like Holladay II and Haigis are generally more accurate than older formulas or regression formulas. They take into account additional parameters like anterior chamber depth. 3. Special considerations are needed for calculating IOL power in children, eyes with previous refractive surgery or conditions like high myopia which can affect biometry measurements. Repeat measurements may be needed if initial values are outside normal ranges.

1. Achieving Accurate IOL
Power Calculations
ABDELMONEM M. HAMED, M.D.
PROFESSOR OF OPHTHALMOLOGY
BENHA COLLEGE OF MEDICINE
FELLOW OF BAYLOR COLLEGE OF MEDICINE,
TEXAS MEDICAL CENTER, HOUSTON, USA

2. Introduction
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

3. Types of Keratometry
1. Manual Keratometer
2. Auto Keratometer
 IOL Master and Lens star
3. Topography
 Placido disc based (tographer)
 Elevation based topography (sheimphflug)

4. Principle of Automated
Keratometer
 Focuses the reflected corneal image on an
electronic photosensitive device
 which records the size
 and computes the radius of curvature

5. Optical Biometer
IOL MASTER 500
1. Number of points
tested = 6 points in
hexagonal pattern
2. Diameter of corneal
Zone tested = 2.3mm

6. LENS STAR LS 900 No of
points tested : 32 points in
two circles (16 each)
1. Zone of cornea tested
 Inner circle
diameter = 1.65mm
 Outer circle diameter =
2.3mm
Optical Biometer

7. Sim K
It is determined from
the power of placido
mires 7,8 and 9 within
3mm zone.
7
8
9

8. Axial Length Measurement
2 types
1. Ultrasound
2. Optical

9. A-scan
 Principle- The ultrasound probe
has a piezoelectric crystal that
electrically emits and receive high
frequency sound waves.
 Gain: Defined as the electronic
amplification of the sound waves
received by the transducer.
 Measurement is from anterior
corneal surface to internal limiting
membrane.

10. Optical Biometer
They don’t work in presence of dense
cataract.
PRINCIPLE OF IOL
MASTER 500:
uses infrared light
(λ = 780 nm)
allowing to view the
complete
longitudinal section
of eyeball. (AL, Ks,
ACD)

11. LENS STAR LS 900
• All in one
• biometer,
• keratometer,
• pachymeter,
• Pupillometer,
• WTW, and
• IOL calculator
• It Uses the diode laser, that
precisely measure the axial length
on the patient’s visual axis.
• Measurement is from anterior corneal surface to Bruch’s
membrane, so we don’t have to assume 200 microns of
retinal thickness.

12. disvantage Of
Optical
Biometer
They don’t work in
presence of dense
cataract.
Advantage Of Optical
Biometer
Non contact biometer

13. Types of formulas calculating
IOL power
 Theoretical
 Regression

14. Theoretical formulas :
It is based on the optics of the eye( i.e.
corneal power, RI of cornea, distance of
cornea to IOL, axial length, etc…)
1st and 2nd generation formulas is less
accurate than regression formulas in normal
length of the eye (22 – 25.5mm)
3rd and 4th generation formulas is more
accurate than regression and 1st and 2nd
generation theoretical ones

15. Regression formulas :
It depend on the regression analysis of
actual results of many uncomplicated
IOL implantation
However it is less accurate than
theoretical formulas in short and long
eyes 22 – 25.5mm)< >

16. 1st and 2nd generation
theoretical formulas like
Colenbrander
Fyodorove
Van der Heijde
Binkhorst
Hoffer
Shammas fudged

17. 3rd and 4th generation
theoretical formulas like
Holladay II
SRK/T
Hoffer Q
Haigis
What is the ACD?

18. Regression formulas like
SRK: P=A-0.9K-2.5L
SRK/T T= theoretical
What is the A-constant?

19. ACD depth
White to white length
Effective lens position
(distance from cornea to
lens or ACD)
All the 3rd and 4th generation
formulas consider:

20. Potential errors leading to the
placement of a wrong IOL include:
 Use of an older IOL formula
 Incorrect measurement of the eye's axial length
 Incorrect keratometry readings
 Mistakes in entering the data into the IOL calculation
program
 Incorrect labeling of the IOL by the manufacturer
 Mistakes in providing the IOL at the time of
surgery

21. Consider repeat
measurements if
 Axial length <22.0 or >25.0 mm
 Average corneal power < 40 D or > 47 D
 Between 2 eyes, there is a difference in:
 Corneal power > 1 D
 AL > 0.3MM
 Emmetropic IOL power > 1 D

22. What you have to do in these
situations?
 If you have an eye filled with silicon oil
 If the patient had lasik
 If you have a child Pt.
 If you have pt. with high myopia.

23. If you have an eye filled with
silicon oil
The RI of SO = 1.42 (higher than
vitreous 1.34)
Velocity of SO = 1000
Do not use silicone IOL if there is
SO inside the eye.

24. If you have pt. who had lasik
 Total corneal refractive power
(Hamed et al., ophthalmology
, thin lens optics), taking in
consideration during
calculating the corneal power,
the corneal thickness, and
power of the posterior corneal
surface.
Ophthalmology J. 2002

25.  The Haigis formula is based on a three-
variable (a0, a1 and a2).
 The a1 constant is tied to the measured ACD,
 The a2 constant is tied to the measured AL.
 This allows all three variables to be optimized
for a wide range of ALs and ACDs using
double-regression analysis.
Haigis Formula

26. B CL = base curve of the contact lens in diopters.
P CL = spherical power of the contact lens in diop.
Rcl = spherical equivalent refractive error with C. lens
R No CL= spherical equivalent refractive error without CL
Ktrue = the estimated corneal power after refractive
surgery
Hard contact lens over refraction
Ktrue = B CL + P CL + (R CL – R NO CL )

27. If you have a child Pt.

28. If you have a child Pt.
Ideal Rx: plano to -1.0 D
Expected induced myopia with development
(myopic shift from age 2 – 6 ys.) up to -10D
Role of thumb: it is much easier to correct
myopia at age 20 than amblopia

29. Pediatric biometry
 Rule of # 7 , up to age 7
 Target refraction = 7 – age in years
 Ex: kids 2 yrs. old= 7- 2 = 5 D of residual hyperopia. If
biometry is 30 D, then insert 25 D IOL.
 A greater under correction can lead to amblyopia.

30. Primary
piggyback
 Formula to be used:
 Haigis 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
 Use rule of # 7 to calculate the power of
the ant. IOL
P.S. Reduce 1 D for sulcus placed IOL

31. Secondary
piggyback IOL
 When to perform 2ry piggyback:
 Patients with refractive error following
the primary IOL implantation.
 Calculated based on refractive
error.

32. Secondary piggyback
IOL
The IOL power can also be calculated by one of these
formulae:
1. Holladay’s refractive formula.
2. Empirical method:
 Myopic correction: P = Refractive Error × 1.0
 Hypermetropic correction: P = Refractive Error × 1.5

33. Eyes with high myopia
 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
Fovea

34. Which formula is accurate

36. https://www.apacrs.org/barrett_universal2/
What to do if you don’t have the new
generation formulas on your machine?

37. https://www.apacrs.org/barrett_universal2/