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.
Achieving Accurate IOL Power Calculations Techniques
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
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.
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?
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 )
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