IOL power
calculation after
refractive
surgery
A. Peyman, MD

IOL power calculation:
IOL
power
AL
ELP
K

Effective lens position (ELP) is
the most challenging part
ELP
K
AL
ACD

Problems with calculations??!
 Problems with AL measurements after
refractive surgery?!
 Errors of keratometry

Cornea power determination
 Our instruments calculate corneal power
indirectly

 Because standard keratometry measures only
the anterior corneal curvature, the posterior
curvature is extrapolated based on the normal
anterior/posterior curvature ratio. This
extrapolation is no longer valid after laser
vision correction

Erroneous effective IOL position
prediction
 Standard IOL power formulae use the axial
length and corneal power to predict the
position of the IOL postoperatively.

 Refractive surgery changes the corneal power
but not the depth of the lens, leading to an
error in ELP prediction
 After myopic laser, the corneal power is
decreased and the ELP estimates become too
low.

 This leads to an underestimation of the IOL
power that is required after myopic laser.
 Overestimation of IOL power occurs after
hyperopic laser.

Methods to Obtain the True
Corneal Power after
Refractive Surgery

Clinical history method
 The corneal power is calculated by
subtracting the change in manifest refraction
at the corneal plane induced by the refractive
surgical procedure from the corneal power
values obtained before refractive surgery.
 K=KPRE-RCC

Limitations:
 unavailability or inaccuracy of data
 interval changes in the corneal curvature or
lens power and clarity

Contact lens over-refraction
method
 Corneal power is calculated as the sum of the
contact lens base curve, power, and over-
refraction minus the spherical equivalent of
the manifest refraction without a contact lens.
 is not suitable for cases of dense cataracts

Topography-Based Post-LASIK
Adjusted Keratometry
 Koch and Wang Formula
 K=1.1141×TK -6.1
 Shammas Formula
 K=1.14×TK -6.8

Net corneal power
measurement
 obtaining accurate corneal power: to directly
measure both anterior and posterior corneal
curvature
 Orbscan
 Pentacam
 OCT

IOL Power Formulae for Post-
Refractive Surgery Eyes

Double-K Formulae
 pre-refractive surgery corneal power (or an
estimate of it) is used in the ELP prediction
formula.
 This reduces the error in post-refractive
surgery ELP calculation.

 Double-K versions are available:
 SRK/T
 Hoffer Q
 Holladay II

Haigis-L Formula
 built-in for IOLMaster.
 Corneal power is calculated by inputting IOL-
Master biometry

Masket Formula
 P=PTARG-0.326×RCC -0.101
PTARG: IOL power calculated by standard
IOL formulas
RCC: surgically induced refractive change
(SRK/T: myopes; Hoffer Q : hyperopes)

Koch and Wang Nomogram
Adjustment
 This nomogram is easy to use by just look up
the axial length of the patient and add or
subtract the adjusted IOL power to the IOL
power.

 This formula is a regression formula based on
statistics.
 Accuracy may decrease if the eye is on the
edge of normal distribution (high myopic or
high hyperopic eyes).

How to use online calculator of
the ASCRS
 Go online and type:
http://iol.ascrs.org/

Enter pre-op data
 Stable refraction 6-12 months following
LASIK/PRK

Post-op data

Phakic IOL
power
calculation

Necessary information
 Pre-op refraction
 Corneal power
 Expected position of the lens

Table