IOL power calculation after refractive surgery

IOL power calculation after refractive surgeryA. Peyman, MD

IOL power calculation:

IOL power

AL

ELPK

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 This extrapolation is no longer valid after laser vision correction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 formulasRCC: 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

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