PostLASIK IOL Calculations[Click For a PrinterFriendly Version (PDF)] At its core, the formula necessary for determining IOL power following cataract surgery relies on the axial length of the eye and the cornea’s total dioptric power. When calculating IOL power for a patient that previously underwent corneal refractive surgery, the resulting values are inaccurate if postsurgery keratometric values are used. This inaccuracy is due to two reasons: (1) there is a corneal curvature discrepancy and (2) there is a change in the cornea’s index of refraction.
Corneal Shape/Power DistortionsThe total corneal power is the sum of the anterior power (convex lens) and the posterior power of the cornea (concave lens). In LASIK and PRK, there is a change in the radius of the anterior cornea’s curvature, but the posterior cornea is relatively unchanged. Traditional keratometry and simulated keratometry from corneal topography estimate corneal power by measuring it around a central 3.2mm ring on its anterior surface. For a normal prolate cornea, calculation of the central power is accurate because the sampling location (3.2 mm) has a power similar to the central power. Following keratorefractive surgery, this relationship is altered, and the sampling location may have a power that is steeper than the center (for a myopic ablation), resulting in a calculated power that is higher than in reality (i.e. the cornea is really flatter than indicated and so the power of an IOL is too low, resulting in a hyperopic surprise after cataract surgery) (see below). The opposite is true for a hyperopic ablation. Instruments that measure both anterior and posterior curvature, such as the Orbscan topographer (Bausch & Lomb, Rochester, NY) or Pentacam (Oculus, Inc., Lynnwood, WA), may decrease these errors of determining the true corneal power. However, errors in keratometry may have other implications as well.
Formulae for IOL PositioningThirdgeneration IOL formulae (ie, SRK/T, HofferQ, Haigis, Holladay 2) account for the position of the lens relative to the cornea (also known as the effective lens position) to increase accuracy. The effective lens position is accomplished by measuring anterior chamber depth (Haigis) or by estimating anterior chamber depth from the keratometry measurements (SRK/T, HofferQ).13 If keratometry measurements are inaccurate, that error is propagated into the calculations for effective lens position, further compounding the problem.
Index of RefractionIn addition, the altered index of refraction in a postrefractive surgery cornea is another source of error. Standard keratometers/topographers rely on an index of refraction of the cornea of 1.3775 to convert radius of curvature data into dioptric power. After keratorefractive surgery, the cornea’s index of refraction is altered, introducing further error.

Preoperative Keratometry Required 

Historical Method 
Calculate IOL power using adjusted preoperative keratometry.

FeizMannis Method 
Use preoperative keratometry to calculate IOL power, but
add a correction factor to the IOL power.

Walter Method 
Calculate IOL power using preoperative keratometry, but use
the preoperative manifest refraction as the postoperative target
refraction.

Aramberri DoubleK Method 
Calculate IOL power using the preoperative keratometry for
the part of the SRK/T equation that determines anterior chamber
depth, and use postoperative keratometry for the part that determines
IOL power. 
Preoperative Keratometry Not Required 

Koch Method 
Calculate IOL power using adjusted postoperative keratometry.

Masket Method 
Calculate IOL power using postoperative keratometry and adjust
final IOL power according to a regression formula.

Shammas NoHistory Method 
Calculate IOL power using adjusted postoperative keratometry.

Latkany FlatK Method 
Calculate IOL power using flattest postoperative keratometry
and adjust final IOL power according to a regression formula.

Latkany AverageK Method 
Calculate IOL power using average postoperative keratometry
and adjust final IOL power according to a regression formula.

FeizMannis Nomogram 
Calculate IOL power using postoperative keratometry and adjust
final IOL power according to a regression formula. 
Methods Requiring Special Equipment or Time 

Contact Lens Overrefraction 
Refract with hard contact lens of known base curve and power.

Orbscan Method 
Approximate central corneal power through direct anterior
and posterior corneal measurements.

Pentacam Method 
Approximate central corneal power through direct anterior
and posterior corneal measurements.

Adjusted Effective Refractive Power (EyeSys) 
Approximate central corneal power through software analysis.

Raytracing Method (Oculis) 
Approximate central corneal power through software analysis.

Mackool Aphakic Refraction Method 
Remove cataract and leave aphakic. Perform a manifest refraction 30 minutes
later and calculate lens power needed via regression formula. Return to OR and implant lens.

Ianchulev Aphakic Autorefraction Method 
Remove cataract. Measure
aphakic autorefraction using handheld autorefractor. Calculate lens power needed via a regression
formula. 
All pages are Copyright ©2006 by Dennis
H. Goldsberry, M.D., P.E.
Reproduction or archival of any protion of these pages is strictly prohibited
except by express written permission.