Choosing the Proper Power Choosing the Proper Power for the IOLfor the IOL

Brannon Aden, MDBrannon Aden, MDMiles H. Friedlander, MD, FACSMiles H. Friedlander, MD, FACS

Goal’s of Surgery Have Goal’s of Surgery Have Changed.Changed. In past the goal was good In past the goal was good visual visual

outcomeoutcome Now an equal goal is a good Now an equal goal is a good

refractiverefractive outcome outcome• Central to that is an accurate Central to that is an accurate

calculation of the correct IOL powercalculation of the correct IOL power• Next came a variety of formulas Next came a variety of formulas

aimed at achieving that accuracyaimed at achieving that accuracy

Possible Sources of Error Possible Sources of Error in IOL Calculationin IOL Calculation Systematic error-weakness in formula or Systematic error-weakness in formula or

weakness in a measurement techniqueweakness in a measurement technique• Example of technique is altering the axial Example of technique is altering the axial

length of the eye by using a contact type length of the eye by using a contact type probeprobe

Random errorRandom error• Not common but tend to produce larger Not common but tend to produce larger

errorserrors– Example is presence of a staphylomaExample is presence of a staphyloma

FormulasFormulas What is the current standard of What is the current standard of

care for accuracy?care for accuracy?• 50% +/- 0.5D50% +/- 0.5D• 90% +/- 1.00D90% +/- 1.00D• 99.9% +/- 2.00D99.9% +/- 2.00D

Is this good enough for refractive Is this good enough for refractive lens surgery?lens surgery?

Factors Needed to Factors Needed to Calculate IOL PowerCalculate IOL Power Axial length of globe (distance Axial length of globe (distance

from anterior corneal vertex to from anterior corneal vertex to fovea)fovea)

Corneal powerCorneal power Location of lens in eye (related to Location of lens in eye (related to

anterior chamber depth)anterior chamber depth)

Axial LengthAxial Length Most important anatomical Most important anatomical

variablevariable Greater deviation away from 22.5 Greater deviation away from 22.5

the greater the IOL power the greater the IOL power calculated especially with short calculated especially with short eyeseyes

Axial Length MeasurementAxial Length Measurement ContactContact

• Very personal dependent Very personal dependent • Average error +/- .2 mm ( .50D)Average error +/- .2 mm ( .50D)

ImmersionImmersion• Technician unfriendlyTechnician unfriendly• Accurate +/- .1 mmAccurate +/- .1 mm

Contact ApplanationContact Applanation

Immersion ScanImmersion Scan

Measurement ContinuedMeasurement Continued Buzard “Touch and Go”Buzard “Touch and Go”

• Table mounted A-scanTable mounted A-scan• Flood eye with tearsFlood eye with tears• Advance probe toward eye until Advance probe toward eye until

retinal spike produced on oscilloscoperetinal spike produced on oscilloscope• Requires skilled and experienced Requires skilled and experienced

examinerexaminer

IOL Master (Humphrey and IOL Master (Humphrey and Zeiss)Zeiss) Uses optical interference (Partial Uses optical interference (Partial

Coherence Interferometry) to Coherence Interferometry) to measure axial lengthmeasure axial length

Keratometry also performed by Keratometry also performed by machinemachine

IOL MasterIOL Master

Corneal CurvatureCorneal Curvature Error of 0.1 mm = 1 Diopter errorError of 0.1 mm = 1 Diopter error Sources of errorSources of error

• Contact lens wareContact lens ware• Refractive surgeryRefractive surgery

Anterior Chamber DepthAnterior Chamber Depth Now refers to final position of IOL or Now refers to final position of IOL or

the distance from the posterior the distance from the posterior vertex of the cornea to the anterior vertex of the cornea to the anterior surface of the IOLsurface of the IOL

ACD shallows 0.1 mm per decade ACD shallows 0.1 mm per decade because of lens growthbecause of lens growth

In myopia deepens 0.06 mm per 1 DIn myopia deepens 0.06 mm per 1 D Of less importance than pastOf less importance than past

Early Formulas (First Early Formulas (First Generation)Generation) Anterior chamber depth (ACD) was Anterior chamber depth (ACD) was

constant valueconstant value Early lenses were iris supported which Early lenses were iris supported which

produced small variations in Post Op produced small variations in Post Op ACDACD

Later with the introduction of PC IOL’s Later with the introduction of PC IOL’s formula was less accurateformula was less accurate• Difference of in the bag vs. sulcus was 1 Difference of in the bag vs. sulcus was 1

mm therefore 1 Dmm therefore 1 D

Next First Generation Next First Generation Regression Formula (SRK Regression Formula (SRK 1)1) Used multiple regression analysisUsed multiple regression analysis Eliminated ACD variable and Eliminated ACD variable and

replaced it with A-constantreplaced it with A-constant• Given by manufacturer and is based Given by manufacturer and is based

on expected position in eye, haptic on expected position in eye, haptic and optic design, and refractive index and optic design, and refractive index of IOL materialof IOL material

Problems With SRK 1 Problems With SRK 1 FormulaFormula Formula assumes 2.5 D refractive Formula assumes 2.5 D refractive

change for each 1 mm of axial change for each 1 mm of axial length regardless the axial length length regardless the axial length of the globeof the globe

Tended to under estimate IOL Tended to under estimate IOL power in globes 25 to 29 mm longpower in globes 25 to 29 mm long

Second Generation Second Generation Regression FormulasRegression Formulas SRK II recognized the non linear SRK II recognized the non linear

relationship between axial length relationship between axial length and IOL powerand IOL power

Binkhorst II, Holladay, Donzis also Binkhorst II, Holladay, Donzis also addressed same problems addressed same problems

Third Generation FormulasThird Generation Formulas Holladay 2, SRK/T, and HofferQHolladay 2, SRK/T, and HofferQ Normal range of 22.0 mm to 24.5 Normal range of 22.0 mm to 24.5

mm- All three do equally wellmm- All three do equally well Short eyes < 22.0 mm Hoffer Q Short eyes < 22.0 mm Hoffer Q

performed bestperformed best Long eyes (24.5 to26 mm) Holladay Long eyes (24.5 to26 mm) Holladay

formulaformula Very long eyes (>26 mm) SRK/TVery long eyes (>26 mm) SRK/T

IOL Design and MaterialsIOL Design and Materials Majority of lenses are convex-Majority of lenses are convex-

plano, biconvex, or plano-convexplano, biconvex, or plano-convex Vitreous pressure, haptic flexibility, Vitreous pressure, haptic flexibility,

and final position of ccc by and final position of ccc by contraction of the lens capsule contraction of the lens capsule effect final refractive erroreffect final refractive error

Choice of Lens MaterialsChoice of Lens Materials In normal, non allergic, disease free eye either In normal, non allergic, disease free eye either

PMMA , silicone, or acrylic okPMMA , silicone, or acrylic ok Eyes with silicone oil or anticipated vitro-Eyes with silicone oil or anticipated vitro-

retinal surgery need heparin surface-modified retinal surgery need heparin surface-modified 100% PMMA -tend to retard adhesion of 100% PMMA -tend to retard adhesion of silicone oil to lenssilicone oil to lens

Uveitis- use heparin surface-modified lensesUveitis- use heparin surface-modified lenses Posterior capsule opacification - Prevent? with Posterior capsule opacification - Prevent? with

acrylic lenses (stick to pc and stop proliferation acrylic lenses (stick to pc and stop proliferation of epithelial cells)of epithelial cells)

Lens PositionLens Position Plus lens- need more power as approach Plus lens- need more power as approach

the retinathe retina Minus lens- need less power as approach Minus lens- need less power as approach

the retinathe retina .Anterior iris plane, sulcus, capsule bag. .Anterior iris plane, sulcus, capsule bag.

• For every 1 mm of displacement- 1 D of For every 1 mm of displacement- 1 D of corrective changecorrective change

• Example If a capsular bag lens is placed in Example If a capsular bag lens is placed in the sulcus then the power is reduced by 1 Dthe sulcus then the power is reduced by 1 D

Good ScanGood Scan

Poor ScanPoor Scan