Post on 31-Dec-2015
description
Choosing the Proper Power Choosing the Proper Power for the IOLfor the IOL
Brannon Aden, MDBrannon Aden, MD
Miles 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
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
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 Plus lens- need more power as approach the retinaapproach the retina
Minus lens- need less power as Minus lens- need less power as approach the retinaapproach the 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