Choosing the Proper Power for the IOL
Brannon Aden, MD
Miles H. Friedlander, MD, FACS
Goal’s of Surgery Have Changed.
•	In past the goal was good visual outcome
•	Now an equal goal is a good refractive outcome
•	Central to that is an accurate calculation of the correct IOL power
•	Next came a variety of formulas aimed at achieving that accuracy
Possible Sources of Error in IOL Calculation
•	Systematic error-weakness in formula or weakness in a measurement technique
•	Example of technique is altering the axial length of the eye by using a contact type probe
•	Random error
•	Not common but tend to produce larger errors
–	Example is presence of a staphyloma
Formulas
•	What is the current standard of care for accuracy?
•	50%          +/- 0.5D
•	90%          +/- 1.00D
•	99.9%      +/- 2.00D
•	Is this good enough for refractive lens surgery?
Factors Needed to Calculate IOL Power
•	Axial length of globe (distance from anterior corneal vertex to fovea)
•	Corneal power
•	Location of lens in eye (related to anterior chamber depth)
Axial Length
•	Most important anatomical variable
•	Greater deviation away from 22.5 the greater the IOL power calculated especially with short eyes
Axial Length Measurement
•	Contact
•	Very personal dependent 
•	Average error +/- .2 mm ( .50D)
•	Immersion
•	Technician unfriendly
•	Accurate +/- .1 mm
Contact  Applanation
Immersion Scan
Measurement Continued
•	Buzard  “Touch and Go”
•	Table mounted A-scan
•	Flood eye with tears
•	Advance probe toward eye until retinal spike produced on oscilloscope
•	Requires skilled and experienced examiner
IOL Master (Humphrey and Zeiss)
•	Uses optical interference (Partial Coherence Interferometry) to measure axial length
•	Keratometry also performed by machine
IOL Master
Corneal Curvature
•	Error of 0.1 mm = 1 Diopter error
•	Sources of error
•	Contact lens ware
•	Refractive surgery

Anterior Chamber Depth
•	Now refers to final position of IOL or the distance from the posterior vertex of the cornea to the anterior surface of the IOL
•	ACD shallows 0.1 mm per decade because of lens growth
•	In myopia deepens 0.06 mm per 1 D
•	Of less importance than past
Early Formulas (First Generation)
•	Anterior chamber depth (ACD) was constant value
•	Early lenses were iris supported which produced small variations in Post Op ACD
•	Later with the introduction of PC IOL’s formula was less accurate
•	Difference of in the bag vs. sulcus was 1 mm therefore 1 D
Next First Generation Regression Formula (SRK 1)
•	Used multiple regression analysis
•	Eliminated ACD variable and replaced it with A-constant
•	Given by manufacturer and is based on expected position in eye, haptic and optic design, and refractive index of IOL material
Problems With SRK 1 Formula
•	Formula assumes 2.5 D refractive change for each 1 mm of axial length regardless the axial length of the globe
•	Tended to under estimate IOL power in globes 25 to 29 mm long
Second Generation Regression Formulas
•	SRK II recognized the non linear relationship between axial length and IOL power
•	Binkhorst II, Holladay, Donzis also addressed same problems  
Third Generation Formulas
•	Holladay 2, SRK/T, and HofferQ
•	Normal range of 22.0 mm to 24.5 mm- All three do equally well
•	Short eyes < 22.0 mm Hoffer Q performed best
•	Long eyes (24.5 to26 mm) Holladay formula
•	Very long eyes (>26 mm) SRK/T
IOL Design and Materials
•	Majority of lenses are convex-plano, biconvex, or plano-convex
•	Vitreous pressure, haptic flexibility, and final position of ccc by contraction of the lens capsule effect final refractive error
Choice of Lens Materials
•	In normal, non allergic, disease free eye either PMMA , silicone, or acrylic ok
•	Eyes with silicone oil or anticipated vitro-retinal surgery need heparin surface-modified 100% PMMA -tend to retard adhesion of silicone oil to lens
•	Uveitis- use heparin surface-modified lenses
•	Posterior capsule opacification - Prevent? with acrylic lenses (stick to pc and stop proliferation of epithelial cells)
Lens Position
•	Plus lens- need more power as approach the retina
•	Minus lens- need less power as approach the retina
•	.Anterior iris plane, sulcus, capsule bag.  
•	For every 1 mm of displacement- 1 D of corrective change
•	Example If a capsular bag lens is placed in the sulcus then the power is reduced by 1 D
Good Scan

Poor  Scan