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Intraocular Lens Calculation - Formulas

Historic Background. First generation formulas: Clinical History Formula Theoretical Formula: Binkhorst I Hoffer's Regression Formula: Binkhorst II SRK ISecond generation formula: SRK II. Historic Background. Third generation formulas: Holladay I Hoffer Q SRK/TFourth generation fo

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Intraocular Lens Calculation - Formulas

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    1. Intraocular Lens Calculation - Formulas Chi-Wah (Rudy) Yung, M.D.

    2. Historic Background First generation formulas: Clinical History Formula Theoretical Formula: Binkhorst I Hoffer’s Regression Formula: Binkhorst II SRK I Second generation formula: SRK II

    3. Historic Background Third generation formulas: Holladay I Hoffer Q SRK/T Fourth generation formulas: Haigis Holladay II

    4. Clinical History Formula Simple formula used before 1975 Power = 18 + (1.25 x Ref) power – power of iris-supported IOL for emmetropia Ref – patient’s pre-op refractive error Accuracy poor >50% had I D error Some huge errors due to the inaccuracy of calculating refractive error prior to cataract formation

    5. Theoretical Formulas First generation formulas Binkhorst I and Hoffer’s Based on a two-lens system focusing images on retina (cornea & IOL) Predict postoperative refractive error at the corneal plane with a given IOL Not very accurate

    6. Regression Formulas First generation formulas Better than Theoretical Formulas Derived from retrospective computer analysis of postoperative data from a large number of patients

    7. SRK Formula Derived by Sanders, Retzlaff and Kraff in 1983 IOL Power = A – 2.5L – 0.9K A = A constant (varies with IOL style and manufacturer L = Axial length (mm) K = Corneal power (D)

    8. SRK and other first generation formulas work well for average axial length but inaccuracies occurred at extremes of axial length Stronger IOLs for long eyes resulting in excess postop myopia Weaker IOLs for short eyes resulting in excess postop hyperopia

    9. SRK II Formula A second generation formula with adjustments made to account for extremes of axial length: IOL Power = A1 – 0.9K – 2.5L A1 = (A – 0.5) for axial length >24.5 A1 = A for axial length between 22 and 24.5 A1 = (A + 1) for axial length between 21 and 22 A1 = (A + 2) for axial length between 20 and 21 A1 = (A + 3) for axial length <20

    10. Third Generation Formulas Also known as Modern Theoretical Formulas - Holladay I, SRK/T and Hoffer Q A merger of the linear regression methods with theoretical eye models Important concept : postop anterior chamber depth is related to IOL placement in the eye, not to preop anterior chamber depth All use two variables to predict ACD : AL and K Greater accuracy than earlier formulas but the reliance on theoretical assumptions led to differences among the three formulas. All have a personalizable factor to improve accuracy

    11. Third Generation Formulas ___XN_____ - ________N_________ IOL Power = A – C – 0.05 __N__ - _C_+_0.05__ K + R X A = Axial Length C = IOL position K = Corneal Power A = Desired Rx N= Index of Refraction X = Constant

    12. Third Generation Formulas Holladay I (1988) S factor – personalized surgeon factor SRK/T (1990) A constant – based on multiple variables (IOL manufacturer, implant style, surgeon’s technique, etc.) Hoffer Q (1993) Personalized ACD value

    13. Third Generation Formulas SRK/T formula Best for eyes longer than 26 mm Holladay I Best for eyes between 24 mm and 26 mm Hoffer Q formula Best for eyes shorter than 22 mm

    14. All third generation formulas predict the effective lens position (ELP) by performing calculation based on two data variables: axial length (AL) and Keratometry (K), and the assumption that the anterior chamber depth (ACD) was a proportion of the axial length (AL) but not a true measurement have led to IOL surprises with post-refractive patients

    15. Haigis Formula A fourth generation formula - 1991 Does not depend on assumptions for the ACD and requires real measurement of it Has 3 adjustable constants derived by multi-variable regression analysis instead of just one (one constant moves the power prediction curve up or down; the second constant is tied to the measured anterior chamber depth while the 3rd constant is tied to the measured axial length) Allows optimization over a larger range of axial lengths

    16. The Holladay II Formula Another 4th generation formula – 1998 Requires measurement of 7 variables (Horizontal white-to-white corneal diameter, ACD, lens thickness, patient’s age, preop Rx, K and axial length) Probably the most precise formula especially for unusual eyes such as post-refractive surgery

    17. Horizontal White-to-White (HWTW) Probably the most important element in judging the size of the anterior segment and predicting the depth of the IOL within the eye Average HWTW: 11.7 mm + 0.46 mm. Axial length and anterior segment size are almost independent of each other

    18. Which Formula To Use? Hoffer Q or Holladay II – <22.0 mm Hoffer Q or Holladay I – 22.0 to 24.5 mm Holladay I – 24.5 to 26.0 mm SRK/T or Holladay II – >26.0 mm

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