Goals: Obtain precise and accurate refractive outcomes; Avoid postoperative surprises.
Parameters: Axial length (AL); Average Keratometry (K) Value; Anterior Chamber Depth (ACD); A-Constant of IOL; Effective Lens Position (ELP); IOL Formulas
Accurate Measurements are Essential:
- Biometry: Recheck AL if difference of > 0.3 mm between eyes, or difference of > 0.1 mm among readings in the same eye.
- Laser interferometry (IOLMaster, Lenstar LS 900) or immersion A-scan.
IOL formulas: Newer-generation theoretical formulas are the most accurate.
Lens position is important: 1 mm error = 1 D change in power.
Empiric: Derived from clinical studies by regression analysis. No longer used.
- 1ST GENERATION: SRK, Gills-Lloyd
- 2ND GENERATION: SRK II, Thompson-M, Donzis
Theoretical: Derived from optics by vergence formulas.
- 1ST GENERATION (ACD is constant): Binkhorst I, Fyodorov, Colenbrander
- 2ND GENERATION (ACD based on AL): Binkhorst II, Shamas
- 3RD GENERATION (ACD based on AL and K): Hoffer Q, Holladay 1, SRK/T
- 4TH GENERATION (multiple variables or function derived): Holladay 2, Haigis, Olsen, Barrett Universal II (http://www.apacrs.org/barrett_universal2/) Available in IOLMaster and Lenstar
Most accurate formulas by AL:
- Long eyes (AL > 26.0 mm): SRK/T, Holladay 2, Haigis (customized)
- Medium-long eyes (AL 24.6–26.0 mm): Holladay 1 or 2
- Medium eyes (AL 22.0–24.5 mm): Hoffer Q, Holladay 1 or 2, SRK/T, Haigis
- Short eyes (AL < 22.0 mm): Hoffer Q, Holladay 2
Newer instruments, numerous formulas and correction factors (Wang et al, JCRS. 2011; 37 (11):2018, and intraoperative wavefront aberrometers have improved ability to select appropriate IOL power, especially after keratorefractive surgery (http://iol.ascrs.org)
Postop Surprise:
- Axial Length Error: Most common. 0.1 mm error = 0.25-0.3 D surprise.
- Keratometry Error: 1 D error in average K value = 1 D error in IOL power.
- After keratorefractive surgery, traditional K readings are inaccurate:
- Corneas flattened with myopic treatments measure falsely steep.
- Corneas steepened with hyperopic treatments measure falsely flat.
- Myopic Surprise: AL measurement too short, K measurement too flat, IOL position too anterior (capsular block, large rhexis with IOL optic anterior to capsulotomy, anterior vaulted Crystalens)
- Hyperopic Surprise: AL measurement too long, K measurement too steep, IOL position too posterior (excessive posteriorly vaulted Crystalens)