Double-K method to calculate IOL power after refractive surgery
Dr Rosa and coauthors are correct highlighting errors in the formula Appendix:
- In equation 4, rpre2 must be changed to Symbol and Cw2/4 must be changed to Cw2/4. It is true that when Symbol the formula fails to predict any effective lens position (ELP) and IOL power as Sqrt (−n)=e. This happens for certain values of K and axial length (AL); ie, K=45 and AL>33.12. This error can be also found in the original formula I copied from IOL power calculation.1 Although I have not found the solution to this error, it is obvious from the predicted IOL powers that available software applies the solution Dr. Rosa and coauthors propose; that is, when Symbol the result will be 0. In my experience, however, this algorithm leads to an overestimation of ELP and thus to underestimation of IOL power, resulting in postoperative hyperopia. I would propose a more accurate solution, which I have been using for several years: to make this result absolute and then square root it. So, Symbol must be changed to Symbol.
- I agree that in equation 10, n must be changed to na.
- It is useful to add the conversion formula from A-constant to ACD constant (ACDconst).
With these changes, the corrected version of the formula appendix would be:
- Equation 1: Preoperative corneal radius of curvature:
- Equation 2: Corrected axial length (LCOR):
- Equation 3: Computed corneal width (Cw):
- Equation 4: Corneal height (H):
- Equation 5: Offset value:
- Equation 6: Estimated postop ELP (ACD):
- Equation 8: Retinal thickness (RETHICK) and optical axial length (LOPT):
- Equation 9: Postoperative corneal radius of curvature (rpost):
- Equation 10: Emmetropia IOL power (IOLemme):
- Equation 11: Conversion from IOL A-constant to IOL ACD constant:
- L=axial length; Kpre=pre refractive surgery K-value; Kpost=post refractive surgery K-value; ACDconst=IOL constant (can be computed from A-constant as shown in equation 11).
I want to thank Dr. Rosa and coauthors for their help in correcting the article and for their nice comments.
Jaime Aramberri MD
© 2005 by Lippincott Williams & Wilkins, Inc.
1. Retzlaff J, Sanders DR, Kraff MC. Lens Implant Power Calculation: a Manual for Ophthalmologists and Biometrists, 3rd ed. Thorofare NJ, Slack, 1990