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IOL Power Calculations

Cataract Surgery Arizona

Clinical History Method.1,2

The Clinical History Method for corneal power estimation was first described by Holladay and later by Hoffer as:

Kp + Rp - Ra = Ka Cataract Surgery ArizonaWhere...

Kp = the average keratometry power before keratorefractive surgery, and...

Rp = the spherical equivalent before keratorefractive surgery, and...

Ra = the stable spherical equivalent after keratorefractive surgery, then...

Ka = the estimate of the central corneal power after keratorefractive Cataract Surgery Arizonasurgery.

Central corneal power by keratometry will be referred to in diopters (D), even though it is better termed keratometric diopters. This is due to the fact that the cornea has a different index of refraction than keratometers, or corneal topographers (1.3333 vs. 1.3375)7

Corrected for an estimated vertex distance of 12 mm, the historical estimation of the central corneal power of the right eye following LASIK would be carried out as follows:

 

If the Ks before LASIK were 45.37 D / 46.00 D, and...

The refraction before LASIK was -6.00 +0.75 x 135, and...

The stable refraction after LASIK was -0.25 sphere, then...

  Kp   +   Rp   -   Ra   =   Ka
               
  (45.69)   +   (-5.27)   -   (-0.25)   =   40.67 D

Corrected for an estimated vertex distance of 12 mm, the historical estimation of the central corneal power of the left eye following LASIK would be carried out as follows:

 

If the Ks before LASIK were 45.25 D / 46.12 D,

The refraction before LASIK was -5.50 +0.75 x 060, and...

The stable refraction after LASIK was -1.25 +0.25 x 070, then...

  Kp   +   Rp   -   Ra   =   Ka
               
  (45.69)   +   (-4.83)   -   (-1.11)   =   41.97 D

If you are unfamiliar with how to correct for vertex distance, follow this link to an Excel spreadsheet for Correcting for Vertex Distance. Be sure to maintain the correct sign (+ or -) when entering the refractive data, or the results will not be accurate.

References

1. Holladay JT. Consultations in refractive surgery. Refract Corneal Surg 1989; 5:203

2. Hoffer KJ. Intraocular lens power calculation for eyes after refractive keratotomy. J Refract Surg 1995; 11:490-493