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IOL Power Calculations

Cataract Surgery Arizona

Feiz-Mannis Method.1

The Feiz-Mannis Method is what we like to use as an aid in determining the overall refractive picture when good historical data is available. This method is least likely to result in a post-operative hyperopic surprise. As you will see, we can use this method to set the upper limit of possible IOL powers as the Feiz-Mannis Method will frequently produce a myopic over-correction.

Using this technique, first the IOL power is calculated using the pre-LASIK keratometry, as though the patient had never undergone keratorefractive surgery. There is no special formula correction that needs to be employed for the Feiz - Mannis Method. This calculated pre-LASIK IOL power is then increased by the amount of refractive change at the spectacle plane divided by 0.7. The Feiz-Mannis Method is outlined as follows:

IOLpre   -   (ΔD / 0.7)   =   IOLpost       Where...

IOLpre = the power of the IOL using pre-LASIK keratometry, and...

ΔD = the stable refractive change after LASIK at the spectacle plane, then...

IOLpost = the estimated power of the IOL to be implanted following LASIK.

The Feiz-Mannis Method for the right eye following LASIK would be carried out as follows:

 

If the calculated IOL power before LASIK is +9.18 D, and... The change in refractive power at the spectacle plane is -5.38 D, then...

  IOLpre   -   (ΔD / 0.7)   =   IOLpost
           
  +9.18 D   -   (-5.38 / 0.7)   =   +16.87 D

The Feiz-Mannis Method for the left eye following LASIK would be carried out as follows:

 

If the calculated IOL power before LASIK is +10.27 D, and...

The change in refractive power at the spectacle plane is -4.00 D, then...

  IOLpre   -   (ΔD / 0.7)   =   IOLpost
           
  +10.27 D   -   (-4.00 / 0.7)   =   +15.98 D

It is helpful to keep in mind that the sign convention for the change in refractive error (ΔD) following myopic LASIK is a negative number. Using the above formula, the new calculated IOL power will always result in a larger number.

References

1. Feiz V., Mannis M.J. Garcia-Ferrer F. Intraocular lens power calculation after laser in situ keratomileusis for myopia and hyperopia a standardized approach. Cornea 2001; 20:792-797.