Calculations of corneal power after corneo-refractive surgery from keratometry and change of spectacle refraction: some considerations on the "clinical history method".
OBJECTIVE
To derive corneal power after kerato-refractive laser surgery (KRS) to be used for a subsequent intraocular lens (IOL) power calculation.


METHODS
Based on the proportion of curvatures of the corneal front and back surface, the central thickness, and the ablation characteristics, we demonstrate a vergence-based formalism to derive the equivalent and back vertex corneal power before and after KRS from the preoperative measured keratometry. As a second option, we demonstrate in the paper how to derive the respective values from the postoperative (instead of the preoperative) measured keratometry.


RESULTS
Initial refraction before/after KRS, -12.0/-2.0 D; corneal thickness, 550/440 microm; front/back surface power 48.20-5.81 D, measured Zeiss keratometry before KRS, 42.5 D. After KRS, we calculate a corneal front surface power of 39.82 D and an equivalent/back vertex power and keratometry of 34.08/34.48/35.11 D (result of the "Clinical History Method" at spectacle/corneal plane 32.50/33.96 D). Calculated corneal power values are around 2-3 D lower than measured Zeiss keratometry (37.0 D), which will lead to an IOL power overestimation of about 3-4 D and subsequent hyperopia.


CONCLUSIONS
This formalism may help to prevent hyperopia after cataract surgery subsequent to refractive surgery for myopia.
Langenbucher
Achim
A
Department of Medical Physics, University of Erlangen-Nürnberg, Henkestrasse 91, D-91052 Erlangen, Germany. achim.langenbucher@imp.uni-erlangen.de
Scholz
Katja
K
Szentmáry
Nóra
N
Seitz
Berthold
B
D003315:Cornea / Q000473:pathology / Q000503:physiopathology* / Q000601:surgery*; D003941:Diagnostic Techniques, Ophthalmological; D005139:Eyeglasses; D006801:Humans; D053685:Laser Therapy; D007910:Lenses, Intraocular; D008954:Models, Biological; D055095:Optics and Photonics; D011184:Postoperative Period; D012029:Refraction, Ocular; D012030:Refractive Errors / Q000503:physiopathology; D054140:Refractive Surgical Procedures