Calculations of Corneal Power After Corneo-Refractive Surgery from Keratometry and Change of Spectacle Refraction: Some Considerations on the “Clinical History Method”

Abstract

Purpose: To derive corneal power after kerato-refractive laser surgery (KRS) to be used for a subsequent intraocular lens (IOL) power calculation. Model: 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. Example: Initial refraction before/after KRS, −12.0/–2.0 D; corneal thickness, 550/440 μ m; 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.

KEYWORDS:

• clinical history method
• IOL power calculations
• keratometry
• paraxial optics
• refractive surgery