The comparison of central and mean true-net power (Pentacam) in calculating IOL-power after refractive surgery

Abstract

Purpose: To compare the accuracy of central true net corneal power (cTNP) and mean true net corneal power (mTNP) of the Pentacam system to give a keratometry (K) reading for calculating IOL (intraocular lens) power in eyes following refractive surgery.

Methods: Refraction, an automated K-reading (Km), cTNP and mTNP were measured for 15 eyes that required cataract surgery and had previously undergone refractive surgery. The difference between postoperative manifest refraction values and target refraction values calculated with the SRK/T formula using cTNP were compared with the one using mTNP.

Results: The mean deviation from the desired post-cataract refractive outcome was 0.60 diopter (D)+/-0.47 (standard deviation) using cTNP; 0.75+/-0.54 using mTNP (p=0.386). The actual refraction was within +/-0.50D of the intended refraction for 60% (cTNP) and 33.3% (mTNP) of eyes, and within +/-1.00D for 93% (cTNP) and 66.7% (mTNP) of eyes.

Conclusions: Although not statistically significant, the cTNP showed better accuracy than mTNP to give a keratometry (K) reading for post-refractive surgery eyes requiring cataract surgery.

Keywords: Corneal true net power; Intraocular lens implantation; Pentacam.

Fig. 1

Screen-map shot of the true net power map of the Pentacam system. The exact central K-value in the true net power map was used in this study.

Fig. 2

(A) Schematic conceptual relationship between the anterior and posterior corneal curvature in the unaffected eye. Note that the two curves are approximately parallel. (B) Schematic conceptual corneal shape change after RK. Note that the two surfaces are near parallel, but they are significantly flatter than the unoperated cornea in A. (C) Schematic conceptualized drawing of corneal shape after excimer photoablation for myopia. Note that the anterior surface is considerably flatter than the posterior surface. (D) Schematic conceptualized drawing of the corneal shape after hyperopic excimer photoablation. Note that the anterior surface is steeper than the posterior surface.