Abstract
PurposeThe present retrospective study was designed to test the hypothesis that the postoperative posterior to preoperative anterior corneal curvature radii (PPPA) ratio in eyes with Fuch’s dystrophy undergoing Descemet membrane endothelial keratoplasty (DMEK) is significantly different to the posterior to anterior corneal curvature radii (PA) ratio in virgin eyes and therefore renders conventional keratometry (K) and the corneal power derived by it invalid for intraocular lens (IOL) power calculation.MethodsMeasurement of corneal parameters was performed using Scheimpflug imaging (Pentacam HR, Oculus, Germany). In 125 eyes with Fuch’s dystrophy undergoing DMEK, a fictitious keratometer index was calculated based on the PPPA ratio. The preoperative and postoperative keratometer indices and PA ratios were also determined. Results were compared to those obtained in a control group consisting of 125 eyes without corneal pathologies. Calculated mean ratios and keratometer indices were then used to convert the anterior corneal radius in each eye before DMEK to postoperative posterior and total corneal power. To assess the most appropriate ratio and keratometer index, predicted and measured powers were compared using Bland-Altman plots.ResultsThe PPPA ratio determined in eyes with Fuch’s dystrophy undergoing DMEK was significantly different (P < 0.001) to the PA ratio in eyes without corneal pathologies. Using the mean PA ratio (0.822) and keratometer index (1.3283), calculated with the control group data to convert the anterior corneal radius before DMEK to power, leads to a significant (P < 0.001) underestimation of postoperative posterior negative corneal power (mean difference (∆ = − 0.14D ± 0.30) and overestimation of total corneal power (∆ = − 0.45D ± 1.08). The lowest prediction errors were found using the geometric mean PPPA ratio (0.806) and corresponding keratometer index (1.3273) to predict the postoperative posterior (∆ = − 0.01 ± 0.30) and total corneal powers (∆ = − 0.32D ± 1.08).ConclusionsCorneal power estimation using conventional K for IOL power calculation is invalid in eyes with Fuch’s dystrophy undergoing DMEK. To avoid an overestimation of corneal power and minimize the risk of a postoperative hyperopic shift, conventional K for IOL power calculation should be adjusted in eyes with Fuch’s dystrophy undergoing cataract surgery combined with DMEK. The fictitious PPPA ratio and keratometer index may guide further IOL power calculation methods to achieve this.
Highlights
Intraocular lens (IOL) power calculation is traditionally based on keratometers that estimate the corneal refractive power from anterior corneal measurements, by using a standardized fictitious refractive index (1.3320) referring to a theoretical single refractive lens representing both corneal surfaces
In eyes with Fuch’s dystrophy, a hyperopic shift has been reported when cataract surgery is combined with a Descemet membrane endothelial keratoplasty [5,6,7,8,9,10,11,12,13]
To assess the validity of conventional keratometry in eyes with Fuch’s dystrophy undergoing DMEK, the RPPPAFECD/DMEK was compared to the PA ratio in the control group consisting of healthy corneas
Summary
Intraocular lens (IOL) power calculation is traditionally based on keratometers that estimate the corneal refractive power from anterior corneal measurements, by using a standardized fictitious refractive index (1.3320) referring to a theoretical single refractive lens representing both corneal surfaces This so-called conventional keratometry (K) is used in IOL Master PCI devices for corneal power estimation and assumes a constant posterior to anterior corneal curvature radii ratio (PA ratio or RPA) [1]. In eyes with Fuch’s dystrophy, a hyperopic shift has been reported when cataract surgery is combined with a Descemet membrane endothelial keratoplasty (triple DMEK) [5,6,7,8,9,10,11,12,13] This has been attributed to regression of the posterior stroma edema associated with a steepening of the posterior corneal curvature [5,6,7,8,9,10,11,12,13]. The decisive ratio for IOL power calculation in eyes with Fuch’s dystrophy, is the ratio between postoperative posterior corneal radius, once stable refraction is achieved, and preoperative anterior corneal radius, when conventional K is performed (Table 1)
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