Prediction of corneal power vectors after cataract surgery with toric lens implantation–A vector analysis

Abstract

Background Intraocular lenses are typically calculated based on a pseudophakic eye model, and for toric lenses (tIOL) a good estimate of corneal astigmatism after cataract surgery is required in addition to the equivalent corneal power. The purpose of this study was to investigate the differences between the preoperative IOLMaster (IOLM) and the preoperative and postoperative Casia2 (CASIA) tomographic measurements of corneal power in a cataractous population with tIOL implantation, and to predict total power (TP) from the IOLM and CASIA keratometric measurements. Methods The analysis was based on a dataset of 88 eyes of 88 patients from 1 clinical centre before and after tIOL implantation. All IOLM and CASIA keratometric and total corneal power measurements were converted to power vector components, and the differences between preoperative IOLM or CASIA and postoperative CASIA measurements were assessed. Feedforward neural network and multivariate linear regression prediction algorithms were implemented to predict the postoperative total corneal power (as a reference for tIOL calculation) from the preoperative IOLM and CASIA keratometric measurements. Results On average, the preoperative IOLM keratometric / total corneal power under- / overestimates the postoperative CASIA keratometric / real corneal power by 0.12 dpt / 0.21 dpt. The prediction of postoperative CASIA real power from preoperative IOLM or CASIA keratometry shows that postoperative total corneal power is systematically (0.18 dpt / 0.27 dpt) shifted towards astigmatism against the rule, which is not reflected by keratometry. The correlation of postoperative CASIA real power to the corresponding preoperative CASIA values is better than those as compared to the preoperative IOLM keratometry. However, there is a large variation from preoperative IOLM or CASIA keratometry to the postoperative CASIA real power of up to 1.1 dpt (95% confidence interval). Conclusion One of the challenges of tIOL calculation is the prediction of postoperative total corneal power from preoperative keratometry. Keratometric power restricted to a front surface measurement does not fully reflect the situation of corneal back surface astigmatism, which typically adds some extra against the rule astigmatism.

Astigmatism Management in Modern Cataract Surgery

“…Such topographers, and manual or automated keratometers, are limited in their ability to measure beyond the anterior corneal surface. As a result, they do not fully capture the astigmatism (usually ATR) contributed by posterior corneal curvature [ 21 ]. Over one-third of ophthalmologists utilize topography automated biometry (IOLMaster/Lenstar) as the primary preoperative measure prior to toric IOL implantation, according to the most recent clinical survey by the American Society of Cataract and Refractive Surgeons (ASCRS) [ 22 ].…”

Vector analysis of corneal astigmatism in cataractous eyes based on IOLMaster 700 biometry

“…From the curvature and axis data of K, BS and TK (K: (FR1, FR2, FA1; BS: BR1, BR2, BA1; TK: TKR1, TKR2, TKA1)) we derived the respective power vector components as: (K: FSEQ = 0.5 (332/FR1 +332/FR2), FC0 = (332/FR2-332/FR1) cos(2�FA1), FC45 = (332/FR2-332/FR1) sin(2�FA1); BS: BSEQ = -0.5 (40/BR1+40/BR2), BC0 = (40/BR2-40/BR1) cos(2�BA1), BC45 = (40/BR1-40/ BR2) sin(2�BA1); TK: TKSEQ = 0.5 (332/TKR1+332/TKR2), TKC0 = (332/TKR2-332/TKR1) cos(2�TKA1), TKC45 = (332/TKR2-332/TKR1) sin(2�TKA1)) based on a keratometer index n K = 1.332 and refractive indices for cornea n C = 1.376 and aqueous humour n A = 1.336 derived from the Liou-Brennan schematic model eye [10]. Since the calculation strategy for Total Keratometry in the IOLMaster 700 software is undisclosed, we calculated the power vector components for the corneal front vertex power (FV)(equivalent power FVSEQ and projections of the astigmatism to the 0/90˚FVC0 and 45˚/135˚meridian FVC45) as a representation of total corneal power at the corneal front vertex plane using a calculation scheme for toric vergences as described in previous papers [7,8].…”

“…As a simplification, assuming bivariate normal distributions for the astigmatic power vector components, 95% error ellipses were calculated based on the covariance matrix and the ellipse area and the centroid were derived [7]. In addition, the 95% confidence region was calculated without assumptions of parametric distributions by iteratively eroding the data clouds according to their convex hull envelope (iterative convex hull stripping) [19,20], and the area of the 95% confidence region and the spatial median (medoid) were derived [20][21][22].…”

“…For generating a prediction model to map the astigmatism vector components K to all eyes were considered as right eyes as outlined before [7,11]. Various models (multivariate (bivariate) linear regression using the ECM algorithm [23], univariate linear regression with a separation of the 2 vector components, and constant model) were defined and analysed in their performance [24].…”

“…However, a decomposition of astigmatism into vector components is appropriate in all cases [6]. In this context, corneal power as clinically described in terms of sphere, cylinder and axis has to be transformed in 3-dimensional vector components with equivalent power' and the astigmatic projections to the horizontal and vertical (0˚and 90˚) and the oblique (45˚and 135˚) meridians [7].…”