Anterior surface-based keratometry compared with Scheimpflug tomography-based total corneal astigmatism.

PURPOSE: Description of total corneal refractive power (TCRP) astigmatism by ray tracing using a Scheimpflug tomographer.

METHODS: Analysis of Scheimpflug tomography measurements to determine 3-mm-zone TCRP astigmatism (CATCRP) by ray tracing calculations, astigmatism in 15°-zone from simulated keratometry (CASim-K), anterior-surface astigmatism (CAant), and posterior-surface astigmatism (CApost). An analysis of changes with age was performed. The error produced by estimating CATCRP using CASim-K was calculated through vector analyses. The percentages of steep meridian aligned vertically, obliquely, and horizontally of CAant and CApost were calculated.

RESULTS: A total of 3818 healthy, previously unoperated eyes were analyzed. The mean magnitude of CApost was -0.33 diopter (D). A vertically aligned steep meridian (60° to 120°) was found in 71.2% of eyes for CAant and in 88.8% for CApost. With increasing age, the occurrence of steep meridian aligned horizontally in CAant increased from 9.1% (age 20-29 years) to 31.8% (70-79 years), while vertical alignment decreased from 77.4% to 48.1%. Alignment of CApost mostly remained vertical (90.9%-80.7%). When CAant is vertical, CApost is vertical in 97% and CASim-K overestimates total corneal astigmatism (mean 0.11 ± 0.22 D). When CAant is horizontal, 18% of CApost is horizontal and CASim-K underestimates (mean 0.26 ± 0.31 D) total corneal astigmatism. The mean vector difference between CATCRP and CASim-K was 0.18 at 178° and exceeded 0.50 D in 9% of eyes.

CONCLUSIONS: Total corneal astigmatism is overestimated by CASim-K in eyes with with-the-rule astigmatism and cannot be predicted safely in eyes that do not have with-the-rule astigmatism by anterior measurements only.