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 (CA(TCRP)) by ray tracing calculations, astigmatism in 15 degrees-zone from simulated keratometry (CA(Sim-K)), anterior-surface astigmatism (CA(ant)), and posterior-surface astigmatism (CA(post)). An analysis of changes with age was performed. The error produced by estimating CA(TCRP) using CA(Sim-K) was calculated through vector analyses. The percentages of steep meridian aligned vertically, obliquely, and horizontally of CA(ant) and CA(post) were calculated. RESULTS. A total of 3818 healthy, previously unoperated eyes were analyzed. The mean magnitude of CA(post) was -0.33 diopter (D). A vertically aligned steep meridian (60 degrees to 120 degrees) was found in 71.2% of eyes for CA(ant) and in 88.8% for CA(post). With increasing age, the occurrence of steep meridian aligned horizontally in CA(ant) 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 CA(post) mostly remained vertical (90.9%-80.7%). When CA(ant) is vertical, CA(post) is vertical in 97% and CA(Sim-K) overestimates total corneal astigmatism (mean 0.11 +/- 0.22 D). When CA(ant) is horizontal, 18% of CA(post) is horizontal and CA(Sim-K) underestimates (mean 0.26 +/- 0.31 D) total corneal astigmatism. The mean vector difference between CA(TCRP) and CA(Sim-K) was 0.18 at 178 degrees and exceeded 0.50 D in 9% of eyes. CONCLUSIONS. Total corneal astigmatism is overestimated by CA(Sim-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.