Assessing Total Keratometry Astigmatism, Simulated Keratometry, and Total Corneal Topographic Astigmatism Against Manifest Refractive Cylinder.

Purpose: To evaluate how the Total Keratometry astigmatism measure from a swept-source optical biometer compares with simulated keratometry astigmatism from the same device, and total corneal topographic astigmatism (CorT Total) derived from a Scheimpflug tomographer.

Methods: For normal virgin eyes, the ocular residual astigmatism (ORA) magnitudes were determined based on Total Keratometry, simulated keratometry from the same optical biometer (SimK), and CorT Total and simulated keratometry (SimK) from a Scheimpflug tomographer. The ORA magnitudes for each type of measure were summarized into the standard deviations (ORAsd) and means (ORAmean). The lower the ORAsd, the less variability there is between corneal astigmatism and manifest refractive cylinder. The ORAmean indicates the amount of vectorial difference between the total corneal astigmatism and manifest refractive cylinder.

Results: The ORAsd for Total Keratometry was not significantly different from the ORAsd for CorT Total ( = .06) or SimK ( = .41). The ORAmean for Total Keratometry was not significantly different from the ORAmean for CorT Total ( = .15), but was significantly lower than the ORAmean for SimK ( < .001).

Conclusions: Total Keratometry astigmatism correlates as well with manifest refractive cylinder as simulated keratometry astigmatism from the same device and CorT Total from a Scheimpflug tomographer. The average difference (as quantified by the ORAmean) between Total Keratometry astigmatism and manifest refractive cylinder was comparable to that of CorT Total, and less than that of simulated keratometry. Both of these results support the use of Total Keratometry over simulated keratometry in the planning of astigmatism surgery when corneal values are required. .