Comparison of Corneal Astigmatism Magnitude and Axis Measured by Ray Tracing, Simulated Keratometry, and Vector Analysis
Purpose
To compare corneal astigmatism derived from total corneal power (TCP) calculated by ray tracing, with simulated keratometry (SimK) and vector analysis.
Methods
This retrospective study was based on corneal power measurements form 62 normal eyes of 40 consecutive patients candidate for cataract surgery with a minimal astigmatism of 1.5 diopter. Measurements were obtained using a Scheimpflug camera (Pentacam HR, Oculus) and included: Total corneal astigmatism from total corneal power (TCP), corneal astigmatism from SimK, anterior corneal astigmatism and posterior corneal astigmatism. All included measurements were recorded from a 3 mm ring centered on the corneal apex.Vector analysis was used to calculate the summation of anterior and posterior astigmatism.
Results
Mean TCP astigmatism (3.1±1.0D) was significantly higher than SimK astigmatism (3.0±1.0D,p< 0.001) with an absolute difference ≤ 0.2D and ≥0.5D in 70% and 14.5% of the measurements, respectively and a mean of 0.21±0.17D.TCP was also significantly higher than mean astigmatism calculated by vector analysis (2.9±1.0D,p< 0.001) with a mean absolute difference of 0.27± 0.16D.The difference in axis between TCP and SimK was ≤ 3.0 degrees and ≥ 5.0 degrees in 81% and 9% of the measurements, respectively with a mean of 2.1±1.9 degrees (range 0-8.7 degrees).Mean difference in axis between TCP and vector analysis was 2.1±2.4 degrees (range 0.1-10.3 degrees).
Conclusion
In this cohort of subjects, the differences between TCP, SimK and vector analysis were statistically but not clinically significant in most of the cases, although in some cases clinically significance was noted. Further studies need to compare the impact of these modalities on outcomes with toric intraocular lenses.