Corneal Topographic Astigmatism and Total Corneal Power
Purpose
To evaluate the performance of corneal topographic astigmatism (CorT) based on total corneal power measurements, as compared to a CorT based on anterior corneal power measurements alone without consideration of posterior power.
Methods
Anterior, posterior, and total corneal power measurements obtained using tomography were analyzed. From each set of data, CorTs were created and assessed using the ocular residual astigmatism (ORA), which quantifies corneo-refractive differences. A low standard deviation of the ocular residual astigmatism (ORAsd) indicates a low variability between corneal astigmatism and refractive cylinder. A low mean of the ocular residual astigmatism magnitude indicates a close correlation of refractive cylinder and corneal astigmatism and greater parameter reliability.
Results
The CorT based on total corneal power measurements had an ORAsd of 0.30 D and a mean ORA magnitude of 0.53 D, and was calculated by using measurements from a corneal annulus of 2.0 – 5.2 mm in diameter. The CorT candidates based on anterior corneal power measurements all had an ORAsd of at least 0.32 D, and the mean ORA magnitudes were all 0.64 D or greater. Both the ORAsd and mean ORA magnitude of the CorT based on total corneal power measurements were significantly less than those of the CorT based on anterior corneal power measurements (both p < 0.001).
Conclusion
The CorT based on total corneal power measurements corresponds better, both in variability and closeness, with manifest refractive cylinder than the CorT based on anterior corneal power measurements. This total CorT provides a more valuable corneal parameter for toric IOL and femto/LRI surgery with further consistency for the effect of posterior corneal power on total corneal astigmatism.