Vector summation of anterior and posterior corneal topographical astigmatism

Abstract

Consideration of the Orbscan measurement of posterior corneal surface toricity may improve the prediction of the magnitude of refractive astigmatism.

Contribution of posterior corneal astigmatism to total corneal astigmatism

“…[7][8][9][10] In these studies, total corneal astigmatism was calculated by vector summation of anterior and posterior corneal astigmatism, assuming parallel rays approaching the posterior corneal surface and ignoring the contribution of corneal thickness.…”

Comparison of corneal astigmatism and axis location in cataract patients measured by total corneal power, automated keratometry, and simulated keratometry

“…15 Since then, studies 16,17 have shown the benefit of using TCP, in particular in the estimation of the corneal power in IOL calculation after refractive surgery. Other studies 18,19 report good IOL power calculation outcomes using the TCP value from a dual Scheimpflug 20 found an improvement in the prediction of the refractive astigmatism when the posterior corneal surface measurement was added to the calculation. Ho et al 10 found an error in corneal astigmatism magnitude estimation of more than 0.50 D in 28.8% of eyes using the anterior corneal measurement only.…”

A review of astigmatism and its possible genesis

“…[17][18][19][20] These studies have found levels of astigmatism for the posterior cornea ranging from 0.18-0.31 D. The curvature of the posterior cornea combined with the refractive index difference between the cornea and the aqueous means that the posterior corneal astigmatism is of opposite sign to that of the anterior cornea. Therefore, the compensation of corneal astigmatism by the eye's internal optics can be attributed, in part, to the astigmatism of the posterior cornea.…”