Posterior corneal astigmatism.

In the optical system of the human eye, total refractive power consists mainly of corneal and lenticular components. Approximately two-thirds of the eye's converging power is at the air–cornea (tear film) interface; one-third is in the lens. Total corneal power is determined by corneal anterior and posterior curvature, corneal thickness, and the corneal refractive indices. Traditionally, anterior corneal curvature is measured with keratometers or computerized videokeratographers that compensate for the posterior corneal surface using a modified refractive index to convert measurements of anterior corneal curvature to the refractive power of the entire cornea by empirical estimations of the corneal thickness and posterior surface curvature. Currently, posterior corneal curvature can be examined by Purkinje images, scanning-slit imaging, Scheimpflug imaging, and optical coherence tomography. With these tools, total corneal refractive power can be determined by measuring anterior and posterior corneal curvatures as well as corneal thickness. Methods for calculating total corneal power include ray tracing and the Gaussian optics thick-lens formula. Astigmatism is an important and often visually significant optical aberration. In an optical system with astigmatism, rays that propagate in 2 perpendicular planes result in 2 different focal lines. To determine the contribution of posterior corneal astigmatism to total corneal astigmatism, Koch et al. performed a study using a Scheimpflug tomographer combined with a Placido disk topographer. They found that in 715 corneas of 435 consecutive patients, the mean magnitude of posterior corneal astigmatism was 0.30 diopter (D). The study showed that with increasing age, the steep anterior corneal meridian tended to shift from vertical to horizontal, especially in eyes with low degrees of astigmatism, while the steep posterior corneal meridian typically did not change. The magnitudes of anterior and posterior corneal astigmatismwere correlated when the steeper anterior meridian was aligned vertically but not when it was aligned horizontally. Koch et al. also examined the error in estimating total corneal astigmatism from anterior corneal measurements only and found that anterior corneal measurements underestimated total corneal astigmatism by a mean of 0.22 D and exceeded 0.50 D in 5% of eyes. Similar values were found by Ho et al., confirming that neglecting the posterior corneal surface measurement may lead to significant deviation in the corneal astigmatism estimation in a proportion of eyes.