## How do you calculate corneal astigmatism from K readings?

Corneal astigmatism that is calculated by the Sim-K: in this method, corneal astigmatism is calculated by the data from keratometry in 1–4 mm of the central cornea and the total astigmatism which is the difference between the steepest and the flattest meridian, measured by keratometry and in line with the steep …

What is against the rule astigmatism?

If we view the eye as a sphere, “against-the-rule” is astigmatism where the steepest curve lies near the 180-degree meridian (imaginary line connecting east and west points of the cornea), and “with-the-rule” astigmatism (line connecting north and south points) is near the 90-degree meridian.

How do you calculate refractive astigmatism?

To quantify the discrepancy between corneal and refractive astigmatism measurements, calculate the vectorial difference between the refractive cylinder—measured by wavefront or manifest refraction—and the corneal astigmatism measured by topography or keratometry.

### What is K1 and K2 in keratometry?

Keratometric changes with cycloplegia. K1: flat meridian of the anterior corneal surface, K2: steep meridian of the anterior corneal surface.

What is Keratometric astigmatism?

Keratometry (K) is the measurement of the corneal curvature; corneal curvature determines the power of the cornea. Differences in power across the cornea (opposite meridians) results in astigmatism; therefore, keratometry measures astigmatism.

How is internal astigmatism calculated?

Internal astigmatism was calculated as refractive minus corneal power values. This calculation is essentially based on a two-refracting- component schematic model of the eye in which the principal planes of cornea and internal optics are coincident.

#### Why is it called with the rule astigmatism?

“With-the-rule” astigmatism occurs when the vertical meridian of the cornea is steepest. Consider a football shape lying on its side, and the vertical meridian of the football is the steepest curve. For these cases, spectacle lenses are fabricated with a minus cylinder placed in the horizontal axis.

What is with the rule vs against the rule astigmatism?

In with-the-rule (WTR) astigmatism, there is greater refractive power (and increased curvature) along the vertical meridian than along the horizontal. Conversely, in against-the-rule astigmatism (ATR), the higher refractive power is along the horizontal meridian.

What is total astigmatism?

The contribution of the posterior cornea is small, given the slight difference between the indices of refraction of the cornea and aqueous compared to that of the cornea and air. Thus, total astigmatism is the sum of corneal and residual astigmatism.

## How do you identify the rule and against the rule astigmatism?

This is when the axis of the positive cylinder in a pair of glasses is oriented at 90 degrees. “With the rule” astigmatism is typically seen in children, while “against the rule astigmatism” is seen in older eyes.

How is keratometry measured?

1. A keratometer. This device measures the curvature of the anterior corneal surface based on the power of a reflecting surface. It does this by measuring the size of an image reflected from 2 paracentral points and utilizes doubling prisms to stabilize the image enabling more accurate focusing.

What is the Keratometric index of cornea?

The keratometric index is an effective index that accounts for the negative power introduced by the posterior corneal surface. Consequently, keratometry attempts to predict the total corneal power based only on a measurement of the anterior corneal surface.