Alpins' optimal treatment

To understand Alpins' optimal treatment, you need to know the difference between with-the-rule (WTR) and against-the-rule (ATR) astigmatism. While surgeons can choose any emphasis they want in the range between treating 100% of the refractive astigmatism and 100% of the corneal astigmatism (see previous discussion), Alpins proposes a method to determine that emphasis based on the supposition that WTR is "better" than ATR astigmatism. There are studies to support this. The default "optimal treatment" setting programmed into the ASSORT program assumes that WTR is preferable to ATR astigmatism, as described below. However, the ASSORT user can change the default so that any orientation is preferred.

At the risk of oversimplifying, the ideal cornea, when viewed from above, would be a perfect circle. For people with regular astigmatism, the cornea is microscopically elliptical.

The base of a cornea with regular astigmatism is an ellipse. In the above example, the long axis of the ellipse (A) at 10° is perpendicular to the short axis (B). In the otherwise normal astigmatic human eye, the long axis can be found anywhere from 1° to 180°. However, astigmatism with the long axis near 180°, called with-the-rule astigmatism, is considered "better"; that is, people having WTR astigmatism can see better and report less handicap than people with similar degrees of ATR astigmatism.

Alpins' "optimal treatment," which is the optimal point of termination of the TIA with the ORA line (see previous discussion), is determined from the surgical emphasis graph shown above. In this example, the meridian of target topography is 147°. As it lies 57° from a with-the-rule orientation of 90°, the surgeon may decide to apportion 57 of 90, or 63% emphasis, to a topography-based goal of zero astigmatism (and the remainder, 37%, to refractive astigmatism). If the meridian of target topography is 90°, a physiologically more favorable orientation, 100% of the treatment will be devoted to the correction of refractive astigmatism. If the meridian of target topography is 180°, or against-the-rule, 100% of the treatment will be devoted to correcting the topographic astigmatism ("sphericizing" the cornea and eliminating the unfavorable against-the-rule astigmatism).

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