Ocular residual astigmatism and surgical emphasis

The three figures below depict a patient having a discrepancy between refractive astigmatism (R) and corneal topographic astigmatism (T), a situation found in most patients with astigmatism.

This DAVD shows a patient having a discrepancy between refractive astigmatism (R) and corneal topographic astigmatism (T), whose targeted treatment is based 100% on T. The vector between R and T is the ocular residual astigmatism (ORA), the minimal amount of astigmatism that can remain in the optical system of this eye. The target refraction is the amount of refractive astigmatism remaining after treatment to eliminate topographic astigmatism; that is, the cornea would be spherical but the patient would have a remaining refractive astigmatism equal to the target refraction (and ORA) shown. The treatment is shown as a vector of equivalent magnitude to T, but 180° away from T on the DAVD (actual steepening treatment on the cornea would be 90° away).

DAVD shows same patient as above, but with correction targeted 100% on correcting refraction. Target topography is the corneal topographic astigmatism remaining after treatment to eliminate refractive astigmatism. The treatment vector has an equivalent magnitude to R, but is 180° away from R on the DAVD (actual steepening treatment on the cornea would be 90° away).

An intermediate TIA can be chosen between the boundaries of the topographic TIA and refractive TIA. The relative proximity of the intersection to either the topographic or refractive endpoints (heavy dashed line) is determined by the emphasis of treatment required (total will equal 100%). Any TIA that achieves the minimum target astigmatism for the prevailing topographic and refractive parameters will terminate on the ORA line. The surgical emphasis (topography versus refraction) can be selected using Alpins' "optimal treatment," which is ultimately determined by the orientation of the ORA and the individual T and R values of each patient. Optimal treatment is described in more detail later in the article, and is depicted graphically here.

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