Vector summation of anterior and posterior corneal topographical astigmatism

Purpose: To determine whether vector addition of posterior corneal astigmatism improves the correlation between topographical and refractive astigmatism. Setting: Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts, USA. Methods: Orbscan topographical maps of 40 eyes (31 patients) showing 1.0 to 6.0 diopters (D) of astigmatism were analyzed. Topographical anterior and posterior corneal surface astigmatism was determined. Refractive astigmatism was compared to topographical astigmatism using 3 methods: anterior topographical astigmatism, overall topographical astigmatism obtained by vector summation of anterior and posterior topographical astigmatism, and simulated keratometry (SimK). Results: Refractive astigmatism ranged from 0.25 to 5.00 D. The mean error in magnitude between topographical and refractive astigmatism was significantly smaller with the overall topographical method (1.06 D ± 0.92 [SD]) than the anterior topographical method (1.37 ± 1.04 D) (P < .0001). The mean error in axis between topographical and refractive astigmatism was smaller with the overall topographical method (10.4 ± 13 degrees) than with the anterior topographical method (15.5 ± 30.6 degrees) and with SimK (13.3 ± 15.1 degrees), but these differences were not statistically significant. Conclusion: Consideration of the Orbscan measurement of posterior corneal surface toricity may improve the prediction of the magnitude of refractive astigmatism.