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Thank you for the opportunity to reply to the letter from Alpins concerning our recent article in the BJO.1,2
Noel Alpins is a widely respected contributor to many international meetings, having written comprehensively on the use of astigmatism vector analysis. His software program assort is widely used for the planning of refractive surgery and provides many derived indices (transformations) from the vector analysis of both refractive and topographic astigmatism. Although the derived indices are summary measures, we have argued that their usefulness for statistical analysis is limited. This is because the perception of astigmatism is a psychophysical phenomenon altered by the orientation of the axis of astigmatism (the power meridians of the cornea and crystalline lens). Unfortunately, the perceptual response means that the measurement of the axis of astigmatism (which is with an arbitrary 180° scale) is non-linear in outcome terms, as related to visual acuity outcome. Astigmatism obliquity is the least desirable outcome but this is separated into two on the scale “with the rule” astigmatism (WTR), which is generally the most desirable outcome. Oblique astigmatism also separates the two groups of “against the rule” astigmatism (ATR) from the WTR astigmatism. Developments of …
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