Astigmatic keratotomy involves placement of arcuate relaxing incisions in the cornea stroma to reduce astigmatism.21-22 The goal is to reduce, not necessarily eliminate, astigmatism to achieve better visual acuity and allow patients to achieve correction with spectacles or contact lenses. A coupling phenomenon is seen in which there is flattening of the incised steep meridian accompanied by steepening of the unincised meridian 90 degrees away, the ratio of which is termed the coupling ratio. A coupling ratio of 1 leads to no change in spherical equivalent, whereas a ratio > 1 will lead to hyperopic shift and a ratio of < 1 will induce a myopic shift.
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