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In patients with corneal astigmatism, a cylindrical defect of about 0.50 diopter (D) to 1.00D may be considered clinically significant, and may influence visual acuity.1 It has been estimated that between 15 and 20% of patients undergoing cataract surgery have at least 1.5D of pre-existing corneal astigmatism.2 Increasing age is correlated with the prevalence and extent of corneal astigmatism. Up to 50% of the US population aged over 60 years has ≤1.00D of astigmatism,3 while pre-existing astigmatism exceeding 1.50D may be present in as many as 22% of potential cataract surgery patients.2,4,5 Also, with increasing age, the shift of the axis tends towards against-the-rule (ATR) astigmatism, in which the greater curvature or refractive power is in the horizontal meridian.6,7
There have been many studies evaluating astigmatism pre- and postcataract surgery. In one study undertaken to evaluate the type and the number of sutures that produce the least post-operative astigmatism, it was found that, on the whole, post-operative astigmatism tended to be of the same type or axis as pre-operatively, particularly in cases with pre-operative ATR astigmatism. There was no correlation between the amount of pre- and post-operative astigmatism, which accords with findings in another study.8,9 In a study to evaluate the factors that affect post-operative astigmatism, the magnitude of the pre-operative astigmatism was found to affect the magnitude of post-operative astigmatism. This indicates that phacoemulsification incisions other than those in the steepest meridian have little modulating effect on astigmatism in patients who have a large pre-existing astigmatism.10
Conventionally, spectacles and contact lenses have been used to improve or correct pre-existing corneal astigmatism. However, there is an increasing demand for distance vision spectacle independence and, furthermore, spectacles may not always be able to provide sufficient power to correct the refractive error or can cause distortion if not aligned properly with the axis of astigmatism. Also, given the high prevalence of pre-existing corneal astigmatism in cataract patients, it has become common practice during cataract surgery to correct both aphakia and pre-existing corneal astigmatism. This may allow the opportunity to improve the accuracy of refractive correction and of uncorrected visual acuity (UCVA), thereby optimizing visual outcomes after cataract surgery.