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Tianyu Zheng, Yi Lu, ; Estimation Errors for Total Corneal Astigmatism Using Keratomatric Astigmatism in Patients before Cataract Surgery. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1904.
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© ARVO (1962-2015); The Authors (2016-present)
Since more and more people are willing to correct corneal astigmatism during cataract surgery, the accuracy of preoperative estimation of corneal astigmatism is becoming increasingly important. Previous studies found the widely used keratomaric method could lead to significant estimation errors for total corneal astigmatism. However, in previous studies, the part of elder subjects was relatively limited. This study focused on the elder patients preparing for age-related cataract surgery, to evaluate the estimation errors for total corneal astigmatism using keratometric astigmatism (KA).
A prospective, observational study.374 eyes of 374 patients (45 to 84 years) preparing for cataract surgery were measured with Pentacam. KA was obtained using the anterior corneal surface measurement and the keratometric index while neglecting the posterior corneal surface measurement. The Pentacam-derived total corneal astigmatism (PA) was derived by vector analysis of the astigmatism on both corneal surfaces.
The magnitude of anterior and posterior corneal astigmatism was 0.99 ± 0.71 D and 0.25 ± 0.14 D respectively. Posterior corneal astigmatism decreased with age. The mean magnitude was 0.89 ± 0.63 D for KA and 0.97 ± 0.69 D for PA. The absolute error of the magnitude (EM) and angle (EA) between PA and KA were 0.18 ± 0.14 D and 7.7 ± 11.0 degree. The magnitude of the error vector (EV) was 0.24 ± 0.14 D. 19 eyes (5.1%) had an EV larger than 0.5 D. KA overestimated total corneal astigmatism in eyes with with-the-rule anterior astigmatism, and underestimated that in the others. EM was significantly larger in eyes with against-the-rule anterior astigmatism, but EA was larger in eyes with with-the-rule and oblique anterior astigmatism. In eyes with KA larger than 2 D, or elder than 65 years, there was a remarkable increase in EM. And EM was higher in eyes with axial length of 20 to 24 mm, than eyes with a longer axial length (>28 mm).
Neglecting posterior corneal astigmatism yielded significant estimation errors for total corneal astigmatism in patients preparing for cataract surgery, especially in patients with larger magnitude of KA, elder age, and relatively shorter axial length.
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