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Ashleigh L. Levison, Ying Han, Cynthia S. Chiu; Comparison of Refractive Results between IOL Master and Immersion A-scan Ultrasonography. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5686.
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Accurate biometry is essential for successful cataract surgery. Immersion A-scan ultrasound (US), the gold-standard for measuring axial length, has been replaced by the IOL Master (IOLM) in many ophthalmic practices. Controversy exists as to which method is superior in regard to measuring axial length. The purpose of this study is to compare refractive results using the IOLM, which is an operator-independent method of measuring axial length and corneal power, against the traditional combination of measuring axial length using the Immersion A-scan and corneal power acquired from autorefractor or manual keratometry.
Preoperative axial length measurements were taken with the IOLM and Immersion A-scan US in 582 cataractous eyes undergoing cataract surgery at the University of California, San Francisco from October 2006 to April 2010. The patients also underwent autorefractor keratometry, manual keratometry, and keratometry measurements using the IOLM. The Holladay 1 IOL formula was used to select the power of the intraocular lens for all methods. Of the 582 eyes in the study, 529 (91%) had a documented best corrected visual acuity (BCVA) determined by manifest refraction at least 1 month post-operatively. The spherical equivalent of each patient’s post-operative refraction was compared to the refractive error predicted by the IOLM and by the combination of Immersion A-scan and manual keratometry.
The axial length is equivalent between the IOLM and the Immersion A-scan US. Although the average corneal power measured by the IOLM and manual keratometry/autorefractor keratometry are highly correlated, on average the IOLM measures steeper power than the autorefractor and flatter power compared to manual keratometry. Refractive outcomes were good -- 214 out of 306 patients (70%) had uncorrected visual acuity (UCVA) within 0.5D of predicted using Immersion A-scan/manual keratometry and 238 of 306 patients (78%) had UCVA within 0.5D of predicted using the IOLM. Likely the source of error in the population who did not achieve UCVA within 0.5D of predicted is either from patient-specific factors (dense cataract, difficult positioning for measurements) or biases in measuring final refractive error.
Axial length measured by the IOLM and immersion A-scan US are equivalent. The IOLM is better at predicting final refractive outcome. The error in predicting refractive result using the IOLM may be due to an error in keratometry measurements. Although refractive outcomes for both methods are good, there is still a large margin of error for both.
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