Managing Astigmatism

Advances in power calculation methods enable good results in routine cases

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Adi Abulafia MD
While toric IOL implantation has become a standard of care for correcting regular astigmatism at the time of cataract surgery, the refractive results following their implantation can be unpredictable, Adi Abulafia MD 
told a session of the XXXV ESCRS Congress in Lisbon. He spoke during a clinical research symposium in which speakers discussed ways to optimise outcomes across a spectrum of cases focused on calculation strategies in routine cases. He told attendees that the available toric IOL calculators are differentiated mainly by two features: 1) use of a fixed ratio versus estimated effective lens position (ELP) to calculate the IOL cylinder power at the corneal plane; and 2) whether or not 
they incorporate an algorithm to estimate the net corneal astigmatism. 
His advice was to use a calculator that both takes into account ELP and estimates net corneal astigmatism. “Currently, it appears that more accurate results can be obtained using mathematical models that account for posterior cornea astigmatism rather than a direct measurement. Hopefully in the near future, however, we can use direct measurements of the posterior cornea,” said Dr Abulafia, Director of Cataract Services, Ophthalmology Department, Shaare Zedek Medical Center, Jerusalem, Israel. SIMILAR ACCURACY Dr Abulafia explained that ELP affects the refractive outcome with a toric IOL because the amount of induced cylinder power at the corneal plane depends on where the lens sits in the eye. Although in two studies Dr Abulafia and colleagues found similar accuracy when comparing calculators using ELP versus a fixed ratio, he said the results with these two approaches can be very different for the individual, non-average eye. To illustrate his point, Dr Abulafia presented a theoretical case involving a highly myopic eye with relatively high astigmatism. When choosing the same IOL model, the residual astigmatism predicted with the two types of calculators differed by 0.7D. ACCOUNTING FOR THE POSTERIOR CORNEA Several mathematical models that calculate net corneal astigmatism from anterior cornea-based measurements have been developed – Barrett Toric calculator, Abulafia-Koch, and Johnson & Johnson (Koch) – and are used by various online and commercial calculators, sometimes as an on/off option. While these models are not perfect – available data show that the predicted residual astigmatism is ≤0.5D in only about 80% of eyes – another study by Dr Abulafia and colleagues indicated more accurate outcomes were achieved when toric IOL calculations were performed with one of the mathematical models (the Barrett Toric calculator) than if Scheimpflug-based direct measurements of the posterior corneal curvature were used. Dr Abulafia said that surgeons should make sure the tear film is normal and that the corneal astigmatism is symmetric and regular when obtaining preoperative measurements. Reiterating the recommendation of Warren Hill MD, he said to use primary and secondary supporting instruments to verify the steep meridian and to repeat the same proses for the power difference between the steep and flat meridians. He also noted that the mathematical models were not built for performing toric IOL calculations in eyes with unusual corneas, such as those with keratoconus or a history of corneal refractive or transplant surgery, and he cautioned that mathematical models should never be used with total corneal measurements. “You will correct the same error twice, and you will be wrong,” Dr Abulafia said. Adi Abulafia: adi.abulafia@gmail.com