The accuracy of IOL power calculations continues to improve with the use of modern instruments for biometry, updated formulas, and constant optimization. However, unusual eyes still present a challenge, and residual refractive error still occurs in a clinically significant proportion of normal cases, said Giacomo Savini, MD.
Nevertheless, it is still necessary to be careful in long and short eyes. In a paper published by Kane et al. [Kane JX, et al. J Cataract Refract Surg. 2016;42(10):1490-1500], a prediction error within 0.5 D of target was achieved in only about 60% of eyes that had an axial length <22 mm or >26 mm. Another recent paper reported that in hyperopic eyes with an axial length <22 mm, the Barrett Universal II was no more accurate than older formulas and resulted in a prediction error ≤0.5 D in less than 50% of eyes [Shrivastava AK, et al. J Cataract Refract Surg. 2018;44(11):1317-1320].
Eyes with keratoconus also present a challenge for achieving accurate refractive outcomes. The problem was highlighted by the findings of a retrospective analysis performed by Dr. Savini and colleagues that compared prediction errors occurring with the Barrett Universal II, Haigis, Holladay 1, Hoffer Q, and SRK/T formulas and dividing eyes according to keratoconus stage [Savini G, et al. J Cataract Refract Surg. 2019;45(5):576-581]. The results showed that all of the formulas were associated with a hyperopic result. The lowest mean arithmetic errors occurred with the SRK/T.
“Although the findings of this study were unexpected, in my opinion, the explanation for why the SRK/T performed best is that it trends toward resulting in a myopic outcome,” Dr. Savini noted. “Therefore, the error with the SRK/T compensates for the hyperopic error that generally occurs in keratoconic eyes.”
Dr. Savini pointed out that a solution will require more sophisticated methods for biometry.
Giacomo Savini, MD
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This article is based on a paper presented by Dr. Savini at the 37th Congress of the European Society of Cataract and Refractive Surgeons. Dr. Savini is a consultant to, a speaker for, and/or receives research support from companies that manufacture IOLs and instruments for biometry.