Steep Corneas Require Unique IOL Power Calculation Assessment

In eyes with long axial lengths, keratometry has a significant affect on intraocular lens (IOL) power calculations, according to results of a study published in International Ophthalmology.

Patients (N=180) who underwent phacoemulsification cataract surgery with Rayner 920H IOL implantation between 2018 and 2021 at the Joint Shantou International Eye Center of Shantou University and The Chinese University of Hong Kong were retrospectively reviewed for axial length. Eyes with an axial length of 26.0 mm or longer were evaluated for accurate IOL power calculation using the Olsen, Barrett Universal II (Barrett II), Sanders-Retzlaff-Kraff trial (SRK/T), and Haigis formulas on the basis of having flat eyes (keratometry ≤42.0 D; n=22), steep eyes (keratometry ≥46.0 D; n=23), or average eyes (keratometry 42.0-46.0 D; n=135).

Patients were divided into 3 cohorts: flat (12 men, 10 women; mean age, 64.77±10.83 years; axial length, ​​28.68±2.58 mm; anterior chamber depth, 3.38±0.31 mm; lens thickness, 4.57±0.46 mm; white to white, 12.04±0.39 mm), steep (2 men, 21 women; mean age, 60.74±11.21 years; axial length, 28.78±2.04 mm; anterior chamber depth, 3.42±0.41 mm; lens thickness, 4.31±0.43 mm; white to white, 11.31±0.28 mm), and normal keratometry  (46 men, 89 women; mean age, 59.79±9.94 years; axial length, 29.56±2.21 mm; anterior chamber depth, 3.51±0.44 mm; lens thickness, 4.44±0.43 mm; white to white, 11.62±0.41 mm).

Among all cohorts, mean prediction errors did not differ significantly between IOL calculation formulas. For the mean absolute errors and median absolute errors, significant differences were observed in all groups (all P ≤.015).

The smallest and greatest mean absolute errors in each group were observed for Olsen and SRK/T among flat eyes (mean, 0.34 vs 0.67), Olsen and Haigis for steep eyes (mean, 0.66 vs 1.02), and Barrett II and Haigis for average eyes (mean, 0.40 vs 0.74), respectively. The smallest and greatest median absolute errors were Olsen and SRK/T among flat eyes (median, 0.22 vs 0.71), Barrett II and Haigis for steep eyes (median, 0.31 vs 0.85), and Olsen and Haigis for average eyes (median, 0.29 vs 0.69), respectively.

The prediction errors of Barrett II (P <.05), SRK/T (P <.001), and Haigis (P <.001) for flat eyes; Haigis (P <.001) for steep eyes; and Barrett II (P <.001), SRK/T (P <.001), and Haigis (P <.001) for average eyes were significantly greater than errors for the Olsen formula.

Using the target refraction of ±1.00 D, 81.22% (SRK/T) to 95.45% (Barrett II and Olsen) of flat eyes, 56.52% (SRK/T) to 78.26% (Barrett II and Olsen) of steep eyes, and 77.04% (Haigis) to 94.07% (Barrett II) of average eyes were within the target range.

The factors that affected predictive power were axial length (β, 0.063; P <.01) for Olsen, axial length (β, 0.211; P <.001) and keratometry (β, -0.075; P <.05) for SRK/T, and axial length (β, 0.091; P <.001) for Haigis formulas. No significant predictors were observed for the Barrett II formula.

The major limitation of this study was the small sample sizes for the flat and steep cohorts.

“Keratometry influences the accuracy of IOL power calculation, and steep cornea has more influence than the other corneal shape in long eyes,” the study authors report. “Overall, both the Olsen and Barrett Universal II formulas are suitable in long eyes with unusual keratometry. The commonly used SRK/T formula, which tends to myopic in steep cornea and to hyperopic in flat cornea eyes, is not recommended and should be adjusted in this situation.”