Accuracy of new intraocular lens power calculation formula for short and long eyes using segmental refractive indices

Purpose: To evaluate the accuracy of a new intraocular lens power calculation formula using segmental refractive index-based axial length (AL). Setting: Chukyo Eye Clinic, Nagoya, Japan. Design: Retrospective observational study. Methods: This study included patients undergoing preoperative examination for cataract surgery with the new Barrett True AL (BTAL) and Emmetropia Verifying Optical (EVO) formulas using segmental refractive index, and conventional Barrett Universal II (BU II) formula using equivalent refractive index. The predicted refractive error of each formula was compared with the postoperative subjective spherical equivalent. Results: The mean prediction error (MPE) in the short AL group (≤ 22 mm; 44 eyes) was 0.32 ± 0.40 D for BU II, 0.22 ± 0.37 D for BTAL, and 0.10 ± 0.37 D for EVO (P < 0.0001). MPE in the long AL group (≥ 26 mm; 92 eyes) was 0.01 ± 0.32 D for BU II, 0.04 ± 0.32 D for BTAL, and 0.09 ± 0.32 D for EVO (P < 0.0001). In patients with an AL ≥ 28 mm, BU II showed a myopic trend in 57.1% of cases, while BTAL and EVO showed a hyperopic trend in 71.4%. The MPE for patients with an AL ≥ 28 mm was -0.16 ± 0.34 D for BU II, 0.18 ± 0.33 D for BTAL, and 0.16 ± 0.32 D for EVO (P < 0.0001). Conclusions: The new EVO and BTAL formulas showed higher accuracy than BU II in short eyes, whereas there was no difference in long eyes.