Use of the internal astigmatism axis to determine the toric intraocular lens axis: Comparison between C-loop and plate-haptic lenses.

PURPOSE: This study aimed to evaluate the agreement between the internal astigmatism axis obtained using the optical path difference-Scan III (NIDEK Co., Japan) and conventional slit-lamp biomicroscopic measurements for determining the toric intraocular lens axis in non-dilated eyes. METHODS: This retrospective study included 56 eyes that underwent toric intraocular lens implantation and were evaluated at postoperative week 6. The toric intraocular lens axis was measured using the optical path difference-derived internal astigmatism axis under mesopic conditions without pharmacologic pupil dilation and compared with slit-lamp biomicroscopic measurements obtained after pharmacologic dilation. Agreement between the two methods was assessed using Bland-Altman analysis, the intraclass correlation coefficient, angular error analysis, and the distribution of clinically relevant deviations. Subgroup analyses were performed for plate-haptic and C-loop haptic intraocular lens. RESULTS: The mean signed angular difference between the two methods was 0.00° (95% CI:, -0.41° to +0.41°), with 95% limits of agreement ranging from -2.99° to +2.99°. No systematic or proportional bias was observed. The mean absolute angular error was 1.21°±0.91°, and no eye demonstrated an intermethod discrepancy exceeding 5°. The difference between the two methods was ≤1° in 73.2% of eyes, ≤3° in 98.2%, and ≤5° in all eyes. Excellent agreement was observed between the two techniques (intraclass correlation coefficient >0.99, p<0.001). Comparable agreement was also observed in both the plate-haptic and C-loop haptic intraocular lens subgroups. CONCLUSIONS: Optical path difference-derived internal astigmatism axis measurements demonstrated excellent agreement with conventional slit-lamp biomicroscopic assessment for postoperative evaluation of toric intraocular lens alignment. The low angular error, narrow limits of agreement, and absence of clinically meaningful discrepancies suggest that this technique may serve as a reliable alternative for determining the toric intraocular lens axis in non-dilated eyes with adequate pupil size.