Longitudinal agreement between retroillumination-based and scleral vessel landmark-based measurement of toric intraocular lens rotational drift using CASIA2: a prospective within-subject comparison study

To evaluate the longitudinal agreement between retroillumination-based and scleral vessel landmark-based measurement frameworks for assessing toric intraocular lens (TIOL) rotational drift using the CASIA2 swept-source anterior segment OCT system. Thirty eyes of 24 patients implanted with toric intraocular lenses were prospectively enrolled. TIOL axis data were obtained simultaneously at postoperative day 1, week 1, month 1, and month 3 using two CASIA2-based methods: retroillumination imaging (Retro) and scleral vessel landmark registration (Vessel). Rotational drift was defined as the angular deviation from the day-1 baseline, calculated using a circular angular difference formula. Agreement was assessed using Bland–Altman analysis, intraclass correlation coefficients (ICC), and a linear mixed-effects model. Mean rotational drift increased progressively over time with both methods. The linear mixed-effects model showed a significant main effect of follow-up time ( P < 0.001), whereas the main effect of measurement method ( P = 0.078) and the method-by-time interaction ( P = 0.438) were not significant. Bland–Altman analysis revealed small mean bias values but wide limits of agreement (LoA), exceeding the ± 5° clinical threshold at months 1 and 3. ICC values were 0.222, 0.303, and 0.443 at week 1, month 1, and month 3, respectively, indicating poor individual-level agreement at all timepoints. The two CASIA2-based measurement frameworks demonstrated comparable temporal trends at the group level in TIOL rotational drift but demonstrated poor individual-level agreement. Given the poor individual-level agreement observed, caution is warranted when switching between measurement frameworks during longitudinal follow up. In the absence of an external reference standard, the present findings do not establish the superiority of either method; rather, they highlight the importance of methodological consistency when monitoring individual patients over time.