Meridian-Dependent Geometric Planning for Pars Plana Scleral Fixation: An Orientation-Adjusted Analytical Model.

To establish a geometric framework for scleral fixation planning in secondary intraocular lens (IOL) implantation that accounts for corneal ellipticity and meridional orientation, and to analyze the limitations of fixed limbus-distance marking. The limbal boundary was modeled as an ellipse defined by the horizontal and vertical white-to-white (WTW) diameters. A target circumferential scleral locus is defined concentrically relative to an anatomical reference center. Fixation points are determined as a function of the meridional orientation θ using an explicit radial model of the elliptical limbus and an orientation-dependent limbal offset. The framework is analyzed for two-, three-, and four-point fixation configurations, including the effect of uniform circumferential displacement. A constant limbus-distance strategy implicitly assumes rotational invariance of limbal geometry. When the cornea is elliptical, this assumption produces predictable meridional deviations from the intended circumferential scleral locus for vertical and oblique fixation. An orientation-dependent limbal offset preserves geometric symmetry across all meridians and maintains fixation points on a consistent target locus, independent of angular configuration. Under representative biometric conditions, the maximal geometric deviation from the intended scleral locus approached approximately 0.58 mm when a constant limbus-distance strategy was applied. Scleral fixation planning can be formalized as a geometric problem governed by limbal ellipticity and meridional orientation. An explicit orientation-dependent model provides a technique-independent basis for reproducible fixation planning and reduces reliance on fixed-distance heuristics.