Needle bevel–guided standardisation of scleral tunnel formation in intrascleral haptic fixation: a quantitative geometric analysis

BACKGROUND: Scleral tunnel formation during Yamane double-needle intrascleral haptic fixation (ISHF) depends on proprioceptive estimation, producing variable tunnel lengths that contribute to intraocular lens (IOL) malalignment. The aim of this study was to evaluate a needle bevel-guided technique using the innate 2 mm bevel of a 26-gauge needle as a visual endpoint for standardising intrascleral tunnel length, and to assess postoperative IOL positional stability using a quantitative geometric landmark framework. METHODS: Retrospective consecutive case series of 24 eyes undergoing ISHF in the horizontal meridian (3- and 9-o'clock positions) by a single surgeon. During scleral needle advancement, the bevel disappearance point was used to standardise tunnel length to approximately 2 mm at each fixation site. IOL centration was quantified by extracting limbus centre (L) and optic centre (O) from intraoperative and 6-month images, and flange coordinates (F1, F2) from intraoperative images only using Fiji/ImageJ (National Institutes of Health, Bethesda, MD, USA; version 2.14.0/1.54f). The primary outcome was longitudinal Euclidean displacement of the optic centre. RESULTS: Mean best-corrected visual acuity improved from 1.28 ± 0.54 to 0.52 ± 0.47 logMAR (p < 0.001). Mean IOL decentration was 0.401 ± 0.240 mm at baseline and 0.397 ± 0.234 mm at 6 months (mean change - 0.004 mm; p = 0.457). Mean Euclidean displacement was 0.029 ± 0.018 mm; no eye demonstrated displacement exceeding 0.06 mm. No haptic extrusion, IOL dislocation, or sight-threatening complications occurred. CONCLUSIONS: The needle bevel provides a simple, reproducible, and cost-free visual cue for standardising intrascleral tunnel length during ISHF and was associated with excellent longitudinal positional stability. Initial centration, governed by entry angle and chord length symmetry, remains a separate challenge requiring complementary strategies.