New Asymmetric Transition Zone Centration Strategy for Refractive Corrections: Geometrical Background

The purpose of this study was to present a novel asymmetric transition zone (aTZ) centration strategy for keratorefractive lenticule extraction and assess its theoretical potential to compensate for fixation-related lateral and torsional misalignments.

Proposed method adapts correction profiles for lateral and torsional errors by decentering optical zone (OZ) relative to the laser system axis while generating an asymmetric, crescent-shaped transition zone (TZ). Workflow includes defining target pupil position and intended treatment center, capturing intraoperative pupil location, computing deviation, and adjusting OZ and TZ geometry. Modeling was used to evaluate profile geometry, tissue consumption, and higher-order aberration (HOA) behavior.

Modeling demonstrated that aTZ profiles maintain a centered symmetric OZ while creating asymmetric peripheral TZs compensating for fixation errors. Central thickness equals symmetric TZs (sTZs), with slightly reduced tissue-consumption volumes. Under large fixation errors, aTZ enlarges treatment diameter with a wider TZ whereas OZ remains unchanged. Spherical component theoretically induces peripheral coma and the astigmatic component peripheral trefoil. Unlike uncompensated sTZ profiles, where fixation-related errors directly translate into OZ decentration, aTZ design preserves OZ centration within its compensation limits and redistributes HOA induction to peripheral TZ.

A theoretical framework for aTZ strategy is presented, addressing fixation-related centration errors in lenticule refractive surgery. This method provides geometric compensation for lateral and torsional docking errors, which is theoretically expected to preserve optical quality and minimize tissue consumption better than standard symmetric strategies.

This approach extends customized TZ concepts from excimer platforms to lenticule-based surgery and may improve centration robustness when fixation inaccuracies are unavoidable.