Should multifocal corrections be placed horizontally or vertically?

What is it about?

Purpose: Evaluate the importance of orientation of multifocal lens designs with angular increments of addition. Methods: Optical properties of one monofocal and three multifocal designs were analysed with the visual Strehl ratio (VSOTF) metric through-focus (
1 to 5 D). Designs were tested in combination with the higher-order aberrations (HOAs) of 782 subjects (1564 eyes). Simulations included one monofocal, one bifocal (eight orientations), one trifocal (four orientations), and a 4-foci design (four orientations). Monocular and binocular performances of all designs were assessed by computing the area under the through-focus VSOTF plots, the through-focus range of acceptable optical performance, and, for binocular combinations, optical disparity between left and right eyes. Results: Under monocular conditions, bifocal designs generated larger areas under the through focus VSOTF than trifocal designs and 4-foci designs. Specifically, bifocal designs divided vertically were optimal for 48% of eyes. Trifocal designs and 4-foci designs offered longer intervals of acceptable through-focus vision. Coma and spherical aberration values were correlated with the optimal orientation of multifocal corrections. The best binocular combination was achieved with a monofocal and a trifocal lens. The orientation of a multifocal design with angular areas affected the final optical properties of the combination (lens plus eye). Conclusions: The optical aberration distributions for our population of physiologically normal eyes demonstrated improved performance for some lens design orientations (i.e., left-right segments for 2-zone bifocals). Taking into account the HOAs of healthy patients, with special attention to coma and spherical aberration, will increase the optical quality of angularly divided multifocal solutions.

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Why is it important?

The study presented in this manuscript serves two main purposes:1- to show the clinician that physiologically normal eyes have a level of HOA that allows for the customisation of the orientation of angularly segmented multifocal profiles, and 2- to show manufactures which designs to prioritise when limited resources are available and deciding what lenses should become part of their product family.

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