MULTIFOCAL (MF) CONTACT LENS options to help patients with changing visual needs are growing at an exciting pace. Improvements in MF technologies have created designs for giving presbyopic individuals more visual freedom from spectacles. Presbyopic MF prescribing and fitting have become more streamlined with increased parameter choices such as daily disposability, wettable high-Dk materials, and even astigmatism correction (Morgan et al, 2024).
Optically, the most common commercial MF design available is aspheric (Morgan et al, 2024); however, extended depth of focus (EDOF) MF lenses are also available and give different options when correcting presbyopia (Tilia et al, 2017). The best optical design to prescribe will depend on both lifestyle and visual needs of each individual. This article will discuss the differences between aspheric and EDOF MF contact lens designs for presbyopia.
While single vision lenses are designed to focus all light at 1 focal point and correct vision for only 1 distant viewing distance (Megiddo-Barnir and Alió, 2023),the optics of of MF lenses are designed such that light is focused for either 2 or 3 discrete points, allowing vision at those distances (Megiddo-Barnir and Alió, 2023). In an aspheric MF, depth of focus is gained by diffractive optics; where light is divided and focused into discrete foci giving more flexible vision (Megiddo-Barnir and Alió, 2023). For example, a center-near aspheric MF lens has a discrete point of near-add power in the middle of the optic zone. From the center, toward the optic zone periphery, a gradual increase in distance power occurs until the full distance prescription power is reached.
Aspheric MF contact lenses create simultaneous image optics, where distance and near optics are both presented within the optic zone (Morgan et al, 2024). A potential limitation to these optics can be symptomatic glare and halos, which are more noticeable in higher add powers (Morgan et al, 2024). These symptoms occur because each focused point of light creates an area of light scatter (Megiddo-Barnir and Alió, 2023).
EDOF optics create a single, continuous, elongated focal point from far to near to enhance the depth of focus. Ideally this optical design provides improved intermediate and near vision without degrading distance vision (Kanclerz et al, 2020). In contrast to an aspheric optical design, EDOF designs create a linear distribution of light where no discrete focal points exist, thus avoiding defocused light scatter, glare, and halos (Kanclerz et al, 2020). Limitations in EDOF optics include degraded retinal image quality (Alió et al, 2019) and the inability to create enough near power to correct higher presbyopic correction (Kanclerz et al, 2020).
The eye itself has inherent higher-order aberrations (HOAs) that cause increased depth of focus, which can change with age (Osuagwu et al, 2017; Rauscher et al, 2023) and refractive error (Osuagwu et al, 2017). Some characteristics of the eye that cause HOA are corneal astigmatism (Trindade et al, 1997), axial length (myopia or hyperopia) (Osuagwu et al, 2017), and pupil size (Nakazawa et al, 1983). Interestingly, both aspheric and EDOF MFs may take advantage of the eye’s natural HOAs to improve vision -- known as pseudoaccomodative optics (Patel et al, 2011). Increased depth of focus, and thus better vision, is gained by summing the eye’s current HOA with the HOA of the contact lens optic (Patel et al, 2011)
Summary
Enabling vision correction at all the distances needed by prebyopic individuals has led to the development of several categories of MF lens designs, each with their own strengths and weaknesses. Knowledge of how these lenses function in addition to clinical findings and vision demands can empower eye care providers to recommend the best possible contact lens options for each patient.
References
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