Multifocal Intraocular Lenses: What Do They Offer Today? Jorge L. Alió and Joseph Pikkel

When considering the latest innovations in ophthalmology, there is no doubt that one of the leading fields is multifocal intraocular lenses. The quest of patients to be free from wearing glass or using contact lenses meets the elongation of life expectancy as well as older people being more active than in previous years, with the improvement of optical technologies and new inventions, which results in a constant improvement of multifocal intraocular lenses. These new lenses and new technologies open a wide variety of solutions for those who seek to get rid of visual aids as spectacles or contact lenses. Though a great advancement has been made in recent years in multifocal intraocular lenses designs and production, there is still no perfect solution for all distances, and there is still a lot to be achieved. Accommodative lenses might be a solution, and this fascinating issue will be described and discussed later in this book. In this chapter, we will describe the current technologies and advances of multifocal intra ocular lenses.

1.1 How Can We Gain Multifocality in Lenses?

A multifocal intraocular lens must incorporate some mechanism to focus light from distant objects and light from near objects at the same time. A redistribution of the light energy will happen, with no single focus receiving all the energy as it happens in normal physiological accommodation. Unlike spectacle multifocal lenses, the multifocal intraocular lens refracts (or diffracts) light from any object for both near and distance vision at the same time. Thus there must always be some light that is not in focus with the light that is in focus. For distant objects, for example, the “add lens” steals some of the light that would have been focused and instead distributes relatively defocused light onto the retina, decreasing image contrast and reducing contrast sensitivity. Multifocal intraocular lenses can obtain multifocality in different ways:

• 1. A combination of two or more different anterior spherical refractive surfaces for distance and near correction such as a combination of an anterior spherical and an anterior aspheric refractive surface for distance and near correction.
• 2. A combination of a posterior spherical refractive surface and multiple anterior aspheric refractive surfaces.
• 3. A combination of an anterior spherical refractive surface and multiple posterior diffractive structured surfaces for distance and near correction.
• 4. A biconvex lens with longitudinal aberrations on the anterior surface (making it aspheric), providing near vision through the center of the lens, distance vision through the periphery, and intermediate vision in between.

Intraocular multifocal lenses can be refractive, diffractive or of a combined design. Refractive lenses use only differing areas of refractive power to achieve their multifocality. They function by providing annular zones of different refractive power to provide an appropriate focus for objects near and far. Refractive bifocal/multifocal IOLs may be affected by pupil size and decentration, to a greater or lesser degree depending on the size, location, and number of refractive zones. The wavefront produced from the refractive lens is non-spherical, i.e., it does not have a focus. In these lenses the inner zone is powered for distance and outer zone is powered for intermediate vision. The middle zone has an add zone for near vision (Fig.?1.1).

The refractive multifocal lens implant provides excellent intermediate and distance vision. The near vision is typically adequate but may not be sufficient to see very small print.

Limitations of refractive multifocal intraocular lenses are:

• 1. Pupil dependence design.
• 2. High sensitivity for lens centration.
• 3. Intolerance to kappa angle which varies from patient to patient.
• 4. Potential for halos and glare due to more nontransition area—rough area between the zones.
• 5. Loss of contrast sensitivity.

The refractive models reach multifocality by their different refractive power annular zones and usually provide proper far and intermediate vision; however, sometimes, near vision is not sufficient. They are dependent of pupil dynamics, very sensitive to their centering, may cause halos and glare, and reduce the contrast sensitivity [1]. In addition, some refractive designs include a continuous change in curvature between zones providing functional vision across all distances [2].

Diffractive lenses are based on the principle that every point of a wavefront can be thought of as being its own source of secondary socalled wavelets, subsequently spreading in a spherical distribution (Huygens-Fresnel principle). The amplitude of the optic field beyond this point is simply the sum of all these wavelets. When a portion of a wavefront encounters an obstacle, a region of the wavefront is altered in amplitude or phase, and the various segments of the wavefront that propagate beyond

the obstacle interfere and cause a diffractive pattern. As the spacing between the diffractive elements decreases, the spread in the diffractive pattern increases. By placing the diffractive microstructures in concentric zones and decreasing the distance between the zones as they get further from the center, a so-called Fresnel zone plate is produced that can produce optic foci. Thus the distance power is the combined optic power of the anterior and posterior lens surfaces and the zero order of diffraction, whereas the near power is the combined power of the anterior and posterior surfaces and the first order of diffraction (Fig.?1.2).

The diffractive multifocal lens implant provides excellent reading vision and very good distance vision. The intermediate vision is acceptable but not excellent as the far and near vision. However, multifocal diffractive intraocular lenses are less pupil size defendant and are more tolerant to differences of kappa angle.

Bifocal diffractive multifocal lenses only provide two focus points—far and near—and no intermediate foci; they have a high potential of producing halos and glare due to more nontransition area; and since they cause an equal distribution of light for both foci, they cause 18% loss of light in transaction. These disadvantages may decrease quality of vision especially in mesopic and scotopic conditions when more zones affect the incoming light rays to the retina. The modern trifocal diffractive IOLs, provided by different mechanisms that will be explained later on this book, are trying to provide intermediate vision by a redistribution of the diffracted light to other foci.

The diffractive models are composed by diffractive microstructures in concentric zones that get closer to each other as they distance from the center. They generally provide good far and near vision, but the intermediate vision may not be satisfactory in some cases. They are not so dependent of pupil dynamics and more tolerant to their centering, but they usually affect the contrast sensitivity in a greater scale [4]. Although contrast sensitivity in patients with multifocal IOLs is diminished compared with those with monofocal IOLs, it is usually within the normal range of contrast [3].

J. L. Alió

Research & Development Department and Department of Cornea, Cataract, and Refractive Surgery, VISSUM Corporation and Miguel Hernández University, Alicante, Spain

e-mail: [email protected]

J. Pikkel Department of Ophthalmology, Assuta Samson Hospital, Ashdod, Israel Ben Gurion University, School of Medicine, Beer-Sheva, Israel e-mail: [email protected]

? Springer Nature Switzerland AG 2019 1 J. L. Alió, J. Pikkel (eds.), Multifocal Intraocular Lenses, Essentials in Ophthalmology, https://doi.org/10.1007/978-3-030-21282-7_1