**3.2 Multifocal IOLs**

Multifocal IOLs have been designed to provide spectacle independence at near, intermediate, and distance vision tasks. The first concept of a truly multifocal IOL was conceived in 1983 by Hoffer, [56] and the first bifocal IOL implantation was performed by Pearce in 1986 [57]. Since then, many modifications and improvements in multifocal IOL concept have been made [58].

Before the analysis of the pre-, intra- and postoperative management of patients implanted with mIOLs, a brief description of the optical design and properties of mIOLs is required.

#### *3.2.1 Optical design and properties of mIOLs*

Two optical phenomena can be utilized to create multifocal optics: refraction and diffraction.

### *3.2.1.1 Fully refractive IOLs*

Fully refractive multifocal IOLs direct light at different focal points using concentric zones of varying dioptric power within the optic. The optical power depends on the local surface curvature, with regions of differing curvatures achieving different powers within the lens. These IOLs are also called *multizonal refractive IOLs* (**Figure 1**) [59]. The central circular zone has a power corresponding to distance vision. The surrounding annular zones alternate between powers corresponding to near and distance to achieve the multifocal effect [60]. As the pupillary size changes, the number of zones that are utilized varies, and, subsequently, the relative proportion of light directed to the distant, near and/or intermediate focal points changes as well [59]. Thus, image quality can fluctuate depending on pupil size [61, 62].

**Figure 1.** *Refractive IOL design – focal points.*

### *3.2.1.2 Diffractive IOLs*

To create multifocality, diffractive multifocal IOLs use the optical phenomenon of diffraction and take advantage of the wave nature of light by selectively delaying the optical path in selected areas and slightly changing its direction when encountering an edge or discontinuity [59–62]. Typical diffractive multifocal IOLs consist of concentric annular zones in their anterior or posterior surface that constitute an asymmetrical zone plate, also referred as "diffractive kinoform" (**Figure 2**) [63]. The spacing between the zones gets progressively smaller from the center towards the edge of the IOL. Abrupt steps appear at the junction of each zone. These microscopic steps on the diffractive surface of the IOL with height of a few microns have a specific phase delay. The area of each zone determines the add power of the IOL and the maximum height of the steps determines the relative amount of light energy distributed on each focus [60, 61]. In fact, the heights of each step are chosen in such a way that approximately 40 – 40.5% of the incident light contributes to the add portion, 40 – 40.5% of the incident light contributes to the distance portion and the remaining light goes into other diffractive foci. Alternative step heights can be chosen to shift more energy to either the distance or near focus. Therefore, the diffractive element of these IOLs enables the splitting of the incoming light into two or three foci for bifocal and trifocal IOLs, respectively [60, 61].

**103**

**Figure 3.**

*Examples of diffractive multifocal IOL designs.*

*Pseudophakic Presbyopic Corrections*

*3.2.1.2.1 Apodized diffractive IOLs*

through a large pupil [59].

plane) [65].

*DOI: http://dx.doi.org/10.5772/intechopen.96528*

and those with non-apodized diffractive optics (**Figure 3**).

A further division of diffractive multifocal IOLs is based on *apodization*, which is the gradual decrease of the height of the steps from the center of the optic to its periphery [61]. Thus, diffractive IOLs can be classified into those with apodized

The characteristic of apodized diffractive IOLs is a decrease in height from the taller central to the shorter peripheral steps of the optic [59]. The lower steps of the periphery send more energy to the far and less to the near focal point. On the contrary, the higher central steps send equal energy to distance and near [64]. The clinical significance of this phenomenon is shown by the fact that the larger pupil diameter in scotopic light conditions, when only the distance vision is utilized, allows more energy to be directed to the distance focal point, while the smaller pupil diameter in photopic conditions, when both distance and near vision are utilized, allows energy to be directed equally to distance and near (**Figure 4**) [60, 64]. Additionally, apodized diffractive IOLs produce fewer optic phenomena (eg. glare, halos etc) than non-apodized IOL during distance vision

Some characteristic apodized diffractive IOL models are the following:

• AcrySof IQ ReSTOR SN6AD1 (Alcon Laboratories, Inc., Fort Worth, TX, USA): a single-piece, bifocal, symmetric biconvex IOL with an aspheric diffractive, apodized, anterior surface (+3.0 D near add power at the IOL

• AcrySof IQ ReSTOR SN6AD2 [SV25T0] (Alcon Laboratories, Inc., Fort Worth, TX, USA): a single-piece, apodized, diffractive aspheric bifocal IOL with a central refractive zone (*hybrid IOL*) (+2.5 D near add power at the IOL plane) [66].

• FineVision IOL (PhysIOL, Liege, Belgium): a single-piece, apodized, diffractive trifocal (+1.75 D intermediate and +3.5 D near add power at the IOL plane), aspheric IOL (aspheric posterior surface and diffractive anterior surface) [67].

**Figure 2.** *Diffractive IOL design.*

#### *Pseudophakic Presbyopic Corrections DOI: http://dx.doi.org/10.5772/intechopen.96528*

*Current Cataract Surgical Techniques*

*3.2.1.2 Diffractive IOLs*

*Refractive IOL design – focal points.*

**Figure 1.**

To create multifocality, diffractive multifocal IOLs use the optical phenomenon of diffraction and take advantage of the wave nature of light by selectively delaying the optical path in selected areas and slightly changing its direction when encountering an edge or discontinuity [59–62]. Typical diffractive multifocal IOLs consist of concentric annular zones in their anterior or posterior surface that constitute an asymmetrical zone plate, also referred as "diffractive kinoform" (**Figure 2**) [63]. The spacing between the zones gets progressively smaller from the center towards the edge of the IOL. Abrupt steps appear at the junction of each zone. These microscopic steps on the diffractive surface of the IOL with height of a few microns have a specific phase delay. The area of each zone determines the add power of the IOL and the maximum height of the steps determines the relative amount of light energy distributed on each focus [60, 61]. In fact, the heights of each step are chosen in such a way that approximately 40 – 40.5% of the incident light contributes to the add portion, 40 – 40.5% of the incident light contributes to the distance portion and the remaining light goes into other diffractive foci. Alternative step heights can be chosen to shift more energy to either the distance or near focus. Therefore, the diffractive element of these IOLs enables the splitting of the incoming light into two

or three foci for bifocal and trifocal IOLs, respectively [60, 61].

**102**

**Figure 2.**

*Diffractive IOL design.*

A further division of diffractive multifocal IOLs is based on *apodization*, which is the gradual decrease of the height of the steps from the center of the optic to its periphery [61]. Thus, diffractive IOLs can be classified into those with apodized and those with non-apodized diffractive optics (**Figure 3**).
