Trifocal IOL – Cataract implant
As a cataract surgeon, I had been using bifocal lenses in patients seeking some spectacle independance during the years 2005- 2007. These patients would often come back to me satisfied with their distance and near vision, but asking for some « screen reading glasses », as they could not really work on their computer, which distance was about 70 cm from their eyes. This was seminal to the project of developing a multifocal IOL which would provide the patients with distance, near AND intermediate vision.
I have been involved in the technical development and hold a patent for this trifocal IOL, along with the RnD department of the Physiol company (directed by Christophe Pagnoulle), and in collaboration with Yvette Houbrechts, optical scientist the Liège Space Center (Centre Spatial de Liège). I was particularly involved in the optical design of this innovative lens, which had to extend on prior art and take into account some physical and physiological features.
Watch :Video recorded at Fyodorov Institute, Moscow, on October 27th, 2012. It depicts the fundamental principles of diffractive IOLs and the concepts that presided to the development of this first and new trifocal IOL design:
Justification of the trifocality
Bifocal diffractive IOLs allow operated patients to some spectacle independence for distant (more than 2 meters) and near (between 35 and 40 cm) vision. These lenses have however not been shown to provide satisfactory intermediate vision.
Intermediate vision relates to activities such as computer work, car driving (instrument panel), music playing (musical chart), etc. In these activities for which good uncorrected vision is required for distances comprised between 60 and 80 cm) glasses may be required despite satisfactory near and distance uncorrected vision. Interestingly, the commercial release of the FineVision IOL was concomittent to the introduction of the iPad in the USA (march 10). Tablets are way heavier than books, and readers usually hold them on their laps, which increases the reading distance to the intermediate vision range (60 – 70 cm).
This was seminal to the pioneering solution represented by design and introduction of the first trifocal diffractive lens (FINEvision). This lens aims to be more than a bifocal, providing distance and near correction only. The FINEvision lens has an additional foci for intermediate vision, to provide treated patients with a full range of correction : hence, it is a true trifocal IOL (The » FINEvision » commercial designation stands for an acronym: Far – Intermediate-NEar Vision).
The following video relates to the reasons and steps incurred for the development of a trifocal IOL:
This video depicts some of the fundamentals for diffractive IOL design:
Designing a trifocal diffractive IOL
The IOL theoretical profile was first designed from theoretical computational equations. Its effect on an incident wavefront was simulated with MathCAD software (PTC Inc., Needham, Massachusetts, USA). The original idea was to combine two independent diffractive bifocal profiles, yielding a single diffractive pattern (this concept was patented in 2010).
From a Fresnel lighthouse lens to a diffractive optic
Augustin Fresnel was a French scientist from the 19th century, who first developed a concept named after him, the « Fresnel optic », for the lighthouse lamps.
The profile of a multifocal diffractive IOL derives from the Fresnel zone plate.
Zone plates are not efficient, mainly because they block part of the incident light. By using Fermat’s principle, progressive modifications to the zone plate can be made to obtain higher efficiency and decrease the number of foci. The plate can be made transparent, and the blocking zone replaced by « phase shifting » zones which selectively modify the optical path to cancel the half wavelength phase shifts and make more light interfere constructively to the desired focal distance. These zones look like little grooves, which can be made using microlithographic techniques on transparent lens materials.
Before going further, it is important to expose a little bit of theory and remind some of the important properties of diffractive IOLs. To understand how to achieve clinically efficient trifocality, it is mandatory to know the basic principles of diffraction for the realization of bifocal optics. At this stage, it is mandatory to realize that diffractive elements are very sensitive to the wavelength.
Basics of a bifocal diffractive IOL
Bifocality can be achieved by the combination of a conventional monofocal optic, and a “kinoform zone plate”, which profile results of the application of the Fermat’s principle (which states that light will always take the shortest path in time between two points). In gross approximation, the profile of the kinoform ressembles that of an asymetric “saw-tooth” profile. In practice, the height of these steps is of the dimensions of a few microns. This is somewhat expected, as this scale is of the same order than that of the wavelength of the incoming light (the visible light average wavelength is close to half of a micron, i.e 500 nanometers).
Properties of kinoform for bifocal IOLs
The following illustration represents schematically the behavior of light waves diffracted through a kinoform.
When properly designed, the kinoform can split incident light into several foci. This will happen when the optical path represented by the height of the steps is not equal to an even number of the considered wavelength in the lens material. Some percentage of the incident light may also not be deviated by the kinoform (0th order), while some other percentage might be to to the near foci (1st order) . Some calculation shows that some other remaining percentage of the incident light energy (20%) is diffracted in other orders than the 0th (non deviated) and the 1st (near foci) when the kinoform is designed to be « bifocal » for a specific wavelength.
Effects of the width of the diffraction steps:
The width of the steps governs the distance to where the 1st (and subsequent) orders will come into focus.
Apodization : reducing the height of the diffractive steps of the IOL
The height of the steps is controlled by the value of the considered design wavelength: optical designers may choose 550 nm, as it is the wavelength to which foveal (precise) vision is the most sensitive to. If the steps have a constant height value throughout the IOL, the repartition of the incident light energy between the various diffraction orders will also be constant. A progressive reduction of the height of the steps can be achieved to modulate the repartition of the incoming light energy into the different diffraction orders with the pupil diameter. This reduction is called « apodization ».
Vergence of higher diffractive orders
It has been mentioned previously that the kinform diffracted light into several orders beyond the 0th and the 1st order; these represent 20% of the initial light energy, provided 40% are diffracted toward the 1st order, and another 40% is diffracted (but not optically deviated) toward the 0th order. The 0th order contributes to distance vision, the first order contributes to near vision and the second and superior orders are lost for vision, as their vergences (the distance of the foci) are not useful.
Interestingly, with such a design, the second diffractive order has a vergence that is double that of the first order.
The envisioned asymmetric distribution of light energy among the 3 foci is rendered possible by the combination of two specific diffractive kinoform patterns.
The final Finevision trifocal IOL profile (patented) can be thought of a combination of two apodized profiles, and displays a full diffractive area with a specific diffractive pattern comprising alternating diffractive steps of different heights.
This diffractive area extends throughout the anterior side of the IOL. The zeroth (0th) order of the two profiles is used for far vision.
In this innovative trifocal IOL design, the first kinoform pattern is designed with an addition of 3.5 D as the first diffraction order. Therefore, the second diffraction order occurs at a vergence of 7D, which corresponds to lost light for useful vision. In contrary, the second kinoform pattern’s first order provides an addition of 1.75 D (intermediate distance); hence, the second order has a vergence of 2X1.75 = 3.50 D.
Therefore, the 2nd order of the second diffractive pattern is used to reinforce approximately 5 % of near vision (Add +3.5D), which is mainly afforded by the first order of the first diffractive pattern. As a result, the percentage of lost energy, which is usually 20% for standard diffractive bifocal lenses, is reduced with this IOL to approximately 14 %. The relative gain in terms of saved energy is approximately 25 % when compared with standard diffractive IOLs.
The IOL diffractive profile is also gradually attenuated throughout the entire optic (apodization), resulting in a continuous modulation of the light energy distribution directed to the three primary foci. The larger the considered zone, the more light is proportionally directed to the distance foci.
This profile is lathed on the surface of a monofocal apsherical optic. The asphericity of the monofocal lens is intendend to optimize the image quality, by balancing physiological levels of the corneal positive spherical aberration.
Apodization of the trifocal profile
As the step height decreases towards the periphery (apodization), when the pupil aperture becomes larger, the peripheral steps are progressively exposed. This results in an increasing amount of light dedicated to the distance vision foci. Hence, less light is recruited for the near and intermediate focal points. This gradual decrease of the step height from center to periphery has been shown to reduce halos, which are generated by defocused light under dim conditions. In contrary to non-apodized IOLs, the apodization provides some degree of customization to pupil movements: near reading tasks trigger pupil constriction, while mesopic distance vision (eg night driving) incurs pupil dilation. Apodization allows more light to be directed into the distance foci (0th order) when the pupil is dilated and uncover a larger IOL surface. This warrants superior optical peformance in mesopic conditions.
Convolution of the trifocal profile
Diffraction can occur when light waves encounter any « abrupt » change in their path. The sharp edges of each of the steps can be causing unwanted diffraction, causing a slight dispersion of incoming light energy. The light scattering on the edges of the steps can be decreased by their smoothing. Theoretically this can be simulated by adding a mathematical smoothing function, called “convolution”. This function was optimized to fit the lens profile as manufactured, according to the geometry of the cutting tool, as in practice, it is not possible to achieve abrupt steps with current lathing technology.
Optical bench results of the Finevision trifocal IOL
The optical bench evaluation (through-focus and through-frequency MTF) has been carried out according to ISO quality standards. These optical bench results echo the theoretical results. In addition to the two major foci at 0 and +3.5 D add power for far and near vision, respectively, the Fine Vision multifocal IOL displays a focus at +1.75 D, which corresponds to intermediate vision. This characteristic should therefore offer enhanced visual performance for intermediate vision relative to that obtained by conventional bifocal multifocal IOLs.
The diffractive profile test has been designed to extend throughout the entire optical surface of the Finevision trifocal IOL. The gradual decrease in step height towards the periphery allows to reinforce the distance vision under mesopic conditions, in which human eye’s pupils dilate. Without this gradual reduction, the contribution to far, intermediate and near vision would be constant across the entire optical surface, for any pupil size : this would not be optimal as all human pupil tend to dilate when ambient light reduces, and to constrict when attempting to perform near visual tasks such as reading. The variations of the through-focus MTF curve of the IOL for different pupil apertures confirms that the Fine Vision IOL is pupil-dependent and designed to favor distance vision under dim light conditions. This change of energy balance with pupil size mimics the natural pupil’s response to various lighting conditions. It is a function of the required distance (near or far vision). It is consistent with the accommodation reflex, in which the pupil tends to constricts for near vision.8 To decrease the risk of glare at night, only 9 % of the energy is devoted to intermediate vision at 4.5-mm pupil aperture.
The Finevision Physiol trifocal IOL was the first IOL ever designed to display a useful additional focus for intermediate vision at +1.75 D. This IOL improves intermediate vision relative to standard bifocal IOLs, while maintaining near and far visual performance owing to its advanced optical features : apodization, convolution, aspherization.
The risk (for the patient) associated with this intermediate focus seems limited with respect to the offered benefit because the diffractive structure of this trifocal IOL was designed to allocate less energy to intermediate vision as compared to far and near vision. Regardless of the pupil size, the limited amount of energy allocated to intermediate vision minimizes the risk of monocular diplopia associated with intermediate focus.