Learning Objective: At the end of the reading, you will get an understanding of the design of the Extended Depth of Focus (EDOF) IOLs in the market, and how they differ from one another.
Depth of focus & Image quality-
The world of Intra Ocular Lens (IOLs) has gone a sea change in the last twenty years. A plethora of lens options is now available starting with the presbyopia correcting bifocal and trifocal IOLs, to more recent introduction of EDOF IOLs. The depth of focus and image quality are the two most important aspects of post operative visual rehabilitation. Unfortunately, the two are antagonistic. There will be a drop in depth of focus if image quality is to be enhanced, an example being the aspheric lenses. On the other hand, if depth of focus needs to be increased, this may come at the cost of image quality. So there is no free lunch in optics. The bifocal and later trifocal presbyopia correcting IOLs were introduced to increase the depth of focus of patients, but this came often at a cost of photic phenomenon and a resultant drop in image quality.
What is depth of focus (DOF)?
Depth of field can be defined as the nearest and furthest image in a picture that are acceptably clear. In the below picture some objects are sharp and clear while others not. The ones that can be clearly seen falls within depth of field of the camera. Depth of focus is often loosely interchanged with the word depth of field. Depth of focus however is on the image side of the retina, while the depth of field on the object side. Both are intertwined. Visually you will have no depth of field if the lens does not offer depth of focus on the retinal side.
What is EDOF?
Theoretically, depth of focus can be provided by two ways - presbyopia correcting IOLs like bifocal or trifocal IOLs, and by extended depth of focus IOLs (EDOF). The American Academy of Ophthalmology task force consensus statement for EDOF IOLs (1) states the following in as many words:
1. The EDF IOL group should consist of a minimum of 100 patients. The control group cohort should be similar for comparisons. The EDOF IOLs need to demonstrate comparable monocular mean best-corrected distance visual acuity (BCDVA).
2. The monocular depth of focus for the EDF-implanted eyes needs to be at least 0.5 diopters (D) greater than the depth of focus for the monofocal IOL controls at logMAR 0.2 (20/32)
3. The EDF IOL needs to have at least 50% of eyes achieving monocular DCIVA of better than or equal to logMAR 0.2 (20/32) at 66 cm. A logMAR visual acuity chart in 0.1 log unit steps should be used (e.g., ETDRS chart) as designed for the intermediate distance
4. The EDF IOL needs to demonstrate comparable monocular mean BCDVA to the monofocal controls through a statistical noninferiority analysis, using a noninferiority margin of 0.1 log-
MAR (1-sided test using significance level of 0.05).
As you can see, though the AAO task force lays down guidelines for EDOF IOLs, yet it does not make any distinction between the optics, or surface geometry that an EDOF lens might need to have to be categorized in the segment. That is, it categorizes EDOF IOLs in terms of vision that it offers to patients, not on the basis of optics embedded in the IOL to achieve it. By that way, all diffractive IOLs could be categorized as EDOF lenses, if it satisfies the above conditions.
Making sense of EDOF IOLs
In line with the objective of this article, I intend to throw some light into the optics of the current EDOF IOLs. I am excluding from discussion all presbyopia correcting diffractive multifocal IOLs which have three or two distinct foci to provide depth of focus. Instead let us focus on all lenses, that do not rely on diffractive step like structures and create distinct focal points other than the distance focal point in the defocus curve (to understand defocus curve, and how to interpret you can visit my article https://www.quickguide.org/post/defocus-curve ).
Thus we are only focusing on lenses that have a genuine refractive optics that create a single elongated focal point.
Different companies have adopted different technologies to provide extended depth of focus, without however not giving an explanation of the optics. Here I would try to demystify and cut the clutter.
Lenses with refractive mono focal IOL optics that provide extended depth of focus can be categorized into the following:
Lenses that have a change in asphericity from the center to periphery - Ray One EMV
Lenses that have a higher radius of curvature (that provides an added power) over the base curvature of IOLs - Eyhance
Lenses that provides a higher order optics ( that cannot be defined by standard aspheric curvature changes) - Isopure and Impress
Lenses that rely on changing incoming wavefront as it passes through the IOL optics - Vivity
1. Lenses that have a change in asphericity - RayOne EMV
This lens induces controlled positive spherical aberration in the central region of the IOL(2). No information is provided on the amount of positive spherical aberration induced in the paraxial region of IOL to provide a depth of focus of around 1.5D with monovision(3).
A slight positive spherical aberration on the central region would help paraxial rays of light (rays that travel close to optical axis) fall anterior to rays that travel through the base curvature of the lens. This will help patients to have an extended depth of focus, and a functional intermediate vision.
There is however no clear definition that would help distinguish between lenses that add positive spherical aberration to the central paraxial region and lenses that have an added power (higher radius of curvature) in the central region of lens.
Aspheric designs have a gradual change in curvature along one surface. If you cut a cross section of the central (paraxial) region of the lens (in this case RayOne EMV) the radius of curvature will not be the same across the central region where the positive spherical aberration is added. In contrast, lenses that add more power in the central region (Eyhance) will have the same radius of curvature across all points in the central region. Or in other words, since a spherical power is added in the central 1 mm of the Eyhance, its Conic constant will be zero, whereas for the RayOne EMV, the conic constant for the central region will be shaped like an oblate ellipse (positive spherical aberration) with a conic constant more than zero. To understand more about conic constants you can read my article on 'Spheric, Aspheric and Freeform lenses'
2. Lenses that have a higher radius of curvature (that provides an added power) over the base curvature of IOLs - Eyhance
Which brings me to the second category of EDOF lenses that claim a 'small continuous increase in lens power within the central 1mm diameter' (4). 'The power profile decreases towards the periphery outside the central 1 mm diameter in a manner comparable to Tecnis 1 piece (Model ZCB00) enabling the same correction of spherical aberration and resulting in comparable distance image quality' (4).
Increasing the power profile in the central region of 1 mm gives patient a functional intermediate vision. Outside the region, the lens would have a negative spherical aberration profile like that of the parent lens Tecnis 1 piece that corrects for the positive spherival aberration of the cornea.
You may think, if adding a positive spherical aberration, or a higher power in the central region would negatively affect image quality. With small pupils in mesopic condition, this is at least theoretically possible. But patients pupil are generally dilated beyond 3-4 mm in mesopic condition, and therefore the image quality may not experience a sharp drop as beyond the central 1 mm the lens would behave like the Tecnis 1 piece correcting for positive spherical aberration of cornea. However, to what extent patients can tolerate decentration with such a power profile remains to be seen. The MTF of Eyhance as well as RayOne EMV (earlier described) will definitely be affected by decentration more than standard aspheric lenses. But this may be well within the tolerance limit of the patients.
3.Lenses that provides a higher order optics ( that cannot be defined by standard aspheric curvature changes) - Isopure and Impress
Before you read this section, I encourage you to read my article on freeform optics (https://www.quickguide.org/post/spherical-aspheric-and-freeform-lenses)
In that article I had described the optics or surface profile of spherical lenses, aspheric lenses as well as a different form of lens profile called the freeform optics.
Briefly summarizing that article, lenses that have spherical profile have the same radius of curvature throughout the optics. Aspheric lenses do not have the same radius. However, the surface profile have a (rotational) symmetry and are convex in shape (for biconvex shaped IOLs). Unlike spheric and aspheric lenses, free form lenses have a customizable shape that can have a concave up or concave down shape simultaneously. Where the concave up shifts to concave down is called its inflection point. Thus a series of inflection point can be designed in the center or through out the lens that can help provide depth of focus.
The Impress from HOYA has been defined to have an even order surface profile in a peer reviewed clinical study by Rita Menucci(5). What this means is that in the central 2 mm region of the lens, there are a series of concave up and concave down or inflection points that helps throw light to an extended area. The light ray that would pass through the concave up area will reach behind the distance focal point and the ray of light that pass through the concave down area will reach before the distance focal point.
This is the same for Isopure. However, the difference from Impress is that Isopure has this design across its entire optics while the Impress design is limited to central 2 mm of the lens.
You will find that BVI claims Isopure comes with a patented complex surface design across the full optic. Their speakers in podiums refer to full optic anterior and posterior aspheric surface
with higher order terms(6).
To clear the fog here, 'aspheric surface' means any deviation from spherical surface (which inflection points are) and higher order terms are simply terms that describe the shape of the inflection points.
4. Lenses that rely on changing incoming wavefront as it passes through the IOL optics - Vivity
By now you may have already read about Alcon's description of Vivity optics. According to the manufacturer, the Vivity has 2 transition elements in the central 2.2 mm range.
The first transition element stretches the wavefront, creating a continuous focus area. The light is stretched in both directions, that is, in the myopic and hyperopic directions. The light in the hyperopic direction is located behind the retina and would not be usable. Therefore, the second transition element moves the wavefront forward, shifting the light from the hyperopic direction to the myopic direction so that the entire light energy is used. The Vivity IOL generates the extended depth of field by means of the aspherical front lens surface and a spherical rear surface.
To simplify this, the Vivity has a raised step like element that would create a phase shift for the light waves that pass through the central region. To understand the concept of phase shift in the below image the point A to E represent one wavelength of wave as it marks the completion of a cycle. Point B is the crest of the wave ( which is 1/4 of one wavelenth). Similarly points C, D and E marks 1/2 of the wavelength, 3/4 of a wavelenth and 1 wavelength respectively. Points A,B,C, D and E are the phases of the wavelength.
In the image on right we see that both green and blue waves are travelling in phase. The crest of the blue wave is in line with the crest of the green wave.
However, as the waves travel and encounter a discontinuity on the optics, a phase shift takes place. In the image on the left, a 180 degree or 1/2 wavelength of phase shift has happened. This will lead to a positive or a negative phase shift. A phase shift is the change in the position of the point in the wave (crest or trough or any other point on the wave) at a given point in time, compared to a similar point in the wave travelling at another place at a given time.
For the Vivity, note both types of movement of wavefront is described by the manufacturer by two optical elements. So I would assume that both positive and negative phase shift is generated by the two optical elements, respectively. Phase shift is typical of diffraction. It occurs when waves encounters an obstacle or a slit (gap or opening). A positive phase shift occurs when the crest or trough (or any given point in the wave) is shifted to occur later in time. A negative phase shift occurs when the waves crest and trough are shifted to occur earlier in time. Shifting of the wave to the left or right would indicate a negative or a positive phase shift.
So demystify let us put Alcon's Vivity promotional statement and understand its phase shift:
Alcon -"The first transition element stretches the wavefront, creating a continuous focus area.
(note the element described could be regarded as a step or a ring or to steer clear of debate let us assume an optical discontinuity that helps the incoming light to spread or diffract)
The light is stretched in both directions, that is, in the myopic and hyperopic directions.
(thus phase shift would occur in both directions- positive and negative phase shift)
The light in the hyperopic direction is located behind the retina and would not be usable.
(positive phase shift)
Therefore, the second transition element moves the wavefront forward, shifting the light from the hyperopic direction to the myopic direction so that the entire light energy is used.
(negative phase shift that makes the crest/trough to occur earlier in time)
The Vivity IOL generates the extended depth of field by means of the aspherical front lens surface and a spherical rear surface.
Alcon's Vivity patent talks about a base profile consisting of anterior and posterior IOL surface, and an auxiliary profile that is added in the central region of the anterior surface of the IOL. This auxiliary profile is a transition region between an inner refractive region and an outer refractive region that induces a phase shift or phase difference (7).
"Some of my patients are comfortably reading with the EDOF"
I am sure you may have come across doctors who find that some patients are reading with the enhanced monofocal or the EDOF that they have been implanting. In a recent study by Nanavaty and co-authors(8), about 9.6% of patients implanted with the RayOne monofocal aspheric non toric IOL reported having pseudoaccommodation. The authors found that spherical aberration, spherical equivalent, pupil size, preoperative ACD and AL as important influencing factors.
Nawa and co-authors suggested that pseudo accommodation is more in smaller preoperative AL due to more IOL movement. They suggested that when IOL optic moves forward by 1mm (due to ciliary muscles contraction) the pseudoaccommodation is .8 D in long eyes and 2.3 D in short eyes.
Denier and co-authors found larger IOL power and shorther axial length to have more pseudoaccommodation.
Nakazawa and Ohtsuk found pupil diameter to have an impact on pseudoaccommodation.
Lim and co-authors found and association between smaller pupil size and shorther axial length with good near vision.
It has long been believed that since English letters have a vertical axis, vertical coma and against the rule astigmatism that refracts in the vertical axis may help in pseudoaccommodation. However, to their surprise Nanavaty and co-authours (8) did not find this positive co-relation in their study.
(to be continued - keep an eye on this article as I clear the fog around other lenses optics)
1. Optical Bench Analysis of 2 Depth of Focus Intraocular Lenses
Andreas F. Borkenstein; DOI: 10.1159/000519139; Biomed Hub 2021;6:77–85
4. Eyhance DFU - Z311524E Rev. A TECNIS Eyhance Revision Date: 01/2021
5. Enhanced Monofocal Intraocular Lenses: A Retrospective, Comparative Study between Three Different Models Rita Mencucci 1,* , Alberto Morelli 1, Michela Cennamo 1, Anna Maria Roszkowska 2 and Eleonora Favuzza; J. Clin. Med. 2023, 12, 3588. https://doi.org/10.3390/jcm12103588
6. Isopure Premium monofocal, providing more for your cataract patient - Rafael Bilbao Calabuig; 25th ESCRS Winter Meeting Virtual 2021.
7. Extended depth of focus (EDOF) lens to increase pseudo-accommodation by utilizing pupil dynamics - US Patent USRE45969E1 https://patents.google.com/patent/USRE45969E1/en?oq=RE45969
8. Incidence and factors for pseudoaccommodation after monofocal lens implantation: the Monofocal Extended Range of Vision study
Nanavaty, Mayank A; Mukhija, Ritika; Ashena, Zahra; Bunce, Catey; Spalton, David J. - Journal of Cataract & Refractive Surgery 49(12):p 1229-1235, December 2023. | DOI: 10.1097/j.jcrs.0000000000001302