Light Adjustable Lens and EDOF - how does it affect image quality

[JULIE66167]

I have a question about the Light Adjustable Lens and extended depth-of-focus (EDOF).  Definition: a single, contiguous, elongated focal point that enhances depth of focus.  My concern, will glasses still work to sharpen my vision?

This is from Presbyopia Physician March 2023 article The Light Adjustable Lens: Reshaping the Premium IOL Landscape
The Monofocal IOL With a Twist
A misconception about the LAL is that it’s a simple monofocal lens aimed at precision distance vision. While this is partly true, it’s also highly oversimplified. It is indeed a monofocal lens; however, given its aspheric design, it does allow for an element of extended depth of focus (EDOF). This aids in extending visual range even in a plano targeted eye. Additionally, patients can elect to add negative spherical aberration to their nondominant eye during the light treatments. This adds an additional 0.50D-0.75D of EDOF, providing a solution for presbyopia and reducing the need for reading glasses.
Thank you

[Ron_AKA]

Julie, that is a good question. I have tried to make some sense of these claims by RxSight before. My understanding is that the average eye has a positive asphericity after lens removal of +0.27 um. In theory the sharpest focal point and clearest image is achieved with a net asphericity of zero. For that reason J&J make the Tecnis 1 lens with a negative asphericity of -0.27 um to neutralize the inherent positive asphericity in the eye. The down side of doing this is that when all light is brought to exactly the same focus point the depth of focus is minimized. Some manufacturers like Alcon put less negative asphericity in the lens and allow some positive asphericity to remain to extend the depth of focus some. They claim of course that provides the optimum overall vision. B+L on the other hand with their enVista lens provide zero ( or neutral as they call it) asphericity in the lens. That leaves the eye at +0.27 um of asphericity. It compromises visual acuity to some degree but it does increase the depth of focus and makes the lens more tolerant to lens position in the eye, and to a less than perfect eye. And several years ago before aspherical lenses. all that was available were spherical lenses. They left even more positive asphericity in the eye in the range of +0.37 um. There were actually advocates of these spherical lenses, and that these zero asphericity lenses were not actually an improvement.

In any case, I have had a hard time reconciling RxSight's claims. They say they add negative asphericity when it would appear one needs to reduce the amount of negative asphericity to increase depth of focus. And they claim a surprisingly large extension of EDOF. The Eyhance is only about 0.3 D, and the enVista is a little more. The Vivity which actually qualifies to be legally sold as an EDOF by having 0.5 D extension is only just over that.

I think you will also find that RxSight use what the call "Blended Vision", which is really just another name for monovision, and that is really where they get the substantial two eye combined extra depth of focus. And last they do not talk much about the cost of extending the depth of focus. Physics says that if you stretch or smear the depth of focus, then visual acuity must be decreased. They seem to gloss over this aspect of their use of asphericity to extend depth of focus.

[JohnHaganMD]

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[Ron_AKA]

I would be interested in Dr. Hagan's comments to what degree EDOF should be pushed. Mini-monovision with a plano distance eye, and -1.5 D in the near eye should give pretty good vision for 95% of one's needs, without any extension of depth of focus beyond that of a normal monofocal. I wonder to what degree EDOF should be used and what the impact may be on visual acuity?

This said I think the LAL is a fine lens that has some very unique and useful advantages:
1. More accurate target achievement for mini-monovision
2. Ability to reduce astigmatism in finer degrees than a standard toric lens
3. Ability to test drive monovision, the degree of monovision, and potentially even which eye should be the near eye

[MrPresley]

Unrelated to IOLs, but I test drove monovision with contact lenses and never cared for it.

[JohnHaganMD]

At this point I would have to say that you are 'over-engineering' this.  There is no single correct answer, as I've said just like there is no perfect car. Lots of good ones, unique advantages, different purposes. I've taken my part of this discussion as far as I can and still provide useful general information. People that go into cataract surgery expecting not to wear glasses for anything are more likely to be disappointed, no matter how much money they throw into the pot. Best of luck Julie

[Ron_AKA]

My thoughts would be to focus on the positive benefits of the LAL lens and not put too much stock in the EDOF claims. I am not aware that RxSight has done any clinical trials to demonstrate the benefits of their EDOF techniques. I can't see a responsible surgeon putting so much asphericity into the lens that it would impact visual acuity to the extent eyeglass correction would be required. If a simple sphere adjustment is required that should be possible as a standard LAL adjustment.

[John C Hagan III, MD, FACS, FAAO]

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