1. Community
  2. Eyes
  3. Cataract

Aspheric IOLs: Does Aberration Correction Matter?

Posted , 5 users are following.

xen42188
xen42188

I wonder if anyone has done some research on this topic and can share their thoughts.

Some context: The cornea has positive spherical aberration (SA) that is compensated by negative SA in natural lens. Corneal SA estimates vary but there seems to be an agreement on +.25 um as being close to reported clinical values (although I’m sure that the number would vary based on which part of the world you come from). When the natural lens is taken out, you’re left with just the corneal SA. The theory is that negative SA in IOL would help compensate.

Aspheric IOLs either are neutral (such as B&L enVista) or add some negative SA (such as Alcon IQ, Tecnis) to compensate for the positive SA of cornea. Everyone has difference amounts of SA (age is a factor too) but aspheric IOLs typically offer 0 to -.27 um. Obviously, all of these manufacturers claim that their approach is the best 😊

Some researchers say that changes of SA between -.17 and +.2 are hardly noticeable. Others say that residual SA of 0 helps with contrast sensitivity and +.1 may help with visual performance. When I look at feedback from this site, it looks like these differences don’t matter. People who are happy with quality of their vision are equally happy whether it is enVista, IQ or Tecnis.

To get the most benefit from SA correcting lenses, the IOL would have to be matched to the corneal topography and aligned exactly to counteract corneal SA. With the post-operative changes (however small) in decentration and so on, does this benefit really materialize in real life? Or people who are happy, did they just get lucky with a good match?

My question is: Was SA a consideration in your IOL decision? If so, how did you ensure that the IOL you decided upon is a good match for your corneal aberration and what tests did the surgeon do to ensure that?

Many thanks!

0 likes, 10 replies

10 Replies

  • RonAKA
    RonAKA xen42188

    Edited

    That is a very thoughtful question, and yes I did consider it when selecting an IOL. That is the good news. The bad news is that I did this research nearly a year ago, and have forgotten a lot of it! That said here is what I do remember to some degree:

    .

    1. I think that trying to reduce spherical aberration to a small amount is useful for the reasons you suggest - better contrast sensitivity in lower light.
    2. I recall that the normal aberration in the cornea is higher in people with an Asian heritage. This would suggest that a Tecnis correction of 0.27 may be more appropriate if one is Asian, and the Alcon -0.21 or so may be more appropriate if one is Caucasian. I have no idea where I read that.
    3. I recall that some surgeons say they measure the cornea and select a lens that it most suitable to their actual aberration. I did not have that discussion with my surgeon. In the end I was choosing between the Alcon AcrySof and the Tecnis, and since I am Caucasian the AcrySoft was my preferred lens for other reasons, and seemed appropriate from an aspherical point of view too.
    4. Aspheric error is not all bad. Some EDOF lenses (including I believe the Alcon Vivity) use aspheric error to improve the depth of field of vision. Of course they don't call it "error" but use a nicer term like X-Wave or something! Interestingly astigmatism has a similar effect of smearing the focal point to improve the depth of field.
    5. Some (like Alcon) have claimed that a slight positive spherical aberration gives better vision, than zero aberration. Alcon calls it "super-vision".

      .

      Here is one of the articles that I had saved from my research on this subject. I have not read it over so I hope it doesn't contradict too many of the points I made above and make a fool of me 😉

      .

      Role of Asphericity in Choice of IOLs for Cataract Surgery *Aman Khanna,**Rebika Dhiman, *Rajinder Khanna, *Yajuvendra Singh Rathore, *Spriha Arun

    • xen42188
      xen42188 RonAKA

      Posted

      Thank you for sharing the article Ron. There are some good nuggets in there.

      For example, for myopia of -.5D (which is what I’d likely consider if I go for monovision), residual SA of +.2 um was found to be ideal. (That can be achieved with a neutral SA IOL, such as enVista).

      I also found Devgan’s decision tree interesting: for non-lasik eyes, when there is no risk of decentralization, neutral SA gives best DoF. This is consistent with what my surgeon said... that 70% of patients when implanted with enVista in his practise also get get intermediate.

      I also read elsewhere else that on average, IOL decenters by .3mm. At that stage, there is no real difference between lens with neutral or varying degrees of negative SA. In fact, at more than .3mm, neutral is much better than negative SAs. (With a theoretical zero decentralization, -.27 is the best).

      I have come to the conclusion that it’s more important to ensure the lens doesn’t move much (decentralization or tilt) and worry less about SA.

      Lastly, somehow I remember the +.27 was the Caucasian corneal aberration average 😃 I’ll try to look for that reference.

      All the best!

    • RonAKA
      RonAKA xen42188

      Posted

      If you read the Vivity lens product description they describe the shape as being aspheric, and then go on to say the aspheric side of the lens also uses the magic X-Wave technology. I have concluded is that they potentially correct to make the lens aspheric, but then modify it to make it not aspheric to stretch the depth of focus. So in other words using a spherical monofocal lens may be the "poor man's" Vivity. But, I suspect Vivity likely does it better as that is what it is designed to do.

      .

      On the decentralization, one thing to consider is that there are studies which have found that the AcrySof lens is more sticky and stays where it is placed better than the Tecnis material. This is probably more important from an angular point of view in a toric lens, but it should also apply to keeping non toric lenses centered as well.

      .

      On the issue of race and spherical aberration I found this reference:

      .

      "Although the average corneal SA for the population is +0.27 µm, the standard deviation is large and approaches 0.10 µm, or one-third of the value (See Figure 1).13,36 (Beiko G. ASCRS May 2004) The implication of this is that there is a wide spread in the population of the value of the SA and it cannot be assumed that the individual patient undergoing surgery has the "average" value. In fact, there is even further evidence of a racial difference in this value; Asian eyes have been measured to have an average corneal SA of + 0.37 µm. (Nakano EM, et al. Performance of Aspherical versus Spherical Intraocular Lens after Cataract Surgery in Asian Eyes. ESCRS, Stockholm, 2007)"

      .

      The article is old, but here it is:

      .

      Review of Ophthalmology George H.H. Beiko, BM, BCh, FRCSC PUBLISHED 18 NOVEMBER 2008 Understanding Corneal Asphericity and IOLs

    • Dapperdan7
      Dapperdan7 RonAKA

      Posted

      question

      is a "neutral SA IOL like envista" the same thing as a spherical lens?

    • RonAKA
      RonAKA Dapperdan7

      Posted

      No, as I understand it based on my memory of B+L information is that a spherical lens will add about 0.10 um of positive asphericity. So the outcome of the neutral enVista would be +0.27 um, while a spherical lens would be around +0.37 um. Spherical lenses have kind of fallen out of favour so there is not much current information on them.

    • RonAKA
      RonAKA Dapperdan7

      Posted

      As you can see from this link the SA of a spherical lens is not just one number. It depends on the brand, the specific design of the spherical lens, the actual power used, and the pupil size considered. However if one was to pick one number to used out of this study based on the Sensar spherical lens you could expect about 0.35 um from a spherical compared to 0.27 um from the enVista.

      .

      https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3381090/

  • xen42188
    xen42188

    Posted

    I have often wondered what contributes to some people getting excellent near vision with monofocals set for distance. I’m sure there’re other factors but the influence of spherical aberration (SA) is an interesting one.

    This has been a topic of interest for me. I came across another article recently and thought I’ll post for the benefit of those also interested in this question.

    Summary: In normal corneas, IOLs that are neutral (introduce 0 aberration) increase Depth of Focus and provide better DCNVA.

    In a study of 110 patients, the difference between two lens types was significant for DCNVA (@33cm): 0.42 logMAR for patients who got IOL with negative SA versus 0.29 for those with neutral SA (J3 or better)! Both set for distance. Hyperprolate eyes (post lasik/prk) were even better at 0.18 logMAR!!

    AOS Thesis 2020 Static and Dynamic Factors Associated With Extended Depth of Focus in Monofocal Intraocular Lenses

    • xen42188
      xen42188

      Posted

      image

      Here's chart from that article. My takeaway from all of this is that (all other things being equal) you want to be left with positive SA after IOL implant for better range of vision - either through choice of IOL or being the lucky one who has greater positive aberration than what is corrected by the IOL!

    • RonAKA
      RonAKA xen42188

      Posted

      It is an interesting article, but a pretty tough read! It seems to me that at the end of the day there are still some pretty limited choices. Perhaps if the surgeon was to measure your actual eye to determine what the positive asphericity actually is, then the options may become more obvious. But, as I see it there is the Tecnis aspheric lens with a -0.27 correction, the AcrySof with -0.20, and a neutral no correction lens. I found it interesting that they suggested one option to consider is a full correction of the aspheric error for the distance eye for best distance vision, and a no correction lens for the close eye, or a spherical aberration monovision. It would seem like a pretty mild version of monovision, but it could help. The other options are the lenses that intentionally use this effect to improve intermediate vision and to a lesser degree the close vision. Those would be the Tecnis Eyhance and AcrySof IQ Vivity. I suspect the tradeoff in this is the potential loss of contrast sensitivity in lower light conditions. Likely the more aspheric error the more loss of contrast sensitivity.

Back to top

Report or request deletion

Thanks for your help!

We want the community to be a useful resource for our users but it is important to remember that the community are not moderated or reviewed by doctors and so you should not rely on opinions or advice given by other users in respect of any healthcare matters. Always speak to your doctor before acting and in cases of emergency seek appropriate medical assistance immediately. Use of the community is subject to our Terms of Use and Privacy Policy and steps will be taken to remove posts identified as being in breach of those terms.