Last Tuesday I got up in the morning, showered and ate some breakfast, took the dog out, did the crossword, got a new pair of eyes, took a nap, watched the Mariners game and went to bed. In case you missed that, I got a new pair of eyes. OK, new lenses to be precise, but still. The technology is amazing and of course I went down a bit of a rabbit hole learning about how it works. It’s hard to believe that we really get to live in this world — just so cool.
At lot of folks know the basics of how vision work, but let’s start there anyways. Light comes in through an opening in the front of our eye called the pupil (the black part). In front of the pupil is the cornea; just behind it is the lens. Both of these are clear and serve to refract (bend) the incoming light so that it lands perfectly aligned on the retina, a grid of cells on the back of the eye that sense light impulses and send them up the optic nerve to the visual cortex, which assembles the signals into a coherent concept of what we’re looking at.
This works great to see things at a distance — like across the room or street or whatever — when the incoming light rays are almost parallel to each other. But we often need to see things that are much closer, like the words in a book. In this case the incoming light rays diverge and enter the eyes at steeper angles, causing the focal point to fall far behind the retina and blur. Evolution compensates for this by allowing us to dynamically change the shape of the lens to bring things into focus. The ciliary muscle squeezes the lens, making it fatter. This fatter lens bends the outer rays more sharply, pulling the focal point back onto the retina so we can read. Just amazing.
Fun fact, this is why squinting actually can help you see better — it’s a crude way of changing the shape of your eye structure, which can impact where the focal point falls. But squinting can only do so much, quickly tires out your facial muscles, and looks pretty goofy — so not a great long-term solution.
I started wearing glasses full-time for myopia (nearsightedness) when I was about twelve — I could see things close up, but not at distance. This happens because the eyeball itself is elongated, or because the cornea or lens is overly-refractive (too strong). Either one causes the focal point to fall in front of the retina, blurring the image received by the brain.
Hyperopia (farsightedness) is the exact opposite — flaws in the eye cause the natural focus point to fall behind the retina. Either “opia” can be fixed relatively easily by placing lenses in front of the eyes in the form of glasses or contacts. The optometrist just keeps trying different lens powers (“Which is better, A or B?”) until they find the one for each eye that lands the image perfectly on the retina at distance. Your ciliary muscle does its job for closeup tasks, and everything is back in business. Woot!
Note I’m basically ignoring astigmatism here, which occurs when flaws in the cornea or lens are asymmetric — e.g., maybe blurring only happens on the horizontal plane. This makes everything way more complicated, and I don’t have much of it myself, so I’m going to pretend it doesn’t exist. Sorry about that.
Glasses are fine, and truth be told I probably look better with them on. But they’re also annoying, especially in the rain or under ski goggles or whatever. And fully recognizing the irony of this given my enthusiasm for surgery, contacts just scare the bejeezus out of me — no way. So just about eight years ago I decided to get LASIK surgery to repair my nearsightedness. Dr Sharpe seemed like a good guy and got solid reviews, so into the breech I went.
LASIK (Laser-Assisted In Situ Keratomileusis) replaces the need for external lenses by reshaping the cornea so that it refracts properly. Because everything is always complicated, the cornea is actually made up of five distinct layers. Starting from the top:
- The Epithelium is exposed to the environment and passes oxygen/nutrients to the rest of the structure. It constantly regenerates itself and contains a ton of nerves, which is why it hurts so much if you scratch your eye, as I did back in high school with plaster dust. Ouch.
- Bowman’s Layer as near as I can tell basically acts as a buffer/sealer between the dynamic epithelium and more static lower layers.
- The Stroma is the thickest part of the cornea (which isn’t saying much at about 500 micrometers) and where most refraction occurs.
- You’ll have to research Descemet’s membrane the Endothelium yourself because they’re not relevant to LASIK.
The procedure is outpatient and other than a boatload of topical numbing drops, the only anesthesia I had was a medium-heavy dose of valium. A suction/stabilizing device is placed over the eye and the laser cuts a circular “flap” through the top two layers of the cornea. The flap is folded back to expose the stroma, the laser nibbles away at the stroma to reshape it for the correct prescription (flatter for myopia; steeper for hyperopia), and finally the flap is folded back in place.
Apparently the epithelium regenerates so quickly that the flap just heals on its own — I have read about stitches being used, but that didn’t happen in my case. The cut itself is positioned over the iris (the colored part of the eye), so even before it heals there’s no impact to your vision. The weirdest part about all of this is the burnt hair smell that is in fact the laser burning away parts of your eye. Yeesh.
But holy crap, I literally sat up in the chair post-procedure and could see great. Right away. Now of course there was some swelling and pain and stuff over the next few days … but it was one of the most shocking things that has ever happened to me, ever. Just brilliant.
My vision was basically perfect for about six years after LASIK. I can’t say enough good stuff about that decision, but I’m taking a long time to get to the really good part of this article, so I’ll leave it at that. Absolutely would recommend LASIK to anybody who qualifies.
But of course time marches on. Near vision starts to degrade for almost everyone sometime in their forties or thereabouts, which is why we need reading glasses and shine our phone flashlights on the menu. It’s called presbyopia, and it happens because the lens becomes less elastic and those ciliary muscles just can’t squeeze hard enough to change its shape for near focus. Folks who already have glasses start buying biofocals, and those of us with good distance vision (naturally or thanks to LASIK) start haunting the drugstore aisles for cheap readers.
A little fine print (see my eye joke there?): during my LASIK consult, I chose the “regular” version which corrects both eyes for distance. There is another option called “monovision” in which the dominant eye is corrected for distance, but the second eye is corrected for reading. That is, the second eye is adjusted so that an object in the near field is projected clearly onto the retina with the lens at rest (vs. “squeezed” as we discussed above). Typically, the brain is able to adjust and automatically swap between eyes based on what you’re looking at, which is utterly amazing.
Because the near-vision eye can focus with the lens at rest, monovision can head off presbyopia — you don’t need to change the lens shape to see close-up, you just need to use the eye dedicated to that purpose. This was tempting, but there are a few downsides, particularly (for me) some loss of depth perception since you no longer have effective binocular vision. And since LASIK removes only a tiny amount of corneal tissue, you can actually have it done more than once — I was assured that I could simply “touch up” my eyes in the future to address presbyopia or other changes if needed.
Indeed, I eventually started to need readers, and it was fine. I’m not sure why, but there was actually something kind of nice about the ritual of pulling out the glasses to read or work the crossword or whatever. That is, it was nice until I started needing them for everything. Cooking instructions on the frozen pizza? Glasses. Seat on my boarding pass? Glasses. Which direction does the HDMI cable go in? Glasses. You get the idea. When I started needing them just to snooze the alarms on my phone, I knew it was finally time to go in for the “touch up.” Procrastinated a bit more thanks to COVID and all, but finally pulled the trigger about a month ago.
After walking around the house taking pictures of everywhere I noticed readers lying around (above), I rolled up to the Sharpe Vision office for my consult only to realize that it was no longer their office — apparently in the almost-decade since I got my LASIK they moved a few streets down. A quick lookup on the phone (with readers) and I made it just in time for my appointment at their new place past Burgermaster on 112th …
… only to find that the world had changed once again. Yes, they could touch up my LASIK, and could even offer a new flavor called laser blended vision that’s like monovision but with improved depth perception. But what I really ought to check out is RLE — Refractive Lens Exchange. And since apparently I’m always up for new ways to mess with my eyes, I was totally in. Here’s the deal.
Along with presbyopia, over time most people eventually develop cataracts, a clouding of the lenses that makes them less able to transmit light energy. This is the other reason we’re all using our phone flashlights to read our menus. The good news is that cataracts are easily fixed by replacement of the natural lens with an artificial one.
An aside: cataracts are a major cause of correctable blindness in the developing world. Doctors Without Borders has conducted free “eye camps” in Somalia for many years and has fixed cataracts for hundreds of people who literally go from blind to normal vision in one day. If you’re able to give a bit, you’re not going to find a better organization — they are awesome.
Because we do what we do, there’s been a ton of innovation in replacement lens technology. The path of that innovation is pretty neat, and recently folks have realized that — hey — maybe these lenses are awesome and safe enough that we don’t need to wait until cataracts form to swap them in! The material lasts well beyond the fifty-odd years that middle-aged humans have before them, so why not? Thus was born the “RLE” (Refractive Lens Exchange) industry, and a new practice for the newly re-named “SharpeVision Modern LASIK and LENS.”
2023: Refractive Lens Exchange
RLE at Sharpe with Dr. Barker is pretty fancy. Even before we get to the lens itself, the procedure alone shocks and awes:
- The CATALYS Precision Laser System identifies key structures in the eye and creates a 3D map at the micron level. Check out the video of this, it’s super-cool.
- The laser cuts small entry slits through the cornea and a round opening in the front of the capsule that holds the lens.
- The laser softens and segments the existing lens so that it can be easily broken up and sucked out through a small vacuum tube.
- The new lens is passed into the now-empty capsule through a small tube. The lens is flexible and can be folded up so it fits through the small entry hole.
- When the lens unfolds, two springlike spiral arms called haptics hold it in place in the center of the capsule.
All of this computer-assisted laser stuff is just incredible. I was awake throughout my procedure and it was pretty crazy to listen to this HAL-like computer voice announcing what percentage of my lens had been sucked out at each step.
Monofocal IOLs (Intraocular Lenses)
OK, finally I get to talk about the intraocular lens itself, which is what sent me down this rabbit hole in the first place. The old-school version of this is the Monofocal IOL, which “simply” acts just like the lens in your glasses or the reshaped cornea in LASIK, using refraction to focus images at distance onto the retina. Monofocals are the workhorse of cataract surgery, but they have some disadvantages. Primarily, since they have only one focal distance and can’t be squeezed / reshaped by the ciliary muscle, readers are basically guaranteed for close-up work. There is a “monovision” option using differently-powered lenses in each eye, but that comes with all the same issues as monovision LASIK.
Today there are basically two kinds of “premium” IOLs that attempt to provide a glasses-free experience. One is the “accommodating” IOL — most famously the Bauch & Lomb Crystalens. The concept makes a ton of sense — just replicate the action of our natural lens. Remember that an IOL has little springy arms called haptics that hold it in place in the eye (the orange bits in the picture here). The same ciliary muscle that squeezes our natural lens can apply force to these haptics, which are designed to change the lens shape and position in response. The rest of your vision system just does what it’s always done, and the focus point adjusts naturally.
Pretty neat, and I’m always drawn to biomimetic solutions, because evolution tends to, well, work. But while it’s a little hard to find good data, it appears that the Crystalens has seriously dropped in popularity over the last decade or so — only 10% of practitioners were using it in 2021 according to this “Review of Ophthalmology” article that claims to know. From what I can find (e.g., here) it seems that the near vision improvements from these lenses just aren’t that great, and may also decline over time. Perhaps our intrepid ciliary muscle just loses some oomph as we get older … who knows.
So at least for now, accommodating IOLs don’t seem to be the favorite child. Even the original inventor of the Crystalens has moved on to new technologies. But don’t blink (another eye joke), because there are true believers still working the problem with a bunch of new stuff in the pipeline.
OK, we’ve finally arrived at my lens, the Clareon PanOptix Trifocal IOL, presently the most popular of the other class of premium IOLs: multifocal. Multifocal lenses have no moving parts but instead divide up the incoming light rays into multiple focal points — two for bifocals, three for trifocals. The “distance” focus typically uses refraction — the same mechanism we’ve seen again and again on this journey. But multifocal lenses are shaped so that light entering from near or intermediate distances is diffracted to provide focus in those ranges.
Diffraction occurs when light hits a discontinuity in material. The actual math is super complicated and a bit beyond me, but at the highest level, a light wave passing through different materials (the lens itself vs the aqueous material surrounding it) creates interference patterns that ultimately bend the light in a predictable way. A multifocal lens has a bunch of concentric circles of varying heights that produce this effect — you can see them if you click to zoom into the picture of the PanOptix on the right.
The end result is that the lens creates clear images on the retina at three different distances:
- Plano or “infinity” for driving and watching whales in Puget Sound (refracted).
- About 24 inches for “intermediate” tasks like computer (and lathe!) use.
- About 16 inches for “near” tasks like reading.
Multifocal Issues and Mitigations
If you’re paying attention, you’re probably asking the same question I did when I first learned about these things. Aren’t you now getting THREE images projected onto the retina at the same time? Well, kind of yes. But two things help you out. First, your brain is just really smart and figures it out in the same way that it does with monovision — paying attention to the stuff you are showing interest in by the direction of your gaze and other clues. More importantly, at any given time there’s usually only one of these three distances that actually has something to look at. For example, if I’m reading I’m not getting much of an image from anything behind the book. Between the two of these, your brain very quickly just makes it work.
It is amusing to experience these artifacts in real life. The most obvious one is the “halos” that appear around point light sources such as headlights or streetlights. I wish I could capture it with a camera, but you actually see the diffraction patterns — the light looks exactly like the rings on the lens itself! It’s a bit annoying — if I were a long haul trucker I might think twice about getting a multifocal — but for me it’s no big deal.
A second issue makes sense in theory, but (least so far) I’m not experiencing it in practice. With a natural lens, pretty much all of the light that comes into your eye is captured by the retina. Of course the iris opens and closes to admit an optimal amount of light, but very little of that is lost passing through the lens. With a multifocal the energy is divvied up between the focal points, plus there is some additional loss inherent in the diffractive process itself.
The PanOptix has a neat feature that tries to minimize this by “collecting” light energy at a (mostly unused I guess?) focal distance of 120cm and diffracting it in reverse so that energy helps power distance vision. The end result is that the PanOptix uses about 44% of incoming light for distance, 22% each for near and intermediate, and loses about 12% to the process. Not bad! And at least so far I can’t detect any loss of contrast or issues in lower-light situations. The effect is surely there, I’m just not aware of it.
The Hits Keep Coming
So far I’m super-satisfied with my new lenses — distance vision feels about the same as it was before, but I can read and use the phone/computer comfortably without my trusty readers. Every day my brain gets more used to the various artifacts that do exist, and my vision should stay pretty stable much until I die. Woo hoo!
At the same time, it’s clear that all three of the broadly-used lens types out there (monofocal / accommodating / multifocal) have pros and cons — none work as well as the natural lenses of a young adult. So researchers keep pushing the envelope. The latest concept I’ve read about is Extended Depth of Focus (really well-explained here). The concept behind EDOF lenses is to extend the range of distances that can provide an acceptably (if not perfectly) focused image on the retina, rather than pinning focus to specific intervals.
There are a few mechanisms being tried to product EDOF; the easiest for me to understand is the pinhole effect, which has been used in photography for years. By shrinking the hole through which light enters, you basically filter out the steeper rays that would spread out over the retina, leaving only the ones that are already mostly parallel anyways (regardless of how far they are in front of the eye). Of course this also filters out a bunch of light energy, so it’s harder to see in low-light conditions. So far these lenses have mostly been used monovision-style — one eye gets the pinhole lens and the other gets a classic monofocal.
It’ll be interesting to see how this new approach plays out. And I could easily keep digging deeper into this stuff forever — but I think we’ve covered more than enough for one article. In case it isn’t clear, I’m fascinated with attempts to repair, build on and improve the capabilities that have been so hard won by evolution over millennia. Getting new lenses and learning about the technology has been super-fun — thanks for coming along for the ride!