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How does LASIK work?

May 14, 2021

Home > How does LASIK work?

How does LASIK work?

This is a fair question, but no, LASIK is not a miracle, it just often seems that way. It’s science. I love explaining to my patients in terms they can understand when they ask, “How does LASIK surgery work?” or “What does laser eye surgery do?”

So if you don’t understand, it’s my fault for not explaining it well enough. I’m going to talk below about:

  • Laser eye surgery facts
  • How the eye works
  • What does LASIK Eye Surgery do
  • How does LASIK corrected vision
  • Who invented all this?
  • How does LASIK laser surgery correct vision.
  • How much cornea would a wood chuck lase if a woodchuck could lase cornea?
  • Good, you’ve been paying attention!

It may help a little to know some LASIK eye surgery facts. I often (pre-Covid) had the home-schooled children of patients who observed their parents having laser eye surgery. I thought, “Wow, there are infinite opportunities to learn science right in the laser room.” There is of course biology, and lots and lots of physics from optics of the eye, laser physics, and much more. In a sentence: LASIK uses an extremely precise laser to shape your cornea so that light is focused perfectly on the retina in the back of your eye. LASIK itself is another acronym that stands for Laser Assisted In Situ Keratomileusis. Keratomileusis is a word that means shaping the cornea. I find it fun that the word keratomileusis means horn carving from its Greek roots κέρας (kéras: horn) and σμίλευσις (smileusis: carving). Horn carving. That’s what I do at work all day!

Who invented lasers and LASIK?

The invention of LASIK should not, in my opinion, be attributed to one single person. Hundreds, if not thousands of amazing scientists have contributed to its development. However, José Barraquer of Columbia is called “the Father of Modern Refractive Surgery” for his advancement of the procedure.

First, I’ll briefly talk about how the eye works.  When light from a distant object enters your eye, it’s “bent” or focused on the back of your eye by the cornea, then the lens of your eye. Imagine the inside of a tennis ball with a small hole cut in it. If the light is bent too much or too little, it will be blurry. If there’s no lens in this hypothetical hole, then the “eye” (tennis ball) would need more focusing power. In someone with a relatively short eye, light doesn’t have as much space to bend. Short eyes then need more focusing power to see clearly and are likely farsighted. Have you ever looked at your prescription for contacts (or glasses)? It often will have a “minus” sign in front of it. This means that your eye focuses too much, and you need a lens that will take away some of the focusing. If it’s a minus sign you’re nearsighted, and if it’s a plus sign, you’re farsighted. Let’s ignore the astigmatism numbers for this article, as that just confuses things.

Ok, but what does LASIK do? Or what does LASIK eye surgery do?

LASIK uses a laser to precisely change the focusing power by the amount you need to make your eye focus perfectly. If your prescription is a minus, and you need less focusing power, the laser flattens your corneal curvature. If your prescription has a plus, meaning you need more focusing power, the laser makes your cornea more curved by removing tissue in a circular shape in the periphery of your cornea. Pretty cool, huh? It’s basically that simple! But not easy.

How does LASIK correct vision?

Answer: Fortunately very well!

There is a super-smart guy named Charles Munnerlyn who got a Ph.D in optical engineering in 1969 who figured out how much tissue would need to be removed to flatten the cornea the perfect amount to make the eye see clearly. Fortunately, the amount required was feasible in the thickness and curvature of our corneas for the vast majority of people. His equations are called…the Munnerlyn Formula (https://en.m.wikipedia.org/wiki/Munnerlyn_Formula)

There is a balance between the amount of tissue required for the laser to change the curvature of our corneas, and the optical zone. That is, the larger the optical zone diameter, the more tissue required. Also, the higher the amount of nearsightedness, the more tissue required. The extremely fortunate part is that our corneas are (roughly 95% of the time) thick enough to allow both removal of enough tissue to focus light perfectly and make the optical zone diameter large enough to allow perfect uncorrected vision in over 96% of people who undergo LASIK or PRK laser vision correction surgery.

How does laser eye surgery work?

Lasers really are about as close to a “miracle” of science that I can imagine. The first scientific paper describing a laser was published in December of 1958, and the first patent was issued two years later. A LASER is called that because it’s an acronym. That is, it’s letters that stand for other words. L.A.S.E.R stands for Light Amplification by Stimulated Emission of Radiation. What that means is that Radiation occurs when a photon (basically a tiny package of energy) is released by a molecule. In the case of LASIK, it’s a combination of Argon and Fluoride- not exactly a dimer, but part of what makes up the words “excimer (excited dimer) laser. Which is the type of laser used in LASIK. Every different molecule in lasers emits a different wavelength of photons. In the excimer laser used in LASIK, the wavelength is 193 nanometers, which is real dang short. So short we can’t see it. It’s in the invisible spectrum of electromagnetic radiation. It’s in the “ultraviolet” range, meaning it’s shorter than the wavelength of purple light. Longer to shorter wavelengths are infrared, then red, orange, yellow, green, blue, indigo, and violet. Then ultraviolet as the wavelengths get shorter still. Remember ole’ Roy from high school? ROY G BIV, that is? What we can see is actually a very small part of the electromagnetic spectrum. Pretty cool I think, and I hope I haven’t lost you. These photons coming from a laser are different from regular light in that they’re all the same wavelength, travel perfectly together in a focused line, and exert an extremely precise amount of energy on its target. In the case of LASIK, the target is the clear collagen of our corneas. Regular light that we see off of objects is disorganized, has different wavelengths, travels in different directions, and has differing amounts of energy.

But here’s the exquisite part. This LASIK laser wavelength, 193 nm, can sculpt the collagen that makes up our clear cornea without heating it! It sculpts with unbelievable precision. It can remove 1/1000th of a millimeter with extreme accuracy and precision. It’s called “photoablation” which occurs when the laser energy breaks the chemical bonds of the corneal tissue, but does not heat it. Most people think that lasers burn and are dangerous. This can be very true! LASERs can burn through solid steel, they can be used to cauterize wounds, drill holes, send signals for thousands of miles. But if we tune it way down, and use the right wavelength, it’s can be as precise a tool as anything ever invented. Wow! Now are you excited?

What was it you asked? Oh, yes: How does LASIK work?

Once we have all the measurements, scans, and have talked to our patients about what to know about LASIK, and how LASIK corrects vision, we begin our procedure. The patient is placed on the LASIK bed, and without going into every detail here (see “What to Expect”) the laser focuses its beam on the center of the cornea, where the most tissue is removed, it removes exactly 0.25µm (microns) per pulse. Each pulse is shaped in a circle of gradually increasing size.  The laser uses infrared tracking to ensure that each pulse is perfectly centered on the pupil. The laser removes tissue in a gradually tapering circular pattern from the center to the periphery of the cornea. It removes about 15 µm per diopter (unit of measure of your glasses prescription). Therefore, if you wear -4.00 contact lenses, the laser will remove roughly 60 µm of tissue at the center of your cornea. The average corneal thickness is about 540 µm, so we remove less than 10% of your total corneal thickness in this example. Then a miracle occurs. This reduced focusing power of your eye allows light to be focused perfectly on the back of your eye (the retina) and you can see clearly without glasses or contact lenses.

This is the ten-minute description of “How does LASIK work,” and I hope it’s been helpful. I love what we do at SharpeVision, and even if you don’t need or want to know how it works, we can still get you out of glasses and contacts. Make your appointment now!

Signature of Dr. Matthew Sharpe, MD
-Dr. Matthew R. Sharpe