How Does Your Cornea Protect Your Eye After LASIK?

The cornea is the clear front structure at the front of the eye that is essentially the “window” into the eye. Dr. Swanic is a board-certified ophthalmologist who completed a fellowship in Corneal Surgery at UCLA following his residency. This training included extensive training in refractive surgery but also training in cornea transplantation along with diagnosis and surgical treatment of rare corneal conditions.

The cornea is a 3 layered structure with a regenerative layer on the surface called the epithelium. The epithelium on the cornea is similar to epithelium all over our body in that is constantly being regenerated. The layer below the epithelium is known as the stroma and is the major portion of the cornea accounting for approximately 90% of its structure. Below the stroma lies a single layer of cells called the corneal endothelium that is responsible for keeping the cornea clear. When we perform LASIK or PRK, we are changing the shape of the stroma. If we changed the shape of the epithelium, there would be no permanent effect since this layer regenerates. The endothelial layer is so deep that we do not alter this layer at all during LASIK or PRK. If you wear soft contact lenses, then your contact lens sits directly on top of your cornea and changes the flow of light into your eye so that you can see images clearly.

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At Las Vegas Eye Institute, we pride ourselves on having not only having a cornea specialist perform all of our procedures but also in having the most advanced corneal diagnostic devices in Las Vegas to evaluate the eye before and after our procedures. This allows us to ensure that we select the best candidates and obtain the best results possible.

Our advanced corneal diagnostic tools are unmatched. All refractive practices should have a reflective device that performs corneal topography to measure the curvature of the cornea. At Las Vegas Eye Institute, we actually have four corneal topographers and evaluate all of our LASIK candidates with all four devices. We are the only practice in the city that uses the Cassini Ambient device on LASIK screens. This device reflects multicolored LEDs off of the cornea and analyzes them to create a detailed map of the front surface of your cornea. We then use the Galilei G4 device to reflect rings off of the cornea and a rotating slit beam to measure the thickness of your cornea at thousands of points.

This device is important because it allows us to get a detailed look at the corneal endothelial surface to make sure we don’t see any signs of corneal weakness before your procedure. We then use the iDesign capture device, which reflects infrared dots off of the cornea to create another corneal map. While the iDesign does this, it also creates a wavefront map of your entire eye by reflecting an infrared beam off the retina. This map allows us to determine if your cornea is currently affecting the optics of your entire eye.

Lastly, we have the Alcon Topolyzer Vario device that creates multiple maps of your cornea and allows us to “register” your unique iris details to our Alcon EX500 laser. These details are later used to center your treatment and ensure that astigmatism treatments are properly aligned. The Topolyzer data can also be used for unique corneal treatments called Contoura. These are primarily helpful in patients we determine have preexisting corneal irregularities that we want to correct at the time of LASIK. This may seem like a lot of devices, and we can attest they aren’t cheap, take significant time to perform, and a significant time to interpret. However, we feel the investment is worth it because all of this data combines to give you the best outcomes possible.

How does LASIK surgery permanently change the shape of the cornea, and what are the implications of this for the cornea’s ability to protect the eye?

LASIK (Laser Assisted in Situ Keratomileusis) eye surgery alters the middle layer of the cornea called the stroma. We have to alter this middle layer because the surface layer, called the epithelium, is constantly regenerating, so LASIK surgery would not be permanent if we only performed laser reshaping on the surface layer. The amount of tissue that is removed during LASIK surgery is measured in micrometers or microns (1000ths of a millimeter) and is typically between 20 and 100 microns for most treatments. In comparison, the thickness of a human hair is approximately 60 microns. The total cornea thickness, on average, is 540 microns, so we are typically removing 5-20% of corneal thickness with the LASIK procedure. The excimer LASER was FDA-approved in 1995, so we now have nearly 30 years of experience performing LASIK and PRK (Photo Refractive Keratectomy.). We know that as long as we don’t remove too much tissue, or perform LASIK on poor candidates, the cornea will remain strong, and the quality of vision will be maintained long-term. Dr.Swanic has laser vision correction performed on his eyes in 2010, and 12 years later (this article was written in 2022), his vision remains the same

When we mentioned that LASIK can be problematic for some patients, we are typically referring to patients with corneas that are too thin or have a condition called keratoconus. Keratoconus is a condition where the cornea has a local area of thinning that makes the cornea bulge forward like a cone. This thinning of the cornea makes the cornea weaker, so we would not want to use LASIK to remove further amounts of tissue and weaken it more. If you are considering LASIK, we strongly recommend being evaluated by an ophthalmologist who is fellowship trained in cornea and refractive surgery. Ophthalmologists with this type of training will have encountered many people with keratoconus in their career, they will have likely performed many corneal transplants to help people with severe keratoconus, and will have helped countless others be fit with specialized contact lenses called RGPs or scleral lenses with the help of an optometrist. Cornea and refractive specialists, like Dr. Swanic, are more likely to have advanced technology to detect keratoconus at its earliest possible stages and counsel them to avoid LASIK and seek appropriate care to correct this condition.

What refractive errors can LASIK correct?

The LASIK reshaping process shows a very high rate of efficacy for the correction of myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. LASIK is FDA-approved to correct up to 12 diopters of myopia (although we typically only correct 8-9), 6 diopters of hyperopia (although we typically don’t correct more than 4), and 6 Diopters of astigmatism (correction of over 5 diopters is rare.)

What are myopia, hyperopia, and astigmatism? How do they relate to LASIK and my cornea?

In myopia, the eye has too much power. This is usually because the eye is too long, and the image is focusing in front of the retina. It can also occur if the cornea has too much power. However, this is less typical. The following video has a great graphical depiction of myopia.

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The power of the eye is measured in diopters. The diopter is a metric unit and is related to meters. A person who has 1 diopter of myopia has their best vision 1 meter from their eye. Further out from this, objects become blurry. The relationship is 1/Diopters. Thus, a patient who has 5 diopters of myopia has their best vision 1/5 meters (or 20cm) from their face. If you are myopic, you can take off your glasses and test how far before objects become blurry for you!

In hyperopia, the opposite is true. The eye has too little power, and images are focused behind the retina. This usually occurs because the eye is too short but can also occur if the cornea has too little power.

Astigmatism is a condition in which images are distorted. If you have astigmatism, doctors will often tell you that your eye is shaped like an American football, where it has two different curvatures.

In LASIK, your doctor will program the laser to reshape your cornea so that the power of the cornea matches the length of your eye. In myopic LASIK, the power of the cornea is decreased; in hyperopic LASIK, the power of the cornea is increased; and in astigmatic LASIK, the curvature of the cornea is equalized so that images are no longer distorted when they reach your retina.

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What happens to cornea after LASIK?

The human eye has a remarkable ability to heal, and the cornea has a built-in healing process that allows the corneal flap made during LASIK to rapidly adhere to the cornea. Due to the previously described endothelial pump mechanism, the LASIK flap is adhering to the cornea within minutes of your treatment. Over the next day, the surface epithelial layer fills in the gap at the edge of the LASIK flap to provide additional support. In the coming weeks and months, the cornea adheres more firmly to the edges cut during LASIK. With modern LASIK flaps made with vertical cuts (not possible with a blade), the LASIK flap is very stable and can withstand substantial trauma without movement.

Does LASIK weaken the cornea?

The short answer to this question is “yes.” But the much longer answer is that Dr. Swanic at Las Vegas Eye Institute will determine how much we can safely remove to provide you with excellent and stable vision.

Through extensive preoperative testing and laser ablation planning software the amount of tissue you have before the procedure and the expected amount that will remain after the procedure will be precisely determined. There are two areas that your LASIK surgeon should look at: Residual Stromal Bed (RSB) thickness and the Percentage of Tissue Altered (PTA). The residual stromal bed thickness will typically be kept above 300 microns of tissue. For example, if your cornea is 550 microns thick preoperatively and your surgeon uses a laser to create a 100 micron thick flap followed by a 60 micron ablation then your RSB will be 390 microns (550 microns-100 microns flap thickness-60 microns LASIK ablation= 390 microns residual thickness.) A more modern approach is to keep the percentage of tissue altered under 40%. In the previous example that would be 29% (160 microns tissue altered from flap and ablation / 550 microns starting corneal thickness= 29%) At Las Vegas Eye Institute we look at both RSB and PTA on every case we perform. We also use the highly accurate Cirrus 6000 OCT and Galilei G4 corneal tomographer to measure your corneal thickness in thousands of locations to be sure that we have an accurate value for your starting tissue thickness.

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Does cornea thickness grow back after LASIK?

The surprising answer to this question is “Yes!” However, if you had asked us this question 20 years ago, we would have thought the answer was no. Advanced corneal imaging devices like the OCT (see above) have given us the ability to precisely measure the surface layer of the cornea called the epithelium. We have found that the cornea actually has a very predictable response to LASIK, and this response explains many of the undercorrections seen in the early days of LASIK in the 1990s.

After myopic LASIK (where we lower the power of the cornea by removing central tissue), the central corneal epithelium thickens by about 10-15 microns in an attempt to fill in some of the tissue it recognizes has been removed. This explains why many of our original LASIK treatments ended up undercorrected over time. As this epithelium thickening occurred, the original treatments were losing their efficacy. Fortunately, researchers recognize this occurs, and modern lasers automatically remove extra tissue to prevent this from occurring today.

How much of your cornea is removed in LASIK?

The amount of tissue removed will vary between hyperopic treatments and myopic treatments. Hyperopic treatments only remove a relatively small amount of tissue in the shape of a peripheral ring to steepen the central cornea. This amount of corneal tissue removed is typically between 20 and 40 microns, depending on the desired effect.

Myopic LASIK ablation is in the shape of a circle on the central cornea and varies based on the level of treatment. The average amount of corneal tissue removed per diopter with modern lasers is 15 microns. Hence if you are 5 diopters myopic, then your LASIK surgeon is likely to remove 75 microns of tissue (5 x 15 microns.)

How can I improve my corneal thickness after LASIK?

We do not know of any ways that you can improve your corneal thickness prior to LASIK or after LASIK. However, we do know of ways to help protect your eye and corneal health. One way is to wear sunglasses when you are outside. Another way is to limit the amount of time you spend wearing contact lenses. Contact lenses often lead to eye discomfort and dry eyes because they are constantly using your eye’s natural moisture to keep the lens soft and wet. This problem is amplified if you sleep in your contact lenses.

The lack of eye moisture combines with the contact lens blocking the flow of oxygen to decrease the health of the corneal epithelium. When the corneal epithelium is unhealthy, you are more prone to develop eye infections called corneal ulcers. A corneal ulcer is typically a bacterial infection in which the corneal tissue is damaged and scarred. Corneal ulcers are a leading cause of full-thickness corneal transplants in the United States and the rest of the world.

I am interested in LASIK. What is my next step to see if I am a candidate? Please contact our office by requesting an appointment at this link or call our office at 702-816-2525.

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