Laser eye surgery is a very popular procedure for correcting near- and far-sightedness as well as astigmatism. Since the first techniques were developed in the early 1980s, many millions of people have gone under the laser hoping to eliminate the need for contacts and glasses. I myself underwent PRK in July, 2011. As is my nature, before I leapt, I looked very closely at the available information on laser eye surgery. What I found was surprising, even disturbing.

It wasn’t until I spent a great many hours investigating the procedure in detail, that I realized how stunningly little information is available to those who are performing their own casual investigation into laser eye surgery. I am dismayed at the lack of information or the outright misinformation provided by practitioners of the surgeries, no doubt in order to prevent scaring off their potential clientele.

The truth is that there are a huge variety of laser eye surgeries, all masquerading under one or two common names. If you think there is only one type (or ten types) of laser eye surgery commonly performed, you are seriously under-informed about the entire procedure. For example, two people, each having had the Lasik variety of surgery performed, could have had completely different surgeries, using completely different procedures, and with completely different risks, side-effects, and expected outcomes.

I hope the following article will provide prospective patients a strong base-level of necessary understanding of the currently available methods of laser eye surgery. Rather than trusting what I provide here as gospel, I prefer that the reader take this article as a jumping-off point for their own education.

For reasons that will become readily apparent after reading this article, I strongly recommend against the corneal flap-based Lasik procedures. Ultimately though, you must find your own answers. Before undergoing surgery, you should educate yourself on the risks involved, you should know what questions to ask of a potential surgeon, and most importantly, it is absolutely critical to know exactly why you would choose PRK, Lasek, Epi-Lasik, or especially Lasik.

While I will discuss some important areas of risk, especially the risk differences between the various procedures, this won’t be an exhaustive risk analysis of laser eye surgery. Each prospective patient should find their own answers to these questions through self-guided research and through conversations with their (hopefully independent) ophthalmologist and their potential eye surgeons. It is my hope however that, after reading this article, those looking into the surgery will have a better idea of just what questions they will need to find answers to.

A Brief Look at Eye Anatomy

Before we begin with the details of laser eye surgery, we need to establish the relevant structure of the eye, specifically the cornea, the transparent part of the eye which covers the iris, pupil, and interior parts of the eye.

The cornea is comprised primarily of the Stroma, with a layer of transparent skin, the Epithelium, covering it. The average corneal thickness in a Caucasian is about 540 µm (about 0.5 mm); it is less in Blacks (520 µm) and even less in Asians (490-510 µm). These are average values, and each individual’s corneal thickness can vary by a significant amount. As I’ll discuss further down, these variances can have very large effects on the risks of the various laser procedures.

In addition to performing about two-thirds of the focusing power of the eye (with the adaptive lens performing the remainder), a very important role of the cornea is to provide structural integrity of the eye. When intraocular pressure is too great, or the cornea is otherwise unable to resist against the pressures within the eye, serious conditions such as ectasia or keratoconus (bulging of the cornea, requiring transplant) can result.

Because all methods of laser eye surgery involve some level of cutting or reshaping of the cornea, the cornea’s structural integrity is always compromised; how much depends greatly on which type of laser surgery is chosen. The difference of just how much cornea is cut between the different methods may surprise you, and should be the paramount consideration when choosing a laser eye surgery method.

The Many Flavours of Laser Eye Surgery

The first step in understanding laser eye surgery is to examine the various procedures that fall under the broad umbrella of the term. The variations between different surgeries can be grouped as follows:

1. Where the correction is made and how the site is accessed.
   • Mid-Cornea – Cornea Flap: Lasik
   • Cornea-Surface
     • No Flap: PRK
     • Skin-Only Flap: Lasek and Epi-Lasik
2. How the correction ‘prescription’ or ‘map’ is developed.
   • General Prescription Map
   • Semi-Custom Preset Map
   • Fully Custom Map
3. How the eye is tracked and the laser is aimed.
   • Outside vs Inside Iris Tracking
     • 3D vs 2D Tracking
   • Tracking Speed
4. How the correction is applied to the eye and the size of treatment zone.
   • Correction Zone Only vs Correction Zone with Tapered Edge
     • Correction/Tapered Zone Larger/Smaller than Dilated Pupil

As you can see, each of the major category groups contains a number of choices (and sub-choices) which can drastically affect the surgery performed. This makes for a dizzying variety of possible procedures falling under the Laser Eye Surgery umbrella. While some discussions on laser eye surgery focus on comparing Lasik, Lasek, PRK, etc, even this level of detail covers only a subset of the important differentiators, and decision points, that make up just the first grouping above. In order to truly understand the procedure, prospective patients need to look at each of these areas in detail.

In this article, I will focus on just the first major question: where the correction is made. This is the major differentiator between Lasik, PRK, Lasek, etc. For more information about the subsequent (important!) questions, see my article: My Laser Eye Surgery, Part I: PRK Pre-Op Preparation.

Critical Decision: Correction Site and Access: Lasik vs PRK

The most critical decision in choosing a laser eye surgery procedure is selecting where the correction is made and how that correction site is accessed. The four primary types of laser eye surgery, PRK, Lasik, Lasek, and Epi-Lasik fall within two main categories of where the correction is made: mid-cornea corrections (Lasik) and cornea-surface corrections (PRK, Lasek, and Epi-Lasik). Each procedure (and their sub-types) are refinements on how that correction site is accessed.

While each of the differences between the various procedures warrants careful consideration, this factor alone, where the correction is made, has by far the greatest impact on overall short-term and long-term risk of the surgery.

Mid-Cornea Correction: Lasik

The Lasik family of eye surgeries, including the various ‘all laser lasik’ procedures (but not epi-lasik, see below) all involve cutting a flap into the cornea (some with a blade, others with a laser), folding that flap back, performing the correction mid-cornea by burning or more accurately vaporizing a lens shape in your cornea, and then finally re-seating the corneal flap. These mid-cornea procedures are relatively new compared to the much older cornea-surface procedures.

As illustrated above, the flap-based correction method involves cutting a significant way through the structure of the cornea, before any corrective shaping is performed. As a result of this, Lasik mid-cornea procedures are always more invasive, significantly compromising the biomechanical integrity of the eye.

This graphic illustrates an average 150µm cornea-flap cut (see the section below on the variability of Lasik cornea flaps), with 1-10 diopter mid-cornea corrections. Note that, since the creation of this graphic, among some practitioners, the 250µm ‘safe’ residual thickness is being revised to 300-350µm.

Cornea-Surface Correction: PRK, Lasek, & Epi-Lasik

The cornea surface procedures, PRK, Lasek, and Epi-Lasik, all involve removing only the epithelium, the skin layer which covers the cornea, and then performing the correction by reshaping the surface of the cornea. With PRK (pictured below), the skin layer is loosened by an alcohol bath, removed completely via a blade, and then allowed to grow back over the cornea after surgery. With Lasek and Epi-Lasik, the skin layer is loosened by an alcohol bath (all Lasek and some Epi-Lasik), a skin-only flap is created via a blade, and then re-seated overtop the cornea after surgery.

As illustrated above, the cornea-surface correction method involves lifting off the epithelium, the skin layer, then correcting the surface of the cornea. As a result, the cornea-surface procedures are the least invasive, retaining the highest degree of biomechanical integrity of the eye.

Each of the surface-ablation methods are quite similar, with the only differences being how the skin is removed and whether or not it is reseated after surgery. Check out the Pros and Cons section below for a more detailed breakdown of each.

This graphic illustrates 1-10 diopter cornea-surface corrections. Note that, since the creation of this graphic, among some practitioners, the 250µm ‘safe’ residual thickness is being revised to 300-350µm.

Serious Concerns with Lasik

If you haven’t already realized the serious concerns with Lasik, concerns arising over the significant difference between Lasik vs PRK (and the other cornea-surface procedures), I urge you to think intuitively about the procedures.

Use your instincts when assessing the wisdom of severing a significant portion of the cornea. Ask yourself what purpose the cornea serves (See: ‘A Brief Look at Eye Anatomy’ above) and what potential consequences can arise from compromising it so significantly. Bear those thoughts in mind as you read the following areas of concern with the Lasik procedure.

1. The Lasik Cornea Flap Never Heals

First, and most importantly, the ugly truth about Lasik is that the cornea flap never actually heals. The cut is permanent. The cornea is forever compromised. You may hear Lasik surgeons repeating a familiar talking-point that: “Within two years, the lasik flap will be strong enough that any force significant enough to dislodge it, would injure an untreated eye.” This is a lie. It’s a lie told over and over again by countless lasik doctors. They sleep at night by telling themselves that the wound does ‘heal’ in that the skin heals and it creates a bond, however the cornea itself never actually heals, never rebinds together, the flap is permanent.

The cornea flap never heals, only the skin layer overtop of it. There are numerous reports of quite mild trauma, even actions as light as finger rubbing, that have dislodged the flap even up to 14 years after surgery.

As you’ll see in the following section, the potential consequences of that permanently thinned cornea are incredibly serious.

2. Higher Risk of Permanent Vision Loss Due to Ectasia with Lasik

Ectasia is a stretching and bulging of the cornea, resulting in rapid vision loss, often requiring corneal transplant to treat. The causes of ectasia are manifold, but a significant rise in post-Lasik ectasia has been seen.

The Lasik community has taken the issue very, very seriously. The most frequent topic being discussed by Lasik surgeons in journals and at conferences is the development of methods of pre-screening patients to reduce the rate of post-lasik ectasia.

The true rate of post-Lasik ectasia may never be known, but estimates have peaked at the 0.6-0.9% rate and with newer screening techniques in place, the current rate is estimated to be about 1 in 2,500.

With millions of lasik surgeries being performed, even a 1 in 2,500 chance of debilitating vision loss is far too high for my taste. And that’s only if Lasik doctors turn away as many patients as possible. The risk factors leading to ectasia are still being studied, but the residual thickness of the cornea, coupled with the amount of correction are universally cited as the prime factors. The rates of ectasia are significantly reduced after PRK and other cornea-surface procedures, where less of the cornea is compromised.

Again, the fundamental problem with Lasik rears its head, the cornea is critically important for maintaining the structural integrity of the eye, yet as the two graphics above illustrate, significantly more cornea is compromised by Lasik than by the cornea-surface procedures such as PRK. Yesterday’s wisdom was that 250µm was a ‘safe’ residual thickness, but today’s best practices suggest 300-350µm. What will tomorrow’s ‘safe’ be? Obviously the Lasik industry is coming around to what should be intuitive to everybody, that the more of the cornea that is compromised (both by the flap cut and by the correction itself), the less it is capable to maintain the proper shape of the eye.

Prospective patients of any medical procedure understand that there are risks. They understand that there is a trade-off between the potential upside of the outcome versus the potential pitfalls of side-effects. However, many prospective Lasik patients aren’t ever aware that there are alternatives that provide those same upsides, without the serious pitfalls. Lasik doctors will tell you that incident rates of ectasia are low, but they don’t tell you that they are many, many times higher with Lasik than with PRK.

I strongly recommend that anybody considering Lasik, do some serious digging into the phenomenon of post-lasik ectasia. It’s an incredibly serious condition that simple common sense (as well as serious scientific study) reveals is more likely to occur with mid-cornea Lasik procedures, than with cornea-surface PRK procedures.

3. The High Variability of Lasik Flap Thickness

With Lasik, before the correction is even applied, a significant amount of the cornea is sliced into. This is done either via a kind scalpel called a “microkeratome”, or via a specialized laser used only to make the flap, an “IntraLase” laser. However, the thickness of this flap is never consistent from patient to patient, indeed it can vary greatly depending upon both the skill of the surgeon and upon which Lasik variant, microkeratome or IntraLase is chosen (with the latter being somewhat more precise).

Even with the best surgeon using the latest flap-cutting technique, there is still quite a bit of chance involved in just how much of the patient’s cornea is going to be cut into to make the flap. For a somewhat frightening exhibit of this, refer to the abstract from the paper below, which describes differences in the ‘reproducibility’ of Lasik flap thickness between mechanical blade and IntraLase laser flap-cutting methods.

What frightens me about that report is that, while IntraLase flaps are off by more than I’m comfortable with, the flaps cut by surgeons using mechanical blades were sometimes off by huge amounts from what was being aimed for; both types of blade had a range of actual cut thicknesses wider than 115µm. Even if you are a ‘good candidate for lasik’, the high variability of the flap thickness could leave you with a much too thin cornea. When aiming to cut only 130μm into the cornea, sometimes cutting over 70µm too deep is an absolutely staggering error. Indeed using one type of mechanical blade, the incidence rate of “thick flaps”, flaps greater than 170μm, which means cutting significantly deeper into the cornea than intended, was over 10%. Scary stuff.

Indeed, due to the unpredictable flap thickness, the Lasik journals recommend surgeons measure the thickness of the patient’s cornea flap mid-surgery (a procedure called pachymetry), aborting the procedure if the flap thickness will result in a too-thin cornea. Makes sense right?

Well most Lasik surgeons don’t do this.

That to me is frightening. There is a huge random variation in flap-thickness, but most doctors don’t even measure the flap. This means most Lasik surgeons have no idea how much corneal thickness their patients have left after surgery.

4. Retreatment is Far More Complicated After Lasik

Because a significant part of the cornea is compromised when cutting the lasik flap, there is obviously significantly less cornea thickness after Lasik. Additionally, because of the high variability of that flap thickness, it is difficult to determine in advance how much flap thickness will remain. This means that, not only is the structural integrity of the eye lessened, and unpredictably so, but the options for retreatment are also severely limited. There is only so much cornea that can be safely burned away, and since Lasik already starts much deeper into the cornea than the cornea-surface options, immediate retreatment to correct errors in the procedure or future retreatment to re-correct changing vision are far more risky, sometimes not even possible.

Even a single Lasik treatment is simply not an option for people with thinner than average corneas or significant vision prescriptions. For these people, the much less risky cornea-surface treatments (PRK, Lasek, epi-Lasik, etc) are still a safe and recommended option. For those with thin, but supposedly ‘safe enough’ corneal thicknesses, they’re rolling the dice as to whether their post treatment eye will still be ‘safe enough’ for retreatment or whether they might be stuck with uncorrectable vision forever.

Not only that, but there can be serious issues with epithelial ingrowth under the flap when a retreatment is attempted. Epithelial ingrowth after Lasik is not exactly unheard of without retreatment, but it is significantly more likely when performing a Lasik retreatment.

Of course, nobody wants a retreatment to correct the first procedure, but it can be necessary. “The re- treatment rate after primary LASIK varies in different studies and is approximately 6% to 20%.” And, because you’re working with a much more compromised cornea after Lasik, the options for retreatment are significantly more complicated.

5. Severe Eye Dryness is Far More Common with Lasik

Many people who have never had laser surgery have dry eyes. It is annoying, sometimes painful. Few of them have found eye dryness severe enough to undergo a surgical procedure to implant plugs to help hydrate their eyes. This is the “dry eye” that is meant in laser eye surgery journals. It is not trivial.

Immediately after my surgery, I got a feel for this sort of severe dry eye. It lasted a few days while my epithelial layer re-grew after PRK. It was not something I would have wanted to live with for the rest of my life. Unfortunately, many Lasik patients have to endure this, or have additional surgical procedures to help mitigate it.

Lasik cuts much deeper into the cornea. It severs a lot more nerves than PRK/Lasek/Epi-Lasik. And of course, the deeper you cut (which is unpredictable due to randomness of corneal flap cuts), the greater chance you have of cutting so many nerves that your eyes no longer produce enough tears. A staggering 1.3% of Lasik patients had to have plugs surgically embedded in their eyes to help offset severe and painful eye dryness.

No PRK patients had to have this done. PRK leaves a more intact, more healthy cornea.

If Lasik is so dangerous, why is it so popular?

This is a question worth asking. If Lasik has higher rates of very serious short- and long-term side-effects, why is it so popular.

Lasik is an easier sell. Lasik patients feel ‘good as new’ just a day or two after surgery. With little discomfort in recovery, Lasik patients tell other prospective patients how easy it was. How great they see the next day. These patients have no idea how much riskier their procedure is over PRK/Lasek/Epi-Lasik, they just know they’re fine now. Lasik is the very definition of a ‘quick-fix’, often it appears easier, but serious problems can occur in the long run.

With PRK, the skin layer of the cornea is removed. Like removing any top-layer of skin, that area is going to be raw until it regrows. PRK has a five-day recovery while your cornea regrows a fresh, seam-free layer of skin under a protective bandage contact lens. These five days are uncomfortable, and painful without medication. Additionally, the skin that grows back is a little swollen for a week or two, this causes slight vision artifacts (ghosting, faint double-images) until the new skin over the cornea completely heals. I describe the preparation and healing process in detail in my series documenting my experience My Laser Eye Surgery, Part I: PRK Pre-Op Preparation.

With the newer Lasek and Epi-Lasik procedures the recovery time and experience matches that of Lasik. In these procedures the epithelial skin layer is not completely removed. Instead the skin layer is folded back in a flap (similar to Lasik, but it’s a skin-only flap, not a corneal flap), and then after the correction is made to the cornea, the skin flap is laid back. This is less traumatic to the cornea’s skin, but has its own minor complications with that flap. These complications are significantly less serious than corneal-flap complications. Often, the worst that happens is the Lasek skin-flap simply has to be removed, effectively turning the procedure into a PRK procedure.

I chose to stick with PRK over Lasek/Epi-Lasik (but never Lasik!) mostly because it was the oldest and most studied procedure and the surgeons were more experienced with it. If I was looking at the procedure today instead of in 2011, I may well have chosen an epithelial-flap procedure like Lasek. I would never contemplate the Lasik corneal-flap procedure.

So, in the past there was a significant difference in recovery between Lasik with its corneal-flap, and the safer PRK, with its epithelial regrowth. Now however, there is little difference in recovery between the corneal-compromising Lasik and the corneal-conserving Lasek/Epi-Lasik.

Of course, doctors are still performing Lasik, when in my opinion there is absolutely zero benefit (even over the short term) to the patient compared the alternatives.

So why is Lasik still performed?

Well, it costs money to upgrade a practice from Lasik. New machines, new training, new techniques. What’s the business incentive for a doctor to do this? There is little. Until Lasik is removed as a medically authorized procedure, doctors can tell themselves that incidents of complication are low (which is true, but many times higher than other, equally effective procedures), and continue earning huge amounts of money offering the Lasik quick-fix.

And Lasik is a powerful ‘brand’. When people think laser eye surgery, they think Lasik. Indeed some PRK and Lasek providers are advertising their clinics as Lasik clinics, capitalizing on the brand recognition, while recommending the safer procedures.

What is a prospective patient to do?

The vast majority of prospective patients will never research laser eye surgery. Will never even be aware of the issues described above. That to me is frightening.

I hope the information I’ve provided helps the people who have found this page. Prospective patients should take this information and go do their own research. They should ask their local Lasik/PRK clinics questions about these complications. They should take those answers and try their best to find out how true they are.

Ultimately prospective patients are going to have to make a decision for themselves. Hopefully I’ve helped to make it a well-informed decision.

Good luck, and keep me posted on your journey!

Continued Reading

Here are my posts on PRK and laser eye surgery, continue reading or, feel free to join the discussion in the comments below!

Laser Eye Surgery Primer: Lasik vs PRK
Lasik vs PRK: What the Lasik Doctor Doesn’t Tell You (You are Here)
My Laser Eye Surgery, Part I: PRK Pre-Op Preparation
My Laser Eye Surgery, Part II: The PRK Operation
My Laser Eye Surgery, Part III: PRK Post-Op Recovery
Halos: A Likely Complication of PRK or Lasik    

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