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TransPRK vs Epi Lasik vs Lasek vs PRK? PRK Types Explained

TransPRK vs Epi Lasik vs Lasek vs PRK? PRK Types Explained

There sure is a lot of variety out there. Over the years, PRK has developed multiple variants to the procedure. Some have stuck around more than others. But overall, all these variants have very confusing names. So let’s cover everything you need to know about the different types of PRK.

During PRK, the surface layer of the cornea called epithelium is removed to perform a laser correction treatment. All the different types of PRK differ in how this epithelium is removed ranging from blunt instruments to alcohol to even using lasers and also whether or not this epithelium is replaced.

But despite all the different versions of PRK, you really don't need to worry much about which version you get (unlike with lasik where there is one type definitely more superior). There can be a few minor differences but ultimately all will get you to your goal of correcting your vision.

Basics Of PRK

Before we get into all the variants of PRK, let’s talk about what PRK actually is. PRK (which stands for photorefractive kerectecomy), is a vision correction procedure similar to lasik. Lasik and PRK are similar in that they both use excimer laser energy to change the shape of the cornea.

The cornea exists in different layers. The bulk of the cornea is the structural layer of the cornea called stroma. The laser energy from the excimer laser in both PRK and lasik is applied to this layer.

Sitting on the surface of the stroma, however, is another layer. This layer is called the epithelium and serves as the protective skin-like layer for our cornea.

Layers of the cornea

Layers of the cornea; Image by StemBook (CC BY 3.0) / modified from original

The difference between lasik and PRK is how we get down to the stroma. Lasik creates a flap within the cornea. PRK instead, removes that epithelium layer. This epithelium layer must regrow. Because of this key difference, PRK has a longer recovery time than lasik.

While this epithelium layer regrows, vision will be blurry and the eye will have some amount of discomfort and pain. So of course, surgeons have always been looking at how to improve this step and allow things to heal quicker and more comfortably. Thus, many different PRK versions were born.

Conventional PRK

In the standard conventional PRK, the epithelium of the cornea is removed and then discarded. But there isn’t a single standardized version of conventional PRK. Different tools or methods to remove that epithelium have been developed.

  • Blade or blunt instrument. With this tool, the epithelium is scraped away. This is a very mechanical process.
  • Using a rotary brush. While still mechanical, this brush allows the epithelium to be removed very quickly and uniformly.
  • Soaking the epithelium in an alcohol solution. Alcohol causes the epithelium to detach. This avoids mechanical irritation to the cornea but does require more time and a little more effort to remove the epithelium.

In general, there aren’t major differences between all of these different techniques. Over the years, the easy and simplicity of the rotary brush and the reduced mechanical irritation from the alcohol assisted removal have allowed those two techniques to become the more popular ones for conventional PRK.

TransPRK

But there is another way to remove the epithelium. This is with the excimer laser itself. Refinements in this approach have caused this technique to branch off of conventional PRK into a whole new type of PRK called Transepithelial PRK or TransPRK for short.

In fact, conventional transepithelial PRK has been around almost just as long as the other techniques. However, it was a cumbersome process that required two steps: one to remove the epithelium and one to correct vision. In addition, these lasers couldn’t adjust for variations in the thickness of the epithelium or uniformly reach the edges of the cornea. The epithelium removal wasn’t as regular and reproducible as the other manual techniques.

It wasn’t until transepithelial PRK could be done in one step that it started to gain traction as its own category of PRK. The laser which performs this one step TransPRK is the SCHWIND AMARIS (however it isn’t FDA approved for use in the United States).

What About Replacing The Epithelium?

It was pretty clear that removing the epithelium with PRK led to discomfort and a prolonged healing time. But could this be reduced by replacing the removed epithelium after the treatment?

This seems like a very logical thought and led to the development of two more branches of PRK with terribly confusing names.

LASEK

LASEK is short for laser epithelial keratomileusis (not to be confused with lasik or laser-assisted in situ keratomileusis); confusing? most definitely.

This branch of PRK is very similar to alcohol-assisted PRK. The difference is that the epithelium is left intact with LASEK in the form of an an epithelium flap. Following the treatment, the epithelium flap is folded back onto cornea.

Epi Lasik

Again, another confusing name. Epi Lasik is short for epipolis laser in situ keratomileusis. So how is the epithelium removed in this one?

Let’s return to actual lasik for a sec. When lasik was first developed, the lasik flap was created using an instrument called a microkeratome. This instrument used a blade to create a thin flap within the cornea at the depth of the stroma.

Epi lasik uses a similar instrument. This instrument is called an epi-microkeratome. But instead of a metal blade, it uses a plastic one. This plastic blade can’t cut into the stroma of the cornea, it can only break through the epithelium. Thus, when the epi-microkeratome cuts into the cornea, it cuts through and then underneath the epithelium creating an epithelial flap.

Again like with LASEK, this epithelial flap is replaced following the laser treatment.

Are these the best?

The idea of replacing the epithelium seems like a great idea. But when results panned out, saving the epithelium didn’t have any benefit.

Using alcohol to remove the epithelium or using the epi microkeratome didn’t keep that epithelial flap alive. These damaged epithelial cells needed to be replaced anyway by new epithelial cells.

In fact, having an epithelium flap can actually slow down recovery. The old damaged epithelium serves as a barrier to the new healthy growing epithelium. In addition, vision is more blurry trying to look through that epithelium flap.

Eventually “flap off” techniques were developed for epi lasik to remove the epithelial flap at the end of the case and LASEK fell out of favor for just simple alcohol-assisted PRK. Bringing things pretty much back to conventional PRK.

So Which Is Best?

Frankly, there aren’t a whole lot of differences between all the different types of PRK. Many many studies have paired the different types together head to head. Some studies favored one procedure or another. But when looked at collectively, all the different types are pretty much the same. All the different types of PRK all have the same visual outcome, accuracy and safety.

But there are a few differences to note.

Many of the different types of PRK exist in order to help improve the healing and recovery afterwards. And some types have less pain and a quicker recovery. In fact, epi lasik is the only type that consistently has more discomfort compared to the other types of PRK.

TransPRK vs PRK

The goal for TransPRK was to reduce discomfort and improve healing by avoiding any touching of the cornea at all. Seems like a nice way to have your cornea treated with PRK.

But the verdict on whether modern TransPRK reigns superior is unclear. While studies have demonstrated less pain and improved recovery for TransPRK when compared to conventional PRK [1][2][3]. Studies have also demonstrated more pain and a longer recovery for TransPRK when compared to the manual techniques [4][5].

The winner is just simply too close to call.

(and also irrelevant if you live in the United States due to a lack of FDA approval)

Summary

PRK is still used very commonly to correct vision and there are many ways the procedure can be done. Some types have fallen out of favor compared to others (especially the ones that try to preserve the epithelium), but overall with the remaining types there aren't a ton of differences.

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