How is LASIK similar to a Ziploc bag and why should you care?

Many people do not understand the revolution in vision correction surgery ushered in by LASIK and why it works so well.  The eye, like the rest of the body, has a defense mechanism that when anything more than touches us we have response to repel the invader.  This is the limitation of PRK, or surface ablation where the outside of the eye is reshaped.  The eye shortly after this knows that something drastic has happened and a healing response begins.  We can blunt this response with things like mitomycin C (off label) and a contact lens, and other medications, but there is a battle to calm the eye while it heals.  LASIK was an evolution from ALK (Automated Lamellar Keratoplasty) where a flap is made and tissue was removed using the same cutting device.  Although not nearly as accurate as the laser method, it was apparent that the eye healed quickly from this procedure, and most patients were pain free and had clear and quiet eyes just one day later.  While doctors were struggling with PRK recovery, it was almost intuitive that if one mixed the benefits of ALK with the accuracy of a laser, that a better hybrid would result.  This was the beginning of LASIK.

There were advances in lasers to do the correction but not much else changed for about a decade until the next big innovation, the femtosecond laser.  We were one of the first few sites in the world to experience this revolution in the first critical step in the LASIK procedure.  Instead of using a vibrating razor blade, a laser was used to cut a very precise flap, and a new name was given to this procedure, Intralasik.  Besides being safer, easier for the patient, and more precise, it turned out that the results were better as well.  Here, what I want to discuss is how these flaps on the front of the eye are different from an architectural standpoint than those created by the vibrating blade, the microkeratome.

Because of the way a blade cuts, it always enters superficially and cuts deeper as it moves across.  In the case of the laser, we can make flaps of any thickness, diameter, and importantly the profile of the entry point.  These flaps are very very thin, about 100 microns, which is not much thicker than a sheet of paper.  With the laser method they are "punched out" and fit like a manhole cover which makes replacement more secure and exact.  In the case of the blade, there is opportunity for sliding and malposition.   We have seen a much lower incidence of what is called slipped flaps since introducing the femtosecond LASIK than we saw with the microkeratome.

Here is the important part.  Recently, some doctors in Europe (I just returned from the ESCRS meeting in Barcelona, Spain) have started using reverse geometry flaps with the VisuMax femtosecond laser.  This means that the shape is the opposite of that created by a microkeratome.  The "hole" is smaller on top and bigger at the bottom.  This is how a Ziploc bag works.  If you think about it, a Ziploc, or similar bag holds two very thin pieces of plastic securely together by snapping them together.  This occurs because of the same principle described here.  Now we can ZIP the cornea back.  It always had sticking properties, even back in 1992 when I first saw this amazing quality.  And this got better with the femtosecond laser, such as the Intralase laser.  But now we can Zip the cornea back together without stitches, glue, or anything else for a more secure fit, quicker healing, and less risk to problems after the surgery.  We may not need as much protective googles, or limitation of activities with this newer method, but we will proceed cautiously for now.

LASIK has been called "flap and zap" and now maybe we should call it "zip and zap"?  This might be of special interest to those where the security and rapid healing really make difference.