A patient who has thin corneas before conventional or custom wavefront Lasik, Bladeless Lasik, PRK, LASEK, or Epi-Lasik, may not be excluded as a candidate for all refractive surgery, but it is a condition that must be reported to the doctor. Pre-existing thin corneas need to be evaluated before surgery and may exclude a patient from all or some refractive surgery techniques.
A primary concern will be if the cornea is naturally thin or is subject to disease such as keratoconus. A thorough examination by a competent doctor can determine the health of the cornea.
Lasik, Bladeless Lasik, PRK, LASEK, and Epi-Lasik all remove corneal tissue to change refractive error. If the patient is myopic, the removal is in the center of the cornea. If the patient is hyperopic, the removal is at the periphery of the cornea. What is most important is that there remains a stable and untouched portion of the cornea to insure corneal integrity and stability.
Studies show and most doctors agree that at least 250 microns of cornea must remain untouched for the cornea to remain stable. More is always better. If too much tissue is removed too deep, the cornea may become unstable and ectasia may develop. This is not something anyone will want to deal with and is most certainly not worth the risk of refractive surgery. It is possible to calculate the amount of tissue that will be removed prior to surgery and take measurements of the cornea to determine with reasonable certainty how much cornea will remain and if this is enough to keep stability.
Lasik requires that a thin layer of the cornea to be cut across the front of the visual axis and laid back like a hinged door. This is called a Lasik flap and the flap is normally around 160 microns thick, give or take. The flap is created with a microkeratome. Mechanical microkeratomes use a ultra-sharp metal blade to create the flap. Mechanical microkeratomes are reasonably predictable, but are unable to make thin flaps and have a significant variability in thickness. Bladeless Lasik is Lasik with a flap created with a femtosecond laser. The Intralase flap can reliably be made thinner and has a smaller variability in thickness when compared to a flap made with a mechanical microkeratome. It may be for an individual with a moderately thin cornea that Bladeless Lasik would be appropriate, but Lasik would not.
With Lasik and Bladeless Lasik, excimer laser energy is applied to ablate tissue from the area under the flap, then the flap is laid back down over the reshaped area. For Lasik or Bladeless Lasik to proceed correctly, the flap needs to be thick enough to remain stable and stay attached at the hinge. Also, there needs to be enough tissue under the flap to allow removal of tissue for the refractive change and keep the structural integrity of the cornea intact.
PRK, LASEK, and Epi-Lasik do not require this thick corneal flap, these are known as surface ablation techniques. With Lasik, the laser ablation starts at about 160 microns down into the cornea. With PRK, LASEK, and Epi-Lasik, the laser ablation starts virtually at the surface of the cornea. A patient with a thinner cornea may find that a surface ablation technique may be appropriate.
Wavefront-guided ablations nearly always remove more tissue than a conventional ablation for the same correction. Although there are many advantages to a wavefront-guided ablation, it may be that wavefront would be excluded because of the lack of corneal tissue to accommodate the deeper ablation.
There are other refractive surgery techniques that add material to the eye. Intacs adds tiny semicircular slivers of plastic into the cornea at the outer edge to correct up to about 3.00 diopters of myopia (nearsighted, shortsighted vision). P-IOLs and RLE add a lens inside the eye to correct refractive error. These techniques may be appropriate for a person with thin, but healthy, corneas.
All refractive surgery techniques have advantages and disadvantages. It may be helpful to read the detailed articles we have about each procedure on our website, and discuss all options with a qualified doctor. It may be that a different procedure will be fine, or it may be that no refractive surgery of any kind is appropriate.
