What Is A LASIK Enhancement Procedure?

Enhancement surgery following LASIK allows the surgeon to fine tune the result to a patient’s individual healing characteristics. Two percent of patients in my practice benefit from an enhancement procedure based on their individual healing.

Enhancement surgery is a “touch up” surgery performed typically six months to one year after LASIK or PRK laser eye surgery. Ultimately, it is a way for the surgeon to fine to the result in response to the individual tissue characteristics and healing patterns of the patient. The laser is capable of a perfect correction, but, of course, everyone heals a bit differently.

When LASIK or PRK laser eye surgery is performed, the laser is set to give a perfect result. Ultimately, the job of the laser is to reshape the cornea, the clear domed cap of the eye, so it focuses light perfectly on the retina in the back of the eye. All patients undergoing LASIK have a flawed shape to their cornea and the goal of LASIK is to give the cornea a perfect shape.

While the laser itself is capable of an almost unimaginable level of precision in reshaping the cornea, it is always the case that the final shape that the cornea will achieve cannot be guaranteed. This is because the laser is being asked to reshape living human tissue. If the cornea were made of an inert material such as plastic or glass, then a perfect shape would probably always result from the laser treatement. Because the corena is alive, however, individual patient healnig characteristics will determine the final optical outcome.

Broadly speaking, there are two types of tissue healnig responses that potentially could alter the perfection of the shape of the cornea after LASIK eye surgery: 1) biomechanical shifting of the cornea and 2) epithelial (surface) healing.

When LASIK is performed, a flap first is made in the eye. This flap will alter the stress-strain physical properties of the dome of the cornea following the engineering principles of the architectural properties of all domes. Depending on the inherent strength of the patients cornea and the thickness of the flap, the creation of the flap alone could cause a shifting of the corneal shape without factoring in any re-shaping performed by the laser. If this biomechanical shift is signficant enough, it could alter the final shape of the cornea enough to affect the focus, even if the laser portion of the procedure itself delivers a perfect corrective shape. If this happens, the optical result could be different from the intended result, although typically still quite close to the intended result.

For this reason, the trend in LASIK technique has been to make ultra-thin flaps which minimize the biomechanical impact of the flap (“thin flap LASIK”). The ideal flap thickness, in my opinion, is between 100 to 120 microns. which is approximately the thickness of a human hair. Making thin flaps is part of a LASIK technique known as “SBK,” or Sub-Bowman’s Keratomileusis.

The second main cause for needing an enhancement is epithelial hypertrophy, or thickening. The human cornea is covered by a very thin skin layer called the epithelial layer. This layer typically is about 55 microns thick in the center. When a LASIK flap is made, the epithelial layer rides on top of the flap so that when the flap is re-positioned after surgery, the epithelial layer remains unaffected. However, in some patients more than others. the epithelial layer responds to the fact that the cornea has been sculpted underneath the flap it sits on. To a limited degree, the epithelial layer can thicken, or hypertrophy, to try to counteract the shap change etched into the cornea by the laser beneath the flap. After LASIK, the epithelial layer can hypertrophy to 80 microns thick in some cases. While this is not dangerous and whil the epithelial layer never really fully “undoes” the laser’s reshaping, it can make a limited attempt to do so. This results in a phenomenon called “regression” after LASIK — a partial loss of effect from the surgery in terms of the overall cornea shape. This partial loss of effect in terms of the final corneal shape, in turn, reverses a small amount of the visual result achieved by the laser.

The treatment for either biomechanical shifting or epithelial hypertrophy is to wait until everything is stabilized, re-lift the flap, and apply a small amount of laser under the flap to counteract the shape changes produced by the healing process. This, of course, is an enhancement procedure.

In general, two percent of my patients will require enhancement procedures. Laser technology has progressed so far that the laser essentially delivers a perfect treatment and these two patient characteristics — biomechanical shift and epithelial hypertrophy — are basically the only two variables left in the equation to limit accuracy. For the two percent of patients in which these come into play, an enhancement surgery can be performed to fine tune the result and take the shape of the cornea back to the shape the laser gave it from the shape that these healing factors took it.

It is important to realize that needing an enhancement per se does not mean a complication has happened. It just means that the patient’s vision is signficantly improved, just not quite perfect due to healing characteristics. If everyone healed identically, there would be no need for enhancements, but this would not be the real world unfortunately.

I am often asked why patients who need an enhancement wouldn’t in turn just need another one since they heal differently than the other 98% of patients. In fact, it turns out that it is exceptionally rare to need more than one enhancement. The reaons for this is that, after the first LASIK surgery, the two healing factors (biomechanical shift and epithelial hypertrophy) are already factored in. There is no new biomechanical shift from the flap since the same flap is used and its biomechanical influence is already part of the result. Similarly, in terms of epithelial hypertrophy on the surface of the flap, this issue will have already reached a stable level. When the flap is lifted for an enhancement, the amount of laser applied is so small that there isn’t enough significance to the new laser re-shaping to stimulate further epithelial hypertrophy.

In sum, enhancement procedures can be a very effective way to fine tune the result to an individual’s healing characteristics. In my practice, about two percent of patients benefit from an enhancement procedure based on healing.

What Is A LASIK Enhancement Procedure?

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