The cornea is covered by a delicate layer of skin (epithelium) which regenerates if damaged. In order to permanently correct the prescription, the excimer laser must alter the shape of the underlying tough layer called the ‘stroma’. The epithelium and the stroma are separated by an insulating layer of inert tissue (Bowman’s membrane).



There are 2 types of excimer laser techniques, and their differences are more than "skin-deep".

The surface skin layer of the cornea is either destroyed by a surgical knife (‘PRK’) or by the laser ('trans-epithelial PRK'), or separated by a solution of weak alcohol ('LASEK') or a mechanical device (‘EPI-LASIK’) revealing the underlying Bowman’s membrane. The laser then removes Bowman’s membrane and a thin layer of underlying stroma. The removal of Bowman’s insulation exposes the healing cells of the stroma to the active skin (‘epithelial’) cells. This can cause scarring and visual distortions which are very difficult to treat.

A very precise instrument called a "keratome" is used to lift of the surface epithelium and Bowman’s membrane of the cornea, and the excimer laser is applied to the stromal bed under the flap.
The main advantage is the preservation of Bowman’s membrane.


Between 1992 and 1995, the Laservue Doctors performed over 4000 surface ablation (PRK) surgeries, and the results were on average very good. This experience however made them realize that the infrequent but major sight threatening side-effect of surface ablation is not related to the use of the excimer laser, but rather to the fact that this Bowman’s membrane is destroyed during surgery: scarring induced by poor healing of the cornea. Also, steroid eyedrops are necessary to modulate the healing of this superficial layer, and can cause significant complications (such as glaucoma, an elevation of eye pressure).

Since LASIK does not remove the surface skin layer, both scarring and infection have essentially been eliminated, and prolonged use of steroid eyedrops is rarely required.