Description

In this article we compare and contrast the structural and functional consequences of photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), the two photorefractive surgical procedures in use today. PRK eliminates the natural interface, the anterior limiting lamina, between the epithelium and the stroma and this corneal layer cannot regenerate. Thus, the cornea must improvise with less than perfect outcomes that include corneal haze and regression of refractive effect. The LASIK procedure largely avoids these problems by ablating tissue at the mid-stromal level.

The laser beam triggers apoptosis of keratocytes and an inflammatory cell invasion, which wanes as the epithelial barrier function, is re-established. The lost keratocytes are replaced by activated new stromal cells. The latter are particularly plentiful in PRK and cause corneal haze and regression of refractive effect by synthesising abnormal matrix containing hyaluronic acid, fibronectin, tenascin and laminin. In addition collagen type III, the form of collagen associated with repair, is also deposited in the vicinity.

It is anticipated that future developments in laser procedures will be derived from new ways to control both keratocyte death and the appearance of the new stromal cells, processes that we may monitor non-invasively in patients using a clinical confocal microscope. Results may be further enhanced by the application of the emerging wavefront optical technology that may allow visual acuity to be improved to 6/3 or even better.