CORNEAL MODULATORS AND THEIR USE IN EXCIMER-LASER PHOTOTHERAPEUTIC KERATECTOMY

The excimer laser has provided a new surgical instrument in the treatm ent of corneal disease [1, 2]. The laser emits ultraviolet radiation a t a wavelength of 193 nm and can remove corneal tissue at a rate of 0. 24 to 0.28 mu m per pulse with minimal damage to the surrounding tissu e [3, 4]. Phototherapeutic keratectomy (PTK) with the excimer laser is effective in removing superficial corneal pathology and in smoothing surface irregularities [5, 6]. The excimer laser consequently may less en the need for more invasive procedures such as lamellar or penetrati ng keratoplasty [7]. Corneal modulators or ablatable mask materials ha ve been used to eliminate irregularities in the corneal surface during laser ablation. In this chapter, we describe the currently available masking materials and discuss their advantages and limitations. Many d isorders that limit vision, including band keratopathy, Salzmann's nod ular degeneration, and various postinfectious and traumatic corneal sc ars, are characterized by irregular corneal surfaces. The excimer lase r is notable for its ability to produce a flat beam profile, owing to beam homogeneity [3, 8]. A direct excimer laser treatment to a rough c orneal surface, however, does not result in smoothing [9]. Instead, th e surface contour is duplicated deeper into the corneal tissue. Theref ore, the production of a smooth surface requires the use of a substanc e during ablation that protects low areas so that high points are abla ted preferentially [10]. These substances commonly are referred to as corneal modulators, masking agents, smoothing agents, or ablatable mas ks.