PRK is very similar to LASIK but in this version of laser vision correction a flap is not made to preserve the epithelium. During PRK there is a broad area of epithelium removed to enable the excimer laser treatment to be performed. This epithelium then regenerates under a bandage contact lens. The disadvantages of PRK compared with LASIK are slow recovery in the initial stages and more discomfort (4-5 days versus 3-4 hours). The results of PRK for myopia with low astigmatism are similar to LASIK but in higher myopia, higher astigmatism and in hyperopia the results are not as precise necessitating a higher enhancement rate. In the early days of PRK pain was a major consideration but with medication now available this is much easier to tolerate. PRK in the past was also associated with an incidence of surface scarring (haze) that is not seen with LASIK. The targeted use of anti-fibrotic medication during PRK has seen this problem all but eliminated.
Generally the indications for PRK are similar to those for LASIK except that only small degrees of hyperopia can be reliably treated. Usually the need for PRK arises when there is a contraindication to LASIK. Such contraindications include:
- Pre-existing corneal disease, especially keratoconus. (stable and very mild degrees only);
- A thin cornea (<490 microns);
- Predicted low residual corneal tissue depth under the flap after treatment(<300 microns);
- Evidence of loose adhesion between the surface epithelium and the underlying cornea;
- Very dry eyes;
- Occupational or recreational risk of post-operative flap trauma, eg rugby players;
- Very small orbit preventing safe access for the suction ring used in LASIK;
- Personal preference for PRK.
Complications for PRK are lower in that there are no potential flap issues. Dry eye incidence is also lower with PRK and ectasia is extremely rare unless there is significant pre-existing keratoconus. Because there is a larger area of epithelium to heal, the incidence of corneal infection and inflammation is slightly higher with PRK.