Corneal thinning from the ablation is less significant when performed at a deeper level in the cornea. LASIK is performed under a flap of cornea, typically 110 to 120 micrometres thick. PRK (phototherapeutic keratectomy), on the other hand, is performed at a more superficial level under the epithelium, which is only 50µm thick. The risk of ectasia, although very low, is slightly greater in relative terms after LASIK compared with PRK.
There are generally accepted safety margins regarding how much cornea should be ablated (or how much short-sightedness should be treated) for different corneal thicknesses. For thinner corneas, consideration should be given to PRK in preference to LASIK. For particularly thin corneas, PRK should be performed.
It is also worth noting that other factors are taken into account when considering whether PRK or LASIK is the best treatment for patients with thin corneas.