Photorefractive keratectomy: implications of corneal wound healing

Photorefractive keratectomy (PRK) with an argon fluoride laser (193 nm) was developed to alter the refractive power of the eye predictably and permanently. At PRK the central corneal epithelium is debrided mechanically and a disc 4 to 6 mm in diameter is ablated into the anterior stroma using computer controlled algorithms to control the depth of the wound. Theoretically for corrections of up to -7 dioptres less than 10% of the axial corneal thickness is removed.' As there is little loss of corneal strength and miniimal tissue distortion during surgery, reproducibility is enhanced, producing a potentially superior refractive technique to radial keratotomy. Initial enthusiasm for PRK was based on the remarkable precision and smoothness of the ablated surface"A which it was hoped would be translated to an accurate change in refraction. An ideal result would be achieved if the corneal epithelium then assumed the contour of the ablated surface and there was no stromal regeneration. However, it is now apparent that an initial period of overcorrection is followed by regression to a relatively stable refraction and that in a proportion of cases stromal scarring can produce glare and a loss of best corrected visual acuity.--' Despite the precision of the PRK ablation the accuracy of the final refraction is at present only comparable to radial keratotomy. Accordingly 5 months after PRK approximately 55% of eyes have an unaided vision of 6/6, while 95% of eyes with -2-00 dioptres of error are within 1 dioptre of their intended refraction,7 but with considerable variation between individuals.578 This error in final refraction is due to the normal response of the cornea to an anterior keratectomy, which is to regenerate tissue to restore the integrity of the eye. Anterior stromal haze after PRK is thought to result from light scattered by scar tissue and has been shown to be more severe with increased ablation depth.5 7'" Gartry et al detected a reduction in corneal transparency in 92% ofpatients, ofwhom 18% had lost one to two lines of best corrected Snellen acuity 1 year after surgery.7 Additionally, if a small ablation zone (4 mm) is used positive spherical aberration (creating a halo effect) occurs in conditions of reduced illumination as the pupil dilates past the edge of the ablation.57 Although these halos can be eliminated by using a larger ablation zone,7 this entails an increased depth of ablation with its associated increased risk of haze.8 A potential loss of transparency in the optical axis of an otherwise healthy eye must give cause for concern. Unfortunately, topical corticosteroid therapy following PRK has not been demonstrated to reduce subepithelial haze or regression significantly.8 Trials are therefore in progress to evaluate different postoperative regimens to minimise scar formation. As modification of the wound healing response may become an integral part of the management of patients following PRK it is opportune to review our current concepts of corneal scar formation.