Age and intended correction dependence of effective ablation rate during photorefractive keratectomy

The long term refractive outcome of a photorefractive keratectomy (PRK) may depend on several factors such as age, intended correction, corneal hydrat ion, and wound healing. Thus we introduce the effective ablation rate which includes both the corneal etching rate during the treatment and the healin g process after the treatment. In this study we define the effective ablati on rate as the etching rate calculated from the difference between intended correction and the achieved correction measured 6 months after PRK. We hav e evaluated our data to find correlation between the effective ablation rat e and the age of treated patients and the intended myopic correction. We ha ve performed photorefractive keratectomy by a Schwind Keratom 2F excimer la ser. 613 eyes of 348 patients were treated. 311 cases were selected where t he follow-up was at least 6 months long. The effective ablation rate was ca lculated for each case and we have found that the effective ablation rate o f human cornea depended on not only the intended correction but also the ag e of patients. The effective ablation rate can be calculated by the followi ng equation: kappa(eff) = 207 + 0.53A - 3.16D(i), where A is the patient ag e in years, D-i is the intended correction in diopters, and kappa(eff) is t he effective ablation rate in nm/pulse. In the meaning of the above equatio n the effective ablation rate is increasing as the intended correction chan ging from low to high myopia (e.g., from -1 D to -9 D) or as the patient's age is increasing. Above 40 years age and in the case of the -6 D or higher intended correction the increased effective ablation rate very likely lead s to hyperopic result, since the effective ablation rate is equal or higher than the nominal etching rate. To reach the best correction for all patien ts the observed dependence must be taken into account during design of PRK.