A Study of Central Islands after Excimer Laser phootorefractive Keratectomy

We analyzed computed corneal topography(EH-270,visioptics, Inc., USA) after excimer laser photorefractive keratectomy(PRK, Omnimed, Summit Technology, Inc., USA) to determine the prevalence f central islands, the factors having an influence on their occurrence, and the corneal ablation patterns. PRK was performed with single zone on 314 consecutive myopias(-1.0D~-11.25D) on which topographic analysis was done at least 3 months after surgery. Corneal ablation patterns were classified as uniform, keyhole, semicircular, and central island. Age, sex, amount of attempted correction, preoperative corneal thickness, and ablation zone(5.0mm vs 6.0mm) were studied whether they can affect on the prevalence of central islands. Topographic results showed a uniform ablation patterns in 213 eyes(67.7%), a keyhole ablation in 54 eyes(17.3%),a semicircular ablation in 18 eyes(5.7%), and a central islands in 29 eyes(9.3%). There was no association between prevalence of centralisland and age, sex, amount of attempted correction, or preoperative corneal thickness(P>0.05). Only one of 81 eye(1.0%) with smaller ablation diameter(5.0mm) presented central islands showing the highly statistical significance(P<0.005) while 28 of the 233 eyes(12%) with larger ablation diameter(6.0mm) presented central islands. In conclusion, we think that the ablation diameter is a risk factor in the prevalence of central island, and the further studies for the etiology of central island, which are based on the optical zone should be needed.

Method of Preventing Central Island after Excimer Laser Photorefractive Keratectomy for Myopia

In order to prevent the development of central island after excimer laser photorefractive keratectomy(PRK), 547 eyes which had undergone PRK were studied to evaluate the incidence, width and height of central island retrospectively. Based on this result, PRK was performed on 63 eyes under conventional protocol(software version 3.20, VISX Twenty/twenty Inc, Santa Clara, CA) and then additional laser ablation with a correction amount of 2.5D and adiameter of 3mm was performed whether the incidence has been decreased and additional complications have occurred. As a result of conventional protocol, central island had occurred in 121 eyes out of 547(22.1%) and the width was 2.94+/-0.22mm and the height was 2.64+/-0.89D. In case of 63 eyes which underwent additional laser ablation, central island occurred in 4 eyes (6.34%) and overcorrection occurred only in 3 eyes(4.76%) at one month after PRK. From the standpoint of the above results, we reached the conclusion that the additional ablation for 2.5D by a 3mm in diameter after PRK under conventional protocol is effective. By using this method, we could reduce the incidence of central island without the aid of the upgraded version that contains the anti-central island program.

The Central Island Occurrence Rate and Correlation between the Central Island and Corrected Visual Acuity after Excimer Photorefractive Keratectomy According to the Myopia Degree

We examined 60 eyes of 60 patients after excimer photorefractive keratectomy(PRK) for myopia and analyzed the changes of central island on corneal topography for 6 months prospectively. These patients were divided into two groups according to the degree of myopia, group 1(26 eyes, 6 diopters or below by spherical equivalent) and group 2 (34 eyes, over 6 diopters). The occurrence rate of central island was 76.4% collectively, 87.5% in group 1, 67.7% in group 2 at 2 weeks after PRK. It decreased to 30.4%, 43.5%, and 21.1% respectively at 6 months after PRK and was lower in group 2 than group 1. However there was no statistically significant correlation between the reduction of central island size and improvement of corrected visual acuity in both groups. In conclusion, the central island disappeared and decreased the amount for six months follow-up period after PRK and seems to be no effect on the corrected visual acuity.

Topographic Central Island after Excimer Photorefractive Keratectomy

We analyzed the topographic findings of central islands following excimer photorefractive keratectomies(VISX Twenty/Twenty, Inc, Santa Clara, CA) for myopia to evaluate whether the size of the central island was significantly different betwaen 1 week and 6months postoperatively and whether central island at postoperative 6months affected the decrease of corrected visual acuity from preoperative level. There were statistically significant differences in the indices related to the size of central island(peak, height, area) between 1 week(40 patients, 41 eyes) and 6 months(55 patients, 64 eyes) follow-up groups. In 64 eyes of 6months follow-up group, we could not find any statistically significant effect of the indices related to the size of central island on the decrease of corrected visual acuity, except SAI.