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Time Course of Changes in Corneal Forward Shift After Excimer Laser Photorefractive Keratectomy
Kazunori Miyata, MD;
Kazutaka Kamiya, MD;
Tetsuya Takahashi, MD;
Tatsuro Tanabe, MD;
Tadatoshi Tokunaga, COT;
Shiro Amano, MD;
Tetsuro Oshika, MD
Arch Ophthalmol. 2002;120:896-900.
Background Excimer laser refractive surgery has been reported to induce forward
shift of the cornea, but its long-term sequelae remain unknown.
Objectives To prospectively investigate the time course of changes in corneal elevation
after excimer laser photorefractive keratectomy (PRK).
Methods We performed PRK on 65 eyes of 34 patients with refractive errors of -1.25
to -10.0 diopters. The anterior/posterior corneal elevation and corneal
thickness were measured with a scanning-slit corneal topography system before
and 1 week and 1, 3, 6, and 12 months after surgery. Twenty eyes of 10 healthy
control subjects underwent similar measurements at 3-month intervals.
Results The posterior corneal surface displayed a mean ± SD forward shift
of 36.6 ± 25.3 µm 1 week after PRK, which gradually increased
to 55.1 ± 46.1 µm at 1 year. All postoperative values were significantly
larger than those of healthy controls (2.4 ± 8.9 µm; P<.001, Mann-Whitney test). The largest forward shift occurred within
the first postoperative week. The progression thereafter was most pronounced
from 1 to 6 months, and nearly stabilized at 6 months. The variance of postoperative
data was statistically significant (P<.001, repeated-measures
analysis of variance). Multiple postoperative comparisons demonstrated significant
differences between measurements at 1 week and 6 months (P = .002, Tukey Honestly Significant Difference), at 1 week and 1 year
(P<.001), at 1 and 6 months (P<.001), and at 1 month and 1 year (P<.001).
Progression of forward shift was more prominent in eyes with less preoperative
corneal thickness and greater myopia that required larger laser ablation.
We observed no progressive thinning and expansion of the cornea during the
1-year follow-up, which refuted the occurrence of true ectasia. A statistically
significant correlation was found between the amount of myopic regression
and the forward shift of the cornea (Pearson correlation coefficient, r = -0.37; P = .005).
Conclusions Photorefreactive keratectomy induced forward shift of the cornea, which
is not true corneal ectasia. The largest forward shift occurred within the
first postoperative week. Changes were progressive up to 6 months postoperatively,
but became almost stable thereafter. Eyes with thinner cornea and higher myopia,
requiring greater photoablation, are more predisposed to progression. Forward
shift of both corneal surfaces added to the tendency toward myopic regression
after PRK.
From the Meiwakai Medical Foundation, Miyata Eye Hospital, Miyazaki,
Japan (Drs Miyata, Takahashi, and Tanabe and Mr Tokunaga); and the Department
of Ophthalmology, University of Tokyo School of Medicine, Tokyo, Japan (Drs
Kamiya, Amano, and Oshika). The authors have no commercial or proprietary
interest in the product or company described in this article.
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