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Forward corneal shift found after PRK
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MIYAZAKI, Japan — Photorefractive keratectomy causes a forward shift of the cornea, but not a true ectasia, a study here found.
Thinner corneas and higher myopic correction are risk factors for progressive forward movement of the cornea after PRK, said Kazunori Miyata, MD, and colleagues. Most of the shift occurs within a week of the surgery, and the cornea becomes almost stable by 6 months, the researchers found.
“This study implies that a forward shift of the cornea can affect the instability of refraction after corneal refractive surgery,” the researchers reported. “This is especially important when considering an enhancement ablation for regression. By subtracting more tissue from the cornea, more anterior shift of the cornea may occur, counteracting the corrective effect of anterior surface flattening.”
The forward shift is not a true corneal ectasia, as progressive thinning and expansion of the cornea during the 1-year follow-up did not occur, Dr. Miyata and colleagues reported in the July issue of Archives of Ophthalmology.
Dr. Miyata and colleagues performed PRK on 65 eyes of 34 patients (mean age 32) with refractive errors ranging from -1.25 D to -10 D. The anterior/posterior corneal elevation and thickness were measured preoperatively and at 1 week, 1, 3, 6 and 12 months postoperatively. Twenty eyes of 10 control patients underwent similar measurements at 3-month intervals.
PRK was successful for all patients, reducing the mean manifest spherical equivalent from -5.3 D preoperatively to 0.27 D at 1 week, 0.35 D at 1 month, 0.11 D at 3 months, plano at 6 months and -0.21 D at 1 year postop.
Of the 65 eyes, 39 showed a tendency to forward progression. Eyes that had forward progression had significantly thinner preop corneas and higher myopia, requiring greater photoablation.
The posterior corneal surface in these patients displayed a mean forward shift of about 37 µm at 1 week, which gradually increased to a mean of 55 µm at 1 year. All postop values were significantly higher than those of healthy controls (P < .001). The largest forward shift occurred within the first week postop, was most pronounced between 1 and 6 months, and seemed to stabilize after that, the researchers said.
A statistically significant correlation between the amount of myopic regression and the forward shift of the posterior corneal surface from 1 week to 1 year postop was found (P = .005).