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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.
ABSTRACT
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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.
INTRODUCTION
EXCIMER LASER refractive surgery modifies the refractive power of the
cornea by means of photoablation of the corneal tissue. There is concern that
the cornea is structurally compromised by the surgical tissue subtraction
and by the loss of integrity of the Bowman membrane after excimer laser surgery.1 Several cases of iatrogenic keratectasia after excimer
laser surgery have been documented.2-10
In a series of patients undergoing laser in situ keratomileusis, forward shift
of the posterior corneal surface has been demonstrated, which correlated with
the residual corneal bed thickness11 and the
amount of laser ablation.12 Increases in posterior
corneal curvature resulting from forward bulging of the cornea after photorefractive
keratectomy (PRK)13-14 and laser
in situ keratomileusis15 also have been reported.
The long-term course of these changes, however, has not been studied. Considering
the expected longevity of patients undergoing refractive surgery, it is critical
to investigate whether progressive anterior protrusion of the cornea occurs
after refractive surgery. In addition, forward shift of the cornea can cause
myopic regression after excimer laser surgery.13-15
Thus, longitudinal assessment of the corneal geometric changes in relation
to refractive regression after PRK is also important. We conducted a prospective
study to evaluate the time course of changes in anteroposterior movement of
the cornea after PRK.
PATIENTS AND METHODS
Unless otherwise indicated, data are given as mean ± SD.
Sixty-five eyes of 34 patients undergoing PRK for myopia were enrolled
in this study. Mean age was 31.6 ± 10.1 years. The preoperative refraction
was -5.29 ± 1.97 diopters (D) (range, -1.25 to -10.00
D). Eyes with keratoconus were excluded by using the keratoconus screening
test of Placido disk videokeratography (TMS-2; Computed Anatomy Inc, New York,
NY). Informed consent was obtained from all patients.
Photorefractive keratectomy was performed with an excimer laser system
(VISX Twenty-Twenty; VISX, Inc, Santa Clara, Calif) using an average fluency
of 160 mJ/cm2 and a repetition rate of 6 Hz. Ablation depth was
60.3 ± 23.1 µm (range, 13-109 µm). In all eyes, we selected
the preoperative manifest refraction as the target correction.
Anterior/posterior corneal elevation and corneal thickness were measured
with scanning-slit topography (Orbscan; Bausch & Lomb, Rochester, NY)
before and 1 week and 1, 3, 6, and 12 months after surgery. Changes in the
elevation of the posterior corneal surface were evaluated at the center of
the difference map generated from preoperative and postoperative elevation
maps. For surface alignment in the difference map, the 3-mm-wide peripheral
annular fit zone was used.11-13
Elevation of the anterior corneal surface was assessed in terms of changes
from the first postoperative week using the difference map made from the first
week and subsequent postoperative maps. The amount of myopic regression was
calculated as the change in refraction between 1 week and 1 year after PRK.
Twenty eyes of 10 healthy control subjects (mean age, 30.5 ±
6.2 years) underwent scanning-slit topographic measurements at 3-month intervals.
Their refraction was -1.87 ± 0.94 D. They had no ocular disease
except mild refractive errors.
RESULTS
The number of eyes examined at each postoperative visit is listed in Table 1. Data were collected from more
than 90% of eyes on each follow-up occasion, and all patients completed at
least 4 of 5 predetermined postoperative examination visits. Time course of
changes in mean refraction is shown in Figure
1. The surgery reduced the mean manifest spherical equivalent from -5.29
± 1.97 D preoperatively to 0.27 ± 0.73 D at 1 week, 0.35 ±
0.79D at 1 month, 0.11 ± 0.57 D at 3 months, 0.00 ± 0.72 D at
6 months, and -0.21 ± 0.83 D at 1 year postoperatively. We found
a trend of gradual myopic regression during the 1-year postoperative period.
The amount of regression of mean manifest spherical equivalent was 0.48 ±
1.03 D from 1 week to 1 year postoperatively (Table 1).
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Table 1. Postoperative Data*
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Figure 1. Time course of mean change in
the manifest spherical equivalent after photorefractive keratectomy (PRK).
We found a trend toward gradual myopic refraction during the 1-year follow-up.
Error bars represent SD. D indicates diopters.
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After surgery, the posterior corneal surface displayed a mean forward
shift of 36.6 ± 25.3 µm at 1 week, which gradually increased
to 55.1 ± 46.1 µm at 1 year (Figure 2). All postoperative values were significantly larger than
those of healthy controls (2.4 ± 8.9 µm) obtained at 3-month
intervals (P<.001, Mann-Whitney test). The largest
forward shift occurred within the first postoperative week. The progression
of the shift thereafter was most pronounced from 1 to 6 months, and almost
stabilized at 6 months. The variance of postoperative data was statistically
significant (P<.001, repeated-measures analysis
of variance [ANOVA]). Multiple postoperative comparisons demonstrated significant
differences between 1 week and 6 months (P = .002,
Tukey Honestly Significant Difference [HSD]), 1 week and 1 year (P<.001), 1 and 6 months (P<.001), and
1 month and 1 year (P<.001).
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Figure 2. Time course of changes in mean
forward shift of the posterior corneal surface after photorefractive keratectomy
(PRK). The largest forward shift occurred within the first postoperative week.
Progression thereafter was most pronounced from 1 to 6 months postoperatively,
and nearly stablized at 6 months. The variation 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).
Error bars represent SD.
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We found a statistically significant correlation between the amount
of myopic regression and the forward shift of the posterior corneal surface
(Pearson correlation coefficient, r = -0.37; P = .005) (Figure 3)
from 1 week to 1 year after surgery.
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Figure 3. Significant correlation between
myopic regression and forward shift of the posterior surface at 1 year postoperatively
(r = -0.37; P = .005).
The amount of regression was calculated as the myopic changes in refraction
from 1 week to 1 year after photorefractive keratectomy.
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Anteroposterior shift of the corneal front and rear surfaces were compared
as changes in the elevation from the first postoperative week. As shown in Figure 4, both surfaces demonstrated gradual
forward shifts, and those changes were symmetric in magnitude and time course
throughout the 1-year observation. The variance of anterior corneal shift
was statistically significant (P<.001, repeated-measures
ANOVA), and the postoperative values at 3 months (P<.001,
Tukey HSD), 6 months (P<.001), and 1 year (P<.001) were significantly larger than at 1 month after
the procedure. Postoperatively, corneal thickness increased significantly
(Table 1) (P<.001, repeated-measures ANOVA), and postoperative values at 6
months and 1 year were significantly larger than those at 1 week and 1 month
(P<.001, Tukey HSD).
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Figure 4. Anteroposterior shift of the corneal
front and rear surfaces calculated as mean changes in the elevation from the
first week after photorefractive keratectomy (PRK). Both surfaces demonstrated
gradual forward shifts, and those changes were symmetric in magnitude and
time course throughout the 1-year observation. Error bars represent SD.
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In each eye, a regression line was created for the postoperative posterior
corneal elevation from 1 week to 1 year (52 weeks), and the inclination of
the line was computed using the least squares method. According to the sign
(positive or negative) of the inclination, eyes were divided into the following
2 groups: eyes that showed progressive forward shift of the cornea during
the 1-year observation (39 eyes) and those that did not (26 eyes). Clinical
data were compared between these groups. As shown in Table 2, preoperative corneas were significantly thinner in the
progression than in the nonprogression group (P =
.01, Mann-Whitney test). Achieved myopic correction was significantly larger
in the progression than in the nonprogression group (P
= .01). No intergroup difference in patient age and preoperative intraocular
pressure was found.
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Table 2. Comparison of Data According to the Occurrence of Progressive
Forward Shift*
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COMMENT
In the present 1-year prospective study, we found a trend toward progressive
forward shift of the posterior corneal surface after PRK. Although the progression
tended to stabilize after 6 months, marked postoperative changes were observed
between 1 and 6 months (Figure 2).
The progressive forward shift, however, did not occur in every case. Among
the 65 eyes, 39 (60%) showed a tendency to forward progression and 26 (40%)
did not. The cases with forward progression were found to have had significantly
thinner preoperative corneas and higher myopia to be corrected. Baek et al,12 who investigated factors affecting the anteroposterior
movement of the cornea 1 month after laser in situ keratomileusis, reported
that eyes with thinner corneas and higher myopia requiring greater laser ablation
are more predisposed to the anterior shift of the cornea. The present study
indicates that less corneal thickness and greater myopic correction are the
risk factors for progressive forward movement of the cornea after PRK.
The reproducibility/accuracy of the scanning-slit topography has been
discussed in previous articles. The reproducibility of 2 consecutive measurements
of spherical power has been reported to be 0.80% in healthy human eyes.16 In eyes with keratoconus, reproducibility indices
were 1.73% and 2.16% for the anterior and posterior surfaces, respectively.17 These values are comparable with the 0.9% obtained
by means of Placido disk videokeratography in healthy controls.16
Kamiya et al13 reported that the instrument
has sufficient sensitivity to detect surgically induced changes in posterior
corneal curvature. Baek et al12 reported that
2 consecutive measurements of the posterior surface elevation in healthy eyes
yielded a mean variation of 2.0 ± 1.7 µm, accounting for less
than 0.4% of the total corneal thickness. Seitz et al15
described that repeating of the measurement of posterior corneal power 3 times
by the same examiner revealed a reliability coefficient of 0.96 (Cronbach  ),
indicating a high reproducibility (small test-retest variability). In the
present study, we measured 20 healthy eyes at 3-month intervals and obtained
a variability of 2.4 ± 8.9 µm for the posterior corneal elevation.
High reproducibility may not correlate with high accuracy. However, in studies
that deal with time changes, eg, preoperative and postoperative changes, it
is not irrelevant to assess reproducibility of the measurements to see the
applicability of data. The current and previous results suggest that the scanning-slit
topography possesses reasonable accuracy in the comparative assessment of
posterior corneal surface.
Because the anterior surface of the cornea after refractive surgery,
especially after PRK, is subject to epithelial and stromal wound healing,
changes in the anterior corneal elevation may not directly indicate general
corneal protrusion. The ability to individually assess the anterior and posterior
corneal surfaces may facilitate the better understanding of surgical physiology
of the cornea after excimer refractive surgery.
Several cases of iatrogenic keratectasia have been seen after excimer
laser corneal surgery.2-10
The term corneal ectasia, however, is not appropriate
for the forward shift of the cornea observed in the present study. Progressive
thinning of the cornea did not occur during the 1-year postoperative period
(Table 1), and forward shift of
the anterior corneal surface was highly similar in magnitude and time course
to that of the posterior corneal surface (Figure 4). The patients even showed gradual increases in the corneal
thickness, which were thought to be attributable to epithelial hyperplasia.18-22
The term ectasia is defined as a dilation, expansion,
or distension, all of which invoke the notion of an increase in surface area
by a process of stretching.23 The present findings
are incompatible with this definition.
Myopic regression is a main factor limiting the predictability and long-term
stability of refraction after PRK. The results obtained herein revealed a
statistically significant correlation between the amount of myopic regression
and forward shift of the posterior corneal surface from the first week to
the first year after surgery (Figure 3).
Steepening in the posterior corneal surface indicates an increase in the negative
power of that surface. Since refractive corneal surgery for myopic correction
aims to reduce the corneal refractive power, an increase in the posterior
corneal negative power adds to the effects of surgery, ie, the possible source
of overcorrection. The anteroposterior corneal shift, however, should affect
both corneal surfaces equally. The steepening of the anterior corneal surface
means an increase in the positive refractive power. When both surfaces bulge
similarly, the anterior surface exerts far greater absolute refractive changes
than does the posterior surface, since the former faces the air and the latter
contacts the aqueous humor. Thus, the forward shift of both corneal surfaces
counteracts the effects of PRK. The forward shift of the cornea alone cannot
fully explain the occurrence of regression after PRK as evidenced by the small R2 value (0.11), and other factors such as epithelial
hyperplasia and/or stromal remodeling should play important roles.18-22
Nevertheless, the current study implies that forward shift of the cornea can
affect the instability of refraction after corneal refractive surgery. 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.
CONCLUSIONS
We found that PRK induced forward shifts of the cornea, which were progressive
up to 6 months postoperatively and became nearly stable thereafter. Progression
of anterior bulging was more prominent in those eyes with less preoperative
corneal thickness and greater myopia requiring larger laser ablation. These
forward shifts, however, did not represent true corneal ectasia as evidenced
by the lack of progressive thinning and expansion of the cornea during the
1-year observation. Anterior movement of the cornea may add to the myopic
regression after PRK.
AUTHOR INFORMATION
Submitted for publication June 28, 2001; final revision received November
27, 2001; accepted February 12, 2002.
Corresponding author and reprints: Tetsuro Oshika, MD, Department
of Ophthalmology, University of Tokyo School of Medicine, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8655, Japan (e-mail: oshika-tky{at}umin.ac.jp).
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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