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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.

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.

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.

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.

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.

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.

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.

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.

Table 1. 
Postoperative Data*
Postoperative Data*
Table 2. 
Comparison of Data According to the Occurrence of Progressive Forward Shift*
Comparison of Data According to the Occurrence of Progressive Forward Shift*
1.
Muller  LJPels  EVrensen  GF The specific architecture of the anterior stroma accounts for maintenance of corneal curvature. Br J Ophthalmol. 2001;85437- 443Article
2.
Shimmura  SYang  HYBissen-Miyajima  HShimazaki  JTsubota  K Posterior corneal protrusion after PRK. Cornea. 1997;16686- 688Article
3.
Seiler  TKoufala  KRichter  G Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg. 1998;14312- 317
4.
Seiler  TQuurke  AW Iatrogenic keratectasia after LASIK in a case of forme fruste keratoconus. J Cataract Refract Surg. 1998;241007- 1009Article
5.
Geggel  HSTalley  AR Delayed onset keratectasia following laser in situ keratomileusis. J Cataract Refract Surg. 1999;25582- 586Article
6.
Schmitt-Bernard  CFLesage  CArnaud  B Keratectasia induced by laser in situ keratomileusis in keratoconus. J Refract Surg. 2000;16368- 370
7.
Miyata  KTakahashi  TTomidokoro  AOno  KOshika  T Iatrogenic keratectasia after phototherapeutic keratectomy. Br J Ophthalmol. 2001;85247- 248Article
8.
Ozdamar  AAras  CUstundag  CBahcecioglu  HOzkan  S Corneal iron ring associated with iatrogenic keratectasia after myopic laser in situ keratomileusis. J Cataract Refract Surg. 2000;261684- 1686Article
9.
Amoils  SPDeist  MBGous  PAmoils  PM Iatrogenic keratectasia after laser in situ keratomileusis for less than −4.0 to −7.0 diopters of myopia. J Cataract Refract Surg. 2000;26967- 977Article
10.
Joo  CKKim  TG Corneal ectasia detected after laser in situ keratomileusis for correction of less than –12 diopters of myopia. J Cataract Refract Surg. 2000;26292- 295Article
11.
Wang  ZChen  JYang  B Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology. 1999;106406- 410Article
12.
Baek  TLee  KKagaya  FTomidokoro  AAmano  SOshika  T Factors affecting the forward shift of posterior corneal surface after laser in situ keratomileusis. Ophthalmology. 2001;108317- 320Article
13.
Kamiya  KOshika  TAmano  STakahashi  TTokunaga  TMiyata  K Influence of excimer laser photorefractive keratectomy on the posterior corneal surface. J Cataract Refract Surg. 2000;26867- 871Article
14.
Naroo  SACharman  WN Changes in posterior corneal curvature after photorefractive keratectomy. J Cataract Refract Surg. 2000;26872- 878Article
15.
Seitz  BTorres  FLangenbucher  ABehrens  ASuárez  E Posterior corneal curvature changes after myopic laser in situ keratomileusis. Ophthalmology. 2001;108666- 673Article
16.
Oshika  TTomidokoro  ATsuji  H Regular and irregular refractive powers of the front and back surfaces of the cornea. Exp Eye Res. 1998;67443- 447Article
17.
Tomidokoro  AOshika  TAmano  SHigaki  SMaeda  NMiyata  K Changes in anterior and posterior corneal curvatures in keratoconus. Ophthalmology. 2000;1071328- 1332Article
18.
Gauthier  CAHolden  BAEpstein  DTengroth  BFagerholm  PHamberg-Nystrom  H Role of epithelial hyperplasia in regression following photorefractive keratectomy. Br J Ophthalmol. 1996;80545- 548Article
19.
Gauthier  CAHolden  BAEpstein  DTengroth  BFagerholm  PHamberg-Nystrom  H Factors affecting epithelial hyperplasia after photorefractive keratectomy. J Cataract Refract Surg. 1997;231042- 1050Article
20.
Moller-Pedersen  TLi  HFPetroll  WMCavanagh  HDJester  JV Confocal microscopic characterization of wound repair after photorefractive keratectomy. Invest Ophthalmol Vis Sci. 1998;39487- 501
21.
Lohmann  CPReischl  UMarshall  J Regression and epithelial hyperplasia after myopic photorefractive keratectomy in a human cornea. J Cataract Refract Surg. 1999;25712- 715Article
22.
Moller-Pedersen  TCavanagh  HDPetroll  WMJester  JV Stromal wound healing explains refractive instability and haze development after photorefractive keratectomy: a 1-year confocal microscopic study. Ophthalmology. 2000;1071235- 1245Article
23.
Smolek  MKKlyce  SD Is keratoconus a true ectasia? an evaluation of corneal surface area. Arch Ophthalmol. 2000;1181179- 1186Article
Clinical Sciences
July 2002

Time Course of Changes in Corneal Forward Shift After Excimer Laser Photorefractive Keratectomy

Author Affiliations

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.

Arch Ophthalmol. 2002;120(7):896-900. doi:10.1001/archopht.120.7.896
Abstract

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.

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.210 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.1315 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.1113 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).

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).

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.

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).

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.

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 α), 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.210 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.1822 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.1822 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.

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Article 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@umin.ac.jp).

References
1.
Muller  LJPels  EVrensen  GF The specific architecture of the anterior stroma accounts for maintenance of corneal curvature. Br J Ophthalmol. 2001;85437- 443Article
2.
Shimmura  SYang  HYBissen-Miyajima  HShimazaki  JTsubota  K Posterior corneal protrusion after PRK. Cornea. 1997;16686- 688Article
3.
Seiler  TKoufala  KRichter  G Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg. 1998;14312- 317
4.
Seiler  TQuurke  AW Iatrogenic keratectasia after LASIK in a case of forme fruste keratoconus. J Cataract Refract Surg. 1998;241007- 1009Article
5.
Geggel  HSTalley  AR Delayed onset keratectasia following laser in situ keratomileusis. J Cataract Refract Surg. 1999;25582- 586Article
6.
Schmitt-Bernard  CFLesage  CArnaud  B Keratectasia induced by laser in situ keratomileusis in keratoconus. J Refract Surg. 2000;16368- 370
7.
Miyata  KTakahashi  TTomidokoro  AOno  KOshika  T Iatrogenic keratectasia after phototherapeutic keratectomy. Br J Ophthalmol. 2001;85247- 248Article
8.
Ozdamar  AAras  CUstundag  CBahcecioglu  HOzkan  S Corneal iron ring associated with iatrogenic keratectasia after myopic laser in situ keratomileusis. J Cataract Refract Surg. 2000;261684- 1686Article
9.
Amoils  SPDeist  MBGous  PAmoils  PM Iatrogenic keratectasia after laser in situ keratomileusis for less than −4.0 to −7.0 diopters of myopia. J Cataract Refract Surg. 2000;26967- 977Article
10.
Joo  CKKim  TG Corneal ectasia detected after laser in situ keratomileusis for correction of less than –12 diopters of myopia. J Cataract Refract Surg. 2000;26292- 295Article
11.
Wang  ZChen  JYang  B Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology. 1999;106406- 410Article
12.
Baek  TLee  KKagaya  FTomidokoro  AAmano  SOshika  T Factors affecting the forward shift of posterior corneal surface after laser in situ keratomileusis. Ophthalmology. 2001;108317- 320Article
13.
Kamiya  KOshika  TAmano  STakahashi  TTokunaga  TMiyata  K Influence of excimer laser photorefractive keratectomy on the posterior corneal surface. J Cataract Refract Surg. 2000;26867- 871Article
14.
Naroo  SACharman  WN Changes in posterior corneal curvature after photorefractive keratectomy. J Cataract Refract Surg. 2000;26872- 878Article
15.
Seitz  BTorres  FLangenbucher  ABehrens  ASuárez  E Posterior corneal curvature changes after myopic laser in situ keratomileusis. Ophthalmology. 2001;108666- 673Article
16.
Oshika  TTomidokoro  ATsuji  H Regular and irregular refractive powers of the front and back surfaces of the cornea. Exp Eye Res. 1998;67443- 447Article
17.
Tomidokoro  AOshika  TAmano  SHigaki  SMaeda  NMiyata  K Changes in anterior and posterior corneal curvatures in keratoconus. Ophthalmology. 2000;1071328- 1332Article
18.
Gauthier  CAHolden  BAEpstein  DTengroth  BFagerholm  PHamberg-Nystrom  H Role of epithelial hyperplasia in regression following photorefractive keratectomy. Br J Ophthalmol. 1996;80545- 548Article
19.
Gauthier  CAHolden  BAEpstein  DTengroth  BFagerholm  PHamberg-Nystrom  H Factors affecting epithelial hyperplasia after photorefractive keratectomy. J Cataract Refract Surg. 1997;231042- 1050Article
20.
Moller-Pedersen  TLi  HFPetroll  WMCavanagh  HDJester  JV Confocal microscopic characterization of wound repair after photorefractive keratectomy. Invest Ophthalmol Vis Sci. 1998;39487- 501
21.
Lohmann  CPReischl  UMarshall  J Regression and epithelial hyperplasia after myopic photorefractive keratectomy in a human cornea. J Cataract Refract Surg. 1999;25712- 715Article
22.
Moller-Pedersen  TCavanagh  HDPetroll  WMJester  JV Stromal wound healing explains refractive instability and haze development after photorefractive keratectomy: a 1-year confocal microscopic study. Ophthalmology. 2000;1071235- 1245Article
23.
Smolek  MKKlyce  SD Is keratoconus a true ectasia? an evaluation of corneal surface area. Arch Ophthalmol. 2000;1181179- 1186Article
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