Thickness after PRK or LASIK for High Myopia
Anders Ivarsen, Walther Fledelius, and Jesper Ø. Hjortdal
PURPOSE. To compare 3-year changes in corneal sublayer thick-ness after photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK).
METHODS. Forty-six patients with spheroequivalent refraction of⫺6.0 to ⫺8.0 diopters (D) were randomly assigned to PRK or LASIK. One eye from each patient was included in the study. Examinations included manifest refraction and confocal mi-croscopy through focusing (CMTF) and were performed pre-operatively and postpre-operatively at 1 week and at 1, 3, 6, 12, and 36 months. From CMTF scans, the thicknesses of the central cornea (CT), epithelium (ET), stroma (ST), LASIK flap (FT), and residual stromal bed (BT) were calculated.
RESULTS. After LASIK, spheroequivalent refraction averaged ⫺0.76 D by 1 week and ⫺1.19 D by 1 month, with no subsequent significant change. ET increased 9.0 ⫾ 7.0 m within 1 week and remained constant thereafter. ST increased 12.9 ⫾ 9.4m within 1 year because of increased BT. One week after PRK, refraction averaged⫺0.23 D and stabilized at ⫺1.42 D by 6 months. By 1 week, ET was reduced by 7.5 ⫾ 5.7 m, reached preoperative thickness by 6 months, and in-creased further 7.3⫾ 6.0m by 3 years. ST increased 25.3 ⫾ 17.2 m during 1 year, correlating with the postoperative refractive regression. After both procedures, changes in CT also correlated with refractive changes. No other correlations were identified.
CONCLUSIONS. PRK and LASIK induce a persistent increase in ET that stabilizes 1 week after LASIK and 1 year after PRK. Stromal regrowth is most pronounced after PRK. After LASIK, regrowth is restricted to the residual stromal bed. Postoperative refrac-tive changes correlate with changes in ST (PRK) and CT (PRK and LASIK) but not with changes in ET. (Invest Ophthalmol Vis Sci.2009;50:2061–2066) DOI:10.1167/iovs.08-2853
Laser in situ keratomileusis (LASIK) and photorefractive ker-atectomy (PRK) remain popular surgical procedures for the correction of myopia. Several studies have demonstrated loss of the initial refractive effect after surgery,1–5
and corneal wound repair is believed to be a contributing factor in the development of this postoperative refractive regression.6,7 PRK has been reported to induce more pronounced wound healing than LASIK and has a higher tendency for change in refraction and development of corneal haze.4,5
in-crease in corneal thickness after PRK has been shown in several studies,8 –10
and changes in stromal thickness have been suggested to be responsible for the postoperative refrac-tive instability.6
Changes in epithelial thickness, however, have also been implicated as a potential cause for myopic regression after PRK.11,12
In contrast to PRK, changes in the sublayer thickness after LASIK are more unclear, though several reports have shown a persistent increase in epithelial thickness.8,13,14 In the present study, patients with high myopia were randomly assigned to PRK or LASIK and followed for 3 years with in vivo confocal microscopy to evaluate and compare long-term changes in corneal sublayer thickness.
Forty-six patients with spherical equivalent refraction from ⫺6.0 to ⫺8.0 diopters (D) were randomly assigned to PRK or LASIK. All patients had stable myopia for at least 2 years, astigmatism less than ⫺1.5 D, and monocular best spectacle-corrected visual acuity (BSCVA) of at least 0.10 (logMAR units). Patients who were pregnant or who had systemic disease or a history of previous ocular disease or surgery were excluded from the study.
The study protocol adhered to the Declaration of Helsinki and was approved by the ethics committee of Århus, Denmark. Informed writ-ten consent was obtained from all patients. Randomization to PRK or LASIK was performed with random numbers, and only one eye from each patient was included in the study. Twenty-five subjects were randomly assigned to LASIK and 21 to PRK. Retreatment was not allowed within the first year after surgery.
All surgical procedures were performed under topical anesthesia with oxybuprocaine 0.8% (3 drops administered at 5-minute intervals). Two drops of pilocarpine 2% were applied before surgery to facilitate centration of the suction ring. All operations were performed by the same surgeon.
In PRK, the epithelium was gently removed in a central 8-mm zone after application of 96% alcohol for 1 to 2 seconds. Excimer laser treatment was performed, and one drop of cyclopentholate 1%, one drop of diclofenac 0.1%, and chloramphenicol ointment were admin-istered. Postoperative treatment consisted of chloramphenicol eye drops (0.5%) three times a day for 1 week and prednisolone eye drops (0.5%) three times a day, gradually tapered over 3 months.
In LASIK procedures, a superiorly hinged 9-mm corneal flap was cut with a microkeratome (Supratome; Schwind, Kleinostheim, Ger-many) with a 130-m cutting head. Laser treatment was performed, and the flap was carefully repositioned. A bandage contact lens (Focus Night and Day; Bausch & Lomb, Rochester, NY) was inserted, and cyclopentholate, diclofenac, and chloramphenicol eye drops (one drop each) were administered. The bandage contact lens was removed 1 day after surgery, and chloramphenicol eye drops were prescribed three times a day for 1 week.
Excimer laser photoablation was performed with a flying spot excimer laser (MEL-70 G-scan; Meditec-Aesclepion, Jena, Germany). All treatments were performed in a 6-mm optical zone, and identical nomograms were used for PRK and LASIK surgery. Astigmatism of less From the Department of Ophthalmology, Århus University
Hospi-tal, Århus, Denmark.
Supported by the Danish Medical Research Council, Aarhus Uni-versity Research Foundation, The Institute for Experimental Clinical Research at Århus University, and The Danish Eye Health Society.
Submitted for publication September 10, 2008; revised November 6, 2008; accepted March 13, 2009.
Disclosure:A. Ivarsen, None; W. Fledelius, None; J.Ø. Hjortdal,
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked
“advertise-ment” in accordance with 18 U.S.C. §1734 solely to indicate this fact. Corresponding author: Anders Ivarsen, Department of Ophthal-mology, Århus University Hospital, DK-8000 Århus C, Denmark; firstname.lastname@example.org.
Investigative Ophthalmology & Visual Science, May 2009, Vol. 50, No. 5
than⫺0.75 D was not specifically treated but was included as part of the spherical correction. Astigmatism from⫺1.0 to ⫺1.5 D was treated with an attempted astigmatic correction of⫺1.0 D.
All subjects were examined before surgery and at 1 week and 1, 3, 6, 12, and 36 months after surgery. Examinations included determination of best spectacle refraction and in vivo confocal microscopy. A tandem scanning confocal microscope (Tandem Scanning Corporation, Re-ston, VA) was used to perform confocal microscopy through focusing (CMTF) for corneal sublayer pachymetry, as previously reported.15,16 CMTF is a highly precise technique for measurement of corneal sub-layer thickness and has an SD of only 2.8m for stromal thickness measurements in sedated animals.16In human subjects, however, pre-cision may be degraded by movement, as previously suggested by McLaren et al.17Repeated measurements facilitate the identification of movement-related artifacts. Thus, in the present study, 10 to 20 two-way scans (in and out through the entire cornea) were performed at the corneal apex, followed by careful review for signs of z-axis move-ment. Subsequent analysis of CMTF scans was performed with custom-made software,16and thickness measurements were calibrated using polymethylmethacrylate contact lenses with well-defined thickness, as described in a previous study.16
One patient randomly assigned to PRK was lost to follow-up after 1 month and was excluded from all analyses. During the first year, follow-up was nearly complete apart from follow-up on one LASIK eye at 3 months, three LASIK and two PRK eyes at 6 months, and one PRK eye at 12 months. After 1 year, reoperation was performed in four LASIK eyes and three PRK eyes because of residual myopia, and these patients were excluded from subsequent examinations. By 3 years, 15 LASIK and 14 PRK eyes were available for follow-up.
CMTF Measurement Analysis
CMTF scans were analyzed by identifying in-focus images of corneal structures (epithelium, subepithelial nerve plexus, most anterior keratocyte layer, LASIK interface, and endothelium). The z-axis positions of these images were then used to calculate corneal sublayer thickness (Fig. 1). Briefly, total corneal thickness was defined as the distance from the epithelial surface to the endothe-lium. Epithelial thickness was defined as the distance from the epithelial surface to the subepithelial nerve layer or, after PRK, to the intensity peak located at the photoablated stromal surface. Stromal thickness was defined as the difference between total cor-neal and epithelial thickness. In LASIK-treated corneas, total corcor-neal thickness was further subdivided into flap and residual stromal bed thickness by the LASIK interface that was easily identified by the presence of brightly reflecting particles.18 Only scans from the photoablation center, defined as the region of minimal stromal thickness, were used for subsequent analyses.
Corneal thickness measurements before surgery and at 1 week, 1 year, and 3 years were compared using two-tailed paired t-tests with adjustment for multiple comparisons using the Bonferroni tech-nique. Normal distribution was confirmed with the D’Agostino-Pearson test. Spherical equivalent refraction at 1 year and 3 years was compared with refraction at 1 week with two-tailed paired
t-tests. Changes in refraction from 1 week to 1 or 3 years were correlated to changes in corneal total and sublayer thickness with Pearson correlation coefficient. In all analyses, P⬍ 0.05 was con-sidered statistically significant.
Patients randomly assigned to PRK or LASIK were comparable with respect to all preoperative parameters, including age, best spectacle-corrected visual acuity, spheroequivalent refraction,
keratometry, intraocular pressure, and stromal and epithelial thickness. Preoperative data are summarized in Table 1.
Laser In Situ Keratomileusis
One week after LASIK, spherical equivalent refraction averaged ⫺0.76 ⫾ 0.78 D. By 1 month, refraction had dropped to ⫺1.19 ⫾ 0.74 D; no further significant change occurred during the first year (Fig. 2). Still, a gradual but insignificant loss of refractive effect was noted from 1 year to 3 years, with refrac-tion averaging⫺1.61 ⫾ 0.58 D.
During the first week, LASIK caused an increase in epithelial thickness of 9.0 ⫾ 7.0 m from the preoperative value of 47.4⫾ 3.4m (paired t-test, P ⬍ 0.001). At all subsequent time points, epithelial thickness remained increased, with no signif-icant change from 1 week (Fig. 3; Table 2).
One week after excimer laser treatment, stromal thickness was reduced to 77.5 ⫾ 15.1 m, which was similar to and correlated with the planned photoablation depth of 79.6⫾ 6.5 m (P ⫽ 0.01; r ⫽ 0.48, Pearson correlation). Stromal thick-ness then increased by 12.9⫾ 9.4m, from 404.4 ⫾ 46.4 m at 1 week to 417.3⫾ 43.4m at 1 year after surgery (P ⬍ 0.001; paired t-test). No significant change occurred between 1 year and 3 years (Fig. 4; Table 2).
Flap thickness averaged 141.7 ⫾ 16.6 m 1 week after surgery, with no significant change during the 3-year follow-up (Fig. 5). In contrast, the thickness of the residual stromal bed gradually increased from 319.0 ⫾ 42.2 m by 1 week to 336.8⫾ 41.6m by 3 years (P ⫽ 0.002; paired t-test; Fig. 5). Changes in corneal sublayer thickness gave rise to an initial reduction in total corneal thickness from 529.3 ⫾ 49.4 m before surgery to 460.8⫾ 47.0m by 1 week, followed by a gradual increase to 477.3 ⫾ 43.0m by 3 years (P ⬍ 0.001; paired t-test; Fig. 6). The increase in total corneal thickness from 1 week to 3 years was weakly correlated to the observed change in spheroequivalent refraction (P ⫽ 0.04; r ⫽ 0.54, Pearson correlation). No other correlations were identified between change in refraction and changes in corneal sublayer thickness at any time point.
FIGURE1. CMTF light intensity profile 3 months after LASIK. Peaks correlate closely with well-defined morphologic features, facilitating the determination of corneal sublayer thickness.
Spherical equivalent refraction averaged ⫺0.23 ⫾ 0.77 D 1 week after PRK. By 1 year, the average refraction was⫺1.51 ⫾ 1.31 D, demonstrating a significant loss of refractive effect during the first 12 months (P ⬍ 0.001, paired t-test; Fig. 2). From 1 year to 3 years, no change in postoperative refraction occurred. In one eye, an initial overcorrection of 2.5 D was observed that was followed by the development of haze and marked regression of the refractive effect.
One week after PRK, epithelial thickness averaged 40.0⫾ 4.0m, which was 7.5 ⫾ 5.7 m less than the preoperative value. Epithelial thickness then gradually increased to 53.2⫾ 4.8m by 1 year and 54.8 ⫾ 5.6 m by 3 years (P ⬍ 0.001; paired t-test), giving a net increase of 7.3⫾ 6.0m over the preoperative thickness (P⬍ 0.001; paired t-test; Fig. 3). Thus, changes in epithelial thickness after PRK were more gradual than after LASIK (Table 2). From 1 year to 3 years after surgery, no significant differences were observed between PRK and LASIK.
Excimer laser treatment caused a reduction in stromal thick-ness of 88.7 ⫾ 20.0m 1 week after PRK, which correlated with the expected photoablation depth of 77.2⫾ 7.0m (P ⫽ 0.002; r⫽ 0.66, Pearson correlation). Stromal thickness then increased by 25.3 ⫾ 17.2m (P ⬍ 0.001, paired t-test) from 383.5⫾ 34.6m at 1 week to 410.4 ⫾ 34.5 m at 1 year after PRK. No significant changes were observed from 1 year to 3 years after surgery (Fig. 4). The initial reduction in stromal thickness by 1 week was significantly greater after PRK than after LASIK (Table 2), even though the expected photoablation depth was similar. Subsequent stromal regrowth from 1 week to 1 year after surgery was also significantly greater after PRK
than after LASIK (Table 2), but from 1 year to 3 years the change in stromal thickness was similar after both surgical modalities.
Total corneal thickness was reduced from 522.0⫾ 32.7m before PRK to 423.6 ⫾ 35.9m 1 week after excimer laser treatment (Fig. 6). Corneal thickness then gradually increased to 463.6⫾ 32.7m by 1 year (P ⬍ 0.001; paired t-test), with no significant changes from 1 year to 3 years. The initial reduction and subsequent increase in total corneal thickness was significantly greater after PRK than after LASIK (Table 2). However, changes in total corneal thickness from 1 year to 3 years after surgery were not significantly different between the two procedures.
Changes in refraction from 1 week to 1 year after PRK correlated with the increase in total corneal (P⫽ 0.002; r ⫽ 0.68, Pearson correlation; Fig. 7) and stromal thickness (P⬍ 0.001; r⫽ 0.73, Pearson correlation), indicating that refractive changes were caused, at least in part, by changes in corneal thickness. No other correlations between corneal sublayer thickness and refractive change were identified after PRK.
In the present study, PRK and LASIK both induced increases in epithelial thickness of approximately 15% to 20% that persisted after surgery. In LASIK, the epithelial changes occurred within 1 week and remained unchanged through 3 years (Fig. 3, Table 2). In PRK, per-operative epithelial debridement caused an initial decrease in epithelial thickness, followed by a gradual epithelial thickening over the next 12 months. Thus, PRK and LASIK induced different initial epithelial responses to surgery, though the end point thickness was similar for the two surgical
FIGURE2. Change in average spherical equivalent refraction after PRK
or LASIK. FIGURE3. Change in epithelial thickness after PRK or LASIK.
TABLE1. Preoperative Characteristics of Patients
PRK (nⴝ 20) LASIK (nⴝ 25)
Age (years) 33⫾ 8 (range, 23 to 49) 30⫾ 7 (range, 21 to 46) BSCVA (logMAR units) ⫺0.02 ⫾ 0.05 (range, ⫺0.10 to 0.05) ⫺0.02 ⫾ 0.05 (range, ⫺0.10 to 0.05) Spheroequivalent refraction (D) ⫺6.91 ⫾ 0.57 ⫺7.12 ⫾ 0.57 Keratometry K1 (mm) 7.73⫾ 0.29 7.67⫾ 0.55 Keratometry K2 (mm) 7.54⫾ 0.29 7.62⫾ 0.24 IOP (mm Hg) 16.8⫾ 2.7 16.1⫾ 3.2 Stromal thickness (m) 522⫾ 33 529⫾ 49 Epithelial thickness (m) 47⫾ 4 47⫾ 3
modalities. In one previous study with CMTF, no changes in epithelial thickness were found 1 year after PRK6
; still, other studies have reported an increase in epithelial thickness after PRK8,11,12
Case reports have demonstrated epithelial hyperplasia with an increased number of cell layers after PRK,11
whereas the nature of the epithelial changes after LASIK are less clear. It has been suggested that epithelial hyperplasia after refractive surgery may contribute to the loss of the postoperative refractive effect.11,12,20,21
In the present study, there was no correlation between change in epithelial thickness and change in refraction after PRK or LASIK. This lack of correlation may be attributed to the relatively few patients in each treatment group. After LASIK, however, epi-thelial thickness had already stabilized by 1 week, whereas the major refractive change was noted between 1 week and 1 month after surgery. This suggested that epithelial changes were not the main cause for refractive instability after LASIK. With respect to PRK, the time course of changes in epithelial thickness and refraction was similar, but, as noted, no correla-tion between the two parameters could be identified.
PRK and LASIK both induced stromal regrowth during the first year after surgery (Fig. 4); however, wound repair after PRK gave rise to significantly more stromal tissue deposition than did LASIK (Table 2), and the increase in stromal thickness correlated with the postoperative loss of refractive effect (Fig. 7). PRK has previously been reported to induce a more aggres-sive wound-healing response than LASIK and to entail more myopic regression and more haze development.4,5
Studies have also demonstrated significant amounts of stromal tissue deposition after PRK.6,8
In contrast, stromal changes after LASIK remain controversial, with one study indicating a minor
(insignificant) increase over time,19
one reporting stability,8 and one even indicating a decrease in total corneal and stromal thickness.13
However, the present randomized study is the first to allow a direct comparison of corneal sublayer thickness between LASIK and PRK in human eyes and shows that LASIK causes less stromal tissue deposition than PRK for identical myopic corrections, supporting well-established clinical obser-vations of differences in wound repair (haze development and myopic regression).4,5,22
Mechanisms leading to more aggres-sive wound repair after PRK remain unclear. However, in previous studies of rabbit eyes, we demonstrated that the integrity of the epithelial-stromal barrier at the basement mem-brane level appeared to be of major importance for the gravity of the subsequent stromal wound repair.23,24
Cell culture stud-ies have supported this observation,25
as have clinical obser-vations of other surgical approaches that destroy the epithelial-stromal barrier. Those approaches include laser subepithelial keratomileusis (LASEK), in which the epithelial sheet is sup-posedly kept intact but in which haze and myopic regression also may be seen.26
Interestingly, the stromal regrowth that was observed after LASIK in the present study was found to be localized entirely to the residual stromal bed, whereas flap thickness remained constant through the 3-year follow-up (Fig. 5). This contrasts with our previous observations in rabbit eyes in which LASIK caused stromal regrowth in both flap and residual stroma.27 The present observation is important in patients who are con-sidered for LASIK retreatment because the thickness of the residual stromal bed is a major safety parameter for the amount of refractive correction that can be applied.28,29
In contrast to
FIGURE4. Change in stromal thickness after PRK or LASIK.
FIGURE5. Change in flap and residual stromal bed thickness after myopic LASIK.
TABLE2. Change in Total Corneal, Epithelial, and Stromal Thickness after LASIK or PRK
Before to 1 Week 1 Week to 1 Year 1 Year to 3 Years
Epithelial thickness (m) LASIK 9.0⫾ 7.0* 0.9⫾ 6.8* ⫺0.7 ⫾ 5.1 PRK ⫺7.5 ⫾ 5.7 13.4⫾ 5.2 1.1⫾ 5.8 Stromal thickness (m) LASIK 77.5⫾ 15.1* 12.9⫾ 9.4* ⫺3.8 ⫾ 7.6 PRK 88.7⫾ 20.1 23.2⫾ 14.9 ⫺3.5 ⫾ 7.7
Total corneal thickness (m)
LASIK 68.5⫾ 12.6* 13.8⫾ 8.1* ⫺4.5 ⫾ 5.6
PRK 96.2⫾ 20.8 36.6⫾ 14.1 ⫺2.3 ⫾ 7.9
PRK, changes in stromal thickness after LASIK could not be correlated with changes in postoperative refraction, possibly because of the relatively few patients in both treatment groups. Still, it should be noted that determination of changes in cen-tral thickness alone does not allow comprehensive evaluation of the relationship between stromal tissue deposition and re-fraction.
Both PRK and LASIK showed very good correlation be-tween initial changes in stromal thickness and expected pho-toablation depth. More stromal tissue was removed by PRK than by LASIK (Table 2), even though the nomogram used for the excimer laser photoablation was identical for the two procedures. The reason for the observed difference in the photoablation depth remains unclear, but may be due to dif-ferences in stromal hydration at the time of surgery,30
either because of a longer stromal exposure during epithelial debride-ment in PRK or because of local variation in stromal hydration with depth.31
In accordance with the observed difference in the amount of ablated stromal tissue, spheroequivalent refrac-tion by 1 week averaged ⫺0.23 D for PRK and ⫺0.76 D for LASIK. On average, 1 D refractive change required stromal ablation of approximately 12 m during LASIK and 13 m during PRK, in accordance with the expected value for a 6 mm ablation.32
Interestingly, this observation suggested that the very large difference between PRK and LASIK in central epi-thelial thickness by 1 week (Fig. 3; Table 2) did not have any major impact on the refractive result. Once again, this indicates that evaluating thickness changes in only the center of the cornea may be insufficient to estimate postoperative refractive changes.33
In conclusion, the present study of patients with myopia randomly assigned to PRK or LASIK firmly demonstrates that the two surgical procedures induce different changes in cor-neal sublayer thickness. Epithelial and stromal wound repair occurs after both PRK and LASIK, but the time course is longer and the amount of tissue deposition is greater after PRK. Still, after 1 year, corneas treated with PRK or LASIK appear to be stable and to have undergone no further significant changes in corneal sublayer thickness. Initial changes in stromal thickness (after PRK) and total corneal thickness (after PRK or LASIK) appear to contribute to postoperative refractive regression. In contrast, the increase in epithelial thickness seems to have no refractive impact. In the present study, all thickness measure-ments were obtained only in the center of the cornea. To better evaluate the relation between wound repair and post-operative refractive changes, topographic variations in corneal sublayer thickness over time should be determined.
Unfortu-nately, this is not realistic with in vivo confocal microscopy, but it is hoped that future technical development will allow such investigations.
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FIGURE6. Change in total corneal thickness after PRK or LASIK.
FIGURE7. Change in refraction from 1 week to 1 year after PRK or LASIK as a function of the increase in total corneal thickness.
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