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(Investigative Ophthalmology and Visual Science. 2004;45:1334-1339.)
© 2004 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.03-1025
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Corneal Healing after Uncomplicated LASIK and Its Relationship to Refractive Changes: A Six-Month Prospective Confocal Study

Avni Murat Avunduk, Carl Joseph Senft, Sherif Emerah, Emily D. Varnell, and Herbert E. Kaufman

From the Department of Ophthalmology, LSU Eye Center, Louisiana State University Health Sciences Center, New Orleans, Louisiana.


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
PURPOSE. To investigate corneal healing and the factor(s) possibly responsible for refractive changes after laser in situ keratomileusis (LASIK).

METHODS. Twenty eyes of 10 patients who underwent LASIK for myopia were examined clinically and by real-time confocal microscopy for 6 months. Epithelial and posterior stromal thicknesses and the thickness of the keratocyte activation zone were measured, and refractive changes were compared with these values. Keratocyte morphology, flap thickness, and subbasal nerve fiber bundle morphology after LASIK were also investigated.

RESULTS. No significant change was detected over time in epithelial thickness after LASIK treatment; however, the posterior stromal thickness was found to be significantly higher 1 month after surgery. A slight but statistically significant negative correlation was detected between the thickness of the keratocyte activation zone and the spheroequivalent refraction after LASIK. The subbasal nerve fiber bundle’s morphology returned to its preoperative appearance 6 months after LASIK, but in the flap stroma the nerve fiber bundle morphology remained abnormal at 6 months after LASIK surgery.

CONCLUSIONS. A weak but significant negative correlation between the thickness of the keratocyte activation zone and spheroequivalent refraction was found after LASIK. The different refractive properties of activated keratocytes may be responsible for the myopic shift after LASIK. Further studies are needed to clarify this hypothesis.

Laser in situ keratomileusis (LASIK) is a relatively new technique for correction of myopia. A hinged flap (consisting of epithelium, Bowman’s layer, and anterior stroma) is created first, and the exposed stroma is photoablated after the flap is folded back. Although many studies have been published on the clinical outcome after LASIK,1 2 3 4 relatively few reports address the biological changes associated with the procedure.5 6 7 8 9 The advent of in vivo confocal microscopy has furnished us with the means of improving imaging of wound healing in the living cornea.8 10 The most dramatic morphologic difference between the pre- and postoperative confocal microscopy examination has been reported to occur in keratocytes that settle immediately behind the flap interface. The oval and brightly reflecting keratocyte nuclei appear larger than preoperative nuclei, and processes can easily be visualized, suggesting that the cells are activated.10 11 Keratocyte activation was strongest at 1 to 2 weeks and persisted until 3 months after LASIK surgery.11 12 Similarly, activated keratocytes have been reported after photorefractive keratectomy (PRK).10 13 Neither LASIK nor PRK has been shown superior in efficacy outcomes14 15 ; however, LASIK has some advantages, such as minimal postoperative pain, faster clinical and functional recovery, less regression of refractive status, and less haze formation.14 15 16 A recent confocal microscopic study revealed that keratocyte-mediated regrowth of the photoablated stroma was a key biological factor responsible for post-PRK refractive instability in humans treated with PRK.17 It is logical to think that keratocyte activation can be a determining factor for the refractive changes after LASIK treatment.

The purpose of this study was to investigate the factor(s) responsible for the refractive changes after LASIK. For this purpose, epithelial thickness, posterior stromal thickness, and the thickness of the keratocyte activation zone were measured by confocal microscopy, and we sought to establish a correlation between refractive changes and these measurements. We also investigated keratocyte morphology, flap thickness, and subbasal nerve fiber bundle morphology after LASIK.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Design
This prospective, interventional cohort study was begun after approval was obtained from the LSU Health Sciences Center institutional review board. Each patient gave written informed consent, and the research followed the tenets of the Declaration of Helsinki.

Patients
Twenty eyes of 10 patients who underwent LASIK for myopia were included in the study. All eyes had normal anterior ocular segments, intraocular pressure (<20 mm Hg), and fundi. Contact lens wear was discontinued 2 weeks (soft lenses) or 3 weeks (hard lenses) before the LASIK operation. There were six women and four men (mean age, 35.4 ± 8.7 years). All patients were 21years of age or older and had stable refractive errors at least 1 year before the laser procedure. Patients who had undergone reoperation, those with diabetes mellitus or glaucoma, or those using any topical ophthalmic medication were excluded. Patients with corneas thinner than 500 µm centrally and/or with a severe systemic disorder that could cause them to miss examinations were also excluded.

The average preoperative spheroequivalent refraction was –5.87 ± 3.45 D (range, –1.75–11.00 D) and the planned ablation depth was 59.8 ± 27.1 µm (range, 16–110 µm). Each patient was examined in the pre- and postoperative period. Preoperative examinations were performed 1 to 3 days before surgery. Postoperative examinations were performed 1day, 3 days, 1 week, 1 month, 3 months, and 6 months after surgery. Each examination included latent and manifest refraction measurement, uncorrected and corrected near and distance visual acuity measurement, slit lamp microscopy, and videokeratography. Confocal microscopic examinations were performed at the preoperative period and 1 week, 1 month, 3 months, and 6 months after LASIK.

PRK and LASIK Procedures
All LASIK procedures were performed in eyes under topical anesthesia, using an excimer laser (20/20; VISX, Santa Clara, CA). A corneal flap was produced with an automated corneal shaper (ACS) microkeratome (ALK-E; Chiron Vision, Irvine, CA). The flap diameter was 8.5 mm and the intended thickness was 160 µm. Suction was monitored during the procedure with a Barraquer tonometer. Patients fixated on a target during the ablation. The stromal bed was irrigated with room temperature balanced salt solution before and after flap replacement to eliminate residual debris. The flap was allowed to dry in place for at least 3 minutes to facilitate adhesion at the end of the operation. After the LASIK procedure, the eyes were not occluded. Antibiotic (tobramycin 0.3%; Tobrex; Alcon, Fort Worth, TX) and corticosteroid (fluorometholone 0.1%; FML; Allergan Inc., Irvine, CA) were prescribed to all patients, four times a day for the first 5 days.

Confocal Microscopy
The eyes were examined with a tandem scanning confocal microscope (Advanced Scanning, New Orleans, LA) with a 20x water-immersion objective. Methylcellulose (Goniosol; CIBA Vision Ophthalmics, Atlanta, GA) was used as an optical coupler between the cornea and the tip of the water-immersion objective. The microscope objective lens was disinfected with 70% isopropyl alcohol wipes before and after the examination. Images were displayed in real time on a monitor (Sony Medical Monitor; Sony, San Diego, CA) and recorded through a CCD camera (Kappa Optoelectronics, Gleichen, Germany) onto digital videotape for later playback and analysis. The video images of interest were printed in color (Epson Stylus Color 800; Seiko Epson, Nagano, Japan) without any image enhancement. Video sequences were reviewed at least twice and evaluated in a masked fashion.

From each scan, the flap thickness, defined as the distance between the surface epithelium, and the flap interface, characterized by accumulation of interface particles (Fig. 1) , were measured. Epithelial thickness, defined as the distance between superficial epithelium and basal epithelial nerve plexus, was also measured, as were posterior stromal thickness, defined as the distance between endothelium and flap interface, and thickness of the keratocyte activation zone, defined as the stromal thickness that contained keratocytes with bright nuclei and visible processes (Fig. 2) .



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  		FIGURE 1. Three days after LASIK, interface particles, which indicate the flap interface, were visible (arrow). Scale bar, 100 µm.

 


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  		FIGURE 2. A confocal micrograph taken 1 week after LASIK from behind the flap interface. The cells with swollen stroma, thick and visible processes, and oval, brightly reflective keratocyte nuclei (arrows) are assumed to be activated keratocytes. Scale bar, 100 µm.

 
Statistical Analysis
Statistical analyses were performed on computer (SPSS for Windows, ver. 10; SPSS Sciences, Chicago, IL). Normality was tested by the Shapiro Wilk test. Flap thickness, epithelial thickness, posterior stromal thickness, and the thickness of the keratocyte activation zone, and the change in spheroequivalent refraction at different examination points were compared with each other by using a one-way ANOVA test. Tukey’s post hoc test was used to detect statistically significant differences between values. The correlation between postoperative spheroequivalent refraction and the epithelial thickness, posterior stromal thickness, and the thickness of the keratocyte activation zone was analyzed by partial correlation. Data are expressed as the mean ± SD, and the differences are considered statistically significant when P < 0.05.

Nerve Fiber Bundles
Nerves appeared as long, narrow structures and those longer than 50 µm were counted. The nerve fiber bundles located in the subbasal region (Fig. 3) , in the stromal flap (distance from the most anterior keratocyte to the flap interface), and in the posterior stroma (Fig. 4) were evaluated. A cornea was considered positive when at least one nerve fiber bundle was noted within any of the areas under study. The difference between the preoperative and the postoperative periods was analyzed with a {chi}2 test. The differences were considered statistically significant when P < 0.05.



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  		FIGURE 3. A confocal micrograph from a patient in the preoperative period. Arrows: subbasal nerves. During the preoperative examination all corneas had a good subbasal nerve plexus. Scale bar, 100 µm.

 


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  		FIGURE 4. Confocal micrograph of a patient 6 months after LASIK. Arrow: Nerve fiber bundle in the posterior stroma. In this period, we detected nerve fiber bundles in the posterior stroma of 8 (50%) of 16 corneas. LASIK surgery had no significant impact on the number of nerve fiber bundles in the posterior stroma at any examination point. Scale bar, 100 µm.

 

   Results
Top
 Abstract
 Methods
 Results
Discussion
 References
 
One patient did not return for the 3- or 6-month post-LASIK examinations, and two patients did not return for the 6-month examination. Thus, for the preoperative period, 1 week and 1 month after LASIK, we analyzed data from 20 corneas of 10 patients, but for the 3-month examination we analyzed data from 18 corneas of 9 patients, and for the 6-month examination we analyzed 16 corneas of 8 patients. All data were distributed normally.

The morphology of the first keratocytes observed behind the flap interface was different from the morphology before surgery. The oval and brightly reflecting keratocyte nuclei and the cell processes could be visualized easily, suggesting that the cells were activated.10 We did not detect any activated keratocytes anterior to the keratome cut (Fig. 5) . Nineteen (95%) of the 20 corneas showed activated keratocytes at 1 week as did 10 (50%) of 20 corneas at 1 month. We were able to detect activated keratocytes 3 months after LASIK surgery in 2 (10%) of 20 corneas. We found that the thickness of the keratocyte activation zone at 1 week was 21.54 ± 3.00 µm; at 1 month, 8.75 ± 1.61 µm; and at 3 months, 0.65 ± 1.38 µm. We did not detect any activated keratocytes at 6 months (Fig. 6) .



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  		FIGURE 5. A confocal micrograph taken 1 week after LASIK treatment, from the anterior flap stroma (the stroma in front of the flap interface), shows normal-appearing keratocytes (arrows). No activated keratocyte (with thick and visible processes, and oval, brightly reflective keratocyte nuclei) were observed in the anterior flap stroma at any examination point. Scale bar, 100 µm.

 


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  		FIGURE 6. The thickness of the keratocyte activation zone decreased exponentially 1-week after LASIK. The highest thickness was measured at 1 week after treatment. *Statistically significant differences (P < 0.05). Error bars, SD.

 
The mean epithelial thickness was found to be 44.77 ± 6.13 µm during the preoperative period. At 1 week after LASIK treatment, the mean epithelial thickness was 45.81 ± 7.01 µm; at 1 month, 43.06 ± 6.37 µm; at 3 months, 46.25 ± 7.26 µm; and at 6 months, 44.00 ± 3.85 µm. We did not detect any significant change in epithelial thickness any time point after LASIK treatment (Fig. 7) .



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  		FIGURE 7. The thickness of the epithelium in the preoperative period and after treatment. LASIK did not have a significant impact on the epithelial thickness.

 
The posterior stromal thickness measured 337.75 ± 18.57 µm 1 week after LASIK treatment. One month after surgery, it was significantly higher (370.42 ± 29.22 µm, P = 0.01). At the subsequent examinations, no further significant increase was observed. At 3 months after LASIK, the posterior stromal thickness was 365.93 ± 29.69 µm and at 6 months, 381.11 ± 30.28 µm. Both the 3- and 6-month measurements were significantly higher than the 1-week postoperative value (P = 0.026 and P = 0.01, respectively, Fig. 8 ).



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  		FIGURE 8. Posterior stromal thickness increased 1 month after LASIK compared with the 1-week postoperative thickness, but at later time points, no further change was seen. *Statistically significant difference from the 1-week postoperative thickness (P = 0.01). Error bars indicate SD.

 
The mean spheroequivalent refraction was found to be –0.34 ± 0.18 D one week after treatment. One month after LASIK treatment, the mean spheroequivalent refraction changed to the myopic side considerably (–0.57 ±0. 22 D), but the difference did not reach statistical significance (P = 0.06). Three months after treatment, the mean spheroequivalent refraction was –0.56 ± 0.18 D, and at 6 months, –0.56 ± 0.24 D (Fig. 9) .



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  		FIGURE 9. Mean spheroequivalent refraction changed considerably after LASIK between the 1-week and 1-month examination points, but the difference is not statistically significant (P = 0.06). At the later examination points, no change occurred. Error bars, SD.

 
The mean flap thickness was 145.40 ± 9.55 µm 1 week after LASIK. The flap thickness did not change significantly between examination points.

We did not detect any significant correlation between spheroequivalent refraction and epithelial thickness, when measured at different examination points (r = 0.068, P = 0.650). Similarly, no significant correlation was found between the posterior stromal thickness and the spheroequivalent refraction (r = –0.099, P = 0.54); however, a slight but statistically significant negative correlation was detected between the thickness of the keratocyte activation zone and spheroequivalent refraction after LASIK (r = –0.278, P = 0.049).

During the preoperative examination, all corneas had a good subbasal nerve plexus. However, 1 week after LASIK, we detected 1 (5%) of 20 corneas with subbasal nerve fiber bundles longer than 50 µm. One month after LASIK, one cornea had subbasal nerve fiber bundles longer than 50 µm. Three months after, 9 (50%) of 18 corneas showed subbasal nerve fiber bundles. Six months after the surgery, all corneas had subbasal nerve fiber bundles. The {chi}2 test revealed significant differences between the percentage of preoperative corneas with subbasal nerve fiber bundles, and the same percentages after LASIK treatment at all examination times except 6 months (all P < 0.01).

Before surgery, we detected that 16 (80%) of 20 corneas contained nerve fiber bundles in the anterior stroma, which would correspond with the flap stroma in the post-LASIK period. One week after treatment, only 5 (25%) corneas had nerve fiber bundles in the flap stroma. The difference was statistically significant (P = 0.0005, {chi}2 test). When confocal microscopic examination was performed 1 month after LASIK, no corneas had nerve fiber bundles in the flap stroma. The difference between the preoperative percentage and percentage at 1 month was statistically significant (P = 0.00000002, {chi}2 test). Three months after LASIK, only 1 cornea showed nerve fiber bundles in the flap stroma (0.4%), which is significantly different from the preoperative finding (P = 0.000018). Six months after the surgery, we detected nerve fiber bundles in the flap stroma of 4 (25%) of 16 corneas, and the difference between this and the preoperative percentage was still significant (P = 0.003, {chi}2 test). We detected nerve fiber bundles in the posterior stroma in 8 (50%) of 16 corneas 6 months after LASIK surgery. The percentage of corneas with nerve fiber bundles in the posterior stroma did not change significantly at any examination point.


   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
The keratocyte morphology behind the flap interface at 1 week after LASIK operation was different from the preoperative morphology. We saw many cells with thick and visible processes, and oval, brightly reflective keratocyte nuclei. These cells were assumed to be activated keratocytes.10 18 19 20 Such activated keratocytes have been associated with the healing process in primates after PRK.20 21 The incidence of activated keratocytes diminished after 1 week, but we still detected activated keratocytes in 10 of 22 corneas 1 month after LASIK surgery. This finding is consistent with the findings of Vesaluoma et al.11

Møller-Pedersen et al. demonstrated that activated keratocyte-mediated rethickening of the photoablated stroma is a key biological factor responsible for post-PRK regression of myopia.17 They demonstrated that the corneal rethickening causes myopic regression mediated almost solely by stromal rethickening; only a minor contribution appeared to originate from restoration of the postoperative epithelial thickness. In the present study, we found a significant thickening in the posterior stroma between 1 week and 1 month after surgery. Meanwhile, the spheroequivalent refraction changed considerably to the myopic side between these time points (–0.34 vs. –0.57), but the difference did not reach statistical significance. It is logical to think that the posterior stromal rethickening seen 1 month after LASIK was related to the activated keratocytes, since the highest value for the thickness of the activated keratocyte zone was found at the 1-week postoperative examination point, and it is well known that activated keratocytes are associated with the healing process after excimer laser treatment.17 21 Normally, it would be expected that a 10- to 15-µm rethickening of the posterior stroma produced a 1-D myopic shift, but the much greater rethickening observed in the present study created only a small amount of refractive change. This finding may suggest that the cornea simply swells after LASIK treatment, and anterior curvature does not change despite a high degree of thickening. However, the proposed mechanism is just a speculation at this time, because we did not have any pachymetry data or corneal curvature measurement to support the hypothesis.

Mitooka et al.22 reported that, although keratocyte density decreases in the anterior half of the retroablation layer (100-µm-thick layer immediately behind the ablation) no decrease was detected in the posterior stroma. Pisella et al.23 reported slightly different results. Keratocyte density was found to be increased 8 and 30 days after LASIK compared with the initial value according to the researchers. However, they reported that 3 months after LASIK the keratocyte density was beginning to decrease and returned to the initial value at 6 months. In the present study, we found that the posterior stromal thickness was highest at the 1-month examination. The increase of the posterior stromal thickness can be caused by activated keratocytes, because the greatest thickness of the keratocyte activation zone occurred 1 week after LASIK and decreased abruptly thereafter.

Although we did not detect a significant correlation between the spheroequivalent refraction and the posterior stromal thickness, we found a weak but significant negative correlation between the thickness of the keratocyte activation zone and spheroequivalent refraction. It is difficult to explain this refractive change with activated-keratocyte–mediated rethickening of the photoablated posterior stroma as caused by PRK; because, if this hypothesis were correct, 1-D refractive change would occur for each 10 to 15 µm of rethickening of posterior stroma. Thus, the most probable explanation is that refractive change is induced by different refractive characteristics of activated keratocytes. However, there may be other explanations. For example, the anterior and posterior curvature and the refractive index may be shifting at the same time. However, we cannot draw fully justified conclusions, because the corneal curvature was not evaluated. Although we do not have any data to support this hypothesis directly it is logical to think that different cellular characteristics of activated cells may also change their refractive properties.

We did not detect any change in epithelial thickness after LASIK treatment compared with the preoperative thickness. Erie et al.24 reported a significant increase of epithelial thickness 1 month after LASIK. According to their data, epithelial thickness did not change thereafter, but remained thicker 12 months after LASIK than before LASIK. We do not know the cause of this conflicting result, but we cannot see any reason for thickening of the epithelium after LASIK treatment.

The ACS keratome significantly undercut corneal flaps as measured at the 1-week confocal examination after LASIK. No single patient had a flap thickness greater than the base-plate thickness. Using ultrasonic pachymetry Perez-Santonja et al.4 reported a mean flap thickness of 114.1 µm, with the 160-µm ACS plate. Similarly, but using confocal microscopy, Vesaluoma et al.11 reported a mean of 112 µm and Gokmen et al.25 reported a mean flap thickness of 133 µm with the 160-µm base plate ACS. Vesaluoma et al. reported that the flaps tended to be thicker with time after LASIK. However, we could not detect such an increase in our study as reported by Erie et al.24 This is not unexpected, because we did not detect any activated keratocytes anterior to the flap cut, and we did not detect any epithelial thickening after LASIK.

Six months after LASIK, all our patients had visible subepithelial nerve fiber bundles in their corneas. Linna et al.26 found that subbasal nerve morphology seemed to degenerate from 1 week to 6 months after LASIK and corneal sensitivity returned to normal 6 months after LASIK. However, Lee et al.27 reported significantly lower numbers of subbasal nerve fiber bundles even 12 months after LASIK compared with the preoperative values with a superior hinge. Our findings are more consistent with the findings of Linna et al.27 The reason for the difference between results may be explained by the hinge position, since most nerves appear to enter the cornea at the nasal and temporal limbus.28 In our study, the regeneration of the nerves in the flap stroma was not complete up to 6 moths after LASIK, as reported earlier.27 We did not find any effect of LASIK on posterior stromal nerve fiber bundles, as expected.

In conclusion, we found a weak but still significant negative correlation with the thickness of the keratocyte activation zone and spheroequivalent refraction after LASIK. The different refractive properties of activated keratocytes may be responsible for the myopic shift after LASIK. Further studies with more subjects and concomitant corneal curvature analysis will help to explain the underlying mechanism of refractive error shifts that occur after LASIK surgery.


   Footnotes
 
Supported in part by Grant EY02377 from the National Eye Institute and an unrestricted departmental grant from Research to Prevent Blindness, Inc.

Submitted for publication September 17, 2003; revised October 30, and December 19, 2003; accepted January 22, 2004.

Disclosure: A.M. Avunduk, None; C.J. Senft, None; S. Emerah, None; E.D. Varnell, None; H.E. Kaufman, None

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Corresponding author: Avni Murat Avunduk, Karadeniz Technical University, School of Medicine, Department of Ophthalmology, Trabzon 61080, Turkey; avunduk{at}ttnet.net.tr.


   References
Top
 Abstract
 Methods
 Results
 Discussion
 References
 

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