Laser in situ keratomileusis (LASIK) for the correction of myopia is associated with more rapid recovery of vision, less postoperative discomfort, less haze formation, and less need for prolonged steroid use than photorefractive keratectomy (PRK).1 However, patients often experience dryness, irritation, and soreness after LASIK surgery. In fact, dry eye is the most common complication after LASIK, with an incidence ranging from 3% to 59%.2–6 Fortunately, post-LASIK dry eye is usually transient, and most patients can be managed with topical medications and punctal occlusion 3 to 6 months after surgery.
The mechanism underlying the development of post-LASIK dry eye is not clearly understood. Laser in situ keratomileusis causes damage to corneal nerves by truncation of the sensory nerves in the anterior third of the cornea during creation of the flap.7–9 Moreover, tear film instability,10 decrease in aqueous tear production, corneal epitheliopathy,11 and reduction in conjunctival goblet cells12 after surgery cause sustained dysfunction of the integrated ocular surface–secretory glandular functional unit and contribute to the development of post-LASIK dry eyes.12,13 It has also been demonstrated that the degree of preoperative myopia, the preoperative presence of dry eye disease, and the depth of laser treatment are risk factors for postoperative dry eyes.5,14
The corneal flap during LASIK can be created with a microkeratome (MK) or a femtosecond laser (FS). Mechanical MKs are commonly used, and a rapid visual recovery is observed with minimum discomfort; however, flap-related complications occur in as many as 5% of cases and occasionally result in delayed visual recovery or permanent vision loss.15 Femtosecond laser is a safe and effective alternative to mechanical MKs.16 It may provide greater safety, better reproducibility and predictability of flap diameter and thickness, and more precise control of hinge size and location.16,17 However, the effects of different flap-creation methods on post-LASIK dry eye have rarely been reported. In this study, we use objective parameters and a subjective questionnaire to analyze the effects of the two methods for creating corneal flaps on corneal sensitivity (CS) and dry eye syndrome after LASIK.
From December 2007 to June 2008, 87 myopic subjects of both sexes with moderate to high myopia (refractive errors ranging from −8 to −15.25 diopters [D]) with or without astigmatism (spherical equivalent values ranging from −1.00 D to −16.75 D) who were scheduled to undergo bilateral LASIK treatment and were willing to participate in the study and return for all follow-up examinations were included in this prospective, nonrandomized, and comparative study. Indications for myopic LASIK surgery included a minimum age of 18 years, a normal ophthalmic examination except for refractive error, a stable refraction, and a minimum calculated residual corneal stromal bed thickness greater than 250 μm. Candidate patients for LASIK surgery were asked to discontinue contact lens wear from the day of first visit, and there was at least 2-week “washout interval” between cessation of contact lens wear and baseline assessments. Patients with previous eye surgery, topical ocular medications before surgery, pregnancy, and specific ocular conditions such as ocular rosacea or autoimmune diseases were excluded from the study. We also exclude eyes that had LASIK flap complications (one eye in the MK group) or postoperative enhancement (three eyes in both FS and MK groups) after LASIK.
The patients were divided into MK group (n = 43) or FS group (n = 44) based on their corneal thickness and spherical equivalent values. Patients would have a greater chance to be allocated to the FS group if the calculated postoperative unablated stromal depth would be less than 300 μm. The left eye of each subject was chosen for inclusion in this study. After informed consent for surgery and inclusion in the study, all subjects underwent bilateral simultaneous LASIK surgery. All procedures were performed by the same surgeon (C.-K.C.) at a private refractive center (Nobel Laser Eye Center, Taipei, Taiwan). All study subjects were evaluated preoperatively, at 1 week, and at 1, 3, and 6 months postoperatively.
The left eye of each subject in both groups underwent flap creation with a superior hinge. In the FS group, a 60-kHz IntraLase femtosecond laser (Abbott Medical Optics Inc., Abbott Park, IL) was preprogrammed for each procedure to produce a flap diameter of 9.0 mm, a flap thickness of 110 μm, a hinge angle of 70 degrees with a raster energy of 1.0 μJ, and a side-cut energy of 1.0 μJ. In the MK group, the flap was created using a Moria M2 microkeratome (Moria SA, Antony, France) with a 110-μm plate depth and a 9.0-mm diameter head. Laser ablation was performed with a Visx S4 (Abbott Medical Optics Inc., Abbott Park, IL) laser with an optical zone of 6.5 mm. All surgeries were performed to leave more than 250 μm of residual stromal thickness postoperatively. Postoperatively, all subjects were instructed to apply prednisolone acetate 1% ophthalmic suspension (Pred Forte; Allergan, Irvine, CA) and 0.3% ciprofloxacin ophthalmic solution (Ciloxan; Alcon Laboratories Inc., Fort Worth, TX) four times daily for 1 week. Moreover, as all of the patients included in this study had signs of dry eyes preoperatively, inferior punctal plug (SOFT PLUG Extended Duration Plugs, OASIS Medical, Glendora, CA) occlusions were performed in both eyes in all patients after surgery. Subjects were also instructed to apply artificial tears (Systane, Alcon Laboratories Inc.) four times daily for 1 week and then as needed. Postoperative examinations were performed at 1 week and 1, 3, and 6 months in the following sequence: tear breakup time (TBUT), ocular surface stainings including rose bengal and fluorescein staining, corneal sensation, Schirmer basic tear secretion test, and a questionnaire (Ocular Surface Disease Index [OSDI]).
Corneal sensitivity was measured with a Cochet-Bonnet esthesiometer (Luneau Ophtalmologie, Chartres Cedex, France) consisting of a 60.0-mm adjustable nylon monofilament.18 The filament is soft when fully extended and becomes firm when retracted into the handpiece, creating a pressure gradient that ranges from 11 to 200 mg/mm2. Patients were asked to look straight ahead and to indicate when the top of the nylon filament could be felt touching the cornea. To measure corneal sensation, the filament was applanated against the corneal surface perpendicularly until a small bend was noted; subsequently, the filament was retracted until the patient felt the filament touching the cornea. “Corneal sensitivity” was defined as the length of the filament that the patient could feel its contact. Longer lengths were indicative of greater CS.
Schirmer Basic Tear Secretion Test
Five minutes after instilling a drop of proparacaine 1% into the conjunctival sac and drying the fornix with cotton tip at inferior lateral eyelid margins, a sterile standardized Schirmer Tear Test Strip (Alcon Laboratories) was placed at the junction of the lateral and middle third of both inferior fornices for another 5 minutes. The length of the wet portion of the strip was recorded in millimeters.
Tear Breakup Time
The TBUT was evaluated 2 minutes after the inferotemporal bulbar conjunctiva was touched with a sodium fluorescein strip (Fluor-I-Strip, Bausch and Lomb Inc., Rochester, NY). All patients were instructed to blink, and the precorneal tear film was examined under a slit lamp with a cobalt-blue light. The time interval (in seconds) from the last blink to the first breakup area was recorded. Three separate readings were taken for each eye, and the results were averaged.
Ocular Surface Staining
Both corneal fluorescein and conjunctival rose bengal staining were used to evaluate ocular surface alterations. The grading system was modified from the Oxford Schema in our previous study.19 The cornea was examined under blue-light illumination with a biomicroscope 2 minutes after 1% fluorescein instillation into the tear film. The cornea was divided into the central cornea and the superior, inferior, nasal, and temporal quadrants, and the intensity of fluorescein-stained punctate epithelial keratopathy was recorded for each eye using a standardized four-point scale (0, none; 1, mild; 2, moderate; 3, severe). Rose bengal staining was graded for the entire cornea and the superior, inferior, nasal, and temporal quadrants of the bulbar conjunctiva. After instillation of 1% rose bengal solution into the inferior cul-de-sac, the degree of staining on the bulbar conjunctiva and cornea was quantified by biomicroscopic examination using a standardized four-point scale (0, none; 1, mild; 2, moderate; 3, severe). The scores from each quadrant were recorded and added together. Staining scores for both methods were scaled from 0 to 15.
Ocular Surface Disease Index
The OSDI was developed by the Outcomes Research Group (Allergan Inc., Irvine, CA) and consists of a 12-item questionnaire designed to assess the symptoms of ocular irritation consistent with dry eye disease and their impact on vision-related functions. The questions are divided into three categories, including vision-related function, ocular symptoms, and environmental triggers. The grading of the OSDI is from 0 to 4, where 0 indicates none of the time; 1, some of the time; 2, half of the time; 3, most of the time; and 4, all of the time. The total OSDI score was calculated on the basis of the following formula: (sum of scores for all questions answered) × 25/(total number of questions answered).20 The results are numerical from 0 to 100, where the higher scores represent a greater disability.
Statistical analysis was performed with commercial software (SPSS, version 11.0; Chicago, IL). Normality was tested using Shapiro-Wilk test. Repeated-measures analysis of variance was performed to determine the effects on independent dry eye parameters between the groups. Analysis of variance was performed with the paired t test for differences from baseline within the group. Pearson correlation coefficients (r) were used to evaluate the correlations between continuous variables. Descriptive statistics were calculated as the mean ± SD. A value of p < 0.05 was considered statistically significant. The study power was 0.95 when α = 0.05.
The preoperative characteristics of the patients and the intraoperative measurements are presented in Table 1. The mean age and sex distribution were similar between groups. There was no significant difference in astigmatism or mean keratometry measurements between the two groups (p > 0.05, both comparisons). The mean spherical equivalent and sphere were significantly higher in the FS group than in the MK group (Table 1). Stromal ablation depth, which was positively correlated with preoperative spherical equivalent (r = 0.839, p < 0.01), was significantly thicker in the FS group than in the MK group (Table 1). The suction time during applanation for flap creation was also significantly longer in the FS group (Table 1).
The statistical results of pre- and post-LASIK dry eye–associated parameters are summarized in Table 2. There were no significant differences between the FS and MK groups preoperatively in all of the dry eye–related tests (Table 2), including ocular surface staining, Schirmer test, TBUT, OSDI scores, and central CS (p > 0.05, all comparisons). Because the depth of laser treatment has been proposed as a risk factor for post-LASIK dry eye,6 we adjusted the calculated ablation depth for analysis. After surgery, CS measured by a Cochet-Bonnet esthesiometer was significantly decreased at 1 week in both groups (p < 0.01). Despite the CS recovered faster in the FS group than in the MK group, there was no significant difference in CS between the FS and MK groups (Fig. 1 and Table 2, p = 0.93).
Because of the preoperative dry eye conditions in the patients enrolled in the study, inferior punctal plug occlusions were performed immediately after surgery in all of the patients. Although the values were significantly higher at months 1 and 6 in the FS group compared with those at baseline values (Table 2, p < 0.05), there was no significant difference from the MK group postoperatively (Fig. 2 and Table 2, p = 0.30). However, it was interesting to find that the mean Schirmer test values were increased throughout the postoperative visits in the FS group rather than in the MK group. It is well known that punctal occlusion may aggravate dry eye syndrome in the presence of inflammation by allowing prolonged contact of the inflamed tear with the surface of the eye. Whether this implicates that a less inflamed ocular surface after FS-assisted LASIK is created certainly deserves further investigation.
Tear breakup time was the only parameter that was statistically higher in the FS group than in the MK group postoperatively (Table 2, p = 0.03). Tear breakup time is a useful diagnostic test to evaluate the tear film stability, with fair disease severity correlation.21 However, there was no within-group differences in both groups postoperatively (Fig. 3). In the FS group, corneal fluorescein staining scores were significantly higher at 1 week (p < 0.01) and 1 month (p < 0.05) and almost completely recovered to baseline by 3 months. In the MK group, corneal fluorescein staining scores were increased at all periods (p < 0.01) postoperatively and did not recover to baseline levels after surgery. There was no significant difference in corneal fluorescein staining scores between the two groups at any time point (Fig. 4 and Table 2, p = 0.11). In both groups, conjunctival staining increased mildly by l week postoperatively. The rose bengal scores did not change significantly at any time point in the MK group. There was a trend toward significant improvement in conjunctival staining in the FS group (p < 0.01 at 6 months). However, no significant difference in conjunctival staining scores was noted between the two groups postoperatively (Fig. 5 and Table 2, p = 0.18).
The OSDI scores were significantly increased after surgery in the FS group (Table 2, p < 0.01). The results were possibly related to the different stromal ablation depths between the groups. In a nonrandomized study design, patients with higher myopia (i.e., more calculated ablation depth) were prone to be enrolled in the FS group to maximize the postoperative stromal depth. Therefore, a higher OSDI score in the FS group is expected. In fact, after adjustment of the ablation depth, we did not find a significant difference between the groups (Fig. 6 and Table 2, p = 0.12).
Previous studies have reported that nearly one-half of LASIK patients complained of occasional ocular dryness lasting more than 3 months after surgery.7,8,11 Patients with post-LASIK dry eye usually experience annoying symptoms such as stinging, pain, photophobia, visual fatigue, and blurred or fluctuating vision.3,7,10,11 These symptoms are particularly worrisome for patients who did not experience them before LASIK surgery. In addition, chronic dry eyes after LASIK also results in a higher risk of refractive regression,22 resulting in an unsatisfactory outcome.
In this study, we have demonstrated that CS was decreased in both groups and gradually recovered (Fig. 1 and Table 2). Truncation of corneal nerves after LASIK has been considered the predominant factor leading to post-LASIK dry eyes.7,8 Loss of neural stimuli may compromise the protective blink reflex, delay epithelial wound healing, and increase tear osmolarity by decreasing lacrimal gland protein, electrolyte, and water secretion. Increased tear osmolarity further induces ocular surface inflammation by activating inflammatory cytokines.23 Despite the fact that there was no significant difference in CS between the FS and MK groups, it is believed that a thinner and planar-shaped corneal flap created by FS may lessen damage to corneal nerves and alleviate the symptoms after surgery.24
Previous studies have reported that decreased corneal sensation and LASIK-induced dry eye seem to correlate well with the degree of preoperative myopia and laser ablation depth.6,14 The increased risk of dry eye may be explained by the effects of these factors on the corneal sensory nerves. If this is the case, we would expect more severe cases of post-LASIK dry eye syndrome in the FS group because the degrees of preoperative myopia and planned ablation depth in that group of patients were significantly greater than those in the MK group (Table 1). However, patients in the FS group seemed to have a more stable tear film than patients in the MK group, as evidenced by a higher TBUT after surgery (Table 2). This outcome indicates that other factors are responsible for the faster recovery of post-LASIK dry eye in patients who had undergone surgery with an FS.
Suction ring and suction time during LASIK flap creation could be important factors in the mechanism of LASIK-induced dry eye.25,26 Suction rings are required for both FS- and MK-assisted LASIK surgery. The ring provides sustained vacuum pressure to maintain a tight grip on the globe to facilitate the smooth creation of the flap. The pressure exerted on the ocular surface varies with different rings used for the FS laser and for the mechanical MK. It has been proposed that the lower incidence of dry eye signs and symptoms after FS surgery may be attributed to the application of lower suction ring pressure on the eye.27 Evidence supporting this hypothesis comes from a recent porcine model28 that showed that the intraocular pressure reached 160.52 ± 22.73 mm Hg during the cutting phase with a Moria 2 microkeratome and 119.33 ± 15.88 mm Hg during the cutting phase with an Intralase femtosecond laser. However, the effect of peak intraocular pressure caused by different suction rings on post-LASIK dry eye remains controversial.
Changes in goblet cell counts may contribute to the development of post-LASIK dry eye.12,26 Rodriguez-Prats et al.29 studied the effect of the LASIK procedure performed with FS and mechanical MK on goblet cell density. They found that all patients in both groups had a decrease in goblet cells after LASIK, although the number of cells returned to normal levels after 6 months. They hypothesized that it was probably caused by a longer suction time in the FS group. In our study, the duration of suction time was statistically longer in the FS group (33 seconds) than in the MK group (8.3 seconds) (Table 1), which concurs with a study from Salomão et al.25 However, if the duration of suction time was important, we would have found greater severity of objective signs of post-LASIK dry eye in the FS group. Moreover, the present study demonstrated that the postoperative conjunctival staining scores (correlated with damage to goblet cell) were significantly lower at 6-month compared with preoperative scores in the FS group (Table 2 and Fig. 5). Therefore, further study is required to verify the effects of other factors, such as suction ring materials, on the damage to conjunctival goblet cells and dry eye parameters.
Although not conclusive, flap thickness is another possible factor for post-LASIK dry eyes. Thinner corneal flaps may result in greater residual stromal bed thickness, a reduction in the tissue volume for nerve regeneration, and accelerated recovery of CS and dry eye symptoms after LASIK.30,31 De Paiva et al.6 reported that greater combined flap thickness and ablation depth was associated with a higher incidence of post-LASIK dry eye in patients who have undergone a mechanical MK procedure; however, the flap thickness alone was not a significant risk factor in their study. Salomão et al.25 also reported a lack of correlation between thick flaps created by a mechanical MK and a higher incidence of LASIK-induced dry eye. A recent study demonstrated that hinge position, hinge angle, and flap thickness had no effect on CS or dry eye syndrome in patients receiving FS-assisted LASIK.31
The present study adopted a combination of diagnostic tests, including CS, basic Schirmer test, TBUT, corneal fluorescein staining, and conjunctival rose bengal staining. This approach is consistent with a recently proposed consensus for diagnosis and evaluation of dry eye syndrome.32 All of the participants enrolled in this study had chronic, albeit mild, dry eye disease as shown by slightly decreased tear production, decreased TBUT, and increased ocular surface staining preoperatively (Table 2). Individuals with preexisting dry eye may experience exacerbation of symptoms, but normal recovery is expected. In our study, the corneal fluorescein staining scores returned to preoperative levels in the FS group within 3 months; however, the corneal fluorescein staining scores was significantly higher compared with the preoperative scores in the MK group at the same time point (Table 2 and Fig. 5). A possible explanation for the difference may be the improved epithelial preservation with the FS.33 The Intralase laser uses a smooth and static lens for applanation of the cornea, thereby reducing the occurrence of epithelial erosion. However, the continuously oscillating MK head may damage the corneal epithelium under high pressures.
There are some limitations in this study. First, this is not a randomized study, and the preoperative spherical equivalent was significantly different between the groups. This is a potentially confounding factor despite the fact that we’ve adjusted the stromal ablation depth for the analysis. Second, the flap thickness was not measured intraoperatively. As such, the causal relationships between the study parameters and flap thickness could not be obtained. Third, most of the patients enrolled in this study had chronic dry eyes. Whether the same study results can be applied in the normal population remains unclear. Therefore, a further randomized controlled study performed in a normal cohort is warranted to verify the potential risk factors, including flap thickness in post-LASIK dry eye.
In conclusion, LASIK surgery induced dry eye–related objective signs such as ocular surface staining and decreased CS in both groups. Our study found a higher postoperative TBUT when surgery was performed with the FS. Further randomized controlled studies are required to investigate the underlying mechanisms and to verify whether FS-assisted LASIK decreases post-LASIK dry eye.
Department of Ophthalmology
Chang Gung Memorial Hospital, Keelung
222 Mai Chin Road, An Leh District
This study was supported in part by grant CMRPG260391 from the Chang Gung Medical Research Foundation, grant NSC 99-2314-B-182A-025-MY3 from the National Science Council, Taiwan, and grant NHRI-EX100-9725SC from the National Health Research Institutes, Taiwan. Clinical Trials registration reference number: ISRCTN43661922; http://isrctn.org.
Chi-Chin Sun had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. The authors have no conflicts of interest to declare in this article.
Received December 11, 2012; accepted May 3, 2013.
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Keywords:© 2013 American Academy of Optometry
laser in situ keratomileusis; femtosecond laser; microkeratome; dry eye; corneal sensitivity