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Results of flap repositioning after laser in situ keratomileusis

Lyle, Andrew W. MDa,*; Jin, George J.C. MD, PhDa

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Journal of Cataract & Refractive Surgery: October 2000 - Volume 26 - Issue 10 - p 1451-1457
doi: 10.1016/S0886-3350(00)00571-X
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Abstract

Flap folds (wrinkles, striae) and displacement (slippage, dislocation, dislodgment, misalignment, shifted flap) are relatively common postoperative flap complications of laser in situ keratomileusis (LASIK) that require flap repositioning. Previous studies report rates of flap folds from 1.0% to 9.5%.1–11 In this study, we retrospectively examined the patients who had flap repositioning for flap folds or displacement to determine the incidence and severity of these flap complications and to assess the outcome of flap repositioning procedures.

Patients and Methods

Patient Population

Flap repositioning procedures were performed in 91 eyes of 87 patients by a single surgeon (W.A.L.) at the Eye Institute of Utah between January 1997 and October 1999. Four patients had bilateral procedures. The indications for a flap repositioning procedure included flap displacement, slippage, and flap folds that caused loss of best corrected visual acuity (BCVA), irregular astigmatism, hyperopic shift, and increased astigmatism. Flap displacement was defined as more than half of the flap shifting and hanging by its hinge. Flap slippage (partial flap displacement) was defined as less than half of the flap shifting, with a gap at the edge of the flap and fluorescein pooling. Flap folds were detected with the slitlamp using direct or retroillumination. Patients with fine striae outside the pupillary area, not affecting visual acuity and not requiring flap repositioning, were not included in this study. The mean age of the patients was 40 years (range 21 to 64 years). Forty-one eyes were in men and 50 in women.

Charts of all patients were retrospectively reviewed. The data of all cases were used for analysis of the cause and characteristics of flap folds and displacement. The data of 87 eyes that had at least 3 months of follow-up after flap repositioning were included for the analysis of visual outcome. This included 5 eyes that had LASIK retreatment after flap repositioning. The mean follow-up was 8.33 months ± 7.12 (SD) (range 3 to 31 months) after the flap repositioning procedure; 52% of eyes were followed 6 months or longer.

Surgical Technique

The initial and retreatment LASIK techniques have been described.12 All initial LASIK procedures were performed bilaterally using the VISX Star laser. The right eye was done first, followed by the left eye. The surgical field was draped with an aperture drape (3M Tegaderm) to cover the upper lid margins. After laser ablation, the flap was replaced and the interface was irrigated with balanced salt solution (BSS®). A Merocel® sponge was used to squeeze excess fluid from the interface by painting the flap and stroking from the hinge toward the periphery. This was followed by drying the edge of the flap. While the left eye was being operated on, the right eye was covered with a shield with the speculum in place. The adhesion was checked after 2 to 3 minutes. Immediately postoperatively, 1 drop of ofloxacin (Ocuflox®), fluorometholone (FML®), and diclofenac sodium (Voltaren®) was instilled. The patient was instructed to close the eyes gently for 10 minutes in the postoperative area. The eye was re-examined with a slitlamp, and then a plastic eye shield was taped in place.

Flap repositioning was performed with an identical preoperative and postoperative regimen. The pupil was dilated preoperatively with a drop of tropicamide 1.0% (Mydriacyl®). The 4.0 mm radial keratotomy zone marker was used to mark the temporal cornea with 2 dye rings. A smooth forceps was used to lift the flap and fold it over the hinge. The flap was irrigated with BSS, and the wrinkles were ironed using 2 smooth forceps to stretch and smooth the corneal flap from the stromal surface. The flap was then repositioned with more irrigation. The wrinkles were again stretched out using corneal forceps. The flap was then floated on a layer of BSS and was repositioned by wiping it into place with moistened Merocel sponges. The flap was allowed to dry 5 minutes.

Patient Evaluation

Preoperatively, uncorrected visual acuity (UCVA), BCVA, and manifest refraction were measured; videokeratography and slitlamp microscopy were done. The location and orientation of flap folds were determined by slitlamp using the pupil as the reference origin. The location of the folds was recorded as superior, inferior, nasal, temporal, and central and the orientation, as vertical, horizontal, and oblique. Postoperative examinations were done at 1 day, 1 week, and 1, 3, 6, and 12 months.

Statistical Analysis

Descriptive statistics, multivariate analysis of variance, and correlation analysis were conducted with the SPSS program for Windows (SPSS Inc.). The paired Student t test was used to evaluate the significance of the difference within groups and the chi-square test to compare frequencies in different categories. The Pearson product-moment regression analysis was used to determine correlation between variables. A P value of less than 0.05 was considered significant.

Results

Incidence of Flap Repositioning

During the study period, 9004 initial LASIK procedures and 984 retreatments were performed. Of the initial procedures, 700 were done with the Hansatome microkeratome and 8304 with the Automated Corneal Shaper (ACS). Flap repositioning procedures were performed in 91 eyes; the overall incidence was 0.91%. Multiple procedures were performed in 17 eyes (18.6%); in 10 eyes, 2 flap repositioning procedures were done and in 7 eyes, 3. Fifty-six procedures (61.5%) were in right eyes and 35 (38.5%), in left eyes (chi-square test, P = .028). The incidence of flap repositioning was 1.32% after retreatment (13/984). The incidence was 2.14% (15/700) in the Hansatome group and 0.76% (63/8304) in the ACS group.

Complete flap displacement occurred in 4 eyes (4.4%). In 1 eye, the flap was totally folded back nasally; in the other 3, the flaps were shifted inferiorly. Flap slippage (partial flap dislocation) occurred in 17 eyes (18.7%).

Time of Occurrence and Surgery

Flap folds or slippage was detected at a mean of 4.76 days postoperatively (range 0 to 57 days). In 7 eyes (7.6%), the flap folds were detected 20 minutes to 5 hours after LASIK; in 51 eyes (56%), they were detected at 1 day; in 20 eyes (22%), at 2 to 7 days; and in 13 eyes (14.4%), more than 7 days after LASIK. Flap displacement occurred at 1 day in 3 eyes and at 27 days in 1 eye. Among the 4 eyes with displaced flaps, 3 had experienced corneal trauma by a finger just before the flap shifted.

Flap repositioning procedures were performed on the day the folds were detected in 64 eyes (70.3%) and within 7 days of LASIK in 71 eyes (78.0%).

Location and Orientation of Flap Folds

Location and direction of the flap folds were evaluated in the eyes in which flap repositioning was done following the initial LASIK (78 eyes). In the ACS group, 45% of folds were located in the superior part of the flap and 6% in the inferior; 42% of folds were oriented horizontally and 17%, vertically. Folds extending from superior nasal to inferior temporal were 3 times more frequent than those from superior temporal to inferior nasal (25% versus 8%). Twenty-three percent of eyes had a superior flap edge gap, indicating downward slippage of the flap. In the Hansatome group, 73% of folds were central, 87% were vertical, and 6% horizontal. No flap slippage was found with the Hansatome. The difference in orientation of the folds between the 2 microkeratomes was statistically significant (chi-square test, P < .001).

Symptoms and Associated Problems

Blurred vision was the most common symptom, occurring in 72.0% of patients. Foreign-body sensation, pain, or stinging occurred in 18.0% of eyes. Other symptoms included ghosting, double vision, and vision fluctuation. All the eyes with flap displacement experienced significant stinging or pain. No symptoms were noted in 19.6% of eyes.

A history of epithelial defect at the time of LASIK was found in 17 eyes (18%). Epithelial ingrowth was found in 5 eyes, and an epithelial removal procedure was associated with flap repositioning. Diffuse lamellar keratitis occurred in 3 eyes. Dry eye was found in 42% of eyes. The diagnosis was based on the presentation of positive punctate or confluent corneal staining.

Refractive Change

Table 1 shows the refractive change before and after flap repositioning in eyes that had 3 months of follow-up after the flap repositioning procedure. The spherical equivalent (SE) was –4.67 ± 2.38 diopters (D) (range −0.75 to –12.50 D) before LASIK. It was 0.58 ± 1.03 D (range −1.75 to +4.25 D) prior to flap repositioning, −0.29 ± 0.76 D (range –3.00 to 1.00 D) at 1 week, and –0.07 ± 0.61 D (range −2.35 to +2.00 D) at the last visit. The improvement in refraction with the flap repositioning procedures was significant (paired t test, P < .001). At the last examination, 72% eyes were within ±0.50 D and 87% within ±1.00 D.

Table 1
Table 1:
Change in refraction and corneal topography indices.

The refractive cylinder was 0.89 ± 0.82 D before LASIK. It was 0.95 ± 0.73 D prior to flap repositioning, 0.56 ± 0.50 D at 1 week, and 0.44 ± 0.45 D at the last visit. There was a significant reduction in refractive cylinder after flap repositioning (paired t test, P < .001).

Visual Acuity

The UCVA was 20/50 or worse in 53% of eyes prior to flap repositioning and in 18% 1 week after flap repositioning. At the last visit, the UCVA was 20/20 or better in 59% of eyes, 20/40 or better in 88%, and 20/50 or worse in 12%.

The BCVA was worse than 20/50 in 14% of eyes prior to flap repositioning. One week after flap repositioning, it was 20/40 or better in all eyes; 42% of eyes gained 2 to 12 lines from their pre-procedural values (Figure 1). At the last visit, the BCVA was 20/20 or better in 91% of eyes and 20/40 or better in all eyes. Three eyes (3%) lost 2 or more lines of pre-LASIK BCVA (Figure 2). All 3 had residual folds at the last examination.

Figure 1.
Figure 1.:
(Lyle) Change in BCVA 1 week after flap repositioning compared with preprocedure values.
Figure 2.
Figure 2.:
(Lyle) Change in BCVA at the last examination compared with pre-LASIK values.

Factors Related to Change of BCVA

Loss of BCVA was mainly caused by corneal irregularity. The correlation was significant between change in BCVA and the postoperative surface regularity index (SRI) (Pearson correlation = −0.504, P < .001) and the postoperative surface asymmetry index (SAI) (Pearson correlation = −0.339, P = .018). Among the factors tested, pre-LASIK SE (Pearson correlation = 0.224, P = .038) and SAI prior to flap repositioning (Pearson correlation = −0.421, P = .032) also correlated with loss of BCVA (Table 2).

Table 2
Table 2:
Factors related to the change in BCVA at last examination compared with the pre-LASIK values.

Corneal Irregularity

The mean SRI was 0.42 ± 0.14 before LASIK, 0.96 ± 0.47 prior to flap repositioning, and 0.62 ± 0.35 at the last visit. There was a significant increase in SRI from pre-LASIK to pre-flap repositioning (paired t test, P < .001) and a significant reduction in SRI after flap repositioning (paired t test, P = .016).

The mean SAI was 0.31 ± 0.14 before LASIK, 0.94 ± 0.73 prior to flap repositioning, and 0.51 ± 0.28 at the last examination. The increase in SAI from pre-LASIK to pre-flap repositioning was significant (paired t test, P < .001), but the reduction in SAI after flap repositioning was not (paired t test, P = .071). All 38 eyes examined prior to flap repositioning with manual keratometry had trace to 2+ corneal distortion.

Multiple Flap Repositioning

Seventeen eyes (18.6%) had multiple flap repositioning procedures; 10 eyes had 2 procedures and 7, 3. The mean age of the patients was 44.13 years; 11 eyes (65%) were in women. The mean pre-LASIK SE was –4.73 ± 2.85 D and +1.08 ± 1.38 D (range −0.50 to +4.25 D) prior to flap repositioning. At the last visit, the mean SE was –0.30 ± 0.83 D (−2.00 to 2.00 D); 12 eyes (70%) were within ±0.50 D and 14 (82%), within ±1.00 D. The UCVA was 20/40 or better in 14 eyes (82%). The BCVA was 20/20 in 13 eyes (77%) and 20/40 or better in all eyes. One eye lost 3 lines of BCVA due to irregular astigmatism and residual folds after 3 flap repositioning procedures.

Discussion

The incidence of flap folds varies from 1.0% to 9.5% in previous studies.1–11 In a retrospective study,11 Lin and Maloney report that the incidence is 9.5% in eyes with “wet technique” versus 2.4% in eyes with “dry technique.” According to their description, with a wet technique, the flap is replaced by floating it onto the stromal bed with BSS. The flap is allowed to settle on the stromal bed without drying. With a dry technique, excessive fluid is squeezed out of the flap with a blunt spatula.

Many surgeons use the technique we described. The flap was positioned back onto the stromal bed using a cannula. Irrigation with filtered BSS was carried out underneath the flap. Then a Merocel sponge was used to stroke the flap away from the hinge. With this technique, the overall incidence of flap displacement or folds in our study was 0.91%.

The occurrence of flap folds and displacement is likely multifactorial. Anything that affects adherence of the flap and adds external mechanical force on the surface of the cornea could cause flap folds and slippage. The adhesion mechanisms, including fiber interlacing, endothelial pumping, capillarity, intracorneal suction, and intracorneal molecular attraction have been studied.13,14 In a recent in vitro bovine study, Perez and coauthors15 found that drying increases stromal–stromal adhesion. The authors speculate this is due to the increased concentration of surface molecules, which have high ionic charge densities and ionic bonding. The clinical observation comparing 2 flap replacement techniques by Lin and Maloney11 supports Perez and coauthors' speculation, as does the reduced rate of flap folds with our technique. In addition to the initial physical replacement of the flap, ensuring adherence of the flap thereafter is also important. Most authors allow the cornea to dry for 2 to 5 minutes.3–6 In our study, a 2 to 3 minute delay followed flap replacement. Currently, upon finishing the procedure, we irrigate with BSS and instill carboxymethylcellulose sodium (Celluvisc®) drops on the central cornea, allowing the flap edge to dry for 3 to 5 minutes before removing the speculum, and then move the patient to a recliner with his or her eyes closed for 20 minutes. In the flap repositioning procedure, we instill a hypotonic BSS 50% on the stromal surface of the wrinkled flap for 2 minutes before the smoothing manipulation. After smoothing, the flap is floated on a layer of BSS 50%.

The action of the eyelids on the cornea causing flap folds and slippage was observed: (1) Orientation and location of the folds: With the ACS and nasal hinge, most of the folds were horizontal and at the superior part of the flap; most extended from superior nasal to inferior temporal. In all eyes with flap displacement, the flap was displaced downward with the nasal hinge. With the Hansatome, a superior hinge was created and most folds were at the center and vertical. These phenomena indicate that when the patient's upper lid comes down, it catches the edge of the flap and pushes the flap downward to cause flap folds and slippage. (2) Most flap folds and displacement occurred on the first postoperative day. During sleep, tear flow decreases and rubbing between the lids and the corneal surface increases. (3) Folds frequently occurred in patients with dry eyes in whom tear breakup time decreased with the need to increase blinking. Also, forcible blinking or squeezingof the lids against the flap increases the risk of flapfolds.

We believe that upper lid rubbing on the flap plays an important role in postoperative flap folds, especially in dry-eye patients. Artificial tears should be administered immediately after surgery, every 15 to 20 minutes on the day of surgery; a thicker artificial tear such as Celluvisc should be used at night to reduce the risk of flap folds. In patients with severe dry eye, insertion of punctal plugs is helpful.

Different microkeratomes create different morphologic features as they cut corneal tissue. Using the Hansatome reduces intraoperative flap complications.16 However, postoperative flap folds are more frequent. Using a similar technique, the incidence of flap folds and displacement was 3 times higher with the Hansatome than with the ACS. This is theoretically related to a larger flap, which may allow the lid to have more effect on the security of the flap, especially if the patient has a dry eye. It is also possible that flap folds and slippage might increase with a thinner corneal flap.17,18 Heng and Chan found consistently thinner flaps were created with the Hansatome microkeratome with both the 160 μm and 180 μm plates. The flaps were an average of 17% and 28% thinner than desired, respectively (L. Charters, “Corneal Flaps Consistently Thin with Microkeratome,” Ophthalmology Times, November 1, 1999, page 68). Although unsupported by data in this study, a thinner flap created by the Hansatome may be another reason for a higher rate of flap folds. Gimbel and coauthors19 observed flap wrinkling in 7 of 73 eyes (10%) in their report on LASIK during the learning curve. In this study, all the ACS procedures were performed prior to the Hansatome procedures and the cases of flap folds and displacement were among the early procedures with the Hansatome. The learning curve might relate to the difference in flap complication rates with the 2 microkeratomes.

Poorer vision compared with the fellow eye was the main complaint of the patients. However, 20% of patients had no complaints. A routine first day postoperative examination is crucial to detect flap folds and displacement. Also, even if there is no discomfort, patients with blurred vision should have a slitlamp examination.

The relationship between epithelial defect intraoperatively and the risk of epithelial ingrowth has been observed by others.2 Our findings suggest that an epithelial defect increases the risk of flap folds and displacement. Poor epithelial adherence may indicate a poor flap adherence on the basis of fluid dynamics.1 In this study, 18% of eyes had a history of epithelial loss at the time of LASIK. Among the 4 eyes with flap displacement, 2 had a history of epithelial loss. Preventing epithelial defects during the procedure could decrease the risk of flap folds and displacement.

Based on this study, the characteristics of flap folds or flap displacement include the following:

1. Reduced visual acuity is the main complaint. Patients experience a poor UCVA and loss of BCVA.

2. Flap folds induce a hyperopic change in refraction and increase astigmatism as well as irregular astigmatism, which can be detected by videokeratography and manual keratometry.

3. Most flap folds and displacement occur 1 day postoperatively.

4. Dry-eye patients and patients with loose epithelium at LASIK are more likely to have flap folds and slippage.

5. Flap folds can be associated with other interface abnormalities such as epithelial ingrowth and diffuse lamellar keratitis.

6. Flap folds are more common following LASIK retreament.

7. The location and orientation of the flap folds are related to the position of the hinge and the diameter and thickness of the flap created by different microkeratomes.

8. A gap can be detected with fluorescein pooling at the edge of the flap when flap slippage occurs. Fluorescein staining enhances the ability to visualize the folds.

9. Most flap displacements have a history of injury.

Flap repositioning significantly restored the vision loss caused by flap folds and displacement: 42.0% of eyes gained 2 to 12 lines 1week following the procedure from the pre-flap repositioning level. However, irregular astigmatism due to permanent residual folds remains the main cause affecting the visual and refractive outcome. In this study, 3.5% of eyes lost 2 lines or more of BCVA from pre-LASIK values. All were caused by residual folds.

Although recent folds can be corrected by a flap repositioning procedure, chronic flap folds may be difficult to eradicate completely. Also, delayed treatment could result in permanent residual folds causing irregular astigmatism. In this study, loss of 3 lines of BCVA occurred in the eye with delayed treatment in which residual flap folds existed 23 months after flap repositioning, with a postoperative SRI of 1.98 and an SAI of 1.75. Managing flap folds is still a challenge for refractive surgeons. Early recognition and prompt treatment of flap folds and displacement may avert loss of postoperative BCVA. For example, when a patient is found to have folds on the day of surgery, it is possible to simply use a dry Merocel sponge to smooth the folds, followed by application of artificial tears and eye closure for 5 minutes. This corrects the problem without surgical re-intervention.

References

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12. Lyle WA, Jin GJC. Retreatment after laser in situ keratomileusis. J Cataract Refract Surg 2000; 26:650-659
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15. Perez EP, Viramontes B, Schor P, Miller D. Factors affecting corneal strip stroma-to-stroma adhesion. J Refract Surg 1998; 14:460-462
16. Walker MB, Wilson SE. Lower intraoperative flap complication rate with the Hansatome microkeratome compared to the automated corneal shaper. J Refract Surg 2000; 16:79-82
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18. Yi W-M, Joo C-K. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkeratome. J Cataract Refract Surg 1999; 25:1087-1092
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