LASIK is a refractive surgery in which a corneal flap approximately 100 μm thick is created under topical anesthesia, excimer laser is applied to the underlying corneal stroma, and the flap is repositioned. Flap complications are a general term for structural, inflammatory, and infectious disorders associated with flap creation, repositioning, and postoperative course.
The Guidelines for Refractive Surgery (8th edition) specify “flap abnormalities” and “diffuse lamellar keratitis (DLK)” as postoperative complications of LASIK, stating that “various flap troubles may occur and require appropriate management” 1).
Flap complications are broadly divided by timing into intraoperative (during flap creation to repositioning) and postoperative (immediately after surgery to late phase). They can also be classified by pathology into structural, inflammatory, and epithelial types.
Flap creation devices include conventional microkeratomes and femtosecond lasers. Currently, most flaps are created with femtosecond lasers, which have improved flap accuracy and significantly reduced intraoperative complications such as free cap and irregular flaps. However, femtosecond laser-specific complications (e.g., OBL, vertical gas breakthrough) are also known.
SMILE is a procedure that creates a lenticule inside the cornea without making a flap and removes it, so flap-related complications do not occur in principle. In evidence-based international guidelines for KLEx (small incision lenticule extraction), the absence of a flap is positioned as one of the safety advantages of SMILE2).
QDoes femtosecond laser prevent flap problems?
A
The introduction of femtosecond lasers has greatly improved flap accuracy, and microkeratome-specific problems such as free caps and irregular flaps have significantly decreased. However, femtosecond laser-specific complications (OBL: opaque bubble layer, vertical gas breakthrough, etc.) may occur. Also, DLK, flap displacement, and epithelial ingrowth are complications that can occur even with femtosecond lasers. Complications are not completely eliminated, and appropriate preoperative evaluation and intraoperative/postoperative management remain important.
Intraoperative flap complications are mainly due to the characteristics of the flap creation device. The complication profiles differ between microkeratome use and femtosecond laser use. Timely recognition and management of intraoperative complications greatly affect the final visual outcome. Surgeons are required to be familiar with the response protocol for complications before surgery and to be prepared to make decisions without hesitation during surgery 1).
Microkeratome-related
Incomplete flap: Incomplete incision due to keratome stopping midway. After flap repositioning, consider reoperation after 3–6 months.
Free cap: The flap is completely detached without forming a hinge. The incidence with microkeratome is reported to be 0.004–1.31% 3). If the stromal bed is smooth, continue laser irradiation, then reposition the flap while keeping it moist with BSS, and fix with a therapeutic contact lens.
Buttonhole: Perforation in the central part of the flap. The incidence is reported to be about 0.2% 3). Reposition the flap and postpone reoperation for 3–6 months.
Thin/thick/irregular flap: Deviation from the planned flap thickness. A flap that is too thin poses a risk of reduced corneal strength.
Femtosecond laser-related
OBL (opaque bubble layer): A phenomenon where bubbles generated during laser incision remain within the corneal stroma. Most resolve spontaneously, but invasion into the pupillary zone may interfere with eye tracking.
Vertical gas breakthrough: Escape of bubbles from the incision plane toward the anterior chamber. Bubbles in the anterior chamber may cause a temporary increase in intraocular pressure.
Irregular flap edge: The edge may become irregular due to problems with laser parameter settings.
Spot cutting error: Deformation of an incomplete flap due to localized laser cutting failure.
When a free cap occurs, the surgeon decides whether to continue or stop laser ablation. If the stromal bed is smooth, ablation can be continued. The free cap is handled in a moist environment with BSS (balanced salt solution) drops to prevent deformation due to drying. After ablation is complete, the free cap is repositioned with the epithelial side up and in the correct orientation. Preoperative asymmetric markings are essential for confirming the correct orientation. If the stromal bed is irregular, the flap is repositioned without laser ablation.
After repositioning, a bandage contact lens (BCL) is often placed. The endothelial pump function of the cornea firmly readheres the cap. Keeping the eye closed with adhesive tape for about 30 minutes postoperatively is effective in preventing dislocation. Flat cornea (less than 40 D) and insufficient suction are major risk factors for free cap; preoperative keratometry and appropriate suction ring selection are fundamental for prevention.
Slit-lamp retroillumination image and AS-OCT cross-sectional image of LASIK flap vertical tear and displacement
BMC Ophthalmol. 2016 Jul 18;16:111. Figure 1. PMCID: PMC4950235. DOI: 10.1186/s12886-016-0288-7. License: CC BY 4.0.
Slit-lamp retroillumination image (A): A LASIK flap with a 5.0 mm vertical tear including the pupil center is observed, with an oblique bright line pattern characteristic of flap displacement and wrinkles in the corneal stroma. Anterior segment OCT image (B): Bowman’s membrane separation at the tear site (black arrowhead) and suspected epithelial ingrowth from the flap edge (black asterisk) are shown in the cross-sectional image. This corresponds to flap displacement and flap wrinkles discussed in the section “Early postoperative flap complications.”
Early postoperative flap complications (within one month after surgery) can be classified into three categories: structural, inflammatory, and epithelial-related.
Flap structural complications
Flap displacement: Occurs early postoperatively due to trauma or eye rubbing. Emergency flap lift, BSS irrigation, and repositioning are required. Special caution is needed during the first postoperative week. The incidence is reported to be 0.1–0.3% within the first week 4).
Flap striae/folds: Macrostriae (visible folds) may affect visual function; if they do not decrease over time, early flap repositioning is performed. Microstriae usually have little effect on visual function.
Inflammatory complications
DLK (diffuse lamellar keratitis): Non-infectious interface inflammation under the flap. Punctate opacities are observed under the flap. The overall incidence in LASIK is reported to be approximately 0.1–1% 5). For Grade 1–2, frequent instillation of prednisolone 1% (or fluorometholone 0.1%) eye drops resolves most cases. For Grade 3–4, emergency flap lift and irrigation may be required.
Interface debris: Foreign material under the flap (e.g., from surgical instruments, talc). Usually harmless, but if located on the visual axis, removal may be considered.
Epithelial-related complications
Epithelial ingrowth: Epithelial cells invade and proliferate into the interface from the flap edge. Incidence is reported as 0.5–2% (up to 20% after enhancement) 6). Mild cases are observed. If progression exceeds 2 mm or visual acuity decreases, flap lift, scraping, and edge treatment are performed.
Flap edge epithelial disorder: Epithelial irregularity or step at the flap edge. Most cases resolve with conservative treatment.
Grade classification of DLK (diffuse lamellar keratitis)
A four-grade classification based on the Linebarger/Goodman classification is used to determine treatment strategy 5).
Grade
Findings
Effect on vision
Treatment strategy
Grade 1
White granular infiltrates only in the periphery (central area normal)
Almost no effect
Frequent instillation of prednisolone 1% eye drops every 1–2 hours
Grade 2
Infiltration spreading toward the center
Mild decrease
Continue or increase steroid eye drops; re-evaluate the next day
Grade 3
Severe infiltration reaching the center; early signs of stromal melting
Moderate decrease
Continue steroid eye drops; strongly consider flap lift and irrigation
Grade 4
Central stromal melting (keratolysis) and scar formation
Marked decrease
Emergency flap lift and irrigation; consider systemic steroid administration
QWhat happens if the flap is displaced?
A
If the flap is displaced in the early postoperative period, a sudden decrease in vision and irregular astigmatism can occur. The cause is often trauma (rubbing the eye, impact during sports, etc.), and the first week after surgery is the highest risk period. If flap displacement is noted, an emergency ophthalmology visit is necessary. Treatment involves lifting the flap, irrigating the interface with BSS (balanced salt solution), accurately repositioning the flap, and fixing it with an air bubble. With appropriate management, vision often recovers, but if left untreated, the risk of irregular astigmatism and infection increases.
Flap-related problems occurring one month or more after surgery are classified as late postoperative complications.
Late flap dislocation: Since the LASIK flap never fully heals even years after surgery, flap dislocation can occur several years later due to trauma (e.g., car accidents, direct ocular trauma during sports) 4). The principle of treatment is lift, wash, and reposition as soon as flap dislocation is recognized; the longer the delay, the higher the risk of epithelial ingrowth. Cases of trauma occurring five or more years postoperatively have been reported, emphasizing the importance of informing the treating physician about a history of refractive surgery from a patient education perspective 1).
Progression of late epithelial ingrowth: Mild epithelial ingrowth recognized early postoperatively may enlarge over several years, causing flap melting and irregular astigmatism. Enhancement surgery (re-treatment) is known to further increase the risk of epithelial ingrowth, and shorter follow-up intervals are recommended for eyes with such history 6). The recurrence rate after flap lift and scraping is reported to be 5–20%, and additional procedures such as edge alcohol treatment or flap edge suturing (Ethibond) are considered effective 6).
Persistent dry eye: Reflex tear secretion reduction due to corneal nerve transection after LASIK recovers in most cases within 6–12 months, but in some cases it persists as refractory dry eye7). The Refractive Surgery Guidelines (8th edition) clearly list dry eye as a postoperative complication of LASIK, requiring preoperative evaluation and active postoperative treatment 1). Comparative studies of SMILE and FS-LASIK have shown that corneal nerve density recovers faster after SMILE, with less impact on tear parameters 7), making it a consideration in surgical technique selection.
Flap margin necrosis: As a rare complication, ischemic necrosis of the flap margin may occur. It is recognized on corneal topography as characteristic marginal undulation.
Postoperative ectasia (corneal ectasia): Corneal ectasia may develop postoperatively due to insufficient residual stromal bed (RST) after flap creation or manifestation of pre-existing subclinical keratoconus. The prevalence of ectasia after LASIK is reported to be approximately 90 per 100,000 eyes 14), about 4.5 times that of PRK (approximately 20 per 100,000) 14). RST < 280 μm is a threshold for a sharp increase in ectasia risk, and PTA (percent tissue altered) ≥ 40% is also considered an independent risk factor 13). The Randleman scoring system enables preoperative risk stratification by comprehensively evaluating five factors: abnormal corneal morphology, low RST, young age, thin cornea, and high myopia11).
Epithelial ingrowth from LASIK flap edge: slit-lamp photograph and OCT image
BMC Ophthalmol. 2016 Jul 18;16:111. Figure 2. PMCID: PMC4950235. DOI: 10.1186/s12886-016-0288-7. License: CC BY 4.0.
Slit-lamp photograph (A): Two weeks after LASIK, epithelial ingrowth and epithelial cysts (white arrows) are seen at the flap interface, with white opacification spreading from the flap edge toward the center. OCT image (B): The flap and Bowman’s layer tear edge (black arrowhead) and the opposite flap edge bending (black arrow) are shown in cross-section, confirming the depth of epithelial invasion into the interface. This corresponds to epithelial ingrowth discussed in the “Diagnosis and Management” section.
According to the Refractive Surgery Guidelines (8th edition), it is a principle to perform slit-lamp microscopy on the day after surgery to check for abnormalities, and to continue follow-up for up to 6 months postoperatively 1).
Slit-lamp microscopy: Most important for diagnosis and monitoring of flap complications. Retroillumination is used to evaluate the infiltration and interface condition under the flap. In Grade 1–2 DLK, fine punctate infiltrates in the periphery are characteristic, and timely recognition of spread toward the center is the basis for treatment decisions. Epithelial ingrowth is recognized as white opacification extending inward from the flap edge, and it is important to quantitatively record the distance (mm) from the edge to the center.
Anterior segment OCT (AS-OCT): Essential for evaluating flap thickness and residual stromal bed, confirming the depth and extent of epithelial ingrowth, and differentiating DLK from interface fluid syndrome (IFS). In IFS, a uniform low-intensity fluid layer is seen under the flap, while DLK shows predominantly punctate or linear high-intensity areas 8). AS-OCT also quantifies the degree of flap displacement (distance and depth of shift), aiding in determining the urgency of treatment.
Corneal topography: Used to evaluate flap striae, irregular astigmatism, and ectasia. Changes can be detected through serial postoperative observations. After stromal melting due to Grade 4 DLK, it is important to check for changes in posterior elevation using tomography (e.g., Pentacam) to assess the development of ectasia.
Corneal culture and smear: If infection under the flap is suspected (e.g., anterior chamber inflammation, hyperemia, dense infiltrates, purulent discharge from the flap edge), cultures should be obtained during flap lift to identify the organism and antibiotic sensitivity. Infectious keratitis can clinically resemble DLK, so early microbiological investigation is essential when suspected.
Protect with BSS, accurate repositioning after ablation, BCL fixation
Intraoperative management
Incomplete flap/buttonhole
Flap repositioning, surgery postponed
Consider reoperation after 3–6 months
QWhat to do if DLK is diagnosed?
A
Treatment strategy differs by DLK grade (severity). For Grade 1–2, treat with frequent instillation of prednisolone 1% eye drops every 1–2 hours. Most cases improve with this steroid eye drop therapy. For Grade 3, consider flap lift and irrigation in addition to the above. Grade 4 (central stromal melting) requires urgent flap lift and irrigation, and systemic steroid administration may also be considered. Do not self-judge; if you notice subflap opacity or vision loss after surgery, contact your ophthalmologist promptly.
The incidence of flap complications varies greatly depending on the surgical technique (microkeratome vs. femtosecond laser), facility experience, and patient selection criteria. The incidence rates obtained from major literature are shown below3)12).
With the widespread use of femtosecond lasers, severe intraoperative complications such as free caps and buttonholes have significantly decreased. However, it is important to note that DLK, flap displacement, and epithelial ingrowth can still occur even with femtosecond lasers 12).
Impact of flap-related complications on postoperative visual acuity
DLK Grade 3–4, severe epithelial ingrowth with flap melting, and large-area flap wrinkles can cause permanent vision loss if appropriate treatment is delayed. In an epidemiological study by Stulting et al.12), approximately 0.1% of DLK cases progressed to stromal melting (Grade 4), and even with appropriate treatment, some residual irregular astigmatism was reported. Daily or every-other-day examinations within the first postoperative week are recommended after LASIK during the high-risk early period 1).
A LASIK flap is created by making a lamellar incision approximately 100 μm deep from the anterior corneal surface using a microkeratome or femtosecond laser. The femtosecond laser creates the incision plane through photodisruption, forming plasma and linear arrays of microbubbles. The microkeratome cuts mechanically. In cases with corneal opacity or after radial keratotomy (RK), a microkeratome may be used, or PRK may be chosen.
DLK is a non-infectious interface inflammatory reaction. Debris from stromal ablation, residues from surgical instruments, and irritants from epithelial cells induce polymorphonuclear leukocyte infiltration at the flap interface. It often resolves spontaneously with transient inflammation, but progression to Grade 4 can cause stromal melting (keratolysis), leading to permanent irregular astigmatism. Early detection and intervention within 1–3 days postoperatively are crucial for prognosis.
Epithelial cells at the flap margin invade and proliferate beneath the flap interface. The invading epithelial cells form cell nests under the flap, and as they enlarge, they cause flap melting, irregular astigmatism, and decreased visual acuity. The risk increases after enhancement surgery involving flap lift. Flap edge position and a history of previous surgery are risk factors 6).
After LASIK, the flap does not fully re-adhere to the corneal stroma and contributes little to the structural strength of the cornea. Thicker flaps reduce the residual stromal bed (RST) more, increasing the impact on corneal biomechanics9). Thinning of the residual stromal bed is associated with an increased risk of corneal ectasia, with risk rising sharply when RST < 280 μm 2). Compared to SMILE, LASIK results in greater corneal biomechanical vulnerability, as evidenced by a significantly greater reduction in Corneal Resistance Factor (CRF) at 12 months postoperatively (MD, −1.13; 95% CI −1.36 to −0.90; P < 0.001) 2).
Finite element analysis simulations have quantitatively demonstrated the impact of flap thickness on biomechanics 9). As flap thickness increases, the anterior displacement of the posterior corneal surface increases, indicating that thinner flaps are more favorable for posterior surface stability. However, excessively thin flaps are prone to issues such as inhomogeneity and OBL. Femtosecond lasers can create flaps with ±10 μm precision, allowing safe creation of thin flaps around 100–120 μm. A design balancing biomechanics and reliable flap creation is recommended. The absolute value of the postoperative residual stromal bed (RST) is the most important indicator for ectasia risk assessment in LASIK; surgery should be reconsidered if RST < 280 μm 2)15).
The closed space under the flap requires management different from that of typical corneal infections. Antibiotic eye drops should be selected based on the causative organism; severe cases may require flap lift and irrigation. The Refractive Surgery Guidelines (8th edition) state that “strict adherence to high-level barrier precautions, sterilization of instruments, and disinfection and draping of the surgical field are essential” 1).
Interface fluid syndrome (IFS) is fluid accumulation under the flap caused by steroid-induced intraocular pressure elevation, and differentiating it from DLK is a critical issue that changes the treatment strategy by 180 degrees. IFS shows elevated intraocular pressure (note that Goldmann applanation tonometry may give falsely low readings), and anterior segment OCT reveals a homogeneous low-reflectivity fluid layer. Since continued steroid administration worsens IFS, differentiation using anterior segment OCT and intraocular pressure measurement is essential 8). Late-onset IFS can occur months to years after surgery; if fluid accumulation under the flap is observed, intraocular pressure measurement (especially with peripheral or dynamic contour tonometry) must be performed 8).
Surgical technique improvements in femtosecond laser LASIK
Changes in flap creation angles (vertical and horizontal bevel angles) and adjustments in flap hinge width and angle have improved flap security and reduced the risk of postoperative flap displacement. In particular, there are reports that a superior hinge provides better postoperative stability than a nasal hinge 10). There is also the view that designing the flap edge side-cut angle to be acute (side-cut angle ≥90°) reduces the risk of epithelial ingrowth at the margin 10). Furthermore, flap thickness uniformity is better with femtosecond laser than with microkeratome, and finite element analysis has quantified the impact of flap thickness on biomechanics 9).
Transition to SMILE and avoidance of flap complications
Since SMILE does not create a flap, the flap-related complications described in this section (flap displacement, DLK, epithelial ingrowth, free cap, buttonhole) do not occur in principle. The KLEx guidelines show that the ectasia incidence rate is 90 per 100,000 eyes for LASIK compared to 11 for SMILE, indicating that SMILE has a lower risk of postoperative ectasia2). However, SMILE also has its own complications such as interface inflammation (DLK-like), lenticule remnants, and capsulectomy.
Corneal cross-linking (CXL) for post-LASIK ectasia
When postoperative ectasia progresses, corneal cross-linking (CXL) is the first-line treatment. The standard method is the Dresden protocol, which involves instillation of 0.1% riboflavin followed by UVA irradiation (3 mW/cm² for 30 minutes), and progression is halted in most cases 15). Accelerated CXL (9 mW/cm² for 10 minutes) can shorten treatment time 15). The efficacy of CXL for post-LASIK ectasia tends to be slightly lower than for keratoconus, but early intervention improves prognosis 15). In Japan, CXL has been covered by insurance since 2022. It is important to maximize prevention of ectasia onset through preoperative risk assessment using the Randleman scoring system 11) and preoperative screening recommended by the AAO Corneal Ectasia PPP 17).
It is important to monitor the DLK incidence rate at the facility level and recognize cluster occurrences (multiple cases concentrated in a short period). Cluster occurrences often suggest contamination in the operating room, inadequate sterilization of instruments, or problems with cleaning solutions 16). If the incidence rate exceeds 0.5%, comprehensive inspection of the operating room, instruments, and cleaning solutions is recommended.
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Johnson JD, Harissi-Dagher M, Pineda R, et al. Diffuse lamellar keratitis: incidence, associations, outcomes, and a new classification system. J Cataract Refract Surg. 2001;27(10):1560-1566.
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Santhiago MR, Smadja D, Gomes BF, et al. Association between the percent tissue altered and post-LASIK ectasia in eyes with normal preoperative topography. Am J Ophthalmol. 2014;158:87-95.e1.
Moshirfar M, Tukan AN, Bundogji N, et al. Ectasia after corneal refractive surgery: a systematic review. Ophthalmol Ther. 2021;10:753-776.
Hersh PS, Stulting RD, Muller D, et al. U.S. multicenter clinical trial of corneal collagen crosslinking for treatment of corneal ectasia after refractive surgery. Ophthalmology. 2017;124:1475-1484.
American Academy of Ophthalmology Corneal/External Disease Preferred Practice Pattern Panel. Corneal Ectasia Preferred Practice Pattern. San Francisco, CA: AAO; 2024.
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