Free Cap after LASIK
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Introduction
Free cap (also termed free flap) is a rare intraoperative complication of LASIK (laser in situ keratomileusis) in which the corneal flap completely detaches from the cornea, losing its hinge attachment. Ideally in LASIK, a hinged corneal flap is created that allows the excimer laser to be applied on the exposed stromal bed. If the hinge of the corneal flap detaches, the flap becomes a free cap.[1] The occurrence of this complication is most commonly associated with the use of a mechanical microkeratome, particularly on flat corneas, which predisposes to a small flap diameter.[2] Free caps are preventable and treatable, and rarely lead to severe or permanent decrease in visual acuity.[1][2] The advent of femtosecond laser technology for flap creation has substantially reduced the incidence of this complication.[3]
Free Cap
Incidence
The incidence of free cap during microkeratome LASIK is generally low, though reported rates vary by study, microkeratome type, and era:
In a large retrospective analysis of 84,711 primary LASIK cases, the overall incidence of free flaps was 0.012% (10 eyes).[4]
In a study of 47,094 LASIK surgeries, the overall free flap incidence was 0.08%, but varied significantly by microkeratome type: 0.04% with the Hansatome, 0.20% with the Automated Corneal Shaper (ACS), and 0.29% with the Nidek MK-2000.[5]
A more recent analysis of 55,700 consecutive LASIK treatments using the Sub Bowman Keratome (SBK) with a 90 µm head reported a free flap incidence of 0.276%.[6]
Free caps have been reported at rates as high as 1.31% in some earlier series.[1]
The transition from mechanical microkeratomes to femtosecond lasers for flap creation has virtually eliminated the free cap complication. In comparative studies, free caps occurred in the microkeratome group but not in the femtosecond laser group.[7][8] The femtosecond laser dissects tissue in a planar pattern that allows precise programming of flap width, depth, diameter, and hinge geometry, making free caps essentially a complication exclusive to mechanical microkeratome LASIK.[8][3]
Pathophysiology
During standard microkeratome LASIK, a motorized oscillating blade traverses across the cornea, creating a lamellar cut. The suction ring elevates and stabilizes intraocular pressure, and a mechanical stopper limits the blade excursion to preserve a hinge. A free cap occurs when the microkeratome blade traverses completely across the cornea without leaving an intact hinge. This typically happens because the effective corneal diameter exposed above the suction ring is too small for the selected blade excursion, allowing the blade to pass entirely through the tissue.
In flat corneas (<40 D), the cornea protrudes less above the suction ring, resulting in a smaller effective corneal diameter available for the keratome pass. This reduces the diameter of the flap and increases the likelihood that the blade will cut through the entire width without leaving a hinge.[9][2]
Risk Factors
Risk factors for free cap are generally anatomic, mechanical, or related to equipment selection:
Anatomic Factors
Flat cornea (<40 D): The most commonly cited anatomic risk factor. Flat keratometry reduces the amount of cornea protruding above the suction ring, resulting in a smaller effective flap diameter and a shorter or absent hinge.[9][10][11]
Low central corneal thickness: In a prediction model study of 55,700 eyes, thin corneas were independently associated with increased free flap risk (P < 0.001), even after controlling for other variables.[6]
Deep orbits and deep-set eyes: These anatomic features can impair suction ring application and centration, increasing the risk of inadequate corneal exposure. PRK may be a better option for such patients.[2]
Mechanical and Equipment Factors
Use of mechanical microkeratome rather than femtosecond laser: Free caps are essentially limited to mechanical microkeratome LASIK.[9][8]
Inadequate suction: Loss of suction during the keratome pass can lead to irregular or free cuts. Adequate intraocular pressure must be verified before initiating the pass.[2]
Decentered ring placement: An off-center suction ring exposes an asymmetric amount of cornea, which may result in the blade cutting through a narrow zone without leaving a hinge.[10]
Improper ring height selection: Both very flat (-1) and very thick (+2) ring heights were associated with increased free flap risk in the Katz et al. prediction model (P < 0.001).[6]
Higher stop values: Higher stop values, which determine the blade excursion length, were significantly correlated with increased free flap incidence (P < 0.001).[6]
Faulty or worn microkeratome blades[10]
Stopper set for a smaller hinge[10]
Surgeon Experience
Interestingly, in the Katz et al. prediction model study, surgeon experience (defined by number of procedures performed) was not significantly correlated with free flap incidence, suggesting that mechanical and anatomic factors are the dominant determinants.[6]
Prediction Models
Katz et al. developed a multivariable prediction model for free flaps using 55,700 consecutive LASIK treatments across 10 centers. The model incorporated central corneal thickness, microkeratome ring height, and stop value, and achieved 76% specificity and 73% sensitivity at a cut-off free flap risk of 0.274%. This type of predictive modeling may help surgeons identify high-risk eyes preoperatively and take additional precautions or consider alternative procedures.[6]
Prevention
As free caps are avoidable, prevention is critical. Strategies include:
Preoperative Measures
Keratometry-based ring and stop selection: The most important preventive step. Appropriate suction ring and stop size should be chosen according to the manufacturer's nomogram based on keratometry values. Larger diameter suction rings (e.g., Hansatome, or a 9.5 mm ring on the Nidek MK-2000) should be selected for flat corneas less than 42 D. Flap creation should generally be avoided on corneas with keratometry <38 D.[2][12]
Consider alternative procedures: For patients with very flat corneas, deep orbits, or other high-risk features, surface ablation (PRK/LASEK) or SMILE should be considered, as these do not involve flap creation and eliminate the risk of free cap entirely.[2]
Corneal epithelial reference markings: Asymmetric ink marks should be placed on the corneal epithelium before flap creation. These marks are essential for repositioning the cap in the correct orientation if a free cap does occur, and can prevent postoperative irregular astigmatism from rotational misalignment.[13][2]
Intraoperative Measures
Verification of adequate suction: Intraocular pressure should be confirmed after suction ring application using a combination of pupil dilation, finger tension, contact applanation, pneumotonometer, and/or the patient's report of dimming of vision.[13]
Re-docking if suction is lost: If inadequate suction is detected, the vacuum should be stopped and the microkeratome re-docked with suction before proceeding.[2]
Microkeratome inspection and blade testing: Prior testing of the microkeratome and inspection of the blade before each case are important to identify mechanical issues.[2]
Patient communication: Good verbal anesthesia (clear instructions and patient communication during the procedure) reduces the risk of unexpected patient movements that can disrupt the keratome pass.[2]
Use of stop rings: For novice surgeons, stop rings can be helpful to prevent a full pass through the cornea.[14]
Microkeratome Selection
The choice of microkeratome has historically influenced the rate of free caps. In the Jacobs et al. study comparing the Hansatome (Bausch & Lomb) to the Automated Corneal Shaper (ACS; Chiron Vision Corp.), the overall complication rate was 0.16% with the Hansatome versus 6.38% with the ACS (P < 0.005), with the Hansatome demonstrating significantly lower rates of free cap and other complications.[4] Nakano et al. confirmed the superiority of the Hansatome, reporting a free flap rate of 0.04% with the Hansatome versus 0.20% with the ACS (P < 0.001).[5]
In a separate comparison, the Hansatome had equivalent rates of free cap to the Moria LSK2 (Moria SA, Anthony, France).[15]
A comprehensive video guide for novice surgeons performing microkeratome LASIK, emphasizing these preventive and management strategies, can be found elsewhere.[2]
Management
If a free cap is created, the surgeon must decide whether to continue with excimer laser ablation or to abort the procedure.[2][16]
Decision to Abort
If the stromal bed is irregular, the flap should be replaced without applying laser ablation. Without ablation, generally there is no change in refractive error or significant loss of visual acuity.[2][17]
If the flap is visibly defective or grossly decentered, surgery should be aborted with as little flap manipulation as possible. The flap should be repositioned and allowed to heal. Surface ablation (PRK) with or without mitomycin-C can be performed at a later time, or the flap can be re-lifted and ablation performed after a healing period of several months.[13]
Decision to Continue
If the stromal bed is regular and the cap is of normal thickness, the surgeon may proceed with excimer laser ablation.[2]
Free caps are usually replaced after laser ablation with good visual results.[18]
Handling and Replacement of the Free Cap
Proper handling of the free cap is essential to prevent complications:
Prevent desiccation: The free cap should be carefully placed epithelial-side up on a drop of balanced salt solution (BSS) on a clean, moist surface during laser ablation. Desiccation can cause permanent stromal scarring and cap contracture.** Identify the epithelial surface: It is critically important to identify the epithelial versus stromal side of the cap. The epithelial surface is smoother and more reflective. If the cap is placed epithelial-side down, epithelialization of the interface can occur, leading to loss of the cap.** Reposition using corneal markings: Pre-placed asymmetric corneal ink marks guide proper rotational alignment. Misalignment causes irregular astigmatism.** Apply the cap: The free cap is placed epithelial-side up on the stromal bed, aligned with the pre-placed markings, and allowed to dry in position for several minutes. The endothelial cell pump mechanism typically allows the cap to re-adhere tightly.** Bandage contact lens: A bandage contact lens should be placed over the cap to provide protection and promote adhesion during the early postoperative period.[2][19]** Lid taping: For approximately 30 minutes after surgery, the lids may be closed using two crossed adhesive strips to prevent cap dislocation.[19]**
Postoperative Monitoring
Anterior segment OCT (AS-OCT): Swept-source AS-OCT can be a valuable tool for assessing flap thickness, interface integrity, and detecting complications such as epithelial ingrowth, striae, or interface debris after LASIK flap-related complications. AS-OCT is useful not only for diagnosis but also for management planning, including assessment of flap and residual stromal bed thickness if further procedures are contemplated.[20]
Corneal topography: Serial topographic imaging should be performed to monitor for irregular astigmatism, which may indicate cap malposition.
Complications
Loss of the Disc
One of the most feared complications is loss or dislocation of the free cap after replacement. To prevent this, lid taping for approximately 30 minutes after surgery is recommended.[19] Patients should be counseled to avoid rubbing or touching the eye. Dislocated discs have been reported to be successfully repositioned with good visual outcomes as late as four days after initial surgery.[21] In a study of post-LASIK flap repositioning, final best spectacle-corrected visual acuity of ≥20/25 was obtained in 80% of eyes following repositioning for various flap-related complications.[22]
Irregular Astigmatism
Incorrect flap orientation is the most common cause of irregular astigmatism following a free cap. Corneal markings placed before flap creation are essential to ensure the free cap is replaced in its original anatomic position. Treatment options for significant irregular astigmatism from cap misalignment include:[23]
Removing and repositioning the cap if the proper orientation is known
Repeating LASIK with a deeper flap after ≥6 months of healing
Placing a homoplastic cap approximately 200 µm thick
Topography-guided PRK for residual irregular astigmatism
Irregular astigmatism can also be caused by LASIK flaps that are irregular, fragmented, truncated, buttonholed, or avulsed.[13]
Epithelialization of the Interface
If the free cap is inadvertently placed with the epithelial side down, the interface may epithelialize. This can lead to progressive loss of the disc and significant visual impairment. Careful identification of the epithelial surface prior to replacement is essential to prevent this complication.
Other Complications
Epithelial ingrowth: Epithelial ingrowth is rare after primary LASIK but is more common following flap-lift retreatments or trauma. In the context of a free cap, the absence of a hinge may allow epithelial cells greater access to the interface. Treatment may include lifting the flap and debriding the interface, with suturing or tissue glue for persistent cases.[13]
Flap micro/macrostriae: Microstriae may result from excessive flap hydration or flap-bed contour mismatch, while macrostriae are caused by poor alignment or postoperative flap shift. If visually significant, the flap should be refloated and repositioned. Antitorque or interrupted 10-0 nylon sutures can be considered for recalcitrant striae.[13]
Diffuse lamellar keratitis (DLK): Interface inflammation can occur in the early postoperative period and may be more likely in the setting of free cap manipulation. DLK is characterized by fine granular inflammatory cells confined to the lamellar interface in an otherwise uninflamed eye.[13]
Loss of best-corrected visual acuity: The risk of losing ≥2 lines of BCVA is 1–2% greater with LASIK than with PRK, owing primarily to flap complications including free caps.[24]
Prognosis
The visual prognosis after a free cap complication is generally favorable when the cap is handled and replaced appropriately. Free caps are usually replaced after laser ablation with good visual results.[18] In cases where the procedure is aborted without laser ablation, there is generally no change in refractive error or significant loss of visual acuity.[2][17] However, outcomes depend on several factors including the regularity of the stromal bed, proper orientation of the cap during replacement, and the development of postoperative complications.
Early identification and surgical repositioning of displaced flaps are successful in both anatomical and visual restoration, with 80% of eyes achieving BCVA ≥20/25 after repositioning.[22]
Femtosecond Laser and the Elimination of Free Caps
The femtosecond laser creates corneal flaps through a fundamentally different mechanism than mechanical microkeratomes. Instead of a blade pass, the femtosecond laser generates thousands of tightly focused laser pulses that photodisrupt tissue at a precisely programmed depth, creating a cleavage plane. The surgeon can independently program the flap diameter, depth, side-cut angle, and hinge dimensions.[3]
This programmable approach essentially eliminates the free cap complication:
The hinge is created as a defined zone that the laser does not photoablate, rather than relying on physical blade stoppage.
Flap thickness is more predictable (standard deviations of 4–18 µm vs. 25–250 µm with microkeratomes).[8][3]
Comparative studies consistently show 0% free cap incidence with femtosecond lasers versus measurable rates with microkeratomes.[7]
However, the femtosecond laser does introduce its own unique set of complications not seen with mechanical microkeratomes, including transient light-sensitivity syndrome, rainbow glare, opaque bubble layer, vertical gas breakthrough, and anterior chamber gas bubbles.[3][8]
An AAO Technology Assessment concluded that available level I and II evidence indicates femtosecond lasers are as good as or better than mechanical microkeratomes for creating LASIK flaps.[3]
References
- ↑ 1.0 1.1 1.2 Lin RT, Maloney RK. Flap complications associated with lamellar refractive surgery. Am J Ophthalmol. 1999;127(2):129-136.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 Paryani M, Israni N, Kochar S, Aggarwal K. Microkera to mechronicles: Management of a free flap. Indian J Ophthalmol. 2023;71(7):2926-2927.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Farjo AA, Sugar A, Schallhorn SC, et al. Femtosecond lasers for LASIK flap creation: a report by the American Academy of Ophthalmology. Ophthalmology. 2013;120(3):e5-e20.
- ↑ 4.0 4.1 Jacobs JM, Taravella MJ. Incidence of intraoperative flap complications in laser in situ keratomileusis. J Cataract Refract Surg. 2002;28(1):23-28.
- ↑ 5.0 5.1 Nakano K, Nakano E, Oliveira M, Portellinha W, Alvarenga L. Intraoperative microkeratome complications in 47,094 laser in situ keratomileusis surgeries. J Refract Surg. 2004;20(5 Suppl):S723-S726.
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Katz T, Druckiv V, Siebelmann S, Frings A, Skevas C. Prediction model of free flaps in microkeratome-assisted LASIK. PLoS One. 2021;16(9):e0256956.
- ↑ 7.0 7.1 Kasetsuwan N, Satitpitakul V, Puangsricharern V, Reinprayoon U, Pariyakanok L. Comparison of performances of femtosecond laser and microkeratome for thin-flap laser in situ keratomileusis. Lasers Surg Med. 2016;48(8):758-765.
- ↑ 8.0 8.1 8.2 8.3 8.4 Kahuam-López N, Navas A, Castillo-Salgado C, et al. Laser-assisted in-situ keratomileusis (LASIK) with a mechanical microkeratome compared to LASIK with a femtosecond laser for LASIK in adults with myopia or myopic astigmatism. Cochrane Database Syst Rev. 2020;4(4):CD011952.
- ↑ 9.0 9.1 9.2 Skuta GL, Cantor LB, Weiss JS, et al. Basic and Clinical Science Course, Section 13: Refractive Surgery. American Academy of Ophthalmology. 2012; p104.
- ↑ 10.0 10.1 10.2 10.3 Buratto L, Brint SF. LASIK: Surgical Techniques and Complications. 2nd ed. Thorofare, NJ: SLACK; 2000.
- ↑ Krachmer JH, Mannis MJ, Holland EJ. Cornea. 3rd ed. Vol 2. St. Louis, MO: Mosby/Elsevier; 2011.
- ↑ Bowman RW. Complications of refractive surgery. University of Texas Southwestern Med Ctr, Dallas, Texas. Jan. 2014. Lecture.
- ↑ 13.0 13.1 13.2 13.3 13.4 13.5 13.6 Jacobs DS, Lee JK, Shen TT, et al. Refractive Surgery Preferred Practice Pattern. American Academy of Ophthalmology. 2023.
- ↑ Brightbill FS. Corneal Surgery: Theory, Technique & Tissue, Chapter 96. 3rd ed. St. Louis, MO: Mosby; 1999.
- ↑ Al-Mezaine HS, Al-Amro SA, Al-Obeidan S. Intraoperative flap complications in laser in situ keratomileusis with two types of microkeratomes. Saudi J Ophthalmol. 2011;25(3):239-243.
- ↑ Sahay P, Bafna RK, Reddy JC, Vajpayee RB, Sharma N. Complications of laser-assisted in situ keratomileusis. Indian J Ophthalmol. 2021;69(7):1658-1669.
- ↑ 17.0 17.1 Tham VM, Maloney RK. Microkeratome complications of laser in situ keratomileusis. Ophthalmology. 2000;107(5):920-924.
- ↑ 18.0 18.1 American Academy of Ophthalmology. Laser In Situ Keratomileusis for Myopia and Astigmatism: Safety and Efficacy. Ophthalmology. 2002;109(1):175-187.
- ↑ 19.0 19.1 19.2 Buratto L, Brint SF. CUSTOM LASIK: Surgical Techniques and Complications. Thorofare, NJ: SLACK; 2003.
- ↑ Abdelazeem K, Sharaf M, Saleh MGA, Fathalla AM, Soliman W. Relevance of swept-source anterior segment optical coherence tomography for corneal imaging in patients with flap-related complications after LASIK. Cornea. 2019;38(1):46-52.
- ↑ Cheng AC, Wong VW, Rao SK, Lam DS. Repositioning of free cap four days after LASIK. J Refract Surg. 2007;23(6):625-627.
- ↑ 22.0 22.1 Dvivedi A, Murthy SI, Akkulugari V, Ali H. Surgical and visual outcomes of flap repositioning for various flap-related pathologies post laser in-situ keratomileusis (LASIK). Indian J Ophthalmol. 2024;72(4):558-563.
- ↑ Kwitko ML, Kremer FB, Lee NT, et al. Induced astigmatism following laser in situ keratomileusis for myopia with a free cap. J Refract Surg. 2000;16(3):375-379.
- ↑ Wilson SE. Clinical practice. Use of lasers for vision correction of nearsightedness and farsightedness. N Engl J Med. 2004;351(5):470-475.
