Free cap after LASIK
|Classification and external resources|
A completely detached flap of the cornea
Free cap is a rare intraoperative complication of LASIK (laser in situ keratomileusis). Ideally in LASIK, a hinged corneal flap is created that allows eximer laser to be applied on the exposed stromal bed. If the hinge of the corneal flap detaches, the flap becomes a free flap/cap. The occurrence of this complication is most commonly associated with flat corneas, which predisposes to small flap diameter. Free cap is preventable and treatable. Rarely does the complication lead to severe or permanent decrease in visual acuity.
Risk factors for free cap are generally anatomic or mechanical.
- Flat cornea (<40.00D)  (Note: Only primary flat corneas carry the risk of free cap as flat corneas secondary to refractive surgery will behave like the original cornea.)
- Deep orbits
- Inadequate suction
- Decentered ring placement
- Faulty microkeratome blades
- Use of mechanical microkeratome rather than femtosecond microkeratome 
- Stopper set for smaller hinge 
If a free cap is created, the surgeon must decide whether to continue with eximer laser ablation or to abort the procedure.
- When to abort: If the suction level is low and the stromal bed is irregular, replace the flap without applying laser ablation. Without ablation, generally there is no change in refractive error or significant loss of visual acuity 
- When to continue: If the stromal bed is regular and the cap is of normal thickness, the surgeon may proceed with ablation. Excellent outcome is usually anticipated. The free cap should be carefully handled by being placed on a drop of balanced salt solution in a chamber to prevent desiccation.
Regardless of whether the cap is placed with or without the excimer laser ablation, the surgeon must ascertain that the epithelial side is up and that the reference marks are well aligned.
Prevention is key. Surgeon must prepare for the possibility of free cap by selecting suction rings of larger diameter (example: Hansatome or a larger microkeratome head such as 9.5mm ring on Nidek MK-2000) for flat corneas less than 42D thick (do not cut if <38D), using stop rings especially for novice surgeons of LASIK, taking care during decentration of the ring, and calibrating the stopper of the microkeratome well. Placing epithelial reference markings (see figure) can be useful when free cap does arise so that the cap may be reapplied without inducing significant astigmatism (see figure).
A surgeon may want to consider which microkeratome to use to reduce the incidence of free cap. According to a study by Jacob et al, the incidence of the complication varied significantly when Hansatome (Bausch & Lomb Surgical) was compared to Automated Corneal Shaper® (Chiron Vision Corp.), favoring Hansatome for lower rates of free cap and other microkeratome-related complications. Superiority of Hansatome in preventing free cap complication was confirmed in a study by Nakano et al. . In a different study, Hansatome had equivalent rates of free cap complication compared to Moria LSK2 (Moria SA, Anthony, France).
- Loss of the disc: One of the most feared complication of free cap is the loss of the disc after replacement of the disc. To avoid this complication, for approximately 30 minutes after surgery the lids may be closed using two crossed adhesive strips. Dislocated disc has been reported to be repositioned with good visual outcome as much as four days after the initial surgery..
- Astigmatism: Incorrect flap orientation likely causes irregular astigmatism. Treatment for this complication include removing and repositioning of the cap if the proper orientation is known, repeat LASIK with a deeper flap after 6+ months, a trial of H-PARK, or placement of a homoplastic cap about 200 micrometers thick.
- Epithelialization of the interface: It is often impossible to differentiate between the epithelial and stromal sides of the disc. If the disc is placed with the epithelial side down, the interface may epithelialize, leading to loss of the disc.
- Epithelial ingrowth
- Flap micro/macrostriae
- Loss of best corrected vision
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