Incomplete Flap During LASIK
Incomplete flaps are the most common corneal flap complication associated with LASIK surgery and, if not properly identified and treated, they can result in suboptimal refractive results. Although both newer microkeratome heads and the femtosecond laser are associated with a lower incidence of incomplete flap, there is still a risk of incomplete flap with every procedure. Proper management is based on the characteristics of the flap itself; many can simply be recut with a second pass, however some incomplete flaps necessitate postponing ablation to a later date.
An incomplete flap is a complication of LASIK surgery in which corneal flap creation ends before reaching the intended hinge position, resulting in a flap that is often not large enough to allow for laser ablation.
Incomplete flaps are created during LASIK as the result of a premature stop of the microkeratome head during its course, or an incomplete lamellar or side cut when using the femtosecond laser. This can be due to obstruction of the path of the microkeratome head or femtosecond laser, loss of suction, or device malfunction.
Overall, corneal flap complications are rare, with a reported incidence between 0.3 – 14%, depending on the type of device used with the highest incidence associated with first generation microkeratome models.
Femtosecond Laser Versus Microkeratome
Evidence shows the femtosecond laser produces a more consistent and uniform flap and there is evidence to indicate the incidence of partial flaps is lower when using the femtosecond laser versus a microkeratome.
There is evidence to show that incidence is highly variable based on a surgeon’s personal experience. Experienced LASIK surgeons have a much lower incidence of incomplete flaps than novice surgeons.
Incomplete flaps represent a significant complication of LASIK surgery. If not identified and handled appropriately, they can lead to a delayed recovery of visual acuity, loss of best corrected visual acuity (BCVA), diplopia and compromised night vision.
The development of an incomplete flap during surgery requires the practitioner to stop the procedure in order to determine possible etiologies, as well as evaluate the state of the flap. If the incomplete flap is the result of a suction break, the surgeon must establish what may have caused this. In cases where suction break is not the suspected etiology, the eye itself should be carefully examined for irregularities, and the device evaluated for potential malfunction.
If it is determined that there is sufficient surface area in the stromal bed to allow for laser ablation, with a reasonable ablation zone, then the procedure can continue as planned without any additional risk of postoperative refractive complications.
If the hinge is beyond the visual axis, but the stromal bed is smaller than intended, it is considered safe to reattempt flap creation with an immediate second pass of the microkeratome or femtosecond laser. There is, however, a slightly increased risk of tissue maceration. It is not recommended to dissect the flap manually, as this is linked to a high incidence of stromal and flap irregularities.If the hinge is within the visual axis, the flap should be carefully replaced and the procedure postponed to a later day.
In cases where an immediate second pass is not feasible and the procedure is postponed, there are two options available for the second intervention.
A second cut may be attempted using either microkeratome head or femtosecond laser after the initial flap has been allowed to heal. There is no consensus on how long the optimum healing period should last; however there is agreement that retreatment should not be attempted before three months.
There are risks associated with attempting a second treatment with either microkeratome or femtosecond laser. The primary risk is of the new cut intersecting with the initial cut, resulting in stromal fragmentation, and the potential for loss of corneal tissue. It is therefore recommended that the new flaps should be larger and deeper than the previous attempt.
Photorefractive Keratectomy or Laser Subepithelial Keratomileusis
The second option is surface ablation performed over the incomplete flap via either photorefractive keratectomy or laser subepithelial keratomileusis (LASEK).These options eliminate the risks associated with a second pass of the microkeratome or femtosecond laser, and allow for simultaneous treatment of any corneal surface abnormalities. The waiting period before attempting this intervention is not as long as it is for retreating with microkeratome or femtosecond laser, and retreatment should be carried out 2-4 weeks after the primary procedure. The disadvantage of this treatment modality is an increased risk of corneal haze, so mitomycin C should be administered to every patient for haze prophylaxis.
LASEK can be an appropriate intervention as long as the new flap hinge is opposite to the original LASIK flap.
Insuring a properly functioning device and securing a safe path for the microkeratome or femtosecond laser are the two most important and effective mechanisms to prevent incomplete flap. Prior to any procedure, the microkeratome head should be thoroughly inspected and carefully cleaned, and the whole system tested prior to use. Proper draping will prevent the lashes and lids from obstructing the path. Lifting of the globe after suction activation can be helpful in clearing the path, especially in patients with deep-seated eyes.
The risk of suction loss can often be detected via intraoperative tonometry to ensure adequate intraocular pressure (IOP) is achieved. Clinical signs, such as pupil dilation and symptoms of dimming vision can also confirm adequate elevation of intraocular pressure.
Generally, patients who experience incomplete flap during LASIK have excellent long-term visual outcome. Several cohort studies have shown that patients who experienced intraoperative incomplete flaps, treated immediately with a second pass (of either femtosecond laser or microkeratome), have visual outcomes no different that the cohort that did not experience incomplete flap.
Rare complications of incomplete flap are visual complaints including loss of BCVA and impaired night vision. There are two case studies detailing incidence of incomplete flap resulting in corneal ectasia, but this outcome is believed to be exceptionally rare.
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