Lasik Flap Dilemmas

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There comes a time in every laser eye surgical procedure when leaving
the battlefield is the best of two (or more) evils. When is it best to
reposition the flap, abandon the procedure and possibly attempt the LASIK
procedure at a later time…?

Obtaining adequate suction to certain globes is occasionally an elusive
target. This includes small hyperopic eyes, flat or small diameter
corneas, narrow palpebral fissures… If the level of myopia permits, one
has to remember that PRK is always an available option and patients
must be made aware of this alternative and be consented for it prior to
the procedure. A surgeon might be tempted to extend an incomplete flap
with a crescent blade or similar instrument . This might lead to an
uneven bed and scarring. The closer the hinge to the visual axis the
riskier this maneuver will be. If the bed is large enough (not more
than 0.5 mm of unexposed stroma at the hinge) laser treatment may be
applied (with adequate protection to the underside of the flap).

A thin flap with an underlying shiny bed probably indicated an uncut
underlying Bowman’s layer. It is not clear if performing laser in this
situation has similar or higher risk of haze formation as PRK. Until
more is known about this issue, it is probably safest to reposition the
flap and abort the procedure especially in high levels of correction.
An irregular flap indicates an irregular stromal bed and is best
allowed to heal back in position rather than risk inducing irregular
astigmatism.

If a buttonhole occurs, immediate laser ablation of a central
epithelial island by scraping or by the laser was reported to lead to
uneven ablation and loss of BCVA.

The Free Cap

    A free cap results from unintended complete
dissection of the corneal flap by the microkeratome head. If the cap is
trapped in the keratome head, it should be gently retrieved, stretched
and kept in a dessication chamber if the diameter of the exposed stroma
allows laser ablation.  A small cap (i.e smaller than the optical
zone) should prompt the surgeon to replace it in position and avoid the
laser ablation. If the cap cannot be recovered, the epithelium will
grow centrally as after other “superficial” keratectomy procedures and
may result in a significant hyperopic shift.

 Intraoperative factors leading to a free cap are the same as
those leading to a thin or perforated flap, a poor blade to cornea
coupling. This is especially true for flatter corneas which are more
prone to a smaller cap. Other maneuvers such as malpositioning and/or
misadjusting of the flap thickness foot-plate during assembly of
certain microkeratomes can lead to a free cap.

In certain instances, the microkeratome can jam preventing head
reversal. This might prompt the surgeon to release the suction thus
lifting the instrument with an incarcerated flap resulting in a free
cap.  

Placing corneal marks with gentian violet is time well spent prior to
cutting a corneal flap. When recovered, a cap can be repositioned using
the preplaced marks to allow proper orientation. A bandage contact lens
is usually helpful to tamponade the cap and prevent slippage upon lid
contact. Suturing is rarely necessary.

 If the cap is lost, the corneal epithelium is allowed to heal as
in PRK with a more profound central applanation effect. Laser treatment
is deferred until refractive stability is achieved.

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