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 Bowmans 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.
