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Surgical procedure

Pre-operational corneal mapping
Several days before the procedure, the surface of the patient's corneas are examined with a computer-controlled scanning device to determine their exact shape. Using low-power lasers, it creates a topographic map of the cornea. This process also detects astigmatisms and other irregularities in the shape of the cornea. Using this information, the surgeon calculates the amount and locations of corneal tissue to be removed during the operation.

Also, the patient is typically prescribed an antibiotic to start taking beforehand, to minimize the risk of infection after the procedure.

The operation
The operation is performed with the patient awake and functional, however, the patient is typically given a mild sedative (such as valium) and anesthetic eye drops. The surgeon operates the lasers, which make all of the incisions. A computer system tracks the patient's eye position 4,000 times per second, redirecting laser pulses for precise placement. A flap is cut in the cornea using a blade (called a microkeratome) or a femtosecond laser. A hinge is left at one end of this flap. The flap is folded back revealing the stroma, the middle section of the cornea.

Then an excimer laser (193 nm) is used to remodel the corneal stroma. The laser vaporizes tissue without causing damage to adjacent stroma in a finely controlled manner. The layers of tissue removed are tens of micrometres wide.

Post-operational complications

A subconjunctival hemorrhage is a common and relatively minor post-LASIK complication.While in nearly all cases the operation is a success, the procedure does have some rare complications. A slipped flap (when the corneal flap detaches from the rest of the cornea) is one of the most common complications. The chances of this are greatest immediately following surgery, so patients are typically recommended to go home and sleep, to let the flap heal.

Infection under the corneal flap is possible. It is also possible that the patient has a genetic condition that causes the cornea to thin out following surgery. While this is screened for in the pre-op exam, it is possible in rare cases (about 1 in 5,000) for the condition to remain dormant until later in life (the mid-40s). If this occurs, the patient will require a corneal transplant.

Factors affecting the surgery
The cornea is typically avascular in that it must be transparent to function normally. Its cells absorb oxygen from the tear film. Low oxygen-permeable contact lenses reduce the cornea's absorption of oxygen which sometimes results in the growth of blood vessels into the cornea - a process known as corneal neovascularization. This can cause a mild increase in inflammation and healing time, and some discomfort during the surgery due to augmented bleeding. Although some contact lenses, notably modern RGP and soft silicone pharmacy health topics hydrogel lenses, are made of materials with higher oxygen permeability that help reduce the risk of corneal neovascularization, patients considering LASIK are cautioned to avoid overwearing their lenses. It is usually recommended that contact lens use is discontinued several days to weeks prior to the LASIK procedure.

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