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LASIK (Laser In Situ Keratomileusis)
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Preoperative
Patients wearing soft contact lenses typically are instructed to stop
wearing them approximately 7 to 10 days before surgery. One industry body
recommends that patients wearing hard contact lenses should stop wearing
them for a minimum of six weeks plus another six weeks for every three years
the hard contacts had been worn. Before the surgery, the surfaces 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 astigmatism 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. The patient typically is prescribed an antibiotic to
start taking beforehand, to minimize the risk of infection after the
procedure.
Surgery
The surgery is performed with the patient awake and mobile; however, the
patient typically is given a mild sedative (such as Valium or diazepam) and
anesthetic eye drops.
Lasik is performed in two steps. The initial step is to create a flap of
corneal tissue. This process is achieved with a mechanical microkeratome
using a metal blade, or a femtosecond laser microkeratome that creates a
series of tiny closely arranged bubbles within the cornea.[4] A hinge is
left at one end of this flap. The flap is folded back, revealing the stroma,
the middle section of the cornea. The process of lifting and folding back
the flap can be uncomfortable.
The second step of the procedure is to use an excimer laser (193 nm) to
remodel the corneal stroma. The laser vaporizes tissue in a finely
controlled manner without damaging adjacent stroma by releasing the
molecular bonds that hold the cells together. No burning with heat or actual
cutting is required to ablate the tissue. The layers of tissue removed are
tens of micrometers thick.
During the second step, the patient's vision will become very blurry once
the flap is lifted. He/she will be able to see only white light surrounding
the orange light of the laser. This can be disorienting.
Currently manufactured excimer lasers use a computer system that tracks the
patient's eye position up to 4,000 times per second, redirecting laser
pulses for precise placement. After the laser has reshaped the cornea, the
Lasik flap is repositioned over the treatment area by the surgeon. The flap
remains in position by natural adhesion until healing is completed.
Performing the laser ablation in the deeper corneal stroma typically
provides for more rapid visual recovery and less pain.
Higher-order aberrations
Higher-order aberrations are visual problems not captured in a traditional
eye exam. In a young healthy eye, the level of higher-order aberrations are
typically low and insignificant. Concern has long plagued the tendency of
refractive surgeries to induce higher-order aberration not correctible by
traditional contacts or glasses. The advancement of lasik technique and
technologies has helped eliminate the risk of clinically significant visual
impairment after the surgery. There has been controversy about the amount of
higher-order aberrations that would lead to significant vision impairment.
In extreme cases, where proper policy was not followed and before key
advances, some people could suffer rather debilitating symptoms including
serious loss of contrast sensitivity in poor lighting situations.
Over time, most of the attention has been focused on spherical aberration.
Lasik and PRK tend to induce spherical aberration, because of the tendency
of the laser to undercorrect as it moves outward from the center of the
treatment zone. This is really only a significant issue for large
corrections. There is some thought if the lasers were simply programmed to
adjust for this tendency, no significant spherical aberration would be
induced. Hence, in eyes with little existing higher order aberrations, "wavefront
optimized" lasik rather than wavefront guided Lasik may well be the future.
Regardless, most patients with even the highest corrections remain highly
satisfied even with conventional lasik.
Wavefront-guided LASIK
Wavefront-guided LASIK is a variation of LASIK surgery where, rather than
apply a simple correction of focusing power to the cornea (as in traditional
LASIK), an ophthalmologist applies a spatially varying correction, using a
computer-controlled high-power UV laser guided by measurements from a
wavefront sensor. The goal is to achieve a more optically perfect eye,
though the final result still depends on the physician's success at
predicting changes which occur during healing. Nor are wavefront aberrations
the factor to degrade vision; especially in older patients, scattering from
microscopic particles plays a major role. Hence, patients expecting
so-called "super vision" from such procedures may be disappointed. However,
surgeons claim patients are generally more satisfied with this technique
than with previous methods, particularly regarding lowered incidence of "halos",
the visual artifact caused by spherical aberration induced in the eye by
earlier methods.
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