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Contraindications to Lasik are the same as
those for LASEK and also the following:
- Cornea too thin: The
international standard is to leave a minimum of 250microns behind under the
flap for long term strength of the cornea. Most microkeratomes cut a flap of
about 160 microns thickness, and this flap adds nothing to the overall
strength of the cornea afterwards. The average cornea is about 550 microns,
so the maximum laser ablation is 140 microns (550-250-160). This equates to
about -12 Dioptres with a reasonable sized optical zone. If your cornea is
thicker than 550microns one can do a bigger prescription, and conversely if
it is thinner. Corneal thickness is measured with an ultrasonic probe and
only takes a few seconds. In my experience, this is the commonest cause of
turning someone down for Lasik. I generally try to be very conservative and
normally leave over 300 microns.
- Abnormal Epithelium: very
loose epithelium is found in some corneal abnormalities, which are
commoner in older people. Because the microkeratome passes over the surface
of the cornea, it can partially "scuff" off this loosely
adherent tissue. Although this will regrow in about 2 days, there is a
higher risk of infection and "sands of the Sahara"
- Very flat or very steep cornea: there
is a higher risk of a free cap or buttonholed flap and the microkeratome
manufacturers give guidelines as to the limits of their own particular
machine.
- Glaucoma: Higher
risk of ischaemic optic neuropathy. (see lasik
complications)
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