Contraindications to Lasik are the same as those for LASEK and also the following:
1. 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 flaps between 100 to 130 microns, and this flap adds nothing to the overall strength of the cornea afterwards. The average cornea is about 540 microns, so the maximum laser ablation is 190 microns (540-250-100). I generally try to be more conservative and never take of more than 150 microns and normally leave over 300 microns. If your cornea is thicker than 540microns 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. We do more LASIK nowadays because we can cut very thin flaps safely.
2. 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" This is less common with femtosecond lasers.
3. 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.
4. Glaucoma: Higher risk of ischaemic optic neuropathy. (see lasik complications)