No. Lasik may not give you perfect vision everytime. The American Academy of Ophthalmology (AAO) reports that seven out of ten patients achieve 6/6 or better vision. About 95% will achieve 6/12 or better vision. 6/12 is the minimum visual requirement for obtaining a driving licence in most countries. It also means reduced dependence on glasses or contact lenses for most daily and sporting activities.

Under or over-correction may occur despite the surgeon's best effort due to individual variation in the healing response. When significant, a second "enhancement" procedure may be recommended by your doctor. Sometimes, a mild under-correction in one eye is deliberately aimed for in older patients likely or already suffering from presbyopia (middle-age farsightedness). This concept is termed "mono-vision" (see Q34).

Enhancement is done in about five to fifteen per cent of cases. It is simply done by re-lifting the cornea flap and using the excimer laser to fine-tune the desired refraction. However, there should be a waiting period of between three weeks to three months, with the refractive status stable, before proceeding. During this interim period, if clear distance vision is desired, low-power spectacles or contact lens can be prescribed and worn when required.

For severely over-corrected eyes, re-treatment maybe difficult. Therefore, to reduce this risk, most doctors deliberately aim for a slight under-correction when treating high to moderate myopia.

Yes. The first Lasik procedure was done in 1989. To date, several millions of cases have been done worldwide, mostly with good results. Research studies have shown the results to be stable as long as established treatment protocols are followed. It is also the general consensus of eye surgeons and researchers worldwide that these results are likely to remain stable in the future.

Unstable results have occurred when the amount of cornea tissue sculpted exceeded recommended treatment guidelines. Occasionally, unstable results have occurred because of treatment of eyes with undiagnosed keratoconus. Keratoconus is a condition characterised by cone-shaped thinning of the cornea. Its presentation is often subtle in the early stages and can only be diagnosed with specialised instruments.