Over and Under Corrections
The most common undesired outcomes of LASIK and other refractive surgery procedures are over-corrections or under-corrections, meaning there is residual refractive error remaining after surgery. Over-corrections and under-corrections are rarely caused by surgeon error or laser problems they are typically due to variances in how a person's eyes heal after surgery. How your eyes respond to laser energy and how they heal after surgery play a dominant role in whether or not you achieve the visual outcome you desire after surgery.
Over-correction and under-correction is more common with high prescriptions. For example, if you are highly nearsighted with a prescription of -10.00 diopters (D), a 10% under-correction would leave you with residual nearsightedness of -1.00 D. This usually causes noticeable blur, perhaps in the range of 20/40 vision. In contrast, if you are only mildly nearsighted with a prescription of -1.00 D prior to refractive surgery, a 10% under-correction results in only -0.10 D of residual nearsightedness, which would not interfere with attaining a very clear 20/20 (or even 20/15) result.
Your eye doctor or refractive surgeon can advise you of your risk of a surgical over-correction or under-correction, based on your preoperative prescription and other factors. In most cases, these undesired outcomes can be corrected with a follow-up refractive surgery (called an "enhancement"). LASIK enhancement procedures are usually performed approximately 6 months after the initial surgery, after your eyes have completely stabilized.
In some cases, optimal correction may not be achieved despite an enhancement procedure. In this case, continued use of eyeglasses or contact lenses may be needed. Your prescription will usually not be as strong as before the surgery. However, contact lens options will probably be limited due to the increased difficulty of properly fitting a cornea that has been compromised by surgery. GP contact lenses and soft or GP custom contact lenses can be useful in these situations.