This is a commonly asked question and quite understandable for those embarking on a procedure to correct their vision.
The answer to this question is for the vast majority the procedure is permanent with a definite reduction in dependency on glasses and contact lenses. Laser eye surgery produces a change in the power of the cornea, in a sense it carves out a permanent contact lens on the cornea. When performed with the correct technology in properly chosen cases where the cornea is mechanically strong, the power change is in the long term stable and accounts for why laser eye surgery is stable long term. However for a small number changes in vision can occur in spite of the cornea remaining the same.
To understand why changes in vision might occur, it is useful to understand how light rays are focussed in the eye and how this is influenced by laser eye procedures like Lasik. Light rays entering the eye must focus on the retina. This focus is accomplished by the cornea (67%) and the remainder by the lens within the eye. The focal power required depends on the length of the eye. After the age of 22 or so, the eye does not get any longer and the cornea hardly changes in overall power. Once the eye refraction (prescription) has stabilised it is unlikely that the power of the eye will change.
However the lens in the eye is dynamic, not just by the fact that it moves and changes shape inside the eye but also because it gets new layers added on every year and becomes thicker. It also as a result becomes harder which is why everyone finds it difficult to read in their 40s and need reading glasses – a condition called Presbyopia. The change in thickness and hardening of the lens within the eye can produce a change in power and in turn alter the focussing power of the eye. This often happens when a cataract is about to commence or well in progress. As a result of this change (varies in terms of age) the power of the eye can change resulting in a need for a small spectacle correction. Further laser eye surgery can be contemplated in some, but as the problem is the lens inside the eye, a more permanent solution especially for those in their 50s will be refractive lens exchange where the changing and dysfunctional lens is removed and replaced with a high performance multifocal (trifocal) lens.
This problem of the lens hardening cannot be prevented, although there are some who believe good living, consumption of anti-oxidants and other goodies can delay the onset and progress of lens hardening and cataract and in turn keep the eyes stable.
Laser eye surgery, in particular Lasik is a wonderful and liberating procedure for the vast majority of those seeking vision correction surgery. For the vast majority (selected well) the procedure is long lasting and for the small unlucky group who develop a change with advancing age, there are other options for lasting vision correction.
The key to a long-term successful outcome is to choose your surgeon well.