The human lens is much like a Smartie or M&M and has a transparent thin outer coating (capsule) which encloses the substance of the lens (cortex and nucleus). During cataract surgery the surgeon creates a small circular opening in the capsule at the front of the lens which allows him/her to remove the substance of the cataractous lens (the chocolate in our M&M analogy) whilst leaving the remaining capsular envelope behind. The new lens implant is then placed within this envelope which is critical for supporting the new lens in a stable position. Over time the capsular envelope will shrink wrap around the lens and can become thickened or opacified in around 1 in 10 people. This is much like a frosting on the lens implant and can happen even years after surgery, usually resulting in a slow decrease in vision.
Posterior capsular opacification only requires treatment if vision is affected. A YAG laser easily removes the thickened posterior capsule. A YAG laser capsulotomy takes just a few minutes to perform as an outpatient procedure and is completely painless. Once the central capsule is cleared away the patient cannot develop capsular opacification again and hence a YAG laser posterior capsulotomy is usually a one time procedure.
The image sequence adjacent shows a lens implant before and after a YAG laser capsulotomy. The central visual axis is completely clear of frosted capsule.