Perspex ring implants placed in the cornea to flatten and make more regular the shape of a keratoconic cornea. There are two main types: Intacs which sit further out and cause less glare and Ferrara rings.
There are now two diameters for Intacs to cover higher levels of astigmatism than was possible before. Each one has different strengths depending on the corneal curvature.
The Ferrara ring sits closer to the centre and has more effect, but tends to have more side-effects such as glare and light sensitivity.
Both require a certain minimum remaining corneal thickness for safety as well as no significant scarring.
Both are implanted through a tiny incision at the edge of the cornea in a procedure that takes about 15 minutes.
Both can be removed if need be to return the eye to its un-operated state.
Approximately 95% of those implanted are still in place 5 years later; glare and light sensitivity are the main reasons for removal.
Both usually improve vision significantly making contact lenses unnecessary or easier to wear.