This is mainly related to how old the patients were when they first underwent cross-linking. In simple terms, the younger the patient is at the first cross-linking, the higher the likelihood that they will need a second treatment after 7−8 years.
The cornea is completely rebuilt every 7−8 years – a fact that has only recently come to light. What this means is that no original cross-links are left after this time. If the patients were still very young at the time of the first cross-linking, they are still young enough after 7−8 years for keratoconus to re-emerge.
The following video explains the connections.