What are porcelain inlays / onlays?
Porcelain inlays are custom made ceramic 'fillings.' Because they are made of ceramic they not only look amazing but are incredibly strong. Porcelain onlays are just the same but cover the biting surfaces too, which can add even more protection to a broken down tooth.
Why use inlays / onlays rather than fillings?
Once a filling becomes large, it can actually weaken a broken down or heavily decayed tooth. Porcelain inlays / onlays actually strengthen the tooth as they bond very firmly to the surrounding tooth.
Why use inlays / onlays rather than crowns?
When a tooth is heavily broken down or decayed it is best to try and save as much as possible of what is left. Crowns often require healthy tooth to be removed whilst porcelain inlays / onlays are much more conservative - this is a huge benefit and recent advances in material science make it a highly predictable long term solution.