Porcelain inlays/onlays often sound more complicated than they really are. The bottom line is this: sometimes decay caused by cavities has reached a point where a filling will not be able to fix the issue. This happens when the tooth has deteriorated to the point where there is simply not enough support for a normal composite filling. If Dr. Ottesen finds this to be true for you, she may recommend a porcelain inlay or onlay. Wondering what the difference is between an inlay and onlay? It is all about how much tooth is being covered for restoration and protection. An inlay will be used to gently cover the top of your teeth down to the cusps, while an onlay has the ability to cover more of your tooth extending beyond your cusps or the tops of your teeth.