To determine the preparation form for most restorations three general rules of thumb are utilized. At this point the cusps are sufficiently weakened from loss of dentine support of the enamel that cusp fracture is imminent. A restoration that extends past the height of contour of a cusp is classified as a crown. If the restoration does not reach the height of contour it is an onlay. A filling is a direct restoration incrementally placed into a prepared cavity in a tooth to restore lost tooth structure.
Fillings can extend into all five tooth surfaces and can cover lost cusps and extend subgingivally.
An inlay is fabricated outside the mouth after an impression has been taken. The restoration is then cemented or bonded into the tooth. An inlay can extend to all surfaces of the tooth. An onlay is fabricated outside the mouth after an impression has been taken. It is then cemented or bonded into the tooth.