Preparation for Inlays and Onlays
Conventional teaching stipulates that tooth preparation be material-orientated, i.e. geometric, to accommodate the properties of the restorative material. Conversely, contemporary techniques dispel this destructive overpreparation in favour of disease- or defect-orientated preparations. Modern restorative materials are capable of restoring teeth that have smaller or minimally invasive preparations, and therefore allow a larger amount of natural dental tissues to be retained.
Indirect inlays and onlays are indicated for extensive cavities in premolars and molars, due to rampant caries, or to replace defective existing restorations. An inlay or onlay preparation conserves more of the natural tooth, removing less tooth substance (6–27%) compared to a full-coverage crown (68–76%).
An inlay is an indirect intracoronal restoration that does not support or replace cusps of a tooth. Furthermore, all occlusal contacts in centric, protrusive and lateral excursions are guided by cusps of the natural tooth.
An onlay is an indirect restoration that incorporates, supports and replaces one or more tooth cusps. The functional occlusion is supported by the restorative material that covers the involved cusps. An onlay can incorporate an inlay preparation or be restricted to the occlusal surface to replace an eroded occlusal table, or to raise the occlusal vertical dimension (OVD).
Various cavity configurations of onlays and veneers are possible; for example, a veneerlay restoration that combines an onlay and veneer preparation.
Guidelines for Inlay Preparation
The conventional guidelines for inlay preparations are:
- Taper – the occlusal and cervical boxes should have 6° tapers for adequate path of insertion/withdrawal;