CROWN & BRIDGE
The placement of an indirect restoration requires preparation of a cavity with undercut-free cavity walls to allow a path of withdrawal and insertion of the completed restoration. This allows a pattern or impression to be removed from the cavity. The finished restoration should be capable of insertion into the tooth without the generation of stress. Preservation of remaining tooth structure is important because the restoration relies on the strength and integrity of the remaining prepared tooth substance for retention. The restoration can be used to protect and reinforce the remaining tooth structure to some extent, but the less remaining enamel and dentine, the greater the risk of mechanical or biological failure.
Indirect restorations must be cemented or bonded into place to provide retention and cavity margin seal. The degree of retention available for a non-adhesive indirect restoration depends upon the surface area of the opposing vertical walls of the cavity and their degree of convergence. Only when the restoration is adhesively luted with a resin-based luting cement combined with an enamel/dentine adhesive is the luting agent a major contributor to retention.
Indirect restorations may be:
• intracoronal (inlays)
• extracoronal (crowns)
• a combination of intra- and extracoronal (onlays).
Restorations may be:
• wholly metallic (precious or non-precious alloys)
• a combination of the above (metal-ceramic crown)
Crowns may cover all available surfaces of the tooth (full veneer crowns), or they may be partial veneer (e.g. three-quarter or seven-eights crowns).
The stages in the clinical procedure involved in an indirect restoration are usually as follows:
1. Decision as to restoration type (full or partial coverage; intracoronal or extracoronal), materials and method of luting (conventional cementation or bonding with a resin-based luting material)
2. Discussion with patient before tooth preparation stage as to type of restoration and aesthetic implications
3. Tooth preparation (this may require prior occlusal adjustment or diagnostic wax-up to facilitate production of provisional restoration)
4. Fabrication of temporary/provisional restoration
5. Impressions and occlusal records
6. Shade selection
8. Cementation or bonding.