Preparation for Full Coverage Crowns and Fixed Partial Dentures
Full coverage crowns or abutments for fixed partial dentures (FPD) on natural teeth are probably the penultimate indirect prostheses before considering extractions and implants. Furthermore, before embarking on these highly destructive preparations, alternative adhesive restorations such as onlays or endo-crowns should be considered. Full coverage crowns are also indicated for implant-supported single-unit crowns and FPDs, which minimises the number of implants required for oral rehabilitation.
Crown preparation is usually for 360° coverage, but various configurations are possible such as 1/2, 3/4, 7/8, etc., depending on the remaining tooth substrate. Essentially, tooth preparation for crowns is material-oriented, i.e. the tooth is prepared to accommodate the properties of specific restorative materials such as gold, resin-based composites or ceramics.
The preparation margins, or finish lines can be placed in three locations:
- Supragingival – indicated for low lip lines, posterior crowns, knife-edge finish lines or thin periodontal biotypes. This is an ideal position since it allows both oral hygiene access, and clinical visibility for monitoring. Furthermore, gingival inflammation is minimal, and biologic width integrity is preserved;
- Equigingival – ideal when the underlying tooth colour is acceptable and an all-ceramic restoration is utilised;
- Subgingival – indicated for high lip lines, profound tooth discolouration, masking or altering the unsightly tooth colour, or for all-ceramic crowns with a dense opaque core, e.g. zirconia, to hide the crown/tooth transition. However, subgingival margins should only be considered for thick periodontal biotypes, which have less propensity for postoperative gingival recession, and located within the gingival sulcus to prevent violation of the biologic width. For subgingival carious lesions,