All Ceramic Crown Preparation
Crowns are an example of indirect restorations, which are recommended to restore and strengthen weakened tooth structure. All-ceramic crowns offer the potential for superior esthetics in comparison to porcelain-fused-to-metal (PFM) crowns, although the latter may also result in a very good esthetic outcome. All-ceramic crowns can be made from numerous materials, but lithium disilicate and zirconia are amongst the most popular ones, due to the combination of excellent esthetics and very desirable mechanical properties. Here we will present an anterior tooth preparation (#11) to receive a monolithic lithium disilicate crown.
The materials needed are:
- Scalpel blade and handle
- Silicone putty impression material
- Metal matrix band
- Wooden wedges
- High-speed handpiece
- Coarse-grit flat-end tapered diamond bur
- Coarse-grit football-shaped diamond bur
- Fine-grit tapered flat-end (torpedo) diamond bur
- Small diameter round end (or long needle) diamond bur
- End-cutting diamond bur
The preparation of an anterior tooth for a lithium disilicate crown should follow this sequence of steps
- Make a putty index prior to tooth preparation which will serve as a guide for the amount of tooth reduction. For this purpose, a silicone putty is properly mixed, and an impression is taken of the tooth to be prepared. This should include approximately two teeth mesial and distal to the tooth that will be prepared.
- Protect the tooth adjacent to the proximal wall that you will be preparing (tooth #21) with a metal matrix band stabilized with a wooden wedge. Under high-speed and water-cooling, slice the enamel off the mesial interproximal contact of tooth #11 using the small diameter round end diamond bur. Keep the tip of the bur away from the gingival tissue.
- After protecting tooth #12 as previously described, slice the enamel off the distal interproximal contact of tooth #11 using the same small diameter diamond bur.
- For the incisal reduction, make 2 or 3 depth orientation grooves (approximately 1-1.2 mm deep) using a flat-end tapered diamond bur. Using the same bur, complete incisal reduction by connecting the grooves until the incisal edge is smooth and in an angle of approximately 45 degrees towards the lingual aspect of the tooth.
- The labial reduction is initiated with the tapered diamond bur. Make three depth orientation grooves in two different planes: the first in the gingival aspect parallel to the long axis of the tooth, and the second in the incisal aspect, slightly inclined towards the incisal plane of the tooth. The initial depth of these grooves should be approximately 0.5 mm. Gradually connect the grooves to complete the labial reduction and ensure that the labial surface is smooth. This includes rounding off all line angles, including the incisal-labial, proximal-incisal and proximal-labial angles.
- For the lingual reduction, start with the same flat-end tapered diamond bur in the gingival third of the tooth. Make three depth orientation grooves approximately 0.5-0.7 mm deep, parallel to the tooth long axis. Connect the grooves while establishing the shoulder finishing line around the gingival margin. Continue moving the bur towards the proximal aspect of the tooth to connect the lingual surface to the proximal walls keeping the depth consistent. Then, with the football shaped bur reduce the lingual concavity (incisal edge to the cingulum) to the same depth as the gingival aspect, approximately 0.5-0.7mm. To avoid over reducing, a good strategy is to reduce one half of the incisal portion, and then reduce the second half following the same depth.
- To finish off the preparation, use the fine grit tapered bur to smooth and blend the totality of the crown preparation, rounding off any remaining sharp line angles.
- The circumferential shoulder margin is defined and finalized with the flat-end tapered diamond bur and the end cutting diamond bur. The labial aspect should be at the gingival margin, while the lingual aspect may be 0.5 mm supragingival.
The final dimensions of the preparation vary according to the restorative material to be used, either lithium-disilicate or zirconia. For lithium disilicate, the overall reduction is 1.0 mm with an incisal reduction of 1.5 mm and a 1 mm deep shoulder finishing line. For zirconia crowns, incisal reduction stays the same (1.5 mm), and overall reduction is approximately 0.7-0.8 mm. A bilayered zirconia crown will require more reduction. Consult the preparation quickguide and flowchart in the restorative manual for the recommended amount of reduction for all crown preparations.
Dr. Greg Anderson & Dr. Joel Rosenbloom