Tooth preparation represents a balance between, conserving tooth structure and pulp health on the one hand, whilst on the other,achieving an aesthetic and strong crown. Guidelines for the amount of tooth reduction for different types of crown have evolved largely as a result of experience rather than scientific evidence. Following tables give an indication of the amount of occlusal and cervical reduction for different types of crowns for posterior and anterior teeth.
| Suggested preparation features for Anterior crowns | ||
| Crown type | Posterior crowns — preparation features | |
| | Occlusal reduction* | Finish line depth and configuration |
| PJC | 2 mm incisally 1 mm lingual aspect | 0.8–1.0 mm shoulder |
| RBPC | 2 mm incisally 0.5–1.0 mm lingual aspect | > 0.4 mm chamfer |
| PFM | 2 mm incisally 0.5–1.0 mm lingual aspect (porcelain guidance requires greater clearance) | 1.2 mm labial shoulder† or heavy chamfer 0.5 mm lingual chamfer |
| *Where the vertical dimension is to be increased, the amount of occlusal reduction required will be less or non-existent † Too deep a reduction for diminutive teeth eg lower incisors or for long clinical crowns where a metal collar is preferable | ||
| Suggested preparation features for posterior crowns | ||
| Crown type | Posterior crowns — preparation features | |
| | Occlusal reduction* | Finish line depth and configuration |
| VMC | 1 mm non-functional cusps 1.5 mm functional cusp | 0–1.0 mm Chamfer, knife-edge, shoulder or shoulder with bevel |
| HSPC | 2 mm non-functional cusps 2.5 mm functional cusps | 0.8-1.0 mm Shoulder or heavy chamfer |
| PFM | As for VMC if metal surface 2 mm non-functional cusps 2.5 mm functional cusps | 1.2 mm labial shoulder† or chamfer 0.5 mm lingual chamfer |
| *Where tooth is tilted or where vertical dimension is to be increased, the amount of occlusal reduction required will vary †Too deep a reduction for diminutive teeth or for long clinical crowns where a metal collar is preferable | ||
It is worth emphasising that there will be times when less tooth should be removed in the interests of conserving tooth structure and pulp health. There are also occasions when little or no tooth reduction is needed, eg from the occlusal surface when the vertical dimension is to be increased, or, from the buccal surface where the tooth is already worn and is to be re-contoured by the crown.
The amount of reduction can be gauged using depth cuts coupled with a knowledge of the appropriate bur end diameter ( for examples of typical bur end diameters). A preparation reduction matrix, formed on a diagnostic wax-up, can be invaluable when the shape of the intended crown differs from that of the original tooth.
Burs used for preparation of full veneer crowns (from left to right): a) Flat-end tapered diamond(occlusal and axial reduction) end Ø = 0.8 mm; b) Long, round-end tapered diamond (as for a. and also shoulder production) end Ø = 1.1 mm; c) Long needle diamond (initial proximal reduction); d) Chamfer diamond (chamfer production) end Ø = 1.0 mm; e) Chamfer tungsten carbide (chamfer and preparation finishing); and f) Large flame or ‘rugby ball’ diamond (lingual concavity production)
The metal margins of VMCs and PFMs can be configured in a variety of ways. It is best to avoid the knife edge as the finish line can be difficult for the technician to detect. Chamfer margins and shoulder with bevel margins allow a fine edge of metal to be cast which, if a suitable alloy is chosen, can be burnished by the technician to improve marginal fit. However, there is always the danger that the die may be damaged by burnishing. Furthermore, the margin should not be too acute. Theoretical arguments have been made in favour of acute margins compensating for seating discrepancy but only before cementation. Once cemented, an acutely bevelled margin may prevent the crown from seating fully, presumably by restricting cement extrusion.
Finish lines with marginal configurations for PFMs: a) Shoulder with porcelain butt fit; b) Deep chamfer with metal collar; c) Shoulder plus chamfer (bevel) with metal collar; d) Knife edge with metal margin; and e) Chamfer with metal margin
As a general rule when using porcelain or PFMs, adequate clearance is required to achieve good aesthetics. Traditionally, this is achieved with a shoulder or heavy chamfer of 0.8–1 mm width for PJCs and 1.2 mm width for PFMs. However, shoulders of these depths may compromise tooth strength and pulp health especially for diminutive teeth such as mandibular incisors. A similar problem occurs on teeth with long clinical crowns because of the narrowing of their diameter in the cervical region. In a long preparation extending onto or beyond the cemento-enamel junction, considerable tooth tissue must be removed to eliminate undercuts. Solutions include using a minimal shoulder, a metal collar on a chamfer or placing the margin at the cemento-enamel junction. Under normal circumstances these options do not compromise aesthetics, being hidden by the lip. Clearly, patients need to understand the benefits of a less destructive preparation and, as stated previously, must be involved in the decision making process pre-operatively.
Teeth with long preparations are prone to pulpal exposure especially if a deep finish line is cut. (a) Teeth with narrow roots are most vulnerable (b) A metal collar reduces the need for a deep finish line
Some operators use what they describe as a mini-shoulder, 0.5–0.7 mm wide, for both ceramic and metal margins. This approach can produce acceptable results for metal margins, although it is easier for the technician to adapt the wax and finish to a chamfer. A much greater problem occurs when ceramics are used as the inevitable consequence is for the crown to be over-bulked resulting in compromised aesthetics and a poor gingival emergence profile.
