TOOTH PREPARATION GUIDELINES

All preparations should have the maximum height and minimum taper for optimal resistance and retention form consistent with the chemical situation. To achieve this and to permit an adequate thickness of restorative material without over-contour, the surface of the preparation should mimic that of the intended restoration, both occlusally and axially. 

Features of preparations for indirect restorations

Undercut-free preparation there must be one point above the preparation from which all the margins and internal line angles can be seen.

A single path of insertion over as great a distance as possible this is achieved by preparing opposing walls to be near-parallel to give maximum retention. The position of the adjacent teeth should be considered as they may overhang the margins of the prepared tooth. The path of insertion is therefore dictated by the adjacent teeth.

Resistance form needs to be provided by restoration to displacing forces which are usually occlusal in origin.

The opposing walls in the gingival half of the preparation should be made near-parallel.The occlusal third to half will usually be more tapered as a result of the two plains of labial reduction required to provide sufficient room for the restorative material within the original tooth contours.

With short clinical crowns there is an increased risk of failure because of the short insertion path. Preparation length can be increased by crown lengthening, and resistance form may be improved by the use of grooves, slots or boxes and by converting sloping surfaces into vertical and horizontal components.

Occlusal reduction should follow cuspal outline to maximise retention and minimise tooth reduction. For porcelain fused to metal crowns and for gold crowns these distances are 2 and 1 mm, respectively.

The finished margin position and type are determined by the gingival contour, the nature of the restorative material, the presence or absence of a core margin and the choice of luting agent.Whenever possible, the margin should be supragingival following the natural gingival contours. Finish margins should ideally extend at least 1 mm past core margins to rest on sound tooth tissue.

The finished margin position and type are determined by the gingival contour, the nature of the restorative material, the presence or absence of a core margin and the choice of luting agent.Whenever possible, the margin should be supragingival following the natural gingival contours. Finish margins should ideally extend at least 1 mm past core margins to rest on sound tooth tissue.

Types of finish margins


Chamfers and shoulders give definite finish margins which may be identified on preparations, temporary crowns and dies. Occasionally, knife-edge preparations may be indicated for full veneer crowns where there are deeply subgingival margins (however, periodontal surgery may be more appropriate here), bulbous teeth or pins close to the preparation margin. Metal ceramic crowns may be constructed with metal collars, especially on long preparations on posterior teeth. Lipline on smiling may indicate whether this is a practical proposition. A chamfer or knifeedge finish may avoid excessive tooth reduction in this situation. 


(a) Knife edge; (b) bevel; (c) chamfer; (d) shoulder; (e) bevelled shoulder.


Full veneer crown chamfer 
Metal ceramic crown buccal shoulder/palatal chamfer normally 
Porcelain jacket crown shoulder

Preparation Stage

Occlusal reduction using depth grooves as a guide to the amount of tooth reduction. Grooves are only of use when the shape of the restoration is intended to match the original tooth. 

Gross buccal and palatal/lingual axial reduction. The preparation is kept near-parallel cervically and the labial reduction is made to mimic the contour of the final restoration in two, or occasionally three, planes. The preparation is extended as far interproximally as possible without risking contact with adjacent tooth structure. 

Initial interproximal reduction is achieved with a narrow tapered diamond. A sliver of tooth substance/restoration may be left to protect the adjacent tooth at this stage. 

Complete axial reduction can determine final finishing line position. Finish margins at least 1 mm past any existing restorations and just below the gingival margin labially if required for aesthetics. 

INDIRECT ADHESIVE RESTORATIONS  Tooth-coloured inlays

The increasing expectation of patients that restorations be tooth-coloured has led to an increasing interest in direct and indirect composite and ceramic restorations in posterior teeth. Ceramic and composite inlays are generally considered to be appropriate for larger rather than smaller cavities, given that direct placement resin composite restorations may provide good service in small- to medium-sized cavities. 

Examples of tooth-coloured ceramic inlay materials 
Feldspathic porcelain 
Reinforced ceramics (such as Fortress: Chameleon Dental, KS, USA ) 
Pressed ceramics (such as Empress II: Ivoclar-Vivadent, Leichtenstein) 

Computer-aided design and manufacture techniques (CAD-CAM, e.g. Cerec, Siemens , Germany ) are capable of producing increasingly accurately fitting inlays from blocks of ceramic material. These techniques have the distinct advantage of producing the inlay at the chairside in a short time (within 15 minutes), thereby obviating the need for placement of a temporary restoration and a second visit for  placement. 


Comparison between typical inlay cavities for gold and composite

Crown & Bridge basic
Restoration Assessment
Restoration choice
Tooth preparation

 

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