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Korea Academy of Occlusion, Orthodontics & Osseointegration.

TOOTH PREPARATION GUIDELINES

Categories: 보철, Date: 2014.04.14 17:43:51

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. 
 


RESTORATION MATERIAL SELECTION 
Gold
This is generally considered to be the most satisfactory extracoronal restorative material. It has a hardness similar to enamel, and occlusal and axial contours can easily be built up in the wax prior to casting. Cast gold alloy restorations include single and multiple surface inlays. The latter may include partial or complete coverage (onlays) of the occlusal surface. Extracoronal gold restorations include full veneer crowns and three-quarter crowns, in which only one surface of the tooth (usually the buccal) is left uncovered . Gold can be used in thin sections but it is not aesthetic. One millimetre of tooth reduction is required occlusally, with the exception of the functional cusp bevel where 1.5 mm is necessary. The choice of restoration and preparation design will depend upon the exact details of each clinical situation.

Indications for use

? In situations of severe occlusal stress
? Following endodontic treatment of posterior teeth 
? Full or partial coverage of posterior teeth where there has been significant loss of coronal dentine 
? In situations where other materials are not suitable for establishing proper proximal and/or occlusal contacts 
? For restoration of adjacent and/or opposing teeth to avoid problems arising from use of dissimilar metals. 
 

Contraindications

? Evidence of active caries/periodontal disease 
? Economic and social factors
? Aesthetics
? Where patient management requires short visits and simple procedures.
Porcelain (ceramic)
This is a brittle material which is liable to fracture in thin section unless appropriate fit surface treatment is performed (etching and silanisation) and the restoration is adhesively luted with a resin-based cement (porcelain veneers and dentine-bonded ceramic crowns). A minimum margin reduction of 0.8 mm is required with 1.5?2.0 mm incisally/occlusally. Crown margins are prepared just below the gingival margin (intracrevicularly) if aesthetics dictates that this is necessary (Fig. 10.12). Adequate retention for non-adhesive ceramic crowns depends on near-parallelism of opposing walls, particularly in the gingival third of the preparation. Porcelain crowns are relatively weak restorations and are restricted to anterior teeth unless a high-strength ceramic (Inceram, Procera, or Empress II) is used.
Indications for use
? Large inadequate restorations on anterior teeth, provided there is enough tooth substance for a strong preparation
? Severely discoloured anterior teeth
? Over an existing post and core substructure.
Contraindications
? Teeth which do not allow ideal preparation form to support the porcelain
? Teeth with short clinical crowns
? Edge-to-edge occlusion
? When opposing teeth occlude on the cervical fifth of the palatal surface.
Porcelain jacket crowns are finished to a shoulder or butt joint margin design unless the preparation is to be bonded (dentine-bonded crowns). All-ceramic crowns arepreferred to metal ceramic crowns on post-crowned teeth where there is a risk of trauma. In this case, the weaker porcelain jacket crown fractures rather than the stressbeing transferred via the post core leading to root fracture.
Indirect composite
Laboratory composites with improved strength and wear resistance are now commercially available and are increasing in popularity. Coupled with improvements in resinbased luting cements and dentine bonding systems, indirect composite restorations (with or without fibre reinforcement) may be considered appropriate for single unit inlays, onlays and crowns (Fig. 10.13).Laboratory composites are generally preferred to porcelain restorations for inlays, whereas the latter offer more permanent form stability in onlay and crown situations. Some prefer a material which is less wear-resistant and as such is sacrificial in nature to a highly wear-resistant ceramic restoration which may ultimately cause excessive wear of the opposing dentition.

Metal ceramic
Metal ceramic crown restorations offer a combination of strength and good aesthetics. Additional tooth preparation (1.5 mm) is required to allow for both the metal substructure and metal overlay. These crowns are frequently overcontoured due to inadequate tooth reduction. Heavy tooth preparation to achieve adequate thickness for both materials may result in an increased incidence of pulp death. If this is a risk then a bevelled shoulder or cervical chamfer may be preferred to the conventional full 1.5 mm axial reduction in cases where the tooth preparation has to be extended down onto root surface or where there is a large pulp. Metal occlusal coverage is generally preferred to maximise retention and resistance form and to minimise tooth reduction. Metal occlusal contacts are easier to create and adjust. Porcelain occlusal surfaces are more aesthetic but demand additional tooth reduction and create the risk of excessive occlusal wear of opposing tooth surfaces.
Indications for use
? Anterior teeth where there is insufficient space for an all-ceramic restoration 
? Repeated failure of porcelain jacket crowns (identify reason first) 
? Posterior crowns where aesthetics is important and full or partial veneer gold crowns are contraindicated on this basis.

Contraindications
? Where excessive wear of teeth opposing porcelain occlusal surfaces may be expected. Either a sacrificial indirect composite approach is preferred or permanent night-time protection with a Michigan splint may be indicated 
? Where pulpal damage risk is high, particularly in a young patient. Dentine-bonded ceramic crowns have provided a more conservative viable option in many of these cases.

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) 
? ceramic/composite 
? 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 
7. Try-in 
8. Cementation or bonding.


TREATMENT ASSESSMENT
Before considering embarking on indirect restorations, patients should be assessed to ensure that their periodontal condition has been stabilised and their caries risk is low.
The restorative assessment of the individual tooth involves:
? sensitivity/vitality tests 
? long cone periapical radiograph
? examination of the quality of any existing restorations
? assessing whether the remaining tooth structure after preparation will have sufficient strength
? assessing the need for crown lengthening prior to treatment
? occlusal considerations.
The occlusal assessment should involve consideration of the tooth position relative to the opposing as well as the adjacent teeth, as this will influence preparation design. If there are occlusal interferences, these may place such a crown under high functional stresses and will require removal at a prior visit.The surfaces of the crown will need to be duplicated so that either the group function or canine guidance occlusion is maintained. The tooth may be a key unit in the arch, i.e. partial denture abutment, and the shape of the surface should be modified to allow the subsequent placement of the denture. In such situations, mounted study casts are a useful aid in planning the preparation design as well as carrying out the occlusal assessment. Any tooth preparation for a crown should follow the appropriate biomechanical principles below , and when planning replacement of a failed indirect restoration, it is important to identify the cause(s) of failure so that this may be corrected at the time of preparation.
Biomechanical principles of tooth preparation
? Preservation of tooth structure and pulp vitality 
? Obtaining adequate retention and resistance form
? Obtaining adequate structural durability of the restoration
? Obtaining adequate marginal integrity
? Preservation of periodontal health
? Appropriate aesthetics
Common causes of failure include:
? poor preparation design/shape resulting in lack of retention and/or resistance form
? insufficient reduction or lack of support/thickness for ceramic or composite
? undercut preparations
? failure to identify and/or correct occlusal problems
? poorly fitting restorations resulting from poor impression procedures or faulty laboratory technique
? inappropriate prescription/planning; no preventive regime
? incorrect shade.
 
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