ADMIN
The Empire
الرتبة : ادارة الموقع
عدد المساهمات : 1203
Level : 2104
Like : 5
تاريخ الميلاد : 29/05/1988
تاريخ التسجيل : 20/12/2010
العمر : 36
 المزاج : مزاج طبيب أسنان ممارس
 |  موضوع: TOOTH PREPARATION GUIDELINES  26/02/12, 11:20 pm | |
| 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 – shoulderPreparation 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 |
| |
|
