Three-quarter (posterior) Crown Restorations : Restorative Dentistry Lecture Note

Indications

·         For posterior teeth as a single restoration for teeth that have lost moderate amount of tooth structure with intact buccal wall or as retainer for posterior FPD

Contraindications

·         Teeth that have short clinical crown because the retention may not be adequate.

·         Extensivlely damaged teeth.

·         as retainer for long span ridges.

·         Patients with active caries or periodontal disease.

·         For endontically treated teeth, because insufficient tooth structure remain

·         Thin teeth of restricted labiolingual dimension

Advantages

·         Conservative because it requires less sound tooth structure reduction

·         Less opportunities for periodontal trauma because less restoration margins r in close proximity to gingival

·         Reduced pulpal and periodontal insult during preparation

·         Magins in accessible area to the dentist for finishing and to patient for cleaning

·         Good seating for the restoration coz the luting agent can escape more easily

·         The uncovered labial or buccal surface permits vitality testing

Disadvantages

·         less retention and resistance than complete coverage crowns

·         more difficult preparation

·         some metal is displayed in the restoration

Steps

·         occlusal reduction

·         functional cusp bevel

·         lingual reduction

·         proximal reduction

·         proximal grooves

·         proximal flares

·         occlusal offset

·         buccal cusp bevel

Occlusal reduction (posterior) or incisal reduction (anterior)

·         Depth grooves done on the occlusal surfaces with round-end tapered diamond bur, the reduction then completed by reducing the tooth structure remaining between the depth grooves

In lower teeth:

·         1-1.5mm reduction from buccal cusps

·         2- 1mm reduction from lingual cusps

In upper teeth:

·         1-1.5 mm reduction from lingual cusps

·          2- 1mm reduction from buccal cusps

Functional cusp bevel

·          Done with round end tapered only on maxillary teeth .the bevel extends on the central groove from the mesial to the distal .the difference between maxillary and mandibular teeth that is the buccal cusps of mandibular teeth are the functional cusps so the location of the occlusal finish line on the buccal surface will be different. It will be 1mm gingival to the occlusal contact this result in metal coverage .The occlusal shoulder in buccal aspect of the buccal cusp served as the same purpose of the offset in maxillary teeth so there is no need for an offset in mandibular preparation.

Axial reduction

Lingual reduction:

·         Eliminate the undercut cervical to the height of contour done with round end tapered.

·         -Mesiodistally follow the contour of the tooth

·         -Occlusocervically parallel to the path of insertion with slight occlusal convergence

Proximal reduction:

·         Started by gaining proximal axes with thin tapered diamond then followed by round end tapered to complete the reduction and form chamfer finish line

·         Final extension to buccal surface by thin tapered diamond .the proximal surfaces must be parallel to each other and path of insertion with slight occlusal convergence

Others

Proximal grooves:

·         It must be placed on the buccal half on the proximal surface at the line between the buccal and middle third of proximal surfaces. This increases the bulk of tooth structure lingual to the grooves to resist lingual displacing force.

·         -The I shape groove is the satisfactory form because the lingual form of the groove is prepared to be at right angle to the proximal surface preventing the restoration to be displaced lingually

·         -The box like form is indicated in case of proximal caries or when is made to receive the female portion of precision attachment

·          -The V shaped groove is rarely used due to its reduced retentive quality but it is mainly used for anterior three quarter preparation.

·         Proximal flare:

·          The buccal wall of the groove is then flared buccally by a flame diamond to remove any unsupported tooth structure. The flare should be extended far enough to be reached by the explorer but not so far as to cause display of metal

Occlusal offset:

·         Done with no.171 bur .the offset is a 1mm wide ledge on the lingual incline of the buccal cusp. It connects the groove and plays a major role in the casting rigidity by tying together the proximal grooves. With end cutting bur smooth the offset insuring that it will be a flat ledge.

Buccal cusp bevel:

·          Place a narrow occlusal finish bevel along the occlusobuccal line angle with a flame diamond about 45 degree relative to buccal surface .the bevel should be no more than 0.5mm wide.

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