FULL VENEER CROWNS

FULL VENEER CROWNS
Full veneer crowns on posterior teeth may be used either as restorations for single teeth or as bridge retainers, the types of full veneer crowns are: complete cast crown, metal crown and all ceramic crown.
INDICATIONS:
1- A tooth which has been attacked by caries to such an extent that amalgam or inlay restoration cannot be retained satisfactorily may often be restored with a pin amalgam core under a full crown.
2- Full crowns are particularly useful for teeth with extensive caries of the buccal and lingual surfaces.
3- For teeth with large amalgam fillings which are showing signs of flow or small fractures.
4- Many otherwise unsavable teeth may be restored to function by a full veneer crown.
5- Full crowns may be placed on abutment teeth where a partial denture is planned. This will ensure that these teeth do not develop caries associated with the clasps of the denture. Crowns may also be designed to aid retention of a denture by guiding its path of insertion or accepting occlusal rests or clasp arms. As a bridge retainer the full veneer crown has certain advantages over other forms of retainers since it offers very good retention. Where appearance is not important therefore, it is preferred to a three-quarter crown as it has a shorter periphery and protects all surfaces of the tooth from caries. If a patient wishes to avoid showing metal buccally it is possible to include a facing of acrylic or porcelain. Allowance must then be made during preparation of the tooth for the greater thickness of material that will cover the buccal surface.
CONTRA-INDICATIONS:
There are no absolute contra-indications to full veneer crowns, but the following points should be noted:-
1- Retention can be difficult to obtain in teeth with very short or tapering crowns. In such cases pins or posts may be used to improve retention.
2- Large pulps in the teeth of young patients are easily damaged and crown preparations in these teeth should be approached with caution.
3- Tilted teeth may be difficult to prepare for crowns.
4- Unless the patient’s oral hygiene is satisfactory advanced conservative procedures are not indicated.
CLINICAL ASSESSMENT OF THE TOOTH:
Before deciding on the construction of a full veneer crown the following factors must be considered:-
1- The vitality of the tooth should be normal as far as can be detected by clinical means.
2- If the pulp is not healthy a satisfactory root treatment should be completed before proceeding further.
3- The extent of caries must be determined. It is important to decide if the pulp may be in any danger of exposure, and how far gingivally the caries has extended.
4- The position of the pulp should be assessed in relation to the amount of tooth substance to be removed. This is especially important if the preparation is for a bridge retainer on a tilted tooth, or if pins are to be used to aid retention. For this purpose a good radiograph is required.
5- The shape of the crown must be examined to assess its retentive potential.
6- The periodontal condition, alveolar support, and the occlusal stress must be considered when planning any crown. The gingivae should be treated, if necessary, and the periodontal condition made satisfactory before the preparation is started.
7- If bone support is poor, selective grinding of opposing teeth may be required to ensure that the restored tooth is not subjected to excessive lateral stress.
8- The occlusal contour of the crown should then be made relatively flat.
PREPARATION FOR FULL VENEER CROWNS:
1- ANTERIOR PORCELAIN FUSED TO METAL CROWN PREPARATION:
Depth orientation grooves:
1-A flat end tapered diamond is first aligned with the incisal portion of the facial surface and two vertical cuts are made to the full diameter of the diamond fading out at the “break” where the curvature of the facial is the greatest.
2- Three similar grooves are made maintaining the same instrument parallel to the gingival segment of the facial surface.
3-Two incisal orientation grooves 2.0 mm deep are made.

Incisal reduction:
1-A flat end tapered diamond is used to reduce the incisal edge by 2.0 mm keeping the plane of the reduced surface parallel to the former incisal edge.
2-A round wheel diamond may also be used for incisal reduction.


Facial reduction, incisal half:
1-A flat end tapered diamond is used to remove the tooth structure remaining between the orientation groove in the incisal portion of the facial surface.


Facial reduction, gingival half:
1-A flat end tapered diamond is used to reduce the gingival segment and extend well into the proximal surface. 1.2 mm to 1.4 mm is the accepted reduction for a porcelain fused to metal restoration.
2-If there is sound tooth structure interproximaly a vertical wall or “wing” of it, is left standing in each interproximal area lingual to the proximal contact.


Lingual reduction:
1-A small round diamond with a head 1.4 mm in diameter is used to make four depth orientation cuts.
2-A small round wheel diamond is used create a concave surface over the lingual surface of the tooth incisal to the cingulum.


Lingual axial reduction:
1-Is done with a torpedo diamond producing a definite chamfer finish line at the same time.
2-If there is limited space between the facio proximal angle of the wing and the proximal surface of the adjacent tooth, use a long needle diamond to reduce the axial wall lingual to the wing.
3-The lingual axial wall should be parallel with the cervical one third of the facial surface.


Facial axial finishing:
1- It s important to smooth the entire facial surface and round over any sharp angles on the incisal angle or along the edges.




2-POSTERIOR PORCELAIN FUSED TO METAL CROWN PREPARATION:
Stage (1): Proximal Reduction:
Proximal surfaces may be reduced using a fine-tapered bur in the air-turbine hand piece. The type without abrasive on the tip, is particularly convenient as it lessens the risk of damage to the gingival margins.
As an alternative, a safe-sided diamond disk may he used to prepare the mesial and distal surfaces. The use of this type of diamond disk often requires the prior removal of tight contacts with flexible steel abrasive strips.


This figure can see the reduction of the proximal surfaces of a tooth as described in Stage (1) of the preparation for a full veneer crown.
A disk-guard to protect the soft tissues is always advised unless its use seriously restricts access to the tooth. The disk is held so that the portion will converge between 2° and 5° towards the occlusal surface. Reduction is continued until all undercut is removed inter-proximally and the prepared surfaces extend to the gingival crest and converge a maximum of 5° towards the occlusal surface.
Stage (2):- Buccal and Lingual Surface Reduction:-
A tapered fissure bur is used for this stage:
1- The objects of this are to remove all undercut from these surfaces and sufficient tooth substance to allow coverage with approximately 1 mm. of metal, except at the gingival margin.
2- It is not essential to remove all the enamel from these surfaces.
3- The reduction should be continued to the gingival crest.
4- If disks are used there will tend to be sharp angles where the buccal and lingual cut surfaces meet the proximal portion.


The upper figures show the diamond bur is used to reduce buccal and lingual surfaces in Stage 2. All undercuts must be removed from these surfaces. B, The angles between the axial walls are rounded
5- In these areas there may still remain small triangular undercut areas towards the gingival margin. The undercuts must be eliminated and the sharp angles rounded, so that the proximal portions blend smoothly into the buccal and lingual surfaces of the preparation.
Stage (3), Occlusal Surface Reduction:-
1- The occlusal surface reduction approximately 1.5mm on functional cusps and 1mm on nonfunctional cusps to get enough space for metal to cover the occlusal surface.
2- A cylindrical diamond may be used in the air turbine or a wheel shaped diamond at conventional speed.
3- The general shape of the prepared surface should be similar to that of the tooth itself,
4- It may be necessary to remove a greater amount of tissue in the fissures than elsewhere to ensure that all free of caries.



5- The occlusal surface is reduced to allow coverage with gold.
6- Functional cusp bevel is done using round end tapered diamondDepth orientation grooves are placed across the facial occlusal line angle of the mandibular molar. The bevel should parallel the inward facing inclines of the cusps of the opposing tooth, at a depth of 1.5 mm usually forming a 45° angle with the axial wall.
7- The gingival finishing line may be defined with a diamond bur.
8- Deepening the fissures also increases resistance of the crown to displacement. This should be done with a tapered fissure tungsten- carbide or diamond bur.
9- After the occlusal surface has been reduced it is important that the occlusion should be checked in all positions of the mandible, to ensure that sufficient tissue has been removed. It is often helpful to get the patient to bite on a sheet of pink wax which will reveal points of inadequate clearance.


Various burs designed to give a 135° cavo-surface angle.
Stage 4, Finishing the Preparation:
1- The junction of occlusal and axial surfaces should be rounded slightly, but excessive reduction in this area will lead to some loss of retention.
2- The axial walls should be re-checked at this stage.
3- The gingival finishing line checked for definition. The clarity of this finishing line is essential to enable the technician to form a wax pattern with a clearly defined margin and thus ensure a perfect marginal seal and contour of the completed crown.
4- A plain-cut tungsten-carbide bur may be used for finishing the margin.
5- The cavo-surface angle must be a minimum of 135°. A dome-ended bur or a Tinker bur or hand instruments such as the Blacks No. 77/78 may be of help to establish this.



ADDITIONAL RETENTION:
If teeth with short or tapering clinical crowns are to be prepared for full veneer crowns it may be decided that some further retention is required in addition to that gained in the standard preparation. The most usual ways of gaining additional retention are:-
1. The use of pins or posts in the occlusal. Pinholes may be prepared as for pin-retained inlays incorporated in the finished casting. Alternative, post holes may be prepared in the occlusal surface and the posts cast as part of the crown. The position of the pulp should be determined accurately before the retention holes are cut.
2. The placing of grooves in the proximal surfaces, grooves may also be made in the buccal and lingual surfaces.
3. Seating groove is made on the axial surface. The groove should be cut to the full diameter and it should extend gingivally to a point 0.5 mm above the chamfer.