Principles that govern tooth preparation for cast metal and porcelain restoration-10/3/2016

Lect. 1- 3th stage Crown & Bridge Dr. Ameer H.AL-Ameedee

Each time a tooth is restored with a ?lling, some of the tooth tissue is removed. Eventually, this compromises the strength of the remaining tooth and it may begin to fracture under normal occlusal forces. This especially occurs when teeth have been root treated, so it is usual for heavily ?lled and root ?lled teeth to be crowned before fracture occurs.
In other cases, a tooth may be poorly shaped and require elective crowning to be more aesthetically pleasing. Similarly, a tooth may be too poorly shaped to assist in the retention of, say, a denture, but can be made so by elective crowning.

A crown and bridge is a fixed restoration used to replace one or a few missing teeth in a dental arch, although in advanced cases multiple bridges can be fitted to provide full mouth rehabilitation. Various designs of bridges are available, and that used in each case has to be determined by the dentist on its merits.
For patients with only a few missing teeth and a low biting force in the area of the bridge, a minimal amount of tooth preparation can be carried out and an acid etch retained bridge placed. Where occlusal forces are likely to dislodge this type of restoration, a more conventional design is used, where the adjacent teeth are prepared as for crowns and the missing teeth incorporated into the whole structure. Crowns and bridges are usually constructed of porcelain bonded to a metallic substructure, although some modern, all ceramic materials are also available.

A crown is a restoration which encompasses coronal tooth tissue, covering remaining tooth substance and restorations. When insuf?cient tooth tissue remains, the root canal can be used to aid retention – a post crown.

GENERAL OBJECTIVES OF TOOTH PREPARATION
1. Remove all defects and provide necessary protection to the pulp.
2. Extend the restoration as conservatively as possible.
3. Form the tooth preparation so that under the force of mastication the tooth or the restoration or both will not fracture and the restoration will not be displaced.
4. Allow for the esthetic and functional placement of a restorative material.

Five Principles that govern tooth preparation for cast metal and porcelain restoration:
1. preservation of tooth structure
2. retention and resistance
3. structural durability of the restoration
4. marginal integrity
5. preservation of the periodontium

Preservation of Tooth Structure
1. Entails more than simply avoiding excessive destruction.
2. Requires designing the restoration so that it will reinforce and protect the remaining enamel and dentin.

Effects of excessive removal of tooth structure:
1. overtapered or overshortened tooth will compromise retention and resistance
2. thermal hypersensitivity
3. pulpal inflammation
4. necrosis

Retention and Resistance:
1. A restoration that does not remain firmly attached to the tooth cannot meet it’s functional, biological and esthetic requirements.
2. The capability for retention and resistance must be great enough to withstand the dislodging forces it will encounter during function.

Factors that influence Retention and Resistance:
1. Geometric form –determines the orientation of the tooth restoration interface to the direction of force encountered.
2. Cements –exhibits their greatest strength under compression. They are weakest under tension. Dental cements holds mainly through the mechanical interlocking of projections of cement into small irregularities of the surface being joined.

Factors under the control of the operator during tooth preparation which influence retention:
1. degree of taper
2. total surface area of the cement film
3. area of cement under shear
4. roughness of the tooth surface






The direction of the force acting upon a segment of restoration determines the type of stress to which the cement film is subjected. A resultant force directed away from the tooth produces tensile stress (A), while a force parallel with the interface produces shear(B). A force directed at an angle toward the tooth will produce a combination of shear and compression (C) When the force is perpendicular to the tooth, compression results (D).


Taper and Retention:
1. The more nearly parallel the opposing walls of the preparation, the greater will be the retention, however in order to avoid undercuts and allow complete seating of the restoration during cementation, the walls must have taper.
2. Three degrees produced on each surface , external or internal by the sides of a tapered instrument, the result would be an over-all taper of six degrees.


As the degree of taper of a preparation increases, its ability to retain a restoration decreases.






To produce an optimal 6-degree taper or convergence angle, each opposing axial wall should have an inclination of 3 degrees to the path of insertion.




A crown (A) depends primarily on external retention to resist removal. External retention is provided by approximation of the restoration to the opposing external axial walls of the preparation (arrow B).





Retention is enhanced by restricting the possible paths of withdrawal or paths of insertion. The excessively tapered truncated cone has an infinite number of paths along which a crown could be withdrawn (A). The addition of parallel-sided grooves (B) limits the path of withdrawal to one direction, thereby reducing the possibility of dislodgement.


Surface area of the cement film:
Total surface area of the preparation is influenced by;
1. size of the tooth
2. extent of coverage by the restoration
3. retentive features such as boxes and grooves that are incorporated in the preparation.
The greater the surface area of the preparation, the greater the retention and resistance.


All other factors remaining constant, the greater the surface area of cement film, the greater the retention. Therefore, a restoration on a long preparation (A) can withstand a force that could remove a restoration from a shorter preparation of equal diameter (B). Doubling the height of a preparation would nearly double the area of its axial walls.


Area under shear:
For shear strength of the cement to be utilized:
1. The preparation must have opposing walls.
2. Two surfaces of the preparation in separate planes must be nearly parallel with each other and the line of draw.
3. The opposing surfaces maybe internal, as the facial and lingual walls of the proximal box of an inlay preparation.
4. External, such as the axial walls of a full veneer crown preparation.



When the line of action of an applied force passes within the margins of the restoration, no secondary lifting forces are produced (A). When the line of action passes outside the margins of the restoration, a torque is produced that will tend to tip or rotate the crown around point on the margin (B).





Decreasing the length of a tapered preparation causes a disproportionate decrease in the resisting area. A crown with relatively long axial walls can resist a strong tipping force (A). Although preparation B still has more than half the length of preparation A, it has less than half the resisting area. Its crown will fail under a force that would be easily resisted by the linger preparation (B).



A short restoration on a short preparation is less likely to fail through tipping than is a long restoration on the same preparation. The resistance of this preparation is adequate to prevent the crown form tipping under the applied force (A). Although the preparation and the applied force in (B) are identical to those in (A), the crown fails because of the greater height of the restoration.



The length of the primary lever arm is the shortest distance from the line of action of the force to the nearest margin. On the short crown the lifting force is small because the primary lever arm is short (A). With a long crown (B), the same force produces a greater torque because its line of action passes farther from the point of rotation.



The resisting area decreases as the preparation taper increases. For a cylinder with no taper, the resisting area would cover half the axial walls (A). For an ideally-tapered tooth preparation the resulting area covers somewhat less than half the axial walls (B). An over-tapered ( 20 degrees) preparation has only a small resisting area near the occlusal surface (C).

Surface roughness:
* Adhesion of dental cements depends primarily on projections of the cement into microscopic irregularities and recesses on the surface being joined, the prepared tooth should not be highly polished.

Path of insertion:
1. Before any tooth structure is cut, the path of insertion should be decided first keeping in mind the principles governing tooth reduction. This is especially important when preparing bridge abutments, because multiple path of insertion must be parallel.
2. Path must be selected that will allow the margins of the retainer to fit against their respective preparation.






The ideal path of insertion for a posterior full or partial veneer crown is parallel with the long axis of the tooth.






The path of insertion for a full veneer crown on a posterior tooth in normal alignment parallels the long axis of the tooth (A). A tipped tooth must be handled differently (B). If the path of insertion on a tipped tooth parallels the long axis, the crown will be prevented from seating by those parts of the adjacent teeth which protrude into the path of insertion (C). The correct path of insertion for such a tooth is perpendicular to the occlusal (D).





A tooth has migrated into the space formed by a long-standing carious lesion (A). A vertical path of insertion will not permit seating without removal of excess ice amounts of tooth structure from the proximal surface of the adjacent tooth (B). The problem can be solved less destructively by inclining the path of insertion slightly to the mesial, and removing small amounts of enamel from both adjacent teeth (C). More severe collapse may require orthodontic uprighting to regain needed space.


When viewed with one eye from a distance of 30 cm, all the axial surfaces of a preparation with an ideal taper or angle of convergence of 6 degrees can be seen.


Binocular vision should never be employed to evaluate a preparation for correct taper. With both eyes open, a preparation that is undercut can appear to have an acceptable degree of taper.




A mirror is used to evaluate a preparation where direct vision is not possible. An unobstructed view of the entire finish line barely outside the circumference of the occlusal surface indicates correct taper.


Occlusal Reduction:
Enough tooth structure must be removed from the occlusal surface of the preparation so that when the restoration is built back to ideal occlusion it will be thick enough to prevent wearing through or distorting.



The interocclusal space over the mesial cusps of a tipped tooth may be sufficient for a crown preparation without any reduction. Uniform occlusal reduction in such a case would produce excessive occlusal clearance and an unnecessarilt shortened mesial axial wall (A). Only enough tooth structure to provide necessary space for the restoration should be removed (B). Some of the original occlusal surface may not need to be cut at all.




Factors influencing variation in amount of occlusal reduction:
1. Type of restorative material used.
2. Amount of occlusal reduction is not always the same as the clearance needed.
3. Occlusal reduction should reflect the geometric inclined planes underlying the morphology of the finished crown and follow the major planes of the opposing facial and lingual cusp.

Functional cusp bevel:
1. As part of the occlusal reduction, a wide bevel should be placed on the functional cusp of the posterior teeth to provide structural durability in this critical area.
2. It also rounds over the occlusal line angle, which can be an area of high stress concentration.
3. Angle on the non-functional cusp is rounded over lightly.





A functional cusp bevel in a plane paralleling that of the opposing cusp allows for adequate restoration thickness without undue sacrifice of tooth structure (A). if the functional cusp bevel is omitted, the restoration is likely to be too thin in this stress-bearing area (B). If restoration thickness is achieved by over tapering the axial wall, retention will be compromised (C). Frequently, in the absence of a functional cusp bevel, the technician will over bulk the crown (D). This can result in super occlusion of the restoration, which could only be corrected by occlusal reduction of the opposing tooth.



When teeth are in a cross bite relationship, the functional cusp bevel is placed on the buccal cusps of maxillary teeth and the lingual cusps of mandibular teeth.


Axial Reduction:
1. When axial reduction is sufficient, restoration walls can have satisfactory thickness without over contouring.
2. When axial reduction is inadequate, a restoration with normal contours would be thin and flexible and would be difficult for the technician to fabricate, invest and cast without distortion.


Adequate axial reduction creates space for a strong bulk of metal within the normal contours of the tooth (A). Inadequate axial reduction can result in a crown with thin, weak walls (B). More probably, a restoration with bulky, plaque-promoting contours will result (C).


Marginal Integrity:
Three requirements for a successful restoration margins;
1. they must fit as closely as possible against the finish line of the preparation to minimize the width of exposed cement.
2. they must have sufficient strength to withstand the forces of mastication.
3. whenever possible, they should be located in areas where the dentist can finish and inspect them, and the patient can clean them as well.

Margin Placement:
1.Supragingival margins-located above the gingival margin.
2.Subgingival margins-located below the gingival margin.
3.Equigingival margins-located on the level as the gingival margin.

Advantages of supra gingival margins:
1.They can be easily finished.
2.They are more easily cleaned.
3.Impression are easily made , less potential for soft tissue damage.
4.Restorations can be easily evaluated at recall appointments.

Justification for use of subgingival margins:
1.Dental caries, cervical erosion, or restoration extended sub-gingivally and a crown lengthening procedure is not indicated.
2.The proximal area contact extends to the gingival crest.
3.Additional retention is needed.
4.The margin of a metal ceramic crown is to be hidden behind the labio-gingival crest.
5.Root sensitivity cannot be controlled by more conservative procedure.
6.Modification of the axial contour is indicated.

Types of margin designs:
1.Featheredge or knife-edge.
2.Chamfer.
3.Shoulder.
4.Shoulder with a bevel.






Featheredge
Chamfer
Shoulder
Shoulder with a bevel


MARGIN DESIGN
ADVANTAGE
DISADVANTAGES
INDICATION
FEATHER EDGE
Conservative of tooth structure
Does not provide sufficient bulk
Porcelain laminate veneer
CHAMFER
Distinct margin, sufficient bulk, easy to control
Care needed to avoid unsupported lip of enamel
Cast metal restoration, lingual margin of metal ceramic crown
SHOULDER
Bulk of restorative material
Less conservative of tooth structure
Facial margin of metal ceramic crown, complete ceramic crown
SHOULDER WITH BEVEL
Bulk of material, advantages of bevel
Less conservative
facial margin of metal ceramic crown with supra gingival margins

TYPES OF CROWNS:
Full coverage: Full veneer crowns (usually made of gold for posterior teeth), porcelain jacket crowns (anterior teeth), metal ceramic crowns.
Post crowns: Cast gold core, prefabricated core.
Partial coverage: Three-quarter crowns and reverse three quarter crowns.


ASSESSMENT OF TEETH FOR CROWNS:
Case selection is important. In order to plan treatment appropriately, when considering crowns, assess periodontal support and gingival condition:
1. caries control.
2. Occlusion.
3. radiographic appearance.
4. tooth vitality.
5. oral hygiene.
6. aesthetics(including patient’s expectations).
7. adjacent teeth.
In some cases study casts, clinical photographs and a diagnostic wax-up of anticipated appearance may be useful.






CLINICAL STAGES IN MAKING CROWNS:
1. PREPARATION
Crown preparation involves:
1. Removal of enough tooth substance allowing suf?cient thickness of material (from which the crown is to be made) to provide strength and aesthetics.
2. Preparation must not damage the pulp.
3. Preparation must provide suf?cient retention for the crown. This can be achieved by taper of 5–200 (especially in cervical third of preparation), and inclusion of retention grooves or slots is useful in teeth of reduced occlusogingival height.
4. Preparation should involve minimal gingival trauma.
5. Preparation should have smooth curves, not right angles or sharp edges.
6. Finishing lines depend on the material from which the crown is to be made.

Options for ?nishing lines:
1. Butt joint: e.g. porcelain jacket crown.
2. Chamfer: e.g. palatal margin metal ceramic crown.
3. Taper: e.g. full veneer gold crown.
Preparation is usually achieved by a selection of high-speed diamond burs.

2. TEMPORISATION
Prepared teeth require temporization for aesthetics, pulpal protection and prevention of overeruption or drifting of opposing
or neighboring teeth.

Types of temporary crowns:
* Anterior teeth: polycarbonate preformed crowns, polyethylmethacrylate crowns, fabricated using an initial irreversible
Hydrocolloid impression.
* Posterior teeth: stainless steel, polycarbonate or polyethylmethacrylate.

Usually temporary crowns are cemented with temporary cement. Occasionally a more permanent luting cement may be used when the temporary crowns are to be worn for a prolonged period or preparations are of reduced occluso-gingival height. Heat-cured acrylic temporary crowns may be used if temporization is for a prolonged period.

3. IMPRESSIONS.
4. PRESCRIPTION TO TECHNICIANS.
5. CEMENTING A CROWN

ANTERIOR CROWNS:
Indications: Protection of heavily restored teeth, aesthetics, bridge retainer, tooth wear.

POSTERIOR CROWNS:
Indications: Aesthetics (some posterior teeth only), bridge retainer, tooth wear, partial denture abutments, protection of heavily restored teeth.







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