Principles of Tooth Preparation: #30 Zirconia Crown Preparation

Resistance depends on:

1. Magnitude and direction of the dislodging forces. 2. Geometry of the tooth preparation. 3. Physical properties of the luting agent.

Retention depends on:

1. Magnitude of the dislodging forces 2. Geometry of the tooth preparation 3. Roughness of the fitting surface of the restoration 4. Materials being cemented 5. Film thickness of the luting agent (cement).

Esthetic Considerations

1. Minimumal display of metal 2. Maximum thickness of porcelain 3. Porcelain occlusal surfaces 4. Subgingival margins

Mechanical Considerations

1. Retention form 2. Resistance form 3. Preventing deformation

Occlusal considerations

1. Supra-erupted or tilted teeth 2. Diagnostic wax 3. Diagnostic tooth preparation 4. Endo treatment 5. Ortho treatment

Indications for Placement of a Crown

1. Tooth with extensive destruction by caries or trauma 2. When maximum retention and resistance needed 3. Correct the axial contour 4. Be used to support a RPD 5. Endodontically treated teeth 6. Correction of occlusal planes

How can we prevent damage to adjacent teeth during tooth preparation?

1. Use a skinnier bur (use 12, 14, 16, 18 when prepping #30) 2. Use fender wedges to separate teeth.

Precise control of the orientation of the diamond bur is very important. Explain what happens when the orientation is not correct.

A. Tilting away from the tooth creates an undercut, thus, opposing axial preparation walls diverge in an occlusal direction. B. Tilting toward the tooth results in an excessive convergence angle of the preparation.

Retention: luting agent

Best to worst: Adhesive resin --> glass ionomer --> polycarboxalate and zinc phosphate --> zinc oxide-eugenol

Visual survey

Counting by eye ensures preparation is neither undercut or over tapered

Retention: film thickness

Effect uncertain

_______ is integral part of occlusal reduction.

Functional cusp bevel

Retention: surface area

Greater retention: Large Lesser retention: Small

Retention: Type of preparation

Greater retention: Molar complete crown Good retention: Premolar complete crown Medium retention: Partial crown Lesser retention: Intracoronal restoration

Retention: taper

Greater retention: parallel Medium retention: 6 degrees Lesser retention: Excessive

Retention: surface texture

Greater retention: rough Lesser retention: smooth

BruxZir® Solid Zirconia crown & bridge restorations exceed the _______ for posterior ceramic bridges.

ISO 6872 flexural strength specification of 800 Mpa

How should a mouth mirror be held in a patient's mouth to evaluate taper?

In patient's mouth, mouth mirror is held at an angle approximately 1/2 inch above the preparation. Image is viewed with one eye.

Inadequate axial reduction may produce several problems:

Inadequate axial reduction will have thin walls subject to distortion or result in over contouring the axial surface which could lead to periodontal problems.

Tooth Preparation Guideline for Monolithic Zirconia Crown

Minimum occlusal reduction of 0.5mm, 1.0 is ideal Feather edge margin is acceptable. However, it is difficult to identify and train the eye and the lab may send back a bulkier crown contour.

T/F: If there is already some clearance with the opposing tooth because of malposition, oriental grooves should not be made as deep.

True.

T/F: Retention and resistance are interrelated and often inseparable qualities.

True.

T/F: A complete crown has the best retention, but it can be associated with periodontal disease and poor esthetics.

True. r

Such additional reduction (secondary reduction) is often unnecessary for ________, because their relatively straight or slightly lingually inclined profile allows fabrication of restoration that has good anatomic form and meets minimal material thickness requirements.

mandibular molars

Functional cusp bevel may extend to include ______ of preparation height.

occlusal third

Insufficient axial reduction --> ____________ --> Plaque control around margin --> ______________

over contoured restoration; periodontal disease

The path of insertion should coincide with ________, which for a mandibular molar is typically inclined ________ degrees lingually.

the long axis of the tooth; 9-14

Failure to place functional cusp bevel can produce ______ or ______.

thin casting; poor morphology

#30 Zirconia Crown Preparation Guidelines

-0.5-1.0 mm chamfer finish line for zirconia crown -10-20 degree of total occlusal convergence -Axial modification to increase resistance if needed -Uniform 1.0-1.5mm occlusal clearance (to ensure you do not under or over prep the tooth) -Functional cusp bevel -Follow normal anatomic planes of occlusal surface

Margin Adaptation

-A rough, irregular margin makes the fabrication of an accurately fitted restoration almost impossible. An accurately fitted margin is possible only if it is prepared smoothly. -Accurately adapted margin leads to less recurrent caries and less periodontal disease. -Goals: 1. Ease of preparation w/o overextension or unsupported enamel 2. Ease identification in impression & die 3. Ease finishing wax pattern (digital design) 4. Sufficient bulk of material 5. Conservation of tooth structure

Axial Wall Reduction (buccal/lingual)

-Buccal and lingual walls are reduced with the round-ended, tapered diamond -As these alignment grooves are placed, verify that the shank of the diamond is parallel to the proposed path of insertion of the restoration. Such positioning automatically produces a convergence between the axial walls of the alignment grooves that is identical to the taper of the diamond.

Inadequate functional cusp bevel may produce several problems:

-Can cause a thin area or perforation. -May result in over contouring and poor occlusion -Over inclination of the buccal surface will destroy excessive tooth structure reducing retention

BruxZir Solid Zirconia

-Full-contour solid zirconia crown or bridge restoration with no porcelain overlay. -Designed and milled using CAD/CAM technology -Available in all the classic shades, then staining and glazing for a final shade match and characteristics.

Bilayered Zirconia Crown (Porcelain Fused to Zirconia) Characteristics

-Mostly for the anterior teeth -Better esthetics -Composed of zirconium oxide milled coping and feldspathic porcelain -Require a lot more restoration and prepping

What is the purpose of the functional cusp bevel?

-Provide adequate bulk of an area of heavy occlusal contact -Further reduce the cuspal height by 0.5 mm on the functional cusp to provide adequate thickness of the restorative material on the area of significant functional loading. Place the round-end tapered bur at 45 degrees to the long axis of the tooth at the line angle created between the second plane of the buccal surface and occlusal surface.

The ratio of the occlusocervical/incisocervical dimension to the faciolingual dimension (OC/IC:FL) should be at least ______ or higher for all teeth.

0.4

For milled restorations, in most esthetic materials, minimal thickness requirements approaching ______ mm apply.

1

Indications for Solid Zirconia

1. Anterior and posterior crowns or bridges 2. Anywhere a full-cast or metal occlusal restoration would be indicated

Considerations Affecting Future Dental Health (6)

1. Axial reduction 2. Margin placement (ideally is supragingival margin except for anterior) 3. Margin adaptation 4. Margin geometry 5. Occlusal considerations 6. Preventing tooth fracture

Biological Considerations

1. Conservation of tooth structure 2. Avoidance of overcontouring 3. Supragingival margins 4. Harmonious occlusion 5. Protection against tooth fracture 6. Considerations affecting future dental health

The angle of convergence formed between 2 opposing prepared axial surfaces (total occlusal convergence) should range between ______ degrees.

10 and 20

Accurately adapted margin: metal margin

10 um (0.01 mm)

Center of the occlusal surface of the preparation is viewed with one eye from a distance of _______.

30 cm (12 inches)

The minimal occlusocervical/incisocervical dimension of incisors and premolars should be _____ mm, and the minimal occlusocervical dimension of molars should be _____ mm.

3; 4

On most posterior teeth, the functional cusp bevel is placed at an angle of approximately ______ degrees to the long axis of the prepared tooth.

45

Marginal gap ranging from _______ um are considered clinically acceptable in terms of longevity.

50 to 120

Accurately adapted margin: porcelain

50 um (0.05 mm)

What is the recommended convergence angle?

6 degrees

The path of insertion should coincide with the long axis of the tooth, which for a mandibular molar is typically inclined _______ degrees lingually. Preparing such a tooth with a path of insertion that is perpendicular to the occlusal plane of the mandibular arch is a common clinical error that results in additional unnecessary removal of tooth structure

9 to 14

How wide is the chamfer margin?

A chamfer margin should not be wider than half the rotary instrument used to form it. Otherwise, a "lip" of unsupported enamel will be left. Rule of thumb: only use half of the 169 tapered diamond bur

Pros and cons of a complete crown:

PRO: -Provides best protection against fracture -Has the best retention CON: -Can be associated with periodontal disease -Poor esthetics

Axial Reduction Purpose

Plays an important role in securing space for an adequate thickness of the restorative material

How can we ensure we have the same path of insertion in a FPD prep?

Read slide. Key: do not move mirror out of mouth to see second prep. Guide mirror from one prep to the next for comparison.

Resistance

Resistance prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces. Mastication and parafunctional activity may subject a prosthesis to substantial horizontal or oblique forces. Rotation is prevented by any areas of the tooth preparation that are placed in compression, called resistance areas. Resistance is a function of the relationship between axial wall taper, preparation diameter, and preparation height. It decreases as taper or diameter increases or as preparation height is reduced. The relationship between preparation height, or diameter, and resistance to displacement is approximately linear. A fairly simple way to quantify this at chairside is to evaluate whether the height-to-width ratio of a preparation is 4 : 10 or greater. If so, resistance is probably adequate.

Retention

Retention prevents removal of the restoration along the path of insertion or long axis of the tooth preparation. Certain forces (e.g., when the jaws are moved apart after biting on very sticky food) act on a cemented restoration in the same direction as the path of placement. The quality of a preparation that prevents the restoration from becoming dislodged by such forces parallel to the path of placement is known as retention. Only dental caries and porcelain failure outrank lack of retention as a cause of failure of crowns and fixed dental prostheses.

What is the most suitable instrument for making a chamfer margin?

Tapered diamond with a rounded tip **the resulting margin is the exact image of the instrument

Second plane reduction

The configuration of the facial wall of the maxillary molars may necessitate slight additional reduction in the occlusal half to prevent overcontouring of the restoration. This reduction is referred to as a second plane reduction.

What are the goals of the proximal reduction?

The goals of proximal reduction are to eliminate proximal contact with the adjacent teeth. **Initial proximal cuts can be made with a smaller diameter round-end tapered diamond bur.

Margin Geometry

Types: 1. Feathered edge (very difficult for lab to tell where is your finish margin; in order to make it stronger, lab would have to bulk out your crown and that will create over contour) i. PRO- conservation of tooth structure ii. CON- does not provide sufficient bulk iii. INDICATIONS- not recommended 2. Chamfer (most common) i. PRO- Distinct margin, adequate bulk, easier to control ii. CON- care needed to avoid unsupported lip of enamel iii. INDICATIONS- cast metal restorations, lingual margin of metal-ceramic crowns 3. Shoulder (not common anymore due to using zirconia crown) i. PRO- Bulk of restorative material ii. CON- Less conservative of tooth structure iii. INDICATION- facial margin of metal-ceramic crowns, complete ceramic crowns

Why is binocular vision avoided?

Undercut preparation can appear to have an acceptable taper when using both eyes instead of one.

When is a shoulder margin indicated?

When esthetic restorations are planned to achieve sufficient material thickness to achieve a lifelike appearance; however, it is much less conservative than a chamfer. In the past when we used weaker material like all ceramic crowns, we needed to prep more for the material so a shoulder margin was popular in the past. Nowadays, we use a lot of zirconia type of crowns and this material is relatively stronger so a chamber margin is more acceptable.

The occlusal reduction should follow the ...?

anatomic configuration of the occlusal surface **if you were to just cut straight across, there is not enough retention and you might be too close to the pulp, thus, causing pulp sensitivity.

When the above criteria for total occlusal convergence and occlusocervical/incisocervical dimension cannot be satisfied, _______ should be added to the tooth preparation that increase resistance form.

auxiliary features

Tooth preparation should be a balance of ______, ______, and ______ qualities.

biologic, esthetic, mechanical

Occlusal clearance can be checked with ________ or _______.

clearance tabs; utility wax

A _______ provides the best protection against fracture.

complete crown

For effective retention, a tooth preparation must constrain the movement of a restoration; therefore, it must be _______.

cylindrical A preparation is cylindrical if the two horizontal cross sections of the prepared axial tooth surface ( 1 and 2 ) are coincident.

Adequate clearance of ______mm exists between the external surface of the proximal chamfer margin and the adjacent tooth.

≥ 0.6 mm