M1 PFM

PORCELAIN FUSED TO METAL CROWNS¹

Principles of Tooth Preparation

Tooth Preparation - Steps²

PART I³

Tooth Preparation for indirect restorations

Clinical process of mechanically altering a tooth to receive a custom-made restoration fabricated outside the mouth.

Contrast with Direct Restorations

Unlike direct fillings (e.g., amalgam), where the cavity must have retentive features like undercuts, preparations for indirect restorations must be free of undercuts to allow the restoration to be seated. Retention is achieved through different mechanical principles.

• Crown prep.
• Bridge prep.
• Inlay/onlay prep.
• Veneer prep.
• Post space prep.

Objective of Tooth Preparation⁴

To optimize the shape and form of a tooth that will receive an indirect restoration:

• Create room for the selected restorative material
• Remove diseased and/or weakened tooth structure

No Remaining Decay

Unlike some direct filling techniques, decay cannot be left near the pulp for crowns due to the cost and complexity of future intervention if symptoms arise.

• Provide retention and resistance to the restoration
• Allow a proper fit, function, aesthetics and longevity
• Facilitate subsequent procedures (provisional restoration, impression, cast pouring, final restoration fabrication)

Tip

A well-defined preparation is crucial, as deep or rough margins can make the impression-taking process difficult and unpredictable.

• Preserve healthy tooth structure

Indications for Tooth Preparation⁵

1) Therapeutic preparations:

Restoration of teeth that have lost tooth structure by caries and non-caries lesions: attrition, abrasion, erosion, abfraction; or fractures.

• ==Attrition: Wear from tooth-to-tooth contact, often seen in patients who grind (bruxism).==
• ==Abrasion: Mechanical wear from foreign objects (e.g., aggressive toothbrushing).==
• ==Erosion: Chemical dissolution of tooth structure from acids.==
• ==Abfraction: Wedge-shaped cervical lesions with sharp internal angles, caused by tooth flexure under occlusal load.==

2) Preparations of healthy teeth:

Preparation of abutments, supports of RPR, occlusal correction, aesthetics.

Aesthetic Indications

An example of preparing a healthy tooth for aesthetics is to mask discoloration (e.g., after root canal treatment) or to alter its shape for a better appearance.

Principles of Tooth Preparation⁶⁷

Principles of Tooth Preparation

Abstract

The preparation of a tooth for any indirect restoration is governed by three interdependent principles. An ideal preparation successfully balances the requirements of all three.

• Biological factors
  • Affect the health of oral tissues
• Mechanical factors
  • Determine the integrity and durability of the restoration
• Aesthetic factors
  • Determine the appearance of the final restoration

Principles of Tooth Preparation⁶⁷

Biological Factors

• Prevention of tooth damage during preparation
• Preservation of tooth structure
• Avoidance of overcontouring the restoration
• Supragingival margins
• Marginal integrity
• Harmonious occlusion
• Protection of remaining tooth structures

Prevention of Tooth Damage During Preparation⁸

1. Proximal teeth
   • Can lead to caries and gingivitis
2. Soft tissues
   • Gingival tissues, tongue & cheeks
3. Pulp
   • Thermal irritation: use copious irrigation

Avoid Overheating

Overheating can occur from inadequate water irrigation, especially with high-speed handpieces (400,000 RPM) or old/finishing burs. Cutting a tooth dry is unacceptable.

- **Chemical irritation:** bases, resins, etch, solvents & luting agent (4-8% of pulps die in the 10 years following crown preparations)

Info

This irritation can happen if the prepared dentin is exposed to various chemicals when the tooth is not properly protected with a provisional restoration.

- **Bacterial irritation:** either left behind or gained access due to microleakage

Info

Bacterial irritation often occurs due to microleakage under a poorly sealed restoration, which can lead to pulp symptoms over time.

Preservation of Tooth Structures⁹¹⁰

• Tooth structures loss due to material selection
• Metal coverage alone requires less reduction

Gold crown

PFM

• Preparation with minimal taper
• Occlusal reduction following the anatomical contour

Tip

This not only preserves more tooth structure but also keeps the preparation further from the pulp.

• Uniform axial reduction
• Conservative margins
• Avoid unnecessary apical extension of the preparation

Avoidance of Over contouring the Restoration¹¹¹²

Cause of Over-Contouring

If the tooth preparation does not provide enough space, the dental technician will be forced to make the restoration bulky or over-contoured to achieve the minimum required material thickness.

• Favors plaque build ups – gingivitis, periodontitis
• Ideal contours of the restoration should follow the natural contour of the tooth and surrounding tissue (emergence profile)

Supragingival Margins¹³¹⁴

Ideal Approach

Placing the restoration margin above the gumline is the ideal approach whenever possible.

• Easily prepared
• Easier to take impressions
• Easily maintained by patient
• Easily evaluated at recalls
• Preserve periodontium

Subgingival Margins¹⁵

Biological Width Violation

The margin can be placed within the gingival sulcus (up to 0.7 mm) but must not violate the biological width (the combined height of the junctional epithelium and connective tissue attachment), as this will cause chronic inflammation.

Indications

• Subgingival dental caries, erosion, fracture or restoration

• Apical proximal contact area

• Additional retention is needed

• Aesthetic margins

• Coverage of root surface

• Modification of axial contour

• To improve the emergence profile of a crown when closing a diastema (gap).

Marginal Integrity¹⁶

• Potential site for caries:
  • Dissolution of cement
  • Inherent roughness leading to plaque accumulation
• Should be even and smooth:
  • Facilitates subsequent steps till the delivery of the restoration

Healthy periodontium:¹⁷

• Mean sulcular depth 0.69 mm
• Mean junctional epithelial length 0.97 mm
• Mean connective tissue attachment length 1.07mm
• JE + CT attachment is known as “biological width”

If you place a margin within the biological width, increase the chance of gingival inflammation. Followed by bone loss as the biological width tries to re-establish itself.

Follow the Gingival Contour

The preparation margin should follow the natural scalloped contour of the gingiva, rising over the interdental papilla to avoid placing it too deep proximally.

Follow the gingival and papillary contour to avoid violation of biological width.

Margin/finishing lines¹⁸¹⁹

Design requirements

• Simplicity of preparation
• On intact tooth surface
• No unsupported enamel
• Ease of identification
• Allows sufficient bulk of restoration
  • Durability
  • Aesthetics
• Conservative

Possible Designs

1. Feather edge
2. Bevel
3. Chamfer
4. Shoulder
5. Shoulder with bevel

1) Feather edge²⁰

• Advantages:

  • Conservation of tooth structure

• Disadvantages:

  • Fail to provide adequate bulk at margins

• ==It is not recommended as it is difficult to identify and often leads to over-contoured restorations.==

2) Bevel²¹

• Advantages:

  • Allow the margin of crown to be burnishing against tooth structure
  • Protect the unprepared tooth structure from chipping by remove unsupported enamel

• Disadvantages:

  • Lead to subgingival extension

• It is not suitable for all-ceramic crowns as it results in a thin, fragile ceramic edge.

3) Shoulder²²

• Margin for all materials
• Easy to prepare and finish well
• Easy to judge impressions
• Frequently used for metal crowns, metal ceramic crowns and all ceramic crowns

Traditional vs. Modern Shoulder

The traditional shoulder has a sharp internal line angle suitable for cast restorations. For modern CAD/CAM restorations, a rounded internal line angle is required to allow the milling bur to fabricate it accurately.

Modified Shoulder CAD-CAM all ceramic

4) Chamfer²³

• Can be used margin for all metal and all ceramic restorations
• Preserves tooth tissue whilst providing distinct finish line
• Easy to read in impressions

Gutter Margin

Be careful not to sink the bur too deep, as this can create a ‘gutter margin’ with unsupported enamel.

5) Shoulder with bevel²⁴²⁵

Harmonious Occlusion

• Confirmative or reorganized approach

• ==Conformative Approach: The restoration is designed to fit into the patient’s existing occlusion without changing it. This is the standard approach in the dental school clinic.==

• ==Reorganized Approach: The entire occlusion is rebuilt, usually in complex cases of severe wear.==

• Decide upon a satisfactory occlusal scheme prior to tooth preparation

• Especially important with overerupted & tilted teeth

  • Eventual occlusal plane adjustment
  • Possibility of elective endodontic treatment
  • Possibility of crown lengthening surgery

• Diagnostic wax-ups can be extremely helpful

Pre-operative Assessment

Occlusion must be assessed before starting the preparation to identify contact points and guidance patterns (e.g., canine guidance, group function).

Protection of Remaining Tooth Structures²⁶

• Ensure all weakened tooth structure is removed prior to core placement

CUSPAL COVERAGE²⁷

Occlusal contacts on the margin of the preparation

CUSPAL COVERAGE²⁷

Supporting vs. Non-Supporting Cusps

Supporting cusps (palatal on uppers, buccal on lowers) bear the primary occlusal load and require more reduction than non-supporting cusps.

CUSPAL COVERAGE²⁷

Info

Covering the cusps with the restoration (as in an onlay or full crown) distributes occlusal forces over the entire tooth surface, preventing individual cusps from flexing and fracturing.

Endodontically treated tooth ↓ Weakness of cusp

Principles of Tooth Preparation⁶⁷

BIOLOGICAL

• Conservation of tooth structure
• Avoidance of overcontouring
• Supragingival Margins
• Harmonious Occlusion
• Protection against tooth fracture

MECHANICAL

• Retention form
• Resistance form
• Deformation

ESTHETIC

• Minimum display of metal
• Maximum thickness of porcelain
• Porcelain occlusal surfaces
• Subgingival margins

Optimal situation

SF Rosenstiel, MF Land, R Walter (2022) Contemporary fixed prosthodontics

Mechanical Factors²⁸

Retention form

• Preparation feature which resists displacement occlusally

Info

This means it prevents the restoration from being dislodged along its path of insertion (i.e., resists pulling forces).

• Avoid relying on cements alone to retain the restoration
• Purpose: placing the cement shear instead of tension
• Retention form depends on:
  • Preparation height
  • Degree of taper
  • Freedom of displacement
  • Roughness of the surfaces
  • Physical properties of luting cement

Geometry of Tooth Preparation²⁹³⁰

• Enhance the cement function

• Cement:

  • Fills the crown-tooth space and creates friction
  • Prevents surface from sliding

• ==Height: Taller preparations are more retentive than shorter ones.==

• ==Width: For a given height, narrower preparations (e.g., incisors) are more retentive than wider ones (e.g., molars).==

• Parallels: Best retention, but difficult to fit

Warning

Parallel walls are clinically impractical, as they make it impossible to fully seat the crown without binding.

• Overtapered: unsatisfactory retention

The axial walls must taper to:

• Permit more complete seating of the restoration
• Prevent undercuts
• Visualize preparation walls and finish line
• Compensate for inaccuracies during fabrication

Degree of taper³¹³²

• ideal taper 6°
• Clinically 6–20° is acceptable

Tip

Short preparations require minimal taper (closer to parallel) to maximize retention.

• Tapered burs are designed to cut a 3° taper on each wall and should be hold parallel to the long axis of the tooth

• One eye vision

• All the margins should be clearly visible

  • In the mouth by mirror
  • On the cast (distance of 30 cm)

Preparation Surface³³³⁴

1. Surface area
   • The greater the surface area, the greater the retention
   • Molars are more retentive than premolars with similar degree of taper
2. Surface roughness enhances retention

Info

A rougher prepared surface provides more microscopic retention for the luting cement.

Resistance form – Stability³⁵³⁶

• Preparation features which resist displacement in all directions other than occlusal (horizontal and oblique forces)

Important

Resistance form is considered more critical than retention form because chewing forces are primarily oblique, not vertical.

• Depends on:

  • Magnitude and direction of dislodging forces
  • Geometry of the tooth preparation:
    • Height
    • Degree of taper
  • Cementation material
  • Resistance to deformation

• Increasing the proportion of axial surfaces: More surface area interfering with dislodgement:

  • Reduced taper
  • Increased preparation length

• Higher preparations show more stability

Mechanism of Resistance

A tall preparation with minimal taper will resist rotational forces, whereas a short, over-tapered preparation will offer almost no resistance, allowing the crown to be easily dislodged.

• Less resistance with partial coverage

Tooth with smaller diameter resists more to rotating movements than preparation with equal height, but greater diameter.

Short perps - Molars require more parallel surfaces than premolars or anterior teeth

Consider additional features

When to Use Additional Features

For short clinical crowns, retention grooves or boxes can be added to enhance resistance form.

Pin holes Boxes grooves

All in natural tooth structure

Resistance to Deformation (Structural Durability)^37383940

Depends on:

• Depth of preparation (space for restoration)

• Type of prep margin

• Type of restorative material

• Sufficient strength to prevent permanent deformation

• Failure at cement-restoration interface or metal-ceramic interface

May result from:

• Inappropriate material selection
• Inadequate preparation
• Poor restoration design

Adequate Tooth Reduction⁴¹

• Sufficient reduction to accommodate the restoration material

Avoid Flat Reduction

A flat occlusal reduction creates thin spots over the cusps and thick spots in the fossae, leading to stress concentration and a higher risk of fracture. The reduction must follow the occlusal anatomy to create a uniform material thickness.

• Controlled by diagnostic procedures
• Occlusal position (MIP) and excursive movements (Lateroprotrusion)

Margin Design⁴²

• Enough restoration bulk to resist distortion
• Avoid occlusal contact on the margin of partial coverage restoration
• Avoid feather edge
• boxes enhance the rigidity

Aesthetics Considerations⁴³

Natural looking restoration depends on:

• Facial tooth reduction
• Incisal reduction
• Proximal reduction
• Material selection
• Substratum color

Aesthetic Considerations

• Maximal Ceramic Thickness: For aesthetic crowns, the preparation must allow for sufficient thickness of porcelain to mask the underlying tooth or metal coping and achieve a natural appearance.
• Minimal Metal Display: In PFM crowns, the design aims to hide the metal collar at the margin.

Labial margin placement⁴⁴⁴⁵

Info

For anterior teeth, the labial margin is often placed slightly subgingivally for superior aesthetics, hiding the transition from crown to tooth. In non-aesthetic areas (e.g., posterior teeth), supragingival margins are preferred.

• Consider intracrevicular placement: especially for high smile line
• Inform the patient of the anticipated margin location
• Healthy periodontal tissues is mandatory
• Sufficient time after periodontal surgery

For each case, select the ideal combination of tooth preparation principles Some principles are non-negotiable

Footnotes

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