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F5 Principles of tooth prep

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Lecture 1 – Principles of Tooth Preparations1

Why does a tooth need a crown?2

As part of DMD1 and DMD2 (Semester 1) you have learnt about:

  • Composite restorations
  • Amalgam restorations

So why do we need crowns?

Considerations for a crown34

  • Destruction of tooth structure
    • This is indicated when there is extensive loss of tooth structure due to:
      • Large carious lesions.
      • Fracture of cusps, often in teeth with existing large restorations (e.g., MOD amalgams). A crown protects the remaining weakened tooth structure from further fracture under occlusal loads.
      • Endodontically treated teeth, especially posterior teeth, which are brittle and have lost structure from the access cavity. Full occlusal coverage is crucial to prevent catastrophic fracture.
      • According to Terry E. Donovan (2006) in Fundamental of Fixed Prosthodontics, a study comparing resistance to removal forces greatest with an all metal crown in comparison to onlays).
  • Aesthetics
    • Crowns, such as PFM crowns, can be used for full mouth rehabilitation in cases of severely worn dentition or to address issues like tetracycline staining.
      • To mask severe discoloration like tetracycline staining, a thin ceramic veneer or crown can provide a superior aesthetic result compared to bulky composite additions.
      • To restore teeth in cases of severe tooth wear, crowns offer better mechanical properties and retention than direct composites for rebuilding incisal edges worn down by bruxism.
  • Plaque control/moisture control
    • Some studies report a lower annual failure rate in indirect restorations (ceramic restorations 1.9%) compared to direct (composite 2.2%, amalgam 3%) restorations.
    • The predominant mode of failure of ceramic restorations is bulk fracture.

    • The predominant mode of failure for composite restorations were reported to be marginal breakdown (secondary caries or deterioration) or fracture.

Clinical considerations

  • In areas with difficult moisture control (e.g., deep subgingival margins), an indirect restoration made from a well-recorded impression may offer a better marginal seal and adaptation than a direct composite.
  • Prerequisite: The patient must demonstrate good oral hygiene. If a patient has poor plaque control, placing an expensive crown is risky, as secondary caries can form at the margins. It may be better to place a large direct restoration and monitor the patient’s hygiene commitment.
  • Retention
  • Finance

Info

The patient’s ability to afford treatment is a significant factor, but it does not change the clinical need for the crown.

  • Other prosthodontic treatment (survey crowns for RPDs)

Clinical procedures56789

graph TD
    subgraph "1. Diagnosis and Prep"
        A[Clinical examination] --> B[Primary Impression]
        
         This is the branching point for digital vs. analog
        C --> D[Definitive Impression]
        C --> E[Intraoral scanning]
        
         Analog Workflow
        D --> H["Wax up (gold crown, PFM)"]
        H --> I[Casting and veneering]
    end

    subgraph "3. Delivery"
         Styling to match the circles in the image
    style B fill:#fff,stroke:#ff0000,stroke-width:3px
    style J fill:#fff,stroke:#e6b400,stroke-width:3px

Slide by Dr Matsubara

graph TD
     --- Conventional Branch ---
    subgraph "Conventional"
        D --> E[Casting]
        E --> F[Manual finishing]
        F --> G["Veneering/Polishing"]
        G --> H[Insert]
    end

     --- Styling for Red Arrows ---
    linkStyle 1 stroke:#ff0000,stroke-width:2px
    linkStyle 3 stroke:#ff0000,stroke-width:2px
    linkStyle 6 stroke:#ff0000,stroke-width:2px

Slide by Dr Matsubara

Workflow101112

Understanding the overall workflow helps contextualize the principles of tooth preparation.

  1. Examination & Treatment Planning:
    • Includes clinical exams, pulp testing, radiographs, and diagnostic impressions to create mounted study models on an articulator.
    • This phase determines the prognosis and confirms the need for a crown.
  2. Tooth Preparation & Temporization:
    • The tooth is reduced according to the principles for the chosen material.
    • A temporary crown is placed to protect the prepared tooth, prevent sensitivity, maintain occlusal and proximal contacts, and provide aesthetics.
  3. Impression / Digital Scan:
    • Conventional: A definitive impression is taken using a material like PVS. The lab then pours a model.
    • Digital: An intraoral scanner (e.g., TRIOS) is used to create a digital model directly, or the conventional impression/model is scanned by the lab.
  4. Laboratory Fabrication:
    • Conventional Workflow: A lab technician waxes up the crown by hand, casts it (for metal/PFM), applies porcelain, and finishes it.
    • Digital (CAD/CAM) Workflow: The crown is designed on a computer (Computer-Aided Design) and then milled from a block of ceramic or other material (Computer-Aided Manufacturing). It is then sintered, finished, and glazed.
  5. Try-in and Evaluation:
    • The temporary crown is removed, and the definitive crown is tried in.
    • Key aspects are evaluated: interproximal contacts, occlusal contacts, marginal fit, shade, and shape.
  6. Cementation:
    • If the try-in is successful, the crown is permanently cemented. Excess cement must be meticulously removed.
  7. Review: The patient should be reviewed to ensure proper function and health of the surrounding tissues.

Explain biological width and its implications in crown preparation

Types of Crowns13

Materials

  • All ceramic
    • Zirconia or Glassy Ceramics (eg Lithium Silicate or Lithium Disilicate)
      • ==Zirconia: Very strong, opaque.==
        • ==Glassy Ceramics: More aesthetic, includes lithium disilicate and feldspathic porcelain.==
  • Metal
    • Varying alloys
  • Porcelain Fused to Metal (PFM)

Margins14

  • Supra-gingival of subgingival
  • Types of margins
    • Feather edge
    • Bevel
    • Chamfer
    • Shoulder
    • Shoulder with bevel

Subgingival Margins

Indications

  • Subgingival dental caries, erosion, fracture line or restoration
  • Additional retention is needed
  • Aesthetic margins
  • Coverage of root surface
  • Modification of axial contour
    • ==To improve the emergence profile and avoid a bulky contour.==

Supragingival Margins

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

Types of Margins1516

Feather edge

  • Advantages:

    • Conservation of tooth structure

  • Disadvantages:

    • Fail to provide adequate bulk at margins

  • The finish line is indistinct and difficult for the lab to read and fabricate to.

  • CONTRA-INDICATED IN MOST CASES!

Indication

Generally contraindicated for crowns; may be used for some feldspathic veneers.

Bevel17

  • Advantages:
    • Allow the margin of the crown to be burnished against tooth structure
    • Protect the unprepared tooth structure from chipping by removing unsupported enamel
    • May reduce marginal discrepancy when complete crown fails to seat completely. (But doesn’t work for oversized crown)
  • Disadvantages:
    • Can lead to subgingival extension
  • Commonly for cast restorations

Shoulder18

  • Advantages:

    • Easy to prepare and finish well
    • Easy to judge on impressions

  • Disadvantages:

    • More aggressive preparation

  • Indication: ==Primarily for all-ceramic crowns.==

  • Can be used for all materials.

Info

It is prepared with a flat-end bur, resulting in a rounded internal line angle.

Chamfer19

  • Advantages:
    • Conserves tooth structure whilst providing distinct finish lines
    • Easy to read in impressions

Info

This is the most common margin type.

  • Commonly used for metal, or ceramic crowns
  • Use a tapered diamond bur with round tip!

Shoulder with bevel20

  • Sometimes used for PFM crowns with a metal labial margin (hidden in the sulcus). But a shoulder/chamber is preferred for biologic and aesthetic considerations

Historical Use

It was historically used for PFM crowns where a metal collar at the margin was hidden subgingivally for a better seal. It is not preferred today due to poor aesthetics and potential violation of biologic width.

  • Uncommonly used.

Principles of Tooth Preparations21

The principles of tooth preparation for an abutment tooth (partial or complete preparation) can be categorized as follows:

The Three Pillars

A successful preparation balances three key principles:

  • BIOLOGICAL: Preserving the health of the tooth and surrounding tissues.
  • MECHANICAL: Ensuring the restoration is stable, retentive, and durable.
  • AESTHETIC: Achieving a natural and pleasing appearance.

BIOLOGICAL

  • Conservation of tooth structure
  • Avoidance of overcontouring
  • Supragingival margins
  • Harmonious occlusion
  • Protection against tooth fracture

MECHANICAL

  • Retention form
  • Resistance form
  • Deformation

AESTHETIC

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

Biological Principles22

Conservation of tooth structure23

  • According to a study by Edelhoff and Ahlers (2018), approximately 70% of the volume of the clinical crown of a posterior tooth is removed for a full-crown preparation, whereas only about 30% is lost for an occlusal onlay.

Clinical Best Practices

  • Excessive removal brings the preparation closer to the pulp, increasing the risk of thermal, chemical, or bacterial damage leading to pulpal complications.
  • Always use sufficient water spray with a high-speed handpiece to prevent thermal damage.
  • Consider more conservative options when possible, such as partial coverage restorations (onlays, 3/4 crowns) or materials requiring less reduction (e.g., metal crowns).

Avoidance of overcontouring242526

  • Patient can’t clean this!

Emergence Profile

The emergence profile (the contour of the tooth or crown as it emerges from the gingiva) must be correct.

  • Over-contoured crowns, often a result of under-preparation, create a ledge that traps plaque and leads to gingival inflammation.
  • Under-contoured crowns can lead to food impaction and direct trauma to the gingiva.

Supragingival margins272829

We risk violation of biologic width!

Biologic Width Violation

Placing a restoration margin within the ~2 mm space of the biologic width causes chronic inflammation, attachment loss, pocket formation, and bone resorption as the body tries to re-establish the space.

Assessment (Bone Sounding)

Under local anesthetic, a periodontal probe is pushed through the tissue to the bone crest. Biologic width = (Probe-to-bone measurement) - (Sulcus depth). A value less than 2 mm indicates a potential problem.

Slide by Dr Matsubara

Biologic Width30

  • Connective tissue: 1.07 mm
  • Junctional epithelium: 0.97 mm
  • Gingival sulcus: 0.5-1 mm
  • Biologic Width: 2 - 2.25 mm

Margin location313233343536

  • If the margins has to be subgingival it should only be placed intracrevicularly

  • Maximum depth around 0.7mm Slide by Dr Matsubara

  • Take Home Message: If you place a margin within 2mm of the crestal bone you will get inflammation, followed by bone loss as the biological width tries to re-establish itself.

Marginal Integrity

Margins must be smooth and continuous. Rough or open margins lead to plaque accumulation, cement washout, secondary caries, and restoration failure.

  • The margins of the restoration can be positioned 0.5 mm below the gingival tissue crest if the probing depth of the sulcus is 1.5 mm or less. If the sulcus depth is more than 1.5mm, then the margins of the restoration should be inserted in the sulcus at a depth that is half its probing depth. Lastly, a gingivectomy may be enacted to extend the tooth and design a 1.5 mm sulcus if the probing depth of the sulcus is greater than 2 mm.

Patient Categories based on Crestal Bone Height

  • Normal Crest (85% of patients): Biologic width is stable. A margin 0.5 mm subgingivally is usually well-tolerated if the sulcus is shallow (≤1.5 mm).
  • Low Crest (13%): The bone crest is further from the CEJ. These patients are prone to gingival recession if the attachment is damaged during procedures.
  • High Crest (2%): The bone crest is very close to the CEJ. These patients are at high risk for biologic width violation and chronic inflammation if an intracrevicular margin is placed.

Harmonious occlusion3738

Note

The final restoration must not create occlusal interferences. Occlusal disharmony can cause pain, temperature sensitivity, tooth mobility, and TMD symptoms.

Protection against tooth fracture3940

  • Cuspal Protection: Coverage of one or more cusps by a indirect or direct restorative material.

Info

This binds the tooth together and redirects occlusal forces along the long axis of the tooth, which it is designed to withstand. This prevents the wedging forces that can fracture weakened cusps.

  • During preparation:
    • Removal of tooth structure less than 2mm thick
    • Removal of any low quality/undermined enamel on the cusps
  • Also avoid occlusal contacts on the margin of the preparations Slide by Dr Nedelcu

Mechanical Principles41424344

Slide by Dr Matsubara

  • Retention

Note

Full crowns offer significantly higher retention compared to other restoration types.

: prevents removal of restoration/crown from coming off the prepared tooth when a force is applied parallel to the path of insertion.

  • Resistance: prevents dislodging of restoration/crown when apical or oblique forces applied, and stops movement of restoration during chewing.

Factors influencing Retention and Resistance45

Taper and Length of axial walls

Important

These are the most critical factors that the clinician controls.

Terminology: Convergence Angle vs. Taper4647

  • convergence angle l: the angle, measured in degrees as viewed in a given plane, formed by the axial walls when a tooth or machined surface on a metal or ceramic material is prepared for a fixed dental prosthesis; Editorial note for usage: the term total occlusal convergence applies to the angle formed between two opposing axial walls; the term taper applies to the angle formed between an axial wall and the path of placement onto the tooth or machined surfaces of a metal or ceramic material; syn, angle of convergence, TOTAL OCCLUSAL CONVERGENCE; comp, TAPER
  • taper: in dentistry, the angle, measured in degrees as viewed in a given plane, formed between an external wall and the path of placement of a tooth preparation or machined surfaces on a metal or ceramic material when prepared for fixed dental prosthesis; comp, CONVERGENCE ANGLE, TOTAL OCCLUSAL CONVERGENCE ANGLE

Key Terminology

It is crucial to distinguish between these terms.

  • Taper: The angle of a single axial wall relative to the long axis of the tooth preparation.
  • Total Occlusal Convergence (TOC) / Convergence Angle: The angle formed between two opposing axial walls. (TOC = 2 x Taper).
  • The ideal TOC is between 6-12 degrees.

Taper and Undercuts484950

Note

Parallel walls (0° convergence) would provide maximum retention. However, this is clinically impossible to achieve without creating undercuts. An undercut is any irregularity or convergence of walls that prevents the seating of the restoration. A slight divergence (taper) is necessary to ensure a path of insertion.

  • Fig 9-13 To examine a preparation for undercuts, one eye should be closed.
  • Fig 9-14 If both eyes are open when the preparation is viewed, undercuts may remain undetected.
  • Fig 9-15 Preparations in the mouth are viewed through a mouth mirror using one eye. Fundamental of Fixed Prosthodontics

Length and Diameter51

  • Fig 9-9 The preparation with longer walls (a) interferes with the tipping displacement of the restoration better than the short preparation (b).
  • Fig 9-10 A preparation on a tooth with a smaller diameter (a) resists pivoting movements better than a preparation of equal length on a tooth of larger diameter (b). Fundamental of Fixed Prosthodontics

Grooves and Boxes52

  • What can we do to increase retention/resistance?
    • Grooves and boxes
  • BUT USE WITH CARE!

Caution

These are difficult to prepare without creating undercuts and are more commonly used for onlays/overlays. They are not generally recommended for crowns unless absolutely necessary.

Clinical Rule

The shorter the axial walls, the more parallel they must be (i.e., a smaller convergence angle is required) to achieve adequate retention and resistance.

Other factors that influence retention and resistance5354

a. Magnitude of the dislodging force b. Geometry of tooth preparation (MOST IMPORTANT, YOU DIRECT IMPACT THIS) c. Roughness of fitting surface of restoration

Info

Roughening the internal surface of a metal or zirconia crown (e.g., by sandblasting) increases surface area and mechanical retention.

d. Material being cemented e. Properties of luting agent

Info

Resin cements, which bond to both tooth and restoration, provide significantly more retention than traditional luting cements that rely on friction.

Deformation55

Note

The restoration itself must have sufficient bulk and thickness to withstand masticatory forces without flexing or fracturing. This is why specific reduction depths are required for different materials. For example, functional cusps (palatal on maxillary, buccal on mandibular) require an additional functional cusp bevel to provide extra space for material in this high-stress area.

Aesthetic Principles56

Material and Margin Considerations

  • Patients dislike visible metal. For PFM crowns, sufficient facial reduction is needed to allow space for the metal, an opaque layer, and the final porcelain layer to achieve a natural look.
  • Inadequate reduction on the facial surface will result in a bulky, over-contoured crown or a crown that is too thin and unable to mask the underlying tooth color or a metal post.
  • A two-plane reduction on the facial surface of anterior teeth is often required to follow the natural tooth contours.

Footnotes

  1. Original PDF page 1: F5 Principles of Tooth Prep, p.1 ↩

  2. Original PDF page 4: F5 Principles of Tooth Prep, p.4 ↩

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