Quartz 4

M2 PFM Prep

10 min read

PART II: PFM Crowns1

Synonyms

  • Metal ceramic
  • VITA Metallic Ceramic (VMK) Crown (historical)

Info

VMK (Vita Metall-Keramik) is a historical term from the company that first introduced layering ceramics.

Porcelain Fused to Metal crown – PFM2

Definition

Complete-coverage cast metal

Info

Modern alternatives exist where the metal infrastructure is replaced with a strong ceramic like zirconia, creating a layered all-ceramic crown that follows similar fabrication principles.

substructure (metal coping), veneered with

  • Smooth all transitions between planes and margin types.
  • Ensure all line angles are rounded, especially for modern CAD/CAM fabricated copings.
  • Verify from an occlusal view that the entire margin is visible, which confirms the absence of undercuts.
  • ==Avoiding Gutter Margins: To prevent creating a weak, unsupported lip of enamel at the margin, avoid cutting with the very tip of the bur. Instead, approach from the side and use the side of the bur to plane the tooth structure down to the finish line.==

a layer of fused porcelain to mimic the appearance of a natural tooth

General features3

  • Complete-coverage cast metal substructure with a mechanically & chemically bonded ceramic veneer

  • Range of designs depending on the material coverage

  • Variable extent of ceramic veneer

  • ==Full Ceramic Coverage: The entire visible surface of the crown is covered in porcelain. This is used when aesthetics are the primary concern, such as on anterior teeth.==

  • ==Partial Ceramic Coverage: Only a portion of the crown (e.g., the buccal/facial surface) is covered in porcelain, while other areas (e.g., occlusal and lingual surfaces) remain as metal.==

    • ==Benefits of Partial Coverage:==
      • ==More Conservative: The metal portion requires less tooth reduction (0.5-1.0 mm) compared to the porcelain portion (at least 1.5 mm).==
      • ==Wear Resistance: A metal occlusal surface is more wear-resistant for the crown itself and is less abrasive to the opposing natural teeth compared to an unpolished or adjusted ceramic surface. This is particularly beneficial for patients with bruxism.==

  • One of the most widely use crown in the past.

  • Long history of clinical success and have been considered the gold standard.

Indications45

When both strength and aesthetic are needed:

  • Posterior crowns and bridges: high occlusal forces occur
  • Anterior crowns: When improved esthetics are desired but full ceramic is not indicated
  • Long-span fixed partial dentures: due to the strength of the metal substructure
  • Tooth preparations with reduced height: Where a full ceramic crown may not have enough support
  • Teeth receiving heavy load: parafunction habits.
  • Abutments for removable or fixed partial dentures

Info

These are called

Contraindications

  • Patients with metal allergies: especially to base metals (nickel, cobalt)
  • Highly aesthetic cases: Where maximum translucency is required
  • Minimal tooth reduction cases: Requires more tooth reduction than all-metal crowns; Young patients whose pulp chambers are large
  • Deep bite or limited interocclusal space: Risk of porcelain fracture

Clarification

While a deep bite with limited space is a consideration, it is not an absolute contraindication. A PFM with a metal palatal/lingual surface can be an ideal choice, as it requires less reduction (0.5 mm) than an all-ceramic crown (at least 1.0 mm).

  • Parafunctional habits without protection: Increased risk of porcelain fracture

Advantages6

  • Strong and durable: metal substructure provides excellent fracture resistance, suitable for posterior regions and bridges.
  • Good Aesthetics: porcelain overlay mimics natural tooth color (though not as translucent as all-ceramic).
  • Versatility: suitable for single crowns, multi-unit bridges, and implant-supported restorations.
  • Predictable Marginal Fit: especially when fabricated using traditional casting techniques and noble alloys.
  • Proven Clinical Track Record: well-documented long-term success.
  • Functional Performance: handles well heavy occlusal loads
  • Adjustable Shade and Contour: through layering and staining techniques.

Disadvantages7

  • Aesthetic Limitations: less translucent than all-ceramic crowns; may show dark margin
  • Metal Allergies: possible hypersensitivity to base metals like nickel or cobalt.
  • Porcelain Fracture Risk: Chipping or delamination of the veneering porcelain can occur, especially under high stress or trauma.
  • Tooth Reduction Requirement: requires more tooth reduction than full-metal crowns overall
  • Potential for Opposing Tooth Wear: porcelain can be abrasive to natural enamel if not properly polished or glazed.
  • Subgingival Margin Challenges: may compromise periodontal health if margins are deep or poorly contoured.
  • Cost: more expensive than other crowns (metal crowns or monolithic ceramic crowns)

Info

This is due to the skilled labor involved in layering porcelain. However, a PFM is typically less expensive than a full gold crown.

Material science8

Metal alloy: requirements

  • ==Sufficient Rigidity: Must have a high modulus of elasticity to resist flexing under load, which would otherwise cause the brittle porcelain to fracture.==

  • Allow for casting in thin section (0.3 mm)

  • Porcelain-metal compatibility

    • Thermal match between metal and porcelain: the metal should have higher melting temperature than porcelain for thermal stability while porcelain firing;
    • Colour preservation: some components of the alloy affect porcelain color;
    • Thermal contraction: if the porcelain shrinks slightly less than metal, the ceramic will be in a state of beneficial residual compressive stress at room temperature.
    • High alloy rigidity is required to prevent flexing (modulus of elasticity)

Metal alloys9

Classification: Noble and base metal

  • Based on the electrochemical character of the alloys
  • Gold-based alloys are the gold-standard alloys for PFM

Noble metal alloys:

  • High-noble metal (gold-based):
    • Noble metal content: 60% (>40% gold)
    • e.g. Gold-Platinum-Palladium; Gold-Palladium-Silver, Gold-Palladium
  • Noble metal (Palladium-based):
    • Noble metal content: 25% (no gold requirement)
    • e.g. Palladium-Silver, Palladium-Copper-Gallium

Key Feature

All noble alloys for PFMs must contain a small amount of non-noble metal (e.g., tin, indium). This is essential for forming an oxide layer on the coping’s surface, which is required for chemical bonding with the porcelain.

Indium, tin and iron promote bonding of porcelain by the formation of stable adherent oxides

Palladium and platinum are generally added to serve as hardening elements in alloys with high gold content

Base metal alloys10

  • Noble metal content: <25% (no gold requirement)

  • e.g. Nickel-Chromium, Cobalt-Chromium

  • Casting is more technique sensitive

  • Higher Hardness and Modulus of elasticity - Burnishing and finishing are more difficult

  • Less flexure – long span bridge

  • ==Clinicians must check for patient nickel allergies.==

  • These alloys are also more difficult for the dentist to adjust intraorally compared to gold alloys.

Bonding to porcelain11

  • Mechanical:
    • rough metal surface (sandblasting/oxidizing) provides micromechanical retention
    • ceramic locking on cooling
  • Chemical: fusion of ceramic to metal oxide layer

Compression Stress Mechanism

During cooling after firing, the metal alloy shrinks slightly more than the porcelain. Because the two are bonded, this differential shrinkage places the porcelain layer under a state of residual compressive stress. This internal compression makes the porcelain significantly more resistant to cracking and chipping.

Fabrication and preparation of PFM1213

Fabrication principles14

  • Wax-up complete anatomical contour
  • Cut back consistent amount for ceramic veneer
  • EVEN THICKNESS OF PORCELAIN

Coping Design

A properly designed coping provides uniform support for the porcelain layer. If the coping is thin or flat, it can create areas of thick, unsupported porcelain that are prone to fracture.

  • Occlusal contact in centric ≠ Metal-porcelain junction
    • at least 1.5 mm away from the junction
  • Occlusal analysis is necessary to determine the PFM crown design

Modern Workflow

Many labs now use CAD/CAM technology to design and mill a wax pattern of the coping, which is then invested and cast using traditional methods. The porcelain layering, however, remains a manual process performed by a skilled technician.

Always check if the patient accepts a compromised aesthetic

Tooth reduction guidelines151617

Reduction depends on coverage material.

PFM: Full coverage with ceramic18

  • Anterior:

    • Incisal reduction: 2mm
    • Margin: Shoulder, at least 1.0-1.5 mm
    • Axial reduction (metal-ceramic): 1.5mm

  • Posterior (Occlusal Surface - metal-ceramic):

    • Non-functional cusp: 1.5 mm
    • Functional cusp: 2 mm

  • Margin: Shoulder, at least 1.5 mm

  • Axial reduction (metal-ceramic): 1.5 mm

PFM: Partial coverage with ceramic

  • Occlusal Surface:
    • Non-functional cusp: 1 mm
    • Functional cusp: 1.5 – 2.0 mm
  • Margin: Chamfer or shoulder: 0.5 and 1.5 mm
  • Axial reduction (metal only): 1 mm
  • Axial reduction (metal-ceramic): 1.5 mm

Margin design19

Metal collar

  • Conservative
  • More accurate fit
  • Unaesthetic
  • Subgingival placement in some cases
  • More suitable posteriorly

Ceramic shoulder

  • Balanced compromise of aesthetic and conservation

  • Suitable for anterior teeth

  • Equigingival labial margin is possible

  • ==Requirement: This design requires a shoulder margin with at least 1.5 mm of reduction to provide enough bulk for the porcelain. It cannot be done with a chamfer or feather edge.==

  • ==Instruction: The dentist must specifically request a

Diagnostic use: cracked tooth syndrome

A provisional crown is an excellent diagnostic tool for suspected cracked tooth syndrome.

  1. A provisional crown is placed over the suspected tooth, providing full cuspal coverage. This holds the cracked segments together and prevents them from flexing during biting.
  2. The patient wears the provisional for several weeks.
  3. If the biting pain resolves, it confirms that cuspal coverage is the correct treatment, and the dentist can confidently proceed with the final crown.
  4. If the pain persists, the crack may be too deep, and the tooth may require root canal treatment or extraction.

Preparation styles20

Classical preparation

(wing preparation)

Contemporary PFM preparation

(Wingless preparation)

Info

Emphasizes smooth, rounded line angles and flowing transitions. This reduces stress concentration within the restoration and is more compatible with digital scanning and milling technologies.

  • Shoulder blending interproximally from wide labial margin to narrow palatal margin
  • Easier to prepare

Lingual reduction 12122

Warning

The lingual surface is reduced to create clearance for the restoration in both centric occlusion and during protrusive movements. It is critical to check that the planned metal-ceramic junction does not fall on a functional contact point. The lab should be instructed to either extend the porcelain past the contact or keep the entire contact area in metal.

Subgingival Margin Refinement

When placing a subgingival margin, a good technique is to first prepare the margin at the gingival level (equigingival). Then, pack a retraction cord to displace the tissue, and carefully refine the margin 0.5 mm below the retracted gumline. When the cord is removed, the tissue will rebound to cover the finish line. This provides better visibility and control than trying to cut subgingivally from the start.

References23

  • Shillingburg, H.T., Hobo, S., Whitsett, L.D., Jacobi, R. and Brackett, S.E. (1997) Fundamentals of fixed prosthodontics, 3rd Edition, Quintessence
  • SF Rosenstiel, MF Land, R Walter (2022) Contemporary fixed prosthodontics
  • Kenneth J. Anusavice (2003) Phillips’ Science of Dental Materials
  • The Glossary of Prosthodontic Terms: Ninth Edition. J Prosthet Dent. 2017 May;117(5S):e1-e105
  • Goodacre et al. Tooth preparations for complete crowns: An art form based on scientific principles J Prosthet Dent 2001;85:363-76
  • Kontakiotis et al. A prospective study of the incidence of asymptomatic pulp necrosis following crown preparation International Endodontic Journal, 48, 512–517, 2015.
  • Edelhoff and Sorensen Tooth Structure Removal Associated with Various Preparation Designs for Posterior Teeth. Int J Periodontics Restorative Dent 2002;22:241–249
  • J Valderhaug et al. Assessment of the periapical and clinical status of crowned teeth over 25 years. J Dent. 1997 Mar;25(2):97-105.

Footnotes

  1. Original PDF page 1: MR2 PFM Prep, p.1 ↩

  2. Original PDF page 2: MR2 PFM Prep, p.2 ↩

  3. Original PDF page 3: MR2 PFM Prep, p.3 ↩

  4. Original PDF page 4: MR2 PFM Prep, p.4 ↩

  5. Original PDF page 5: MR2 PFM Prep, p.5 ↩

  6. Original PDF page 6: MR2 PFM Prep, p.6 ↩

  7. Original PDF page 7: MR2 PFM Prep, p.7 ↩

  8. Original PDF page 8: MR2 PFM Prep, p.8 ↩

  9. Original PDF page 9: MR2 PFM Prep, p.9 ↩

  10. Original PDF page 10: MR2 PFM Prep, p.10 ↩

  11. Original PDF page 11: MR2 PFM Prep, p.11 ↩

  12. Original PDF page 12: MR2 PFM Prep, p.12 ↩

  13. Original PDF page 13: MR2 PFM Prep, p.13 ↩

  14. Original PDF page 14: MR2 PFM Prep, p.14 ↩

  15. Original PDF page 15: MR2 PFM Prep, p.15 ↩

  16. Original PDF page 16: MR2 PFM Prep, p.16 ↩

  17. Original PDF page 17: MR2 PFM Prep, p.17 ↩

  18. Original PDF page 18: MR2 PFM Prep, p.18 ↩

  19. Original PDF page 19: MR2 PFM Prep, p.19 ↩

  20. Original PDF page 29: MR2 PFM Prep, p.29 ↩

  21. Original PDF page 32: MR2 PFM Prep, p.32 ↩

  22. Original PDF page 33: MR2 PFM Prep, p.33 ↩

  23. Original PDF page 37: MR2 PFM Prep, p.37 ↩

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