Notes on Metals and Alloys in Dentistry

OBJECTIVES

• Describe differences among types of gold alloy used for dental restorations.

• Differentiate between high-noble, noble, and base-metal alloys.

• Detail properties needed for porcelain bonding alloys.

• Discuss properties of metals used for casting partial denture frameworks.

• Explain biocompatibility issues associated with various alloys.

• Explain how solders are utilized in dentistry.

• List metals commonly used for solders.

• Describe differences between wrought metal alloys and casting alloys.

• Discuss uses of wrought wire in dental applications.

• Explain types of metal used in orthodontic arch wires.

• Describe the purpose of an endodontic post.

• Detail materials used for preformed endodontic posts.

INTRODUCTION TO METALS AND ALLOYS

• Metals are essential in restorative and corrective dentistry.

• Pure metals alone may not suffice for restorations, hence alloys are formed by combining metals with other elements.

• Key properties of metal alloys:

  • High strength

  • Durability

CLASSIFYING CASTING ALLOYS

• The American Dental Association classifies casting alloys based on noble metal content:

  • High-noble alloys:

  • At least 60% noble elements (with at least 40% being gold).

  • Remaining 40% can be base metals.

  • Noble alloys:

  • At least 25% noble elements (no gold requirement).

  • Remaining 75% are base metals.

  • Base-metal alloys:

  • No gold requirement and contains less than 25% noble metal.

OTHER NOBLE METALS FOR CASTING ALLOYS

• Gold:

  • Highly corrosion-resistant but too soft for dental use alone.

  • Alloys with platinum and palladium enhance strength.

• Palladium:

  • Widely used for its good corrosion resistance and promotes hardness.

  • More cost-effective compared to gold.

• Platinum:

  • Less common due to high costs and high melting point.

BASE-METAL DENTAL CASTING ALLOYS

• Common base-metal alloys include chrome-cobalt and nickel-chrome, which possess:

  • Corrosion resistance due to chromium.

  • Increased hardness from copper, nickel, silver, zinc, tin, and titanium.

  • Stiffness double that of gold-based alloys (requires twice the stress to deform).

  • Challenges: Higher casting temperatures and potential biocompatibility issues.

BIOCOMPATIBILITY

• Noble metals exhibit better biocompatibility compared to base metals due to lower corrosion rates.

• Metal corrosion products can cause allergic reactions:

  • Nickel identified as having the highest incidence of allergic responses.

  • Allergies typically manifest around free gingival tissues, notably near margins of base metal crowns.

PORCELAIN BONDING ALLOYS

• Classified similarly to casting alloys (high-noble, noble, base-metal).

• Important to have lower thermal expansion than gold alloys to prevent porcelain cracking.

PORCELAIN-BONDED-TO-METAL RESTORATIONS (PFM)

• Metal substructure (coping) covered with porcelain:

  • First layer: Opaque porcelain to conceal the metal base.

  • Heating process: Metal and porcelain fired at temperatures ranging from 870-1370º C, followed by a final firing for surface glazing.

REMOVABLE PROSTHETIC CASTING ALLOYS

• Frameworks for partial dentures primarily made from base-metal alloys (e.g., chrome-cobalt).

• Desired properties include:

  • Fatigue resistance to avoid breaking during use and removal.

SOLDERS

• Two metals joined by:

  • Soldering: Metal joining at 450º C.

  • Brazing: Metal joining above 450º C.

  • Welding: Fusing metals without filler using high heat.

GOLD AND SILVER SOLDERS

• Gold solders for crowns and bridgework include:

  • Components: Gold, Silver, Copper, Zinc, Tin.

  • Utilized in orthodontics for various components and appliances.

• Silver solders melt at lower temperatures, used where appropriate.

WROUGHT METAL ALLOYS

• Formed post-casting via mechanical processes, applicable in orthodontics and endodontics, known for:

  • Enhanced structural properties compared to casting alloys.

METALS IN ORTHODONTICS

• Orthodontic wires typically made of base metals or wrought alloys:

  • Common materials: Stainless steel, Cobalt-chrome-nickel, Titanium, Nitinol (Nickel-titanium).

  • Archwires designed for specific resistance to deformation, contributing to "memory" properties.

• Metal brackets/bands:

  • Bonded to teeth, retaining archwires, made from stainless steel alloys.

METALS IN ENDODONTICS

• Endodontic files and reamers:

  • Wrought metals designed for root canal cleaning and shaping.

• Endodontic Posts:

  • Retaining rods placed after root canal treatment, holding core buildup for final restoration.

SUMMARY

• Key categories of alloys and their properties:

  • Noble alloys for corrosion resistance.

  • Most common base-metal alloys and their applications.

  • Biocompatibility profile of noble vs. base metals.

  • Allergic responses, particularly from nickel.

  • Soldering processes and relevant materials for dental joinery.

  • Prevalent materials for orthodontic arch wires and endodontic applications.