Understanding Dental Materials: Properties and Clinical Significance

Chemical Bonding in Dental Materials

Primary Bonds

Factors Influencing Material Properties:

1. Type of bond between atoms and molecules
2. Inter-atomic distance
3. Atomic packing

Matter: Any substance that has mass and occupies space.

Atom: The fundamental unit of matter, consisting of a central nucleus (protons and neutrons) surrounded by electrons.

Types of Primary Bonds:

• Ionic Bond: Attraction between positive and negative ions. Characteristics: Strong, heat resistant, insulator, insoluble in organic solvents, basic bond in ceramics.
• Covalent Bond: Sharing of electrons between two atoms. Characteristics: Strong, insulator, water insoluble, basic bond in polymers.
• Metallic Bond: Attraction between positive atomic cores and free electrons. Characteristics: High thermal and electrical conductivity, opaque, metallic luster.

Secondary Bonds

• Temporary Dipoles: Attraction between the positive pole of one atom and the negative pole of another atom.
• Permanent Dipoles: Attraction between the negative oxygen pole of one water molecule and the positive hydrogen pole of another water molecule. Characteristics: Weak, low melting temperature, high coefficient of thermal expansion, basic bond in waxes.

States of Matter in Dental Materials

Crystalline Solids

Regular arrangement of atoms, definite melting temperature. Examples: All metals, 14 types of space lattices.

Dental Examples:

• Cubic: Simple (1 atom per unit cell), Body-centered cubic (2 atoms), Face-centered cubic (4 atoms)
• Hexagonal: Simple (3 atoms), Closed packed (6 atoms)

Amorphous Solids

No regular unit cell, no definite melting temperature. Examples: Wax and polymers.

Physical Properties of Dental Materials

Weight

Force of gravity acting on a mass.

Density

Mass per unit volume (g/cm3). Examples:

• Acrylic resins (plastics) = 1.2 g/cm3
• Porcelains = 2.4 g/cm3
• Gold alloys = 17 g/cm3
• Cobalt-chromium alloys = 8 g/cm3

Clinical Significance:

• Upper dentures: Low density materials provide good retention (advantage).
• Casting metals: Molten metals with low density require higher pressure to fill the mold during casting (disadvantage).

Specific Gravity

Ratio of the density of a material to the density of water at 4°C. It is a unitless value equal to density.

Thermal Conductivity

The ability of a material to conduct heat.

Clinical Significance:

• Metallic filling materials (disadvantage): Metals conduct heat to the pulp, causing severe pain.
• Metallic denture base (advantage): Transfers heat to underlying tissues, maintaining a healthy state.

Thermal Expansion

Solids expand upon heating and contract upon cooling.

Coefficient of Thermal Expansion: Change in length per unit length per 1°C.

Clinical Significance: Marginal percolation, bonding, crazing.

Color and Appearance

Color Parameters

• Hue: The color itself.
• Chroma: The intensity or saturation of the color.
• Value: The lightness or darkness of the color (most important parameter in dentistry).

Primary Colors

• Additive (light): Blue, green, red
• Subtractive (pigments): Cyan, magenta, yellow

Metamerism

Two objects appear the same color under one light source but different under another light source.