Dental Biomaterials I - Physical Properties

Dental Biomaterials I - Lecture 5: Physical Properties of Dental Materials

• Physical properties are based on laws of physics (mass, heat, light, electricity, energy, force, and other phenomena).
• Mechanical properties are a subgroup of physical properties dealing with force.
• This chapter provides a brief overview. More detailed discussions will use these properties to describe the characteristics of specific dental materials.

I - Mass-Related Properties: Density

• Density is the mass per unit volume of a material.
• Units: gm/cm³ and pound/in³
• Clinical Importance in Dentistry:
  • Upper dentures require low-density materials for retention.
  • Low-density molten alloys during casting require higher pressure to fill the mold cavity.

II - Thermal Properties

• Thermal Conductivity: the ability of a material to transfer heat.
  • Heat transfer is slower in materials with low thermal conductivity than in materials with high conductivity.
• Coefficient of Thermal Conductivity (k): the amount of heat transported in one second across a unit cube when one face is hotter than the other.
• Thermal Coefficient of Expansion (α): the change in length per unit length of a material for a 1°C change in temperature.
• Melting and Freezing Temperature: the temperature at which a material melts into a liquid state or freezes into a solid state. Important in dentistry:
  • Casting metals and alloys determines the melting machine and investment material selection.
• Heat of Fusion: amount of heat (joules or calories) needed to change 1 gram of a substance from solid to liquid state.
• Specific Heat: amount of heat in calories needed to increase the temperature of 1 gram of a substance by 1°C.
  • Metals have low specific heat; non-metals have high specific heat.
  • Low specific heat of dental gold alloys means less prolonged heating during casting is needed.
• Thermal Diffusivity: the rate at which a material goes from non-uniform temperature to thermal equilibrium (uniform temperature). Important in Dentistry:
  • High thermal diffusivity in gold or amalgam restorations may cause thermal shock (due to high thermal conductivity and low specific heat).

III - Optical Properties

• Optics is the study of light, sight, and vision (including color).
  • Restorative dentistry aims to restore the color and appearance of natural teeth.
• Nature of Light:
  • Light is a beam of photons.
  • Color is the wavelength of light.
• Production of light:
  • Thermal (e.g., the sun, hot metal)
  • Electrical (e.g., passing electric current through inert gas like neon)
  • Chemical (e.g., fluorescence, phosphorescence)
• Interaction of Light and Matter:
  • Reflection (specular and diffuse)
    • Specular reflection occurs on smooth surfaces (e.g., mirrors), reflecting light in one direction.
    • Diffuse reflection occurs on rough surfaces, reflecting light in all directions.
    • Smooth, polished surfaces of restorations are desirable for specular reflection.
  • Refraction (change in the direction of light as it enters a different medium)
  • Scattering (light is redirected and attenuated due to obstructions like inclusions within a material)
    • Opacifiers added to composite materials, air bubbles in restorations, cause scattering.
• Properties of materials in relation to light transmission and absorption:
  • Transparency: allows light to pass through without distortion (e.g., glass).
  • Translucency:allows some light through while scattering or reflecting the rest (e.g., tooth enamel, porcelain).
  • Opacity: prevents light from passing through (e.g., opaque materials).
• Luminescence (fluorescence and phosphorescence): emission of light after absorbing electromagnetic radiation
  • Immediate emission (fluorescence): objects glow after excitation.
  • Delayed emission (phosphorescence): objects continue to glow after the excitation source is removed.
  • Teeth emit fluorescent light when excited by UV light.
• Laser (Light Amplification by Stimulated Emission of Radiation):
  • A device that uses stimulated emission of light to produce high-intensity, monochromatic, and coherent light.
  • A special type of light that can be produced by solid, liquid or gas mediums excited by high-energy. Characteristics of laser light: monochromatic, coherent (same speed), collimated (parallel). Various types of lasers exist (e.g., CO2, Argon, Diode lasers).
• Clinical applications of lasers:
  • Surgery (tissue removal, regeneration, coagulation)
  • Initial carious lesions removal
  • Composite curing
  • Endodontics
  • Filling pits and fissures (hydroxyapatite)
• Laboratory applications of lasers:
  • Welding (joining alloys)
  • Sintering (ceramics)

IV - Color

• Color perception (vision):
  • For objects to be seen, they reflect or transmit light from an external source.
  • Light from objects is focused on the retina and converted into nerve impulses that are transmitted to the brain.
  • Cone cells in the retina are responsible for color vision.
  • Defects in color sensing receptors can result in various types of color blindness.
• Color definition and dimensions: Munsell color system (three dimensions)
  • Hue: name of the color (e.g., red, blue, green) related to the dominant wavelength
  • Chroma/Saturation: intensity or strength of the color
  • Value: lightness or darkness of the color
• Light source and color appearance:
  • Metamerism: Change in color matching between objects under different light sources (e.g., daylight vs. laboratory light)
    • Metameric pairs: Two objects match in color under one light source, but not under another. Isomeric pair- color match under all light sources
• Shade color matching:
  • Dental shade guides consist of color standards mimicking natural teeth.
  • Used to match artificial restorations to natural tooth color.
  • Methods for shade matching: Photocolorimetry, Chair-side colorimeter.
  • Popular shade guides: VITA Classical, Chromoscop, VITA 3D-Master.