Dental Biomaterials Lecture 5

Dental Biomaterials Lecture 5: Physical Properties of Dental Materials

• Physical properties are based on laws of physics (mass, heat, light, electricity, energy, force, and other physical phenomena).
• Mechanical properties are a subgroup of physical properties that deal with force.
• This chapter provides a brief introduction to physical properties, preparing for a more detailed discussion later, where these properties describe specific dental material characteristics.

I - Mass-related Properties

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

II - Thermal Properties

• Thermal Conductivity: Material's ability to transfer heat.

  • Low conductivity materials transfer heat slowly.
  • High conductivity materials transfer heat rapidly.

• Coefficient of thermal conductivity (K): Amount of heat transported in one second across a unit cube when one face is hotter than the other.

• Thermal Coefficient of expansion (α): Change in length per unit length for a 1°C change in temperature.

  • Clinical significance:
    • Close matching of thermal expansion coefficient between tooth and restorative materials prevents marginal leakage (bacteria, fluids, ions seeping between the restoration and tooth). This avoids discoloration, recurrent caries, and hypersensitivity.

• Melting and freezing temperature: Temperature at which a material melts (liquid) or solidifies (solid).

  • Important in casting metals and alloys: choosing the correct melting machine and type of investment materials.

• Heat of Fusion: Amount of heat needed to convert 1 gram of a substance from solid to liquid.

• Specific Heat: Amount of energy needed to raise the temperature of 1 gram of substance by 1°C.

  • Metals have low specific heat, while non-metals have high specific heat.
  • Clinical importance: Dental gold alloys have low specific heat, meaning prolonged heating during casting is unnecessary.

• Thermal diffusivity: Rate at which a material changes from non-uniform temperature to thermal equilibrium (uniform temperature).

  • Gold or amalgam restorations have high thermal diffusivity, potentially causing high thermal shock due to high thermal conductivity and low specific heat.

III - Optical Properties

• Optics: Science of studying light, sight, and vision (color).

  • Restorative dentistry aims to restore teeth's color and appearance; therefore, understanding light and color science is essential.

• 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: Passing electric current through inert gases like neon
  • Chemical: Fluorescence and phosphorescence

• Interaction of light and matter:

  • Reflection: Specular (smooth surfaces) reflect light in a single direction, while diffuse (rough surfaces) reflect light in multiple directions. The restoration should have a highly smooth and polished surface for specular reflection.

• Refraction: Change in light direction when entering a different medium. Refractive index is the ratio of light velocity in vacuum to light velocity in the medium. Matching the refractive indices of the dispersed and matrix phases is important.

• Scattering: Light is redirected and weakened when passing through a medium with obstructions (inclusions, etc.).

• Transparency: Material allowing light to pass through without significant distortion (e.g., glass).

• Translucency: Material letting some light pass and scattering or reflecting the rest (e.g., tooth enamel, porcelain).

• Opacity: Material that prevents light from passing through (e.g., amalgam or composite).

• Luminescence: Emission of light from a material due to excitation by other energy types (e.g., UV).

  • immediate emission: fluorescence
  • delayed emission: phosphorescence
  • Clinical importance: Natural teeth emit fluorescent light under UV irradiation.

• Color:

  • The human eye, with specialized cells (cones), perceives light in three dimensions (hue, value, and chroma, using the Munsell color system) which describes color perceived.
  • Important to match color to adjacent teeth
  • Light source and color appearance: color matching should be done under similar lighting conditions to avoid metamerism.

• Shade color matching: Used to determine the color of natural teeth, aiding restoration replication.

  • Common methods: dental shade guides (VITA Classical, Chromoscop, and 3D-Master), photocolorimetry, chair-side colorimeters.

Laser

• Light Amplification by Stimulated Emission of Radiation (LASER)
• Production: An element or compound ("medium," solid, liquid, or gas), excited by high energy, produces special light.
• Laser light characteristics:
  • Monochromatic: all photons have the same wavelength .
  • Coherent: all waves are in phase (same speed).
  • Collimated: all waves are parallel.
  • Named according to the medium: CO2 laser, argon laser, diode laser.
• Clinical applications: surgery for soft tissue removal/regeneration, ulcer healing, coagulation, curing composites, endodontics, fissure fillings. Laboratory applications: welding alloys, sintering ceramics.