Dental Biomaterials Lecture 5

Dental Biomaterials Lecture 5: Physical Properties of Dental Materials

• Dental biomaterials are materials used in dentistry
• Physical properties are based on the laws of physics (mass, heat, light, electricity, energy, force, other phenomena)
• Mechanical properties are a subgroup of physical properties and deal with force
• This lecture covers mass-related, thermal, and optical properties

I- Mass-related properties

• Density: mass per unit volume (e.g., gm/cm³, pound/in³)
  • Low density materials are important for upper dentures
  • Low density alloys require higher pressure during casting
• Clinical importance in dentistry:
  • Denture retention: Low density is beneficial.
  • Casting: Low density alloys require more pressure

II- Thermal properties

• Thermal conductivity: ability of a material to transfer heat.
• 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
• Melting and freezing temperature: temperature where material melts or freezes
• Heat of fusion: amount of heat needed to convert 1 gram of substance from solid to liquid state.
• Specific heat: amount of heat in calories used to increase 1 gram of substance by 1°C
  • Metals have low specific heat, non-metals high.
  • Low specific heat of dental gold alloys minimizes prolonged heating during casting.
• Thermal diffusivity: rate at which a material converts from non-uniform temperature to thermal equilibrium.

II.1 - Importance of Thermal properties

• Metallic fillings near the pulp: use insulating cement bases (e.g. zinc phosphate cement) from poor conductors
• Metallic denture bases: High thermal conductivity helps maintain tissue health through vasoconstriction/vasodilation due to thermal changes
  • Prevents patients swallowing hot substances

II.2 - Coefficient of Thermal Expansion (α) - Importance

• Importance in dentistry: -Matching α between restorative materials and teeth to prevent marginal leakage or microleakage (fluid, bacteria, ions entering the space between restorative material & cavity walls).
  • Mismatches can lead to: i. Marginal leakage; ii. Discolouration; iii. Recurrent carries; iv. Hypersensitivity
  • Porcelain & metal in ceramo-metallic restorations: used for bonding (crowns/bridges)
  • Artificial teeth and denture bases: avoid crazing (cracking)

II.3 - Melting and Freezing Temperature

• The temperature at which a material melts (liquid state) or freezes (solid state)
• Important in casting: determining the melting machine/type of investment materials

II.4 - Heat of Fusion

• Amount of heat needed to convert 1 gram of a substance from solid to liquid state
• Important in casting operations: selecting appropriate melting machines and investment materials

II.5 - Specific Heat

• The amount of heat needed to increase the temperature of 1 gram of a substance by 1°C.
• Metals have lower specific heat than non-metals
• Important in dental casting: reduces prolonged heating of gold alloys

II.6 - Thermal Diffusivity

• Rate at which a material converts from non-uniform temperatures to thermal equilibrium.
• High thermal diffusivity and conductivity in gold or amalgam restorations can cause high thermal shock (given low specific heat).

III- Optical properties

• Optics: the science of studying light, sight (eye function) and vision (processing of visual information by brain). -Restorative dentistry: to restore the color and appearance of natural teeth, knowledge of light and color is key.

III.1 - Nature of light

• Light is a beam of photons
• Color is the wavelength of light

III.2 - Production of light

• Thermal (e.g., sun, hot metal)
• Electrical (e.g., current passing through inert gases like neon)
• Chemical (e.g., fluorescence, phosphorescence)

III.3 - Interaction of light and matter: Reflection

• Specular reflection: light reflects off a smooth surface in one direction (e.g., mirrors)
• Diffuse reflection: light reflects off a rough surface in multiple directions

III.4 - Interaction of light and matter: Refraction

• Light changes direction (bends) when passing through materials with different densities (e.g. from air to water).
• Refractive index is the ratio of the speed of light in a vacuum to its speed in a medium.
• Important that refractive index is matched when placing restorations so they don't appear different in color.

III.5 - Interaction of light and matter: Scattering

• Light changes direction when it encounters imperfections (e.g., inclusions, air bubbles) in a material & is attenuated (less intense)
• Causes light to emerge in all directions (e.g., opacifiers in composite materials)

III.6 - Properties of materials in relation to light transmission and absorption:

• Transperancy: allows light with minimal distortion
• Translucency: allows some light, scatters/reflects rest
• Opacity: prevents light passage

III.7 - Luminescence (Fluorescence and Phosphorescence)

• Luminescence occurs when the emitted wavelength is longer than the exciting light.
• Fluorescence: immediate emission of light when excited by UV light
• Phosphorescence: delayed emission of light after UV stimulation
• Importance in dentistry: human teeth are fluorescent, materials (restorative/porcelain) can be formulated with fluorescing agents to match appearance

III.8 - Importance in Dentistry of light and shade

• Color matching of restorations (shade matching) is important to match the appearance with natural teeth.
• Techniques for shade matching include shade guides, photocolorimetry, and chair-side colorimeters.

IV- Lasers

• Light Amplification by Stimulated Emission of Radiation
• Different mediums to create lasers (e.g., CO2, Argon, Diode lasers)
• Characteristics of lasers: monochromatic, coherent, collimated
• Applications in dentistry: surgery to removesoft tissue, tissue regeneration, curing composite, endodontics, dental restorations