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

Explore the physical properties of dental materials focusing on mass-related and thermal properties. This lecture discusses important concepts such as density and thermal conductivity, essential for understanding their application in dentistry. Gain insights into how these properties influence clinical practices, especially in denture retention and casting processes.