Dentophysics (Part – 3)

Optical properties :

Colour : the colour of dental restorative materials is most commonly measured in reflected light using a colour measuring instrument or a visual method.

Colour measuring instruments

  • Spectrophotometers are amongst the most accurate, useful and flexible instruments for overall colour matching in dentistry. They measure the amount of light energy reflected from an object at 1-25 nm intervals along the visible spectrum
  • Colorimeters measure tristimulus (relating to values giving the amounts of the three colored lights red, green and blue that when combined additively produce a match for the colour being considered) values and filter light in red, green and blue areas of the visible spectrum.

Visual method : A popular system for the visual determination of colour is the Munsell colour system. A large set of colour tabs is used to determine the colour.

  • Value (lightness) is determined first by selection of a tab that most nearly corresponds with the lightness or darkness of the colour.
  • Chroma is determined next with tabs that are close to the measured value but are of increasing saturation of colour.
  • The hue of the colour is determined last by matching with colour tabs of the value and chroma already determined.

Transparence, translucence and opacity:

Opacity is the property of the materials that prevents the passage of light. Translucency is the property of substances that permits the passage of light but disperses the light, so objects cannot be seen through the material. Some translucent materials in dentistry are ceramics, resin composites and acrylics. Transparent materials allow the passage of light so little distortion takes place and objects may be clearly seen through them.

Teeth and oral tissue are translucent and allow passage of some of the incident light. In some individuals, the enamel may be translucent or in rare circumstances transparent.

Flourescence : It is the emission of luminous energy by a material when a beam of light is shone on it. The wavelength of the emitted light is usually longer than that of exciting radiation.

Generally, natural tooth structure also absorbs light of wavelengths which are too short to be visible to the human eye. The energy that the tooth absorbs is converted to light with larger wavelengths in which case the tooth actually becomes a light source. This phenomenon is called flourescence.

In UV light a natural tooth emits a weak whitish-blue flourescence. This should be taking into account when selecting restoratives. If restorative material do not offer this property, they will look dark in UV light and the restored tooth will stand out against the other teeth in the mouth eg. in some stage shows and discotheques.

Metamerism : The appearance of an object depends on the type of light by which the object is viewed. Objects that appear to be colour matched under one type of light may appear very different under another light source. This phenomenon is called metamerism. Hence, selecting a shade of tooth must be done under two different sources of light.

Radiological properties :

Radiopacity may be defined as the quality of a material to obstruct the passage of radiant energy, such as X rays. Thus the materials that inhibit the passage of electromagnetic radiation are called radiopaque. Those that allow radiation to pass more freely are called radiolucent.

Manufacturers add certain elements in many dental materials in order to make them radiopaque. Commonly used elements are heavy metal glasses and metal oxides (zirconium dioxide and ytterbium oxide). Pure polymers like acrylic resins and BISGMA are radiolucent. Composites, ceramics and metals are radiopaque, with metals showing the highest radiopacity. Within the tooth, enamel is more radiopaque than dentin. If the radiopacity of a material is too low, it will not be visible. Generally, a restorative material should have radiopacity slightly greater than that of enamel in order to present a contrast. Radiopacity increases with increase in thickness of a material. Aluminum is used as a standard to measure radiopacity.

Use of magnets in dentistry : they have been used for various applications in orthodontics and prosthodontics. They can be placed within prostheses without being obtrusive.

  • In orthodontics : their main use has been for tooth movement
  • In prosthodontics : they are primarily used as retentive aids in maxillofacial prosthesis and in tooth and implant supported.

source : craig’s and manapalli textbook of restorative materials

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