Textbook of Dental Materials PDF

# Textbook of Dental Materials ## Classification of Dental Materials **Prophylactic** is a medical term that refers to anything that is used to prevent disease. According to the purpose of their use, dental materials are divided into: 1. **Basic Dental Materials**: Materials remaining in the bod...

# Textbook of Dental Materials ## Classification of Dental Materials **Prophylactic** is a medical term that refers to anything that is used to prevent disease. According to the purpose of their use, dental materials are divided into: 1. **Basic Dental Materials**: Materials remaining in the body for a long time as a prophylactic and healing substances, such as metal alloys, resins and porcelains for fixed and removable prosthetic constructions, as well as restorative materials as cements, composite materials, amalgams, used for the permanent restoration of the destroyed dental tissues. 2. **Auxiliary Clinical Materials**: Materials remaining in the patient's mouth for a short time; they are not remedies but are used in some clinical stages for the fabricating of prosthesis as impression and abrasive materials. 3. **Auxiliary Laboratory Materials**: Materials which do not come in contact with the patient's body, used mainly in dental technician laboratories for fabricating prosthetic constructions as waxes, model and refractory investment materials. According to their **biological properties**, the basic dental materials can be regarded as: 1. **Biologically Tolerated Materials**: have some known adverse effects on the tissues, but they can be overcome by the defensive reactions of the organisms. This biological tolerance is closely related to the general and the local toxicity of dental materials. Biologically tolerated are most of the dental materials as metal alloys, resins and restorative materials. 2. **Biologically Inert Materials**: do not have any impact on the tissues. Biologically inert is the dental porcelain, for example. 3. **Biologically Active Materials**: have a definite beneficial effect on the healing and regenerative processes, or they can increase the resistance of the tissues to pathogenic agents. The biological active materials have the properties of medicaments with a long lasting effect. These are zinc oxide eugenol, calcium hydroxide and glass ionomer cements. ## General Requirements for Dental Materials The basic dental materials must meet the following general requirements: 1. To have mechanical and physical properties equal or similar to those of the tissues they restore. 2. To be biologically compatible with the body tissues. 3. Not to degrade but to retain their shape and volume in the conditions of the oral environment. 4. To have an easy and convenient technology and a low price. The dental materials created by now do not meet all of the specified requirements. The achieving of the optimal therapeutic results requires combinations of many materials with appropriate physical, mechanical, biological and technological properties. ## Structure of Materials Materials can exist in different aggregate states: - **Gaseous**: when they have a low density and take up the entire volume of the container in which they are located. - **Liquid**: when they have a definite volume and ability to flow, but not a fixed shape because they take the shape of the container. - **Solid**: when they retain their shape under normal loads and temperature. Dental materials are used mainly in **liquid** and in **solid** aggregate state. According to their structure, solid substances can be divided into **crystal** and **amorphous**. In the **crystalline substances**, the building elements - atoms, molecules and ions - are arranged in a crystal lattice which is a set of periodically arranged building elements in the space. If the crystal body has a single continuous lattice, it is called **monocrystal**. Monocrystals with very large dimensions and a weight up to hundreds kilograms are found in the earth, in the nature. When the crystal body is composed of many monocrystals located relatively one by another in no particular order, connected with a thin boundary layer, such body is determined as a **polycrystal** or a **crystalline material**. During the crystallization in a humid environment, some crystal substances include a number of water molecules, so they are called **crystal hydrates**. Their formation is associated with a significant increase in the volume and with a porous structure, after the evaporation of the water. The crystal lattices may have various spatial positions and according to their shape are divided into seven types: cubic, rhombic, monoclinic, hexagonal, tetragonal, triclinic, and rhombohedral. Metals and alloys used in the dentistry have mostly cubic or hexagonal crystal lattices (See Figure 1. ). **Figure 1**: * **A** - Cubic body-centered * **B** - Cubic face-centered * **C** - Hexagonal The cubic crystal lattice can be: 1. **Body-centered**: composed of 9 atoms - by one on the eight tops of the cube and one in its centre. Body-centered crystal lattices have chromium, molybdenum, vanadium, tantalum and others. 2. **Face-centered**: composed of 14 atoms - by one on the eight tops of the cube and one in the middle of the six walls of the cube. The face-centered crystal lattices have more atoms per unit volume. Metals with such packing have a high relative density and a high corrosion resistance. Such lattices have the noble metals, nickel, aluminum copper, tin, and others. The hexagonal crystal lattice is built of 12, 14 or 17 atoms, depending on the location and centering of the atoms. It is typical for cobalt, zinc, magnesium, and others. Some crystal substances have the ability to change the crystal lattice, depending on their temperature when they are in solid aggregate state. The phenomenon is called **allotropy** or **allotropism**. The iron to 910 °C is in **alpha(α)** modification, have a 9-atom cubic body-centered crystal lattice, magnetic properties and are not corrosion resistant. Over 910 °C the iron transforms into a **gamma(γ)** modification with 14-atoms cubic face-centered crystal lattice with no magnetic properties, and a corrosion resistance as noble metals. Above 1390 °C to the melting point - 1535 °C the **delta(δ)** modification of iron has again a not corrosion resistant 9-atoms lattice (See Figure 2). **Figure 2**: * **α** - 910° * **γ** - 1390° * **δ** - 1535° *magnetic* *no-magnetic* The amorphous substances have a random molecular formation as fluids, and can be considered as super cooled liquids with a high density and **viscosity**. The amorphous substances are thermodynamically unstable. They have no fixed melting temperature and when heated gradually, soften with the lowering of the viscosity. The amorphous substances are mainly materials of organic origin. The exact distinction between the amorphous and the crystal substances is unreal. There are not perfectly homogenous and pure crystal or amorphous substances. The crystal bodies have amorphous areas, but the amorphous ones have crystal elements; some elements change their structures with the temperature alteration. For example, the waxes under 50 °C are similar to the crystal bodies, and above 52 °C are amorphous. The feldspar porcelains at 1100-1200 °C are amorphous, and by cooling one part of them crystallizes. The amorphous dental glass-ceramics undergo a heat treatment in order to convert a part of their structure into crystalline, for improving their mechanical strength.