Written exam Dental Materials PDF

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Summary/Essays for the written exam in dental materials....

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EXAMINATION

DENTAL MATERIALS

1. HISTORY OF MAIN DENTAL MATERIAL Dentistry is believed to have begun in 3000 BC Historically, wide range of materials have been used as crown and root replacement: ! Animal teeth, bone, human teeth, ivory, seashells, ceramics After 2500 BC gold bands and wires were used by Phoenicians 700 BC: Etruscans carved ivory/ bone for construction of partial dental teeth –fastened to natural teeth by gold wires/bands Earliest documented evidence of tooth implant materials is credited to the Etruscans Around 600 AD: Mayans used implants consisting of sea shell segments placed in anterior tooth sockets Hammered gold inlays and stone or mineral inlays were placed for aesthetic purposes or traditional ornamentation Cavities in teeth were replaced or restored with stone chips, ivory, human teeth, turpentine resign, gums and metal foils ⋅ Pare (1509-1590) used lead or corck for tooth fillings ⋅ Queen Elizabeth I (1533-1603) used cloth to fill cavities in her teeth ⋅ Fauchard, father of modern dentistry (1678-1761) used tin foil or lead cylinders 1746: Mouton described gold shell crowns 1774: Duchateau –French pharmacist and de Chemant –dentist, designed a process for production of hard porcelain dentures 1844: Gypsum was used as impression material 1890: First amalgam was developed in USA as filling material After end of 19th century: development of special materials for dentistry began 1912-1932: Stainless steel introduced 1929: Co-Cr-Mo Alloys were used for casting of dentures 1935: Ni-Cr alloys for metal ceramic application 1957: Silicone was introduced as impression material 1966: Composites were developed as filling material 1969: Carboxylate cements were developed

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DENTAL MATERIALS

2. DENTAL MATERIALS CLASSIFICATION → MECHANICAL, PHYSICAL, BIOLOGICAL AND TECHNOLOGICAL REQUIREMENTS Place and usage period − Basic material –remains a long time in oral cavity e.g. porcelain, polymers, filling materials, cement −

Auxiliary material –remains in the mouth for a short duration of time e.g. impression material

−

Laboratory material –Extra orally in dental labs e.g. gypsum for models

Biological properties − Tolerable material Contains a small amount of toxic material –however does not influence health Small quantity of monomer remains in product after polymerisation − Inert materials Does not exert any influence on surrounding tissues e.g. porcelain − Active material Does not exert any harmful effects Some components of cement have antibacterial, preventative and therapeutic effects Aesthetic properties − Aesthetic materials –identical colour of replaced tissues e.g. porcelain, polymers, composites, cement −

Un-aesthetic materials: colour differs to the original tissues ∴ needs to be covered by aesthetic material e.g. metal alloys

Requirements MECHANICAL → High mechanical properties ⋅ Strength ⋅ Hardness ⋅ Wear-resistance → High elastic properties → Minimal residue deformation Wear of dental materials influences the durability Magnitude and intensity depends of the materials hardness; hardness is usually compared with the hardest tissue –tooth enamel Wear of natural tooth is more intensive that porcelain tooth, hardness of porcelain is twice higher than enamel PHYSICAL For better aesthetic and function of dental restorations should possess the following properties: constant shape, volume and stable colour Visible regions of fillings and dentures have to be manufactured from material whose colour is close/ same as surrounding tissue

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BIOLOGICAL → Harmless → Chemically inert → Corrosion resistant Two main biological effects of dental treatment: toxic and allergic reactions Harmlessness of the materials is defined by their composition; they shouldn’t be toxic in oral cavity both in free and in bound state The oral environment is a chemical active electrolyte Metal dentures, fillings and apparatus can provoke galvanic current ∴ triggers galvanic corrosion which can change the materials structure and decrease their strength TECHNOLOGICAL ⋅ Durability ⋅ Fluidity ⋅ Welding ability ⋅ Forging ability ⋅ Short manipulation time

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DENTAL MATERIALS

3. STRUCTURE OF DENTAL MATERIALS. CRYSTALLINE AND AMORPHOUS MATERIALS. MAIN TYPES OF CRYSTAL LATTICES AND THEIR DEFECTS. POLYMORPHISM AND ALLOTROPY. Materials used in dentistry are mainly in solid state –atoms are bonded to each other by either primary or secondary forces (H bond, vDWs) In solid state they combine in a manner which ensures minimal entire energy Can possess amorphous or crystalline structures Amorphous material ⋅ Inorganic/ organic ⋅ Molecules distributed at random, no ordered arrangement ⋅ Isotropic materials –same properties in all directions ⋅ Low heat and electrical conductivity E.g. waxes, porcelain, polymers, composites Crystalline materials ⋅ Have an ordered arrangement ⋅ Inorganic/ organic ⋅ Anisotropic materials –different properties in different directions ⋅ Possess regularly spaced configuration “space lattice” or “crystal forms” Space lattice –any arrangement of atoms in space in which every atom is situated similarly to every other atom May be the result of primary or secondary bonds Types of space lattice: The type of space lattice is defined by the length of each of the 3-unit cell edges, axes, and angles between edges ⋅ Cubic –simplest ⋅ Tetragonal ⋅ Orthogonal ⋅ Monoclinic ⋅ Triclinic ⋅ Rhomboedral ⋅ Hexagonal Type of crystalline lattice: ⋅ Body centred cubic (bcc) ⋅ Face centered cubic (fcc) ⋅ Hexagonal Fcc is the closest packed lattice because it has the highest amount of atoms in a unit volume Materials with face covered cubic lattice have: ⋅ High density ⋅ High mechanical properties ⋅ Corrosion resistance Defects in crystal lattices Crystal defect is imperfection in the regular geometric arrangement of the atoms in the crystalline solid → result from deformations of the solid, ⋅ Rapid cooling from high temperature ⋅ High energy radiation –x-ray or neutrons striking the solid The defects influence the mechanical, electrical and optical behaviour

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3 classes of defects: Point defects → atoms missing or in irregular places in the lattice Vacancies → occur at high temperatures when atoms frequently and randomly change their positions leaving empty lattice sites Diffusion can only occur because of vacancies, in most cases Interstitial → atoms that are squeezed in-between regular lattice sites Self-interstitial atom -if the interstitial atom is of the same species Interstitial impurities Foreign, usually smaller atoms –C, N, H Introduce less distortion to lattice, more common in real materials and more mobile Substitutional impurity → foreign atom replaces/ substitutes a matrix atom Linear defect → groups of atoms in irregular positions Dislocation Areas where atoms are out of position in the crystal structure, it is generated and move when a stress is applied Motion of dislocation allows slip –plastic deformations to occur Planar defects → interfaces between homogenous regions of material Polymorphism or Allotropy –property of some materials to change their crystalline structure at different temperature → different properties

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DENTAL MATERIALS

4. THERMAL PROPERTIES OF DENTAL MATERIALS –HEAT CONDUCTIVITY, HEAT CAPACITY, COEFFICIENT OF THERMAL EXPANSION AND SHRINKAGE, MELTING AND SUBLIMATION Thermal conductivity –property of material to conduct heat Conduction of heat through metals occurs through interaction of crystal lattice vibrations and by the motion of electrons and their interactions with atoms. Conductors: crystal materials –metals and alloys; Ag possesses highest thermal conductivyt Insulators are materials with low conductivity Amorphous materials e.g. wax, porcelain, glass, polymers are insulators Heat is transferred more rapidly away from the tooth when cold water contacts a metallic restoration compared to resin-based composite Increased conductivity of metal induces greater pulp sensitivity Heat capacity –property to absorb heat → c, the heat necessary for increasing the temperature of 1g substance with 1oc Water has the highest s.h.c Low thermal conductivity of enamel and dentin helps reduce thermal shock and pulp pain when hot/ cold foods are in the mouth Presence of oral restorations change the environment, in metal restorations it is necessary to insert a thermal insulator between the restoration and teeth structure ∴ this exhibits low thermal conductivity → more desirable Artificial teeth in denture base is constructed with synthetic resin –insulator ∴ patient partially loses sensation of hot and cold whilst eating/ drinking Coefficient of thermal expansion Linear dimensions and volume of bodies change with changing temperature due to increased fluctuation amplitudes of the atoms and molecules during heating α, characterizes the change in length per unit of original length when its temperature is raised 1oK During cooling, contraction of metals occurs; differs in different stages of the process If molten metal is cooled, liquid phase shrinks with the temperature decrease Solid shrinkage of noble metals is lower than base metals Tooth restorations may expand or contract more than the tooth during a change of temperature ∴ there may be marginal micro-leakage adjacent to the restoration or the restoration may de-bond from the tooth

Melting temperature, Tm –the equilibrium temperature at which heating of a pure metal, compound or alloy produces a change from solid to liquid state , it is specific for different materials Pure metals are melted at a constant temperature (melting point) Alloys are melted at temperature intervals which is lower than the melting temperature of its components Metals with lower heat capacity melt faster and at lower temperatures

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Melting temperature, Tm Low

> 650oc

Medium

650 – 1200oc

High

1200oc >

Sublimation –some substances change from solid directly to gas phase during heating Melting alloys: components with low Tm can evaporate ∴ changing the alloy’s composition and its properties

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5. PHYSICAL PROPERTIES OF DENTAL MATERIALS – DENSITY, VISCOSITY, ELECTRIC CONDUCTIVITY. Density, ρ –mass of a substance in a unit of volume ρ= m/ν Viscosity, η -resistance of a liquid to flow → controlled by internal frictional forces within the fluid Viscosity is a measure of consistency of the fluid, and its inability to flow Viscosity of liquids decreases rapidly with increasing temperature Thixotropic –liquid becomes less viscous and more fluid under pressure In casting of metal construction, it is necessary to increase alloy temperature to decrease viscosity and increase fluidity Electrical conductivity –ability of metals to conduct electric current due to the presence of electron ‘gas’ Crystalline materials: good electric conductors Amorphous materials: electrical insulators Using metals with different electrode potential and different electric conductivity in the oral environment provokes the galvanic current leading to galvanic corrosion → destroys construction ∴ metals are insulated with porcelain and polymers

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DENTAL MATERIALS

6. PHYSICAL PROPERTIES OF DENTAL MATERIALS – ADHESION, WETTABILITY, IMBIBITION AND SOLUBILITY. Adhesion –surface attachment process When 2 substances are brought into intimate contact, molecules of the first substance adhere/ attract to molecules of the second substance Cohesion is when molecules of the same kind are attracted, whereas adhesion occurs between unlike molecules The material or film used to cause adhesion –adhesive; material to which it is applied to –adherend Wettability –ability of liquid to flow easily over entire surface and adhere to solid Adhesion depends on wetting –If the liquid does not wet the surface of the adherend, adhesion between liquid and adherent will be non-existent Contact angle, β -angle formed at interface of the adhesive and adherend, it characterizes the wetting When β∼0 –liquid contacts surface completely and spreads freely When β >> / large angle is formed → determines poor wetting Material with good wettability are preferable for dental purposes → spread well on adherend ∴ better adhesion ⋅

Hydrophilic materials: contact angle is 0, in hydrophobic: contact angle > 90oc

Imhibition –process of absorbing the liquids resulting in dimensional changes (↑ volume) Polymers, hydrocolloid impression materials and composite cements possess this property; porcelain and alloys do not Volume of the polymer base of a denture increases in oral environment after absorption of liquid, if stored in a dry place –volume decreases These multiple actions lead to internal stresses, fatigue, crackling and destruction of material ∴ unfavourable –property of dental materials Solubility –degradation of material in liquids

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7. STRESS AND STRAIN. TENSILE STRENGTH OF DENTAL MATERIALS. “STRESSSTRAIN” DIAGRAM. Stress, σ: force per unit area σ = F/s → applied force / area Types of stress according to nature of applied forces: ⋅ Tensile stress –produced by tensile force ⋅ Compressive stress –produce by compressive force ⋅ Shear stress –produced by shear/ bending force Strain: relative deformation of an object subjected to a stress ε (%) –change is length(Δl), per unit original length (l0) ε = Δl / l0 Types of strain: ⋅ Elastic strain/ reversible –when the force is removed object restores its original shape ⋅ Plastic/ irreversible –permanent deformation of material, does not decrease when force is removed

Tensile strength Tensile test is the most fundamental type of mechanical test which can performed on materials –it is simple and standardised The reaction of material to forces being applied in tension can be quickly determined by pulling on the material As the material is pulled –its strength along with how much it elongates can be determined A complete tensile profile will be obtained as you pull on the material until it breaks:

Yield stress –stress required for the material to shift from the elastic to the plastic range UTS –maximum load specimen sustained during test

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8. TENSILE STRENGTH OF DENTAL MATERIALS - ELASTIC MODULUS, STRENGTH PROPERTIES (DIAGRAMS) OF RESILIENT, DUCTILE AND BRITTLE MATERIALS. Strength –ability to withstand applied stress without failure Tensile strength The reaction of material to forces being applied in tension can be quickly determined by pulling on the material As the material is pulled –its strength along with how much it elongates can be determined A complete tensile profile will be obtained as you pull on the material until it breaks Point P, proportional limit, σP → Relationship is defined as “Hooke’s Law” –stress to stain ratio is constant σ/ε = E σ: stress ε: deformation

Elastic limit → The greatest stress the object can be subjected to in which it can restore its original dimension upon release of the force ⋅ O-E elastic region, E-R plastic region For dental appliances and restorations, high value for elastic limit is required for materials from which they are fabricated because structure is required to return to its original shape after it has been stressed and force is removed In some cases, a larger strain/ deformation is needed with a moderate/ slight stress In an orthodontic appliance a spring is bent a considerable distance under small stress ∴ structure is “flexible”

Elastic modulus, E → Measure of relative stiffness/ rigidity of material, specific for each material E = σ/ε = PP1 / OP1 = tan POP1 Moderately high modulus of elasticity, E, is desirable as only a small deformation develops under considerable stress Resilience –ability of a material to absorb energy under elastic deformation and to recover this energy upon removal of load - Property of a material that allows it return to its original shape after being deformed.! ⋅ Measured by the area bounded by elastic region, materials with a larger elastic area → higher resilience Modulus of resilience –maximum energy that can be absorbed per unit volume without creating a permanent distortion. Ductility –ability of a material to deform under tensile stress. It is a measure of how much strain a material can take before rupturing.

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Brittleness –opposite of ductility Refers to the ability of materials to rupture upon application of tensile force –without any elongation or plastic deformation.

Area under curve represents energy absorbed up to fracture/ energy required to fracture material The greater the size under the curve, the more fracture resistant the material is

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DENTAL MATERIALS

9. HARDNESS OF DENTAL MATERIALS. MEASURING METHODS: MOHS HARDNESS, BRINELL HARDNESS AND VICKERS HARDNESS. Hardness –resistance of the solid material to different permanent shape changes when a force is applied → Mohs hardness –scratch test Measure of how resistant a sample is to fracture/ permanent deformation due to friction from a sharp object Not so accurate and not used to investigate dental materials → Brinell hardness –for metals and alloys Steel ball is pressed under a specified load into polished surface of a material for a definite time Diameter of impression is measured Smaller the indentation → larger the number ∴ harder the material 185 BHN, BHN is related to proportional limit and ultimate tensile stress of metal alloys, especially gold dental alloys Designation of HB: 185 HB 5 (ball diameter)/ 7500N (load)/ 20 (s) HB = F / S N/mm2 load/ impression area → Vickers hardness Same principle as Brinell test; square-based pyramid used instead of a steel ball Method is used for testing brittle and hard materials e.g. porcelain Length of both diagonals of impression is measured and an average value, 160, is used in calculation of Vickers hardness HV = F / S N/mm2

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DENTAL MATERIALS

10.HARDNESS OF DENTAL MATERIALS. MEASURING METHODS: ROCKWELL HARDNESS, KNOOP HARDNESS AND SHORE HARDNESS. → Rockwell hardness –for metals and alloys, similar to Brinell test Steel ball or diamond cones are used as indenters Depth of penetration measured instead of diagonals of the impression Smaller the penetration depth → harder the material Directly read by a dial gauge- instrument → Knoop hardness Diamond-tipped tool, cut in configuration of extended pyramid to obtain a rhombic impression Length of largest diagonal, d, is measured HK = F / S N/mm2 Load / impression area Designation of Knoop Hardness: 160 HK /KHN After removing indenter, stresses are distributed so that only the minor diagonals are subject to change by relaxation ∴ Hardness value is independent of the ductility of tested material Hardness of tooth enamel is compared with gold, porcelain, resin and other restorative materials → Shore hardness –tests elastic materials, where impression cannot be obtained after the load is removed E.g. rubber, impressions, and types of plastic Pin indenter is connected to a scale gauge from 0 -100, the higher the value, the harder the material Tester measures the depth of an indentation in the material created by given force Depth is dependent on the hardness of material, its viscoelastic properties, shape of presser foot and duration of test - 0 HSh –whole indenter penetrates in sample Soft material - 100 HSh –indenter does not penetrate in material Hard material

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