Dental Materials - mechanical PDF

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DENTAL MATERIALS CLASSIFICATION OF DENTAL MATERIALS 

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Basic dental materials:  Remain in the body for a long time as prophylactic and healing substances  E.g. metal alloys, resins and porcelains for fixed and removable prosthetic constructions  Indirect fixed prosthetics: crowns, gold alloy porcelain  Indirect removable prosthetics: dentures, acrylic Auxiliary clinical materials:  Materials remaining in patients mouth for a short time  Used in some clinical stages for fabricating of prosthesis as impression and abrasive materials Auxiliary laboratory materials:  Materials that don’t come in contact with patients’ body  Used in dental lab for fabricating prosthetic constructions as waxes, model and refractory investment materials Biologically tolerated materials:  Have adverse effects on tissues, overcome by defensive reactions of organisms.  Include metal alloys, resins and restorative materials Biologically inert materials:  Don’t impact tissues e.g. dental porcelain Biologically active materials:  Beneficial effect on healing and regenerative processes  Can increase resistance of tissues to pathogenic agents  Properties of medicaments with a long-lasting affect e.g. zinc oxide, calcium hydroxide and glass ionomer cements Non-deformable materials Elastic materials: susceptible to reversible or elastic deformation Plastic materials: susceptible to plastic or irreversible deformation

GENERAL REQUIRMENTS FOR DENTAL MATERIALS 1. 2. 3. 4.

Have mechanical and physical properties equal to or similar to tissues to restore Be biologically compatible with body tissues Not to degrade, but retain their shape and volume in oral environment Easy and convenient technology and low price

2. STRUCTURE OF DENTAL MATERIALS 

Materials exist in different aggregate states  Gaseous: when they have a low density  MECHANICAL PROPERTIES OF DENTAL MATERIALS

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Increasing mechanical loading-> plastic deformation/elastic deformation Then destruction of dental material Strength  Resiststance of a material against deforming or destructive loading  Static, dynamic, cyclic loading  Compressive strength: measures the stress which deform materials under the influence of opposing axial forces  Dentin has less compressive stress than enamel  Composite material has less compressive strength than enamel  Tensile strength: measures the strength which causes deformation……  For brittle materials-> amalgams, composites, cements, porcelain, gypsum products. Use diameter compression test  Dentin has a higher tensile stress than enamel  Gold alloy v high tensile stress  Flexure strength: measures the stresses which deform materials under the action of opposing non-axial forces  1.5 x  Zirconium dioxide highest max flexure strength  Enamel bigger flexure strength than dentine  Shear Strength: measures stresses which destroy materials under the action of opposing non-axial shear forces  Applied in determining the forces of adhesion between materials  Insert stress-strain diagram here  If the stresses during mastication exceed the elastic limit dental materials change their shape as a result of permanent (plastic) deformation  For deformation to be avoided materials with high strength properties should be selected or prosthetic constructions with greater thickness should be fabricated  Young modulus graph inset here  Non elastic rigid materials have a high modulus of elasticity while elastic materials have a low elastic modulus  Resilience: resistance against plastic deformation. The energy needed for the deformation of a material up to the proportional limit  Toughness: resistance against destruction. Energy necessary for destruction of a material  Impact strength: measures the stress which causes fracture of a material under the action of a single sudden maximum load (insert formula)

ABRASIVE MATERIALS AND INSTRUMENTS:      

Abrasion: wearing away or removal of material by the act of rubbing, cutting or scraping Object doing the abrading-> abrasive Finishing: a restoration is contoured to remove excess material and smooth the surface Polishing: final minimum removal of the material resulting in the production of a very smooth shining surface Abrasive materials used are harder than the dental tissues and prosthetic constructions, they can be natural or artificial origin Natural abrasives are environmental materials most common ones are:  Diamond: pure carbon by the Mohs hardness 10  Corundum: aluminium trioxide, mohns hardness 9 one of the most solid and strong oxides  Emery: mix of corundum and other metal oxides  Garnet: gemstone, mohs hardness 7.5  Quartz: silicon dioxide, mohs hardness 7  Artificial abrasives are produced synthetically  Have a permanent composition and well-defined mechanical properties  Artificial diamond, carborudum-silicon carbide by Mohs hardness 8-9, artificial corundum  Artificial instruments-> group of tools that rotate on axis to perform their function; fixed on devices such as hand pieces and motors  parts of an abrasive rotary instrument:  head: working portion made in different sizes and shapes  neck: narrow portion, connects the shank and head  shank: part that fits into the hand piece  abrasive grinders are divided into:  dental burs: smallest rotary instruments, have bladed cutting heads with diameters up to 4mm.  steel burs: cheapest but quickly wear out and corrode. Made out of alloyed steel of high strength  tungstene carbide: very expensive; heads of carbide, corrode easily during disinfection. Can cover with corrosion resistant titanium nitride  diamond burs: have one or more layers of diamond chips attached to a steel shank that inserts into the hand piece. Diamond crystals are bonded by a galvanic plated nickel alloy/thermostable alloy. Diamond turbine burs are coded with colours according to crystal size  cutters are metal or carbide grinders with a diameter of the working heads more than 4mm

 abrasive stones:  used in dental lab and mouth cavity  some are directly mounted on mandrels and some are separate and need

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fixed  abrasive particles of corundum, garnet and carborudum are incorporated through a binder of bakelite or ceramic materials  some of the mounted abrasive stones are colour coded according to the size of the abrasive particles or to the materials they are intended to abrasive separators:  thin discs with diameters more than 10mm, used to separate objects  mounted on screw-type mandrels, made of corundum, having brown colour, or of steel covered with diamond crystals  small elastic discs of paper or plastic, only used once  Covered with thin layer of abrasive material in diff particle sizes Abrasive rubbers:  Working heads of diameter more than 4mm  Made of synthetic rubber of silicone with embedded abrasive crystals  Available as mounted and unmounted in diff colours according to material Size and shape of abrasive instruments are-> spherical, cylindrical, conical, flameshaped, needle-shaped, ovoid, pear-shaped, inverted cone Every abrasive instrument has a 13 digit identification number:  First three: material of the working head  Second three digits: length of the shank and type of attachment to hand pieces  Next four digits: shape and size of abrasive particles of working part  Last three: largest diameter of the working head in 1/10 of a mm two bodied abrasive polishing:  bound particles are solidly fixed to a substrate  TBA products have rubber cups impregnated with abrasive agents so don’t require polishing paste Three bodied abrasion  Loose abrasives move between the surface, being polished and the polishing application device The hardness, size and shape of particles are what make them abrasive Abrasive particles must be harder than the surface or polishing material; the harder particles abrade faster Polishing agents are available in two basic forms:  dry powders (flours) factors determining the rate of abrasion when finishing and polishing:  hardness of abrasive: directly proportional to rate of abrasion  particle shape: sharp irregular particles abrade a surface more rapidly, however the former will produce deeper scratches  speed of rotating cutting tools...