Non-metallic biomaterials (9).pptx
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BIOMINERALIZATION ⦿ Tooth enamel is crowned as one of the hardest of biominerals available. ⦿ It is also the only biomineral in vertebrates to be almost fully deprived of organic components as 96-98% weigh t is accounted for mineral content only. ⦿ Tooth enamel unlike typical ceramics as it has an e...
BIOMINERALIZATION ⦿ Tooth enamel is crowned as one of the hardest of biominerals available. ⦿ It is also the only biomineral in vertebrates to be almost fully deprived of organic components as 96-98% weigh t is accounted for mineral content only. ⦿ Tooth enamel unlike typical ceramics as it has an exceptional toughness and only moderate brittleness. ⦿ Enamel is composed of assembled bundles of apatite fibers and thus rod-shaped aggregates. BIOMINERALIZATION Biomineralization is the process by which living organisms produce minerals, often to harden or stiffen existing tissues. Such tissues are called mineralized tissues. ENAMEL AND DENTINE BONDING FOR ADHESIVE RESTORATIONS Concept of adhesion – word adhesion consists of word ad (to) and haerere (to stick). ⦿ Adhesion or adhesive strength is the measure of the load bearing capability of an adhesive joint. ⦿ When considering adhesion one must take into account 1) The contact between the adhesive and the adherend surface 2) The stress concentration at the interface must be reduced 3) The interface must be protected from the oral environment. ⦿ BONDING SUBSTRATES ENAMEL Microleakage is a problem when considering adhesion in dentistry Phosphoric acid demineralizes enamel and dentine by removing calcium and thus creating microporosities Irregular surface results thus increasing its surface free energy. There is selective hydroxypatite dissolution creating microporosities that will be then filled with resin monomers BONDING SUBSTRATES DENTINE ⦿ Dentine tubules run continuously from the DEJ to the pup in coronal dentine and from the CEJ to the pulp in the root. ⦿ Pashley described dentine as being a porous biological composite made up of apatite crystal filler particles in a collagen matrix. ⦿ Etch and rinse adhesives vs self-etch adhesives ETCH AND RINSE ADHESIVES VS SELF-ETCH ADHESIVES ⦿ ⦿ ⦿ Etch and rinse remove smear layer and superficial Hap while self etch make smear layer permeable without removing it completely. Etch and rinse – universal use, high bonding achieved, but over-etching decreases dentine bond and hydrolytic degradation of the bonds occur when the margins are in dentine. Self-etch – easy and quick, acidity not as strong as phosphoric acid so etching does not occur to the same depth, water may become trapped if not properly evaporated, phase separation occurs rapidly, degradation of the resin-dentine interface by hydrolysis. DENTAL ADHESION MECHANISMS ⦿ Mechanical interlocking – adhesive monomers diffuse into etched dentine to permeate into the collagen mesh and displace water. ⦿ Chemical bonding – interaction of acids and acidic monomers with hydroxyapatite as acids demineralize dental hard tissues and open a pathway for the infiltration of the resin monomers into the microporosities previously occupied by the Hap. DENTAL CERAMICS FOR TOOTH REPAIR AND REPLACEMENT ⦿ ⦿ ⦿ ⦿ ⦿ Wide variety of materials that reaches from filled glasses to nearly dense sintered ceramics. High biocompatibility and aesthetics Ceramics do not have a natural affinity for the teeth so the tooth enamel has to be etched prior to adhesion. Even the ceramic surface should be etched (usually HFhydrofluoric acid) Ceramics can be classified according to their firing temperature: low-fusing, medium-fusing and high-fusing All-ceramic restorations can be fabricated by sintering, heat-pressing, slip-casting and machining. Most common and traditional type of porcelain is feldspar based that contains silica (SiO2-glases) – high resistance to chemical attack and biocompatibility. DENTAL CERAMICS ⦿ ceramic refers to any product made essentially from a non-metallic inorganic material usually processed by firing at a high temperature to achieve desirable properties. ⦿ As a class, ceramics are hard, low in toughness compared to metals, stiff, poor thermal and electrical conductors, and can be cast or machined to fabricate restorations. PROPERTIES ⦿ High compressive strength ⦿ Low tensile strength – brittle nature ⦿ Coefficient of thermal expansion matches that of enamel and dentine. ⦿ One must consider the hostile oral environment including pH fluctuations, temperature changes, tensile stresses etc. ⦿ Good aesthetics ⦿ Combined structures eg PFM can be obtained. CHARACTERISTICS ⦿ Biocompatibility ⦿ Aesthetics ⦿ High hardness ⦿ Wear resistance ⦿ Chemical interness CERAMIC MATERIALS USED IN DENTISTRY Type Glass-ceramics Hybrid ceramics Poly-crystalline ceramics Quality SiO2 based, high aesthetic qualities but usually weak. Glass-infiltrated porous alumina. Infiltration glass is used to fill the porosity associated with alumina Alumina and zirconia Indications Veneering ceramics, laminate veneers, inlays and onlays Core, frameworks Frameworks, now most commonly used ALUMINA ⦿ Aluminium trioxide Al2O3 occurs in nature as bauxite, corundum, gibbsite and diaspore. ⦿ Native alumina is crystalline, hard and insoluble in water. ⦿ Alumina can be called the predecessor of zirconium dioxide. ⦿ Also used in orthopaedics in ball and socket replacements of the hip joint. ⦿ If zirconia particles are added to alumina, alumina becomes harder. ZIRCONIA ⦿ Dense and hard surface ideal for wear resistance and contact damage. ⦿ Highly biocompatible ⦿ White, heavy, odourless, tasteless, virtually insoluble in water, slightly soluble in HCl and HNO3 but slowly soluble in HF. ⦿ Young’s modulus of elasticity, compressive strength and hardness are lower for zirconia than for alumina. ⦿ Zirconia has a higher bending strength though. ZIRCONIA ⦿ Occurs as a natural mineral element called baddeleyite USES OF ZIRCONIA ⦿ Crowns and bridgework ⦿ Inlays and onlays ⦿ Veneers ⦿ Endodontic post and cores ⦿ Dental composite fillers ⦿ Implants ⦿ Orthodontic brackets SURFACE CONDITIONING METHODS The surface conditioning of restorative materials is an important preliminary step in clinical practice to modify surface properties for durable and hydrolytically stable adhesion. Grit blasting The surface of materials such as metals, alloys and some ceramics is sand-blasted with alumina. This process is intended to increase the surface roughness of the materials. It also enhances micromechanical retention for bonding. Pyrochemical silica coating In these systems, a tetraethoxysilane solution is injected into a flame and burned with butane in oxygen. The silane decomposes and forms reactive SiOx-C fragments, which are deposited on the substrate surface. A glass-like silica layer is thereby formed on the surface. Tribochemical silica coating It enhances the adhesion of a silane coupling agent to a silica-coated material by forming a durable siloxane bond (Si-O-Si). This surface treatment also increases the surface roughness that provides micromechanical retention for resin bonding, that is, for the resin to penetrate pores on the surface. Hydrofluoric acid etching When a porcelain surface is etched with hydrofluoric acid etching gel, the acid dissolves the glassy matrix of the porcelain. A microscopically porous and micro-retentive surface is thus produced and micromechanical interlocking for resin bonding is enhanced. ZIRCONIA SURFACE TREATMENT ⦿ Zirconia is very inert and resistant ⦿ Acid etching with hot acids – combination of nitric, sulfuric and hydrofluoric acid or H2O2 + sulfuric acid ⦿ Abrasion with specific burs ⦿ Coating with porcelain micro pearls to have coatings with a lower melting temperature ⦿ Selective infiltration etching – Heat + etching with HF ⦿ Air abrasion with alumina ⦿ Lasers
