Dental Materials Lecture 1 PDF

College of Dentistry 2nd Stage Dental Materials 2023-2024 Lecture 1 Dr Azal Hadi Type Materials Science and Dentistry Materials science seeks to explain the properties and performance of mater...

College of Dentistry 2nd Stage Dental Materials 2023-2024 Lecture 1 Dr Azal Hadi Type Materials Science and Dentistry Materials science seeks to explain the properties and performance of materials by examining their internal structure. Materials science is a combination of chemistry, physics, and engineering rather than a separate scientific domain. ‫مفصل كله‬ ‫ من مجال علمي‬-‫بد‬ - ‫يعني مو كل مجال وحده‬ ‫مندمج تحت مسمى علوم ا;واد‬ In dentistry, a subgroup of materials science has developed. This subgroup, called dental materials, is part of the larger field of biomaterials and, at times, is called dental biomaterials. Rationale for Studying Dental Materials A. To Understand the Behavior of Materials B. To Handle Materials Properly C. To Assess and Treat the Patient History of Dental Materials For centuries humans have attempted to improve their appearance with adornments, such as jewelry and makeup. The replacement of lost teeth is also an ancient practice. At that time, it was more likely for aesthetics than for function. As dentistry developed throughout the ages, more and more materials were used for example, ivory, porcelain, wax and zinc oxide eugenol. By the 1800s, dentistry was becoming a scientifically based discipline. The pace of development of new materials quickened. Amalgam, a silver filling material, was widely accepted and frequently used. Porcelain was also used for inlays and crowns. Now, new ceramic materials and processing technologies have been adapted by dentistry. The pace of dental materials development is so fast that some dental materials books become outdated before they are published. Luckily, the basic concepts of materials science and their use do not change. Dentists need to understand the behavior of the materials they use. After all, they are the ones who must select a product from a rather long list of possibilities. 1/11  Atomic Bonding How do teeth withstand the forces that occur when we bite and grind food? To understand the strength of teeth, we need to understand the nature of atomic bonds. Teeth and restorative materials need to be stronger, and to have stronger atomic bonds, than the food we eat. Properties of materials are a result of their atomic bonds. Atoms of materials are held together by two types of atomic bonds: primary and secondary. Primary bonds are those that involve the transfer or sharing of electrons between atoms. There are three kinds of primary bonds: 1. Ionic bonds: result when an electron is given up by an atom and accepted by another. The positive and negative ions attract each other by the electromagnetic force and form an ionic bond. 2. Covalent bond is formed when two atoms share a pair of electrons 3. Metallic bond involves sharing many electrons by all the atoms in the material. Materials can be classified into three categories based on their primary atomic bonds: Metals are ductile, yet retain strength when bent because the metallic bond allows atoms to slide by one another and not disrupt the bonds in the metal. Ceramic materials are strong when compressed, but weak and brittle when pulled or bent, based on the ionic bond. Polymers or plastics have a range of properties because they have predominantly covalent bonds and a range of secondary bonds in their atomic structure. Composites are a combination of two solid materials and may be considered a fourth category. Enamel is a composite of apatite (a ceramic material) and protein (a polymer). 2/11  Physical Properties of Dental Materials Dental materials properties should be tested in specialized labs before clinical use, for the sake of benchmarking and to check the suitability of the material to be ‫ ولكن في بعض‬،‫واد الترميمية‬4‫دائما ميزة في ا‬ ً ‫تعتبر الخفة‬ employed in its intended application. Some of these properties: ‫حيان يتم استخدام القصدير أو الرصاص داخل طقم أسنان‬N‫ا‬.‫ للتحكم في حركته‬S‫ثقي‬ ً ‫ لجعله‬20 ‫سفلي كامل‬ 1. Density: The amount or mass of a material in a given volume is the density of the material. A common unit of density is g/cm3. Lightness is nearly always an advantage in restorative materials, but sometimes tin or lead is used inside full lower denture to make it heavy to control its mobility. Density of gold: 14 gm/cm3. Acrylic: 1.2 gm/cm3. Chromium/cobalt: 8.3 gm/cm3. Water: 1gm/cm3. 2. Thermal conductivity: Ability of a material to transmit heat. Generally, metals are better heat conductors than non-metals. Metal filling materials like amalgam, sometimes cause pulp pain by transmitting heat or cold more than natural tooth especially in deep cavities. Thus they require a heat insulating layer between the filling and the pulp, here it is undesirable property. On the other hand, the thermal conductivity of metallic denture base material is an advantage as it gives a feeling closer to the normal condition and the patient will feel normal also it will protect him from drinking very hot drinks which may burn his mouth. Silver: 1 cal/sec/cm2 Amalgam: 0.055 Cal/sec/cm2 Zinc oxide eugenol 0.011 Cal/sec/cm2. Enamel: 0.0022Cal/sec/cm2. 3. Coefficient of Thermal Expansion: change in length per unit length of material for 1°C change in temperature. Filling materials should have the same coefficient as the tooth, if it does not, it will press too hard against the cavity wall on expansion and may cause pressure on the pulp, or pull away from the wall when chilled by cold water. The later effect will cause the filling to leak temporarily, which may lead to further caries. Polymers tend to have a larger thermal expansion compared to metals. Thermal expansion in metals is less than in polymers because the chemical bonds in polymers are weaker, causing the molecules and atoms to be further apart. 3/11  Coefficient of thermal expansion for different materials 4. Electrical Activity: It is the ability of metals to ionize by losing electrons.If there is a high difference in the electrode potentials of two metals in contact with the same solution like gold and aluminum, an electrolytic cell may develop and the patient may feel discomfort. Enamel: 2.9*106 Ohm.cm Zinc oxide eugenol 109 -1010Ohm.cm Galvanism: electrical current generated when two dissimilar metals are brought into contact in the oral environment that causes pain. Electrochemical properties: Corrosion: is the deterioration and loss of a material and its critical properties due to chemical, ‫احيانا حشوة‬ electrochemical and other reactions ‫ملكم يتحول‬1‫ا‬ of the exposed material surface with ‫سطحها الى اللون‬ ‫سود والسبب هو‬1‫ا‬ the surrounding environment. ‫تأكسدها واحيانا‬ Tarnish: is a surface discoloration on ‫يسبب تآكل للحشوة‬ a metal or a slight loss or alteration of the surface finish or luster. 5. Viscosity: of a material is its resistance to flow. When placing materials, the handling characteristics of those materials are important. Some materials should flow easily and wet the surface. Other materials need to be more like putty, which can be adapted or formed into a desired shape. 6. Solubility is the amount of a material that dissolves in a liquid. Restorative material should not dissolve in the mouth. And if it dissolves, it should not release toxic substances. It can be measured in μg/mm3. 4/11 7. Color is a complex phenomenon that is a visual response to a physical stimulus. Three dimensions of color: 1. Hue: The dominant color of an object, for example red, green, or blue. 2. Value: Value is also known as the gray scale. 3. Chroma: Chroma is the degree of saturation of a particular hue. For example, red can vary from “scarlet” to light pink. Filling materials have their own set of color tabs or shades, called a shade guide such as the Vita shade guide. Recently, handheld devices have been developed that measure the color of teeth through a digital camera. For good esthetics; the interaction of light with restorative materials must mimic the interaction of light with natural teeth. The dental restorative materials should be translucent in order to look like natural teeth. Should not be stained or changed the color by time; ( anterior filling and artificial teeth). Also should look like a natural tooth substance at different light conditions, such as daylight and artificial light. For dentures, the material should have the same natural appearance as natural gum (Acrylic material can be made with various shades of pink to look as normal gum). *Translucency: is the optical property that allows the light to go short way in the material before being reflected out again. 5/11 8. Adhesion and cohesion: Adhesion is the force which causes two or more different substances to attach when they are brought in contact with one another. When the molecules of the same substances hold together, the forces are said to be cohesion. Mechanical Properties of Dental Materials Stress: force per unit cross-sectional area. These forces can be compressive, tensile, shear, twisting movement and bending movement (flexure). An example of compressive stress: a chewing force of 72 kg spread over a quadrant 4 cm2 in area exerts a stress of 18 kg/cm2. However, the same force on a restoration high spot or a 1 mm2 hard food fragment produces a stress of 7200 kg/cm2, a 400-fold increase in loading. This stress effect is one reason that occlusal balancing is essential in restorative dentistry. ‫ قوة مضغ‬:‫نضغاطي‬,‫جهاد ا‬1‫مثال على ا‬ 4 ‫ كجم موزعة على ربع مساحته‬72 ‫مقدارها‬ ‫ ومع‬.²‫سم‬/‫ كجم‬18 ‫ تؤثر إجها ًدا قدره‬²‫سم‬ ‫ؤثرة على نقطة ترميم‬Q‫ فإن نفس القوة ا‬،‫ذلك‬ ²‫ مم‬1 ‫عالية أو قطعة طعام صلبة بمساحة‬ ‫ أي‬،²‫سم‬/‫ كجم‬7200 ‫تنتج إجها ًدا قدره‬ ‫ تأثير‬.‫ ضعف في التحميل‬400 ‫زيادة قدرها‬ ‫سباب التي تجعل‬k‫جهاد هذا هو أحد ا‬1‫ا‬ ‫سنان‬k‫طباق ضرورية في طب ا‬1‫موازنة ا‬ ‫الترميمي‬ 6/11 Types of Stress: 1. Tensile stress: it results from two sets of forces directed away from each other in the same straight line or when one end is constrained and the other end is subjected to a force directed away from the constraint; it is accompanied by tensile strain. Examples: enamel: 10 MPa, dentin: 106 MPa, amalgam: 32 MPa. 2. Compressive stress: It results from two sets of forces directed toward each other in the same straight line or when one surface is constrained and the other is subjected to a force directed toward the constraint. lt is accompanied by compressive strain. Investment material, restorative materials and models should have high compressive strength. Examples: enamel: 384 MPa, dentin: 297 MPa, amalgam: 388 MPa. 3. Shear stress: Shear is the result of two sets of forces directed parallel to each other (not along the same straight line) which is applied to one part of the body in one direction, and the rest is being pushed in the opposite direction. The result is sliding of the molecules over each other. It is accompanied by shear strain. Examples: enamel: 90 MPa, dentin: 138 MPa, amalgam: 188 MPa. Shear force is the force which causes tearing a paper or a card. If one part of the crown is in occlusion while the rest is not, shear stress will develop. 4. Flexural stress (bending stress): it is the force per unit area of a material that is subjected to flexural loading. It results from an applied bending moment. Usually, three types of stress occur at the same time. If a piece of metal is being bent, it will exhibit tensile stress on the outer surface, compressive on the inner and shear stress in the middle. 5. Torsion stress: Force per unit area of a material that is subjected to twisting of a body. When external force or load is applied to a material the phenomena of strain occurs; i.e. the change in the dimensions of the material. Strain: ratio of deformation to original length under stress, ΔL/L; measures deformation at failure. It is either elongation under tensile stress or shortening under compressive stress. 7/11 Types of the strain 1. Temporary of elastic or recoverable strain: the material is returned to its original length after removal of the applied force. 2. Permanent or plastic or unrecoverable strain: the material is not returned to its original length after removal of the applied force. The material may remain deformed. Stress–strain curves: A plot generated by applying a progressively increasing force while measuring applied stress and material strain until fracture occurs. The shape of the stress–strain curve indicates the properties of the material. The properties that can be seen on the stress-strain curve Elastic modulus: A measure of the rigidity of a material, defined by the ratio of stress to strain (below elastic limit) also known as Young’s modulus. Resilience: The energy absorbed by a material undergoing elastic deformation up to its elastic limit. Given by the area under the elastic portion of the stress–strain curve. Toughness: The amount of energy absorbed up to the point of fracture. Given by the total area (i.e., both the elastic and plastic regions) under the stress–strain curve. Proportional limit: The maximum stress that the material can sustain without deviation from linear stress–strain proportionality. Elastic limit: Maximum stress that can be applied without permanent deformation. 8/11  Ultimate Strength: is the maximum stress that a material can withstand before failure. Examples: acrylic: 8000 PSI, Co/Cr: 100000 PSI, stainless steel: 15000 PSI Flexibility: The higher strain which occurs when the material is stressed to its proportional limit (the amount of strain up to the elastic limit), so flexibility is the total amount of elastic strain in a material. Ductility: It is the ability of the material to withstand permanent deformation under tensile stress without fracture; it depends on plasticity and tensile strength. It's the ability of the material to be drawn into a fine wire. Examples: gold: most ductile. Malleability: It is the ability of the material to withstand permanent deformation under compressive stress without fracture. It's the ability of the material to be drawn into a sheet. Examples: gold: most malleable. Elastic strain = flexibility. Plastic strain = ductility or malleability. Brittleness: It is the opposite of ductility; it requires lack of plasticity. 9/11 Hardness: Resistance to penetration; a measure of scratch resistance. Hardness is measured by several techniques, including the Brinell, Knoop, and Vickers tests. Enamel is the hardest biological tissue in the human body. In dentistry, we are interested in the abrasion resistance (wear resistance) of dental restorations to food, opposing teeth, and other dental materials such as ceramic crowns or porcelain denture teeth. Examples: Brinell hardness number: acrylic: 22, dentin: 65, gold: 250. Knoop hardness number: enamel: 343, dentin 68, Co/Cr: 391.Kg/mm2 Creep: is the slow plastic deformation that occurs with the application of a static or dynamic force over time. Fatigue: is the fracture of a material when subjected to repeated (cyclic) small stresses below the proportional limit. The repeated application of small stress (below the Proportion limit) to an object causes tiny (very small) cracks to be generated within its structure. These tiny cracks do not cause failure immediately, with each application of stress, the cracks grow until the material breaks. 10/11  Biocompatibility of Dental Materials It is the ability of a material to elicit an appropriate biological response in a given application in the body. Inherent in this definition is the idea that a single material may not be biologically acceptable in all applications. For example, a material that is acceptable as a full cast crown may not be acceptable as a dental implant. Also implicit in this definition is an expectation for the biological performance of the material. In a bone implant, the expectation is that the material will allow the bone to integrate with the implant. Thus an appropriate biological response for the implant is osseo-integration. In a full cast crown, the expectation is that the material will not cause inflammation of pulpal or periodontal tissues, but osseointegration is not an expectation. Whether or not a material is biocompatible therefore depends on the physical function for which the material will be used and the biological response that will be required from it. Tissue reaction: some restorative materials are damaging to the living tissue which is in contact with, like silicate filling and zinc phosphate cement which is acidic and may damage the dental pulp unless a protective lining is used. Dental material should not show any allergic reaction to the tissue and also should not provide good culture to the growth of bacteria and Candida albican to grow and cause infection, like soft lining materials. References: 1. Phillips' Science of Dental Materials. 2. Craig’s Restorative Dental Materials. 3. Introduction to Dental Materials, Richard van Noort. 4. Oxford Handbook of Clinical Dentistry, Laura Mitchell. 11/11