PROPERTIES OF THE MATERIALS • Biological • Physical • –mechanical • –thermal • –electrical • –volumetric
BIOLOGICAL PROPERTIES • Dental materials should be biocompatibile, that means –non toxic for patient, dentist and staff –non irritating to oral cavity and tooth tissues –non allergenic –non mutagenic or cancerogenic
Figure 1: Sequence of events that may lead to biological side effects to dental materials • Dental Materials | Release of Components and Degradation Products | Biological Effects? | Disease?
BIOCOMPATIBILITY • The ability to perform with an appropriate host response in a specific application • Features of biocompatibility • •Not only a property of a material, but also of the location used • •Not a single property, but a collection of attributes • •Acknowledges that no material is truly inert • •States that functionis critical
BIOMATERIAL • A substance which • –is not a drug • –can be used for some period of time as a part of a system used for • •treatment • •enlargement • •replacement of tissue, organ or body function
Adverse effects of tissues on materials • Material type Polymer • Effect Depolymerization,Crosslinking, Oxidation, Hydrolysis, Leaching of additives
Adverse effects of tissues on materials • Material type Metals • Effect Corrosion Ion-release
Adverse effects of tissues on materials • Material type Ceramics • Effect Ageing,Dissolution
Adverse effects of materials on tissues • Hypersensitivity • •Toxicity • •Carcinogenicity 1, HYPERSENSITIVITY • Adverse reactions caused by exposure to a particular substance. • •May present as: –local effects •egeczema •itchiness and/or •redness of skin –or systemic effects •egasthma • •Can be immediate or delayed
Agents causing hypersensitivity • Metals –Ni, Co, Cr • •Latex gloves –thiocarbamate curing agents • •Resin monomers - HEMA, TEGDMA
TOXICITY • May cause death or injury to the cells • •Depends on a dose • •May be acute or chronic • Types of toxicity • Acute •Arises soon after administration •Results either in death or in complete recovery • •Chronic •Takes time to develop •Needs prolonged exposure or repeated doses •Results in prolonged ill-health •Most likely type for biomaterials
Target organs • Usually a toxic agent affects a particular organ of the body • •For dental materials it is –dental pulp –gingiva –periodontium and periaplcal bone
CARCINOGENICITY • •Cancer: Malignant tumors that invade neighboring tissues and which can spread to remote sites of the body. • •Rare to find biomaterials implicated.
Biological interaction of restorative materials • None for amalgam or composite resin fillings;| • •Glass-ionomer cements may promote remineralization.
MECHANICAL PROPERTIES • Restorative materials should –be strong to be able to withstand biting forces without fracture –be rigid to maintain their shape under load
STRESS • When an external force is applied on a test specimen an internal force, equal in magnitude but opposite in direction is set up in the body • •STRESS= FORCE/AREA,UNITS= Pascal (Pa) –Tensile(a) –Compressive(b) –Shear(c )
In materials containing • –cracks • –cuts • –fissures • –scratches • –inclusion bodies • –or having irregular dimensions • •the concentrationof stress and strain can occur leading to failure
Maximum compressive strength • MATERIAL MPa • •zinc –polycarboxylate cements 90 • •resins (unfilled) 97 • •porcelane 150 • •glass-ionomer cements 180 –220 • •DENTIN 297 • •composites 200–345 • •ENAMEL 400 • amalgam 310-483
HARDNESS • Resistance to penetration • •It allows to predict the resistans to scratching and wear
Abrasion resistance • Wear can occur by one or more mechanism -due to abrasive forces -intermittent stresses -chemical degradation • •The rate of wear of dental materials are difficult to measure since they are complex and laboratory assessments often give misleading results
FATIGUE • Materials when subjected to cyclical or intermittent loading over a period of time can lead to failure • •Small microcracks may occur caused by stress concentration and the crack is likely to propagate leading to fracture
DIMENSIONAL CHANGES • Can be a result of: –chemical reaction -during setting •eg. polymerisation shrinkage –temperature changes in oral enviroment •set material and tooth structures
Thermal expansion • Is most often different for a material and for tooth structures • •that leads to formation of microfissure between filling and tooth • •that leads to microleakage
When the coefficient of thermal expansion of a restorative material does not match that of the tooth structure, uneven expansion and contraction occur. In turn, gaps and leakage occur at the interface of the restoration and the tooth.
Thermal conductivity • Describes an amount of conducted thermal energy • •Is different for various materials and tooth structures –dentine, enamel, cements, composites and ceramics are good thermal insulators –gold, casts, amalgam and other metals are good thermal conductors
ELECTRICAL PROPERTIES • Galvanic currents • Corrosion
GALVANISM IN DENTISTRY • Phenomenon of electric cell and flow of electrons in oral cavity which is perceived by patient as pain or discomfort and metalic taste • •Requires presence of two metals of different electric potential and an electrolyte (saliva)
Example • Electric potential of aluminium +1.33V • •Electric potential of gold -1.36V • •in case of contact of these two metals in oral cavity the potential difference will be 2.69V and a mild current will be conducted • •patient will complain of discomfort and metalic taste
CORROSION • Metals and amalgams in oral cavity can undergo corrosion - roughening of the surface due to ion exchange • •Corrosion is a result of –galvanic currents –chemical interaction with oral fluids and food
SOLUBILITY AND ABSORPTION • Solubility –% of mass of dissolved material –bigest for cements • •Absorption –describes absorption of a liquid by a solid –bigest for resins –for unfilled resins absorption of water is 2%
PROCESS OF MATERIAL SELECTION • Analysis of the problem • -Direct filling material: Is the restoration in a high stress bearing area? Is it visible when smiling? Is it a deep or a shallow cavity? • •Based on the analysis identify a set of requirements and make an informed choice from the available materials