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Introduction to Dental Materials Testing
Properties of Dental Restorative Materials INTRODUCTION Dentistry is unique among the biomaterials specialties in the wide ranging variety of materials used and the nature of the challenges they must resist. Intra-oral service demands that materials be amenable to a warm moist environment and resist attack by digestive acids and enzymes, be impervious to staining, and have pleasing esthetics. The materials must be non-toxic and affordable, and be easily placed by the clinician without undue technique sensitivity. Other requirements include a thermal expansion characteristic that matches the native tissue, and a thermal conductivity scenario that does not create patient discomfort. Materials that are subject to mechanical forces must maintain strength, fatigue and wear characteristics that support the services required of them. MATERIALS The wide range of materials available for restorative dentistry requires a knowledge of their relative strengths and trade-offs, and offers the opportunity for many interesting lines of research. The spectrum ranges highly elastic polyvinylsiloxane impression materials to the very stiff metal-core ceramic crowns and bridges, so a familiarity with a variety of mechanical testing situations is required from a well-rounded dental materials laboratory. Impression Materials These are used to replicate the contours of a dental surface in preparation of a restorative prosthesis. Formulations containing Polyvinylsiloxane have pre-dominated this area due to their lower polymerization shrinkage, amazing dimensional accuracy and resistance to the effect of water during curing. Their important properties include viscosity, working time, hydrophobicity and sensitivity to detail. Composite Restoratives These tooth colored materials were developed as an alternative to silver amalgam fillings, so their main advantages are improved esthetics and reduced toxicity. They are mainly composed of a mixture of very fine glass particles and acrylic monomers. The monomers are often a mixture of long chain oligomers the ends of which are both capped with methacrylate moieties. This difunctionality provides a highly cross-linked polymer network that has high stiffness and low water uptake. The monomer backbone is often relatively long (MW~500) to reduce the shrinkage that takes place during polymerization. One important feature of the filler particles is a surface treatment that enables it to reactively bind to the polymer matrix, improving strength and stiffness. The composite polymerization is commonly initiated through the action of photo-sensitive free-radical initiators. The rate and extent of this reaction is important to the development of favorable mechanical properties in the composite restorative. The factors that influence this conversion and its effect on crucial material properties is central to the body of research that has been performed on these materials. Adhesives Modern dentistry is focused on using newly developed polymer technology to assure a strong lasting bond between dental restoratives and the native tissue. However, this effort has been met with many challenges. Hard dental tissues are a moist composite structure of crystallized organometallics (calcium hydroxyapatite) and hydrophilic collagen fibers, which is not an ideal surface to which a strong bond can be formed. A multi-step approach has been developed in which the soft organic components are etched away, the remaining crystalline structure is functionalized through a primer, then the reactive adhesive is applied. This procedure has been abbreviated to two and even one step though preparations that combine the different agents, with varying success. One critical measure of adhesive interface viability is the adhesive bond strength, often measured in shear. Ceramics The replacement of hard dental tissues with ceramic prostheses has been increasingly successful. While the materials offer hardness and wear resistance, their esthetics, brittleness and the need for a metal substructure as compromised their desirability. Recently, improved formulations have offered near life-like appearance and fracture resistance high enough to obviate the need for metal cores. The lower frictional characteristic reduces the wear they impose on opposing enamel surfaces. Fiber Composites These are polymer composites including glass or polyethylene fibers that can be used to construct multi-unit prostheses. They are potentially less brittle than ceramic/metal or all ceramic units, though their preparation is more labor intensive. Their resistance to flexural fatigue is especially critical to their long-term clinical success. PROPERTIES A well-equipped dental materials laboratory requires a wide range of testing capabilities. Universal Testing Machine The heart of most dental materials laboratories is one, or several, single axis universal testing machines. These are important for evaluating the strength and stiffness of restorative materials, structures and prostheses. Proper fixturing must be provided for testing in compression, tension and shear. In compressive mode, loading to failure along the axis of cylindrical specimens provide compressive modulus and strength measures. The diametral tensile strength is also evaluated by loading a cylinder in compression across its diameter. The failure occurs in tension, providing a tensile strength measure while avoiding the complications associated with gripping a specimen that arises in tensile loading mode. In flexural mode bars that are supported on a span and loaded centrally provide a convenient repeatable measure of strength, though this mode may occur in the mouth only in certain situations. The load can be applied through two evenly-spaced tips, in which case the four-point loading provides a more consistent flexural strength value, being less sensitive to flaws in the specimen. Another test performed on the flexural span is the loading of single-edge-notched bars to failure to determine the critical stress intensity factor K Ic as a measure of fracture toughness. Tensile mode is mainly used for the popular micro-tensile bond strength test, in which layers of material are bonded together, then fine prisms of the composite structure are sectioned out and the interface is loaded to failure in tension. This procedure offers lower scatter that tensile loading of large bonded structures and produces more results per specimen. Shear modes are also applied to adhesive bond strength testing. Specialized fixtures apply the load in a direction perpendicular to the adhesive interface. This test offers ease of specimen preparation, and repeatable results. Fatigue Cycler Fatigue cycling is characterized by repeated application and removal of sub-critical load levels. Because the specimens must be cycled for hours or even days, it is accomplished in the lab by multi-station cycler that can accommodate multiple specimens, or less desirably by dedicating a Universal Testing Machine to such a long term project. Fatigue cycling can be applied in either an evaluative mode or a conditioning mode. In evaluative mode, recording the number of cycles at which failure occurs for a given load level allows the construction of an S-N curve, though high scatter makes this a less desirable technique. Altenatively, a “staircase” method is employed in which a series of specimens are tested and the load level applied to a specimen is determined by the success or failure of the previous specimen for a given number of cycles. The advantage of this technique is the vastly reduced scatter in results that can be obtained. The technique is often applied to flexural bars. In the conditional mode, fatigue cycling is applied to the specimen and an auxiliary test mode is applied, such as monotonic load to failure or interfacial staining, to determine extent of fatigue damage. Intra-Oral Wear Simulation One of the major challenges that dental restoratives face is the need to resist intra-oral wear. While modern restoratives offer improved resistance to wear, they still have not matched the wear resistance of the natural dentition. The principal wear mechanisms are three-body wear, caused by the excursion of the food bolus under pressure during chewing, and attrition wear, generated by localized contact with the antagonist surface. Composites respond to these two wear mechanisms differently depending upon their compositions. Generally, materials with very fine filler particles offer improved abrasion wear resistance because of their low inter-particle spacing. However, their compromised mechanical properties leave them susceptible to attrition wear through fatigue micro-cracking. On the other hand, more-heavily-filled large particle composites have the higher mechanical properties to resist attrition wear. However they are often sensitive to high abrasive wear. This is why it is important to completely characterize the wear resistance of materials through multi-mode wear testing. Polymerization Shrinkage and Stress The shrinkage the polymer composites undergo during their setting can compromise the integrity of their adhesive interface. Debonding at this interface provides a site for ingress of fluids and bacteria that can lead to sensitivity, staining and recurrent decay. Polymerization shrinkage has been evaluated through a large number of methods, the results of which do not all agree. Generally, materials with lower inorganic filler levels exhibit large amounts of shrinkage during curing, as could be expected. While shrinkage is important, it is the subsequent stress imposed by the material on the interface which can lead to failure of the adhesive bond. This stress and shrinkage do not go hand-in-hand but are inter-related through the developing stiffness of the materials, the viscosity and capacity for plastic deformation of the material, and the stiffness of the adhesive layer itself. Polymerization stress testing has also been performed in a variety of settings, most commonly a cylindrical geometry is used. One descriptor of this test is the ratio of the bonded area to the free surface of the composite volume, termed the C-factor. Another key test descriptor is the compliance of the system, which may be fully or partially corrected by the motion of the test frame actuator during polymerization. Other Properties The ability to evaluate wear and fracture surfaces through SEM is a key capacity to have in the dental materials lab. Thermal cycling is a conditioning mode that simulates the temperature changes that take place in the mouth, revealing weaknesses in the materials due to the thermal stresses imposed. The degree of conversion of polymeric materials is an essential measure, often determined by FTIR (Fourier Transform Infrared Spectroscopy). |