4: The role of the manufacturer

Chapter 4 The role of the manufacturer

Introduction

It is obvious that the manufacturer has a major role in the production of new materials which are suitable for use in the mouth. Over the past 40 years there has been a large increase in both the generic types of material and number of manufacturers producing them. This proliferation of products with their varied applications, particularly in restorative dentistry, has contributed significantly to the way modern dentistry is now practised compared with traditional techniques. The most obvious example is the ability of the dentist to prepare a tooth as conservatively as possible and then select the most appropriate material for the application. This compares with times gone by when the reverse was true as the few materials available determined the clinical preparation, often resulting in the needless loss of tooth tissue (Figure 4.1). This has resulted in a change in philosophy in clinical dentistry. Each advance in dental materials science commonly leads to the production of new products. This increases the treatment options for the clinician, thus theoretically contributing to better care for the patient.

That said in the majority of cases, dental materials manufacturers use previously developed materials as a guide to what may be expected in clinical performance. These performance parameters are used in the development of new materials. New prototype materials which are subsequently produced are then judged against these criteria.

Material Development

Mechanical Tests

Compressive, tensile and flexural strengths together with the modulus of elasticity of the material can be measured using a series of standard mechanical tests. Compressive strength is the ability of a material to withstand axially loaded pushing forces (i.e. applied along the long axis of the object, Figure 4.3) and has some relevance in demonstrating reproducibility and reliability of materials from batch to batch during production.

Firstly, a sample of the material to be tested is constructed in the shape of a cylinder of known dimensions, usually 6×4 mm in diameter. It is then conditioned for up to 24 hours at mouth temperature and often in water to simulate intraoral conditions after placement. The cylinder is placed between the platens of a load testing machine and the upper platen is driven down at a constant speed until the specimen fractures. The ultimate load applied over the surface area of the end of the cylinder is then calculated. To reduce the variation that may occur during specimen preparation, a number of specimens are produced and tested. The mean of the values at fracture are used to determine the compressive strength of the material.

The test for compressive strength is not a very good discriminator between different material types. The manner in which the test is carried out is obviously not representative of what occurs in the mouth. When a masticatory load is applied to the restoration, the tooth tissue surrounding the restoration will act as a support and help to withstand the occlusal forces encountered.

Tensile strength is the ability of a material to withstand pulling forces in an axial direction. There are two types of tensile test, one for brittle materials which do not deform (Figure 4.4) and the other for elastic materials which will elongate before breaking. The diametral tensile strength test, as its name implies, uses a cylinder of material of similar dimensions to the compressive test specimen, but in this case it is loaded along the long axis across its diameter. The compressive forces which are applied at the surface form stress concentrations within the specimen, which is equivalent to pulling the cylinder apart along the midline. This test is used for brittle materials, but it is not used very frequently as the reproducibility with dental materials is low (Figure 4.5).

The diametral tensile strength test is unsuitable for materials that are easily compressible as the material distorts before fracture. The calculation of the strength value relies on the measurement of the diameter of the sample prior to commencement of the test remaining the same throughout the test. As can be seen from Figure 4.6, an easily compressible material such as an exercise ball will deform with the horizontal diameter increasing by a substantial amount, while the vertical diameter is reduced, without any fracture occurring.

Tensile strength testing has greater significance with materials such as elastomeric impression materials as they may tear or stretch when withdrawn from between the teeth. If they stretch but do not return to their original shape, then the cast restoration constructed may not fit the prepared tooth. For these materials, a paddle-shaped specimen is prepared and connected to the two crossheads of a loading machine. The upper crosshead is then moved away from the lower and the paddle-shaped strip of material is stretched until it fractures. The elongation of the sample and the ultimate breaking strength are noted (Figure 4.7).

Flexural strength is the ability of a material to resist load when unsupported. It is most closely related to a load being applied to a restorative material as the upper surface of the specimen is placed in compression whereas the underside is bent downward and is in tension. Flexural strength is also called transverse strength. Figure 4.8 shows the three-point bend test. Load is applied to the unsupported beam. Matchstick-shaped specimens are prepared, 2 × 2 × 25 mm in dimension. The ends of the sample are supported on two knife-edged blades which are positioned on the lower platen of the load testing machine. A knife edged blade is brought down onto the centre of the sample at a constant loading rate and the load is recorded when the sample breaks. This is then used to calculate the flexural strength.

Figure 4.9 demonstrates a biaxial system where the specimen is a disc supported on a ring. As with the three-point bend text, the top of the sample is put under compression while the underneath surface is placed in tension on the specimen. Here, however, any edge defects of the sample will not affect the test result since the edge of the sample is outside the area of material being tested, i.e. the supported area. In this case, a sample is prepared 1 mm thick and approximately 1 to 1.2 cm in diameter. The sample is positioned on an annular ring placed on the lower platen of a load testing machine and a load applied at the midpoint of the sample, the centre of the diameter of the ring, using the upper crosshead. The flexural strength is calculated from the ultimate breaking strain recorded. As with the test for compressive strength, it is difficult to extrapolate any results to the clinical situation as the floor of any cavity will provide support to this surface and as such this cannot be easily simulated in the laboratory (Figure 4.10).

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 4: The role of the manufacturer
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