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.

Fig. 4.1 (A) A large cavity relative to the size of the tooth after the removal of an old amalgam restoration showing substantial loss of tooth tissue. Compare this with (B), where the excision of only the small carious lesion is now necessary due to the availability of more modern materials. In this case, there is no need to remove excess tooth tissue to accommodate a traditional restorative material such as dental amalgam where mechanical retention is required to retain the material. This cavity was restored with a flowable resin composite material.

(Photo on the right courtesy of Dental Update.)

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.

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.

Fig. 4.3 (A) Diagrammatic representation of measurement of compressive strength. A load is applied vertically and the ability of the sample to resist the axial load is measured. (B) A Universal Load Testing Machine being used to measure the compressive strength of foam rubber (ZWICK).

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).

Fig. 4.4 Measurement of tensile strength. A load is applied vertically downward on a cylindrical object. As the cylinder is compressed, its centre is placed under tension. Failure is a result of this tensile stress.

Fig. 4.5 A sample of Portland cement undergoing a diametral tensile strength test: (A) prior to loading and (B) after fracture.

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.

Fig. 4.6 An exercise ball under compression. Note distortion rather than fracture. Diametral tensile testing is unsuitable for materials of this type.

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).

Fig. 4.7A,B (A) A laboratory test jig set up for tensile testing with specimen in place. (B) A paddle-shaped sample used in tensile tests for elastomeric materials.

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.

Fig. 4.8 Flexural strength being measured either using an unsupported beam as shown here or a disc supported in a ring, as shown in Figure 4.9.

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).

Fig. 4.9 A specimen under biaxial loading.

Fig. 4.10 An intracoronal restoration under load, showing support from the cavity floor and the risk of pull away from the walls.

Modulus of elasticity

Modulus of elasticity describes the relative stiffness or rigidity of a material. Brittle materials have a high modulus of elasticity while flexible materials have a low value (Figure 4.11). Glass is a brittle material that exhibits little distortion before cracking when a load is applied to it (Figure 4.12).

Fig. 4.11 A stress/strain diagram showing the effect of loading a brittle and an elastic material. Curve A illustrates a rapid increase in stress with very much less change in strain (the specimen does not bend). The point at which fracture occurs is where the plot drops to zero. Curve B represents an elastic material such as a rubber band. As a load is applied the rubber band stretches. A rapid increase in the strain occurs but the stress only rises slowly. After a period of time the rubber band passes the proportional limit.

Fig. 4.12 A sheet of glass that has been subjected to a load, causing it to fracture. The force applied therefore rises quickly to a point where the glass breaks. This is explained in Figure 4.11.

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