1.1 Introduction

The science of dental materials involves a study of the composition and properties of materials and the way in which they interact with the environment in which they are placed. The selection of materials for any given application can thus be undertaken with confidence and sound judgement.

The dentist spends much of his professional career handling materials and the success or failure of many forms of treatment depends upon the correct selection of materials possessing adequate properties, combined with careful manipulation.

It is no exaggeration to state that the dentist and dental technician have a wider variety of materials at their disposal than any other profession. Rigid polymers, elastomers, metals, alloys, ceramics, inorganic salts and composite materials are all commonly encountered. Some examples are given in Fig. 1.1 along with some of their uses in dentistry.

This classification of materials embodies an enormous variation in material properties from hard, rigid materials at one extreme to soft, flexible products at the other.

Many dental materials are fixed permanently into the patient’s mouth or are removed only intermittently for cleaning. Such materials have to withstand the effects of a most hazardous environment. Temperature variations, wide variations in acidity or alkalinity and high stresses all have an effect on the durability of materials.

Normal temperature variations in the oral cavity lie between 32ºC and 37ºC depending on whether the mouth is open or closed. The ingestion of hot or cold food or drink however, extends this temperature range from 0ºC up to 70ºC. The acidity or alkalinity of fluids in the oral cavity as measured by pH varies from around pH 4 to pH 8.5,

whilst the intake of acid fruit juices or alkaline medicaments can extend this range from pH 2 to pH 11.

The load on 1 mm2 of tooth or restorative material can reach levels as high as many kilograms indicating the demanding mechanical property requirements of some materials.

Many products, for example direct filling materials, are handled entirely by the dentist and their chairside assistant and are rarely encountered by the dental technician. Other materials are generally associated with the work of the dental laboratory and in this case both technician and dentist require a thorough knowledge of the materials in order that they may communicate about selection, manipulation and any problems which arise. A third group of materials link the dental surgery and the laboratory. The most obvious example of such products is the impression materials. Whilst the latter are under the direct control of the dentist it is essential that the dental technician also has a sound knowledge of such materials.

1.2 Selection of dental materials

The process of materials selection should ideally follow a logical sequence involving (1) analysis of the problem, (2) consideration of requirements,

(3) consideration of available materials and their properties, leading to (4) choice of material. Evaluation of the success or failure of a material may be used to influence future decisions on materials’ selection. This selection process is illustrated in Fig. 1.2. Many experienced practitioners carry out this sequence with no apparent effort since they are able to call upon a wealth of clinical experience. However, when presented with new or modified materials even the most experienced dentist

Fig. 1.2 Flow chart indicating a logical method of material selection.

Fig. 1.1 Diagram indicating the wide variety of materials used in dentistry and some of their applications.

should return to a more formal type of selection process based on the criteria mentioned.

Analysis: The analysis of the situation requiring selection of a material may seem obvious but it is of paramount importance in some circumstances. An incorrect decision may cause failure of the restoration or appliance. For example, when considering the selection of a filling material it is important to decide whether the restoration is to be placed in an area of high stress. Will it be visible when the patient smiles? Is the cavity deep or shallow? These factors and many more must be evaluated before attempting materials’ selection.

Requirements: Having completed a thorough analysis of the situation it is possible to develop a list of requirements for a material to meet the needs of that situation. For the example mentioned in the previous section, it may be decided that a filling material which matches tooth colour and is able to withstand moderately high stresses without fracture is required. Some tooth cavities are caused by toothbrush/toothpaste abrasion. In this special case the restorative material used should naturally possess adequate resistance to dentifrice abrasion. Hence, it is possible to build

a profile of the ideal properties required for the application being considered.

Available materials: The consideration of available materials, their properties and how these compare with the requirements is carried out at two levels. The dentist, faced with the immediate problem of restoring the tooth of a patient in his surgery, must choose from those materials on hand at the time. Previous experience with materials in similar circumstances will be a major factor which influences selection. On a wider scale, the practitioner is able to consider the use of alternative materials or newly developed products where these appear to offer a solution to cases which have proved difficult with his existing armoury of products. It is of paramount importance that the practitioner keeps up to date with developments in materials whilst taking a conservative approach towards adopting new products for regular use in his surgery until they are properly tested.

Choice of material: Having compared the properties of the available materials with the requirement, it is possible to narrow the choice to a given generic group of products. The final choice of material brand is often a matter of personal preference on the part of the dentist. Factors such as ease of handling, availability and cost may play a part at this stage of the selection process.

1.3 Evaluation of materials

As the number of available materials increases, it becomes more and more important for the dentist to be protected from unsuitable products or materials which have not been thoroughly evaluated. It should be emphasized, however, that most manufacturers of dental materials operate an extensive quality assurance programme and materials are thoroughly tested before being released to the general practitioner.

Standard specifications: Many standard specification tests, of both national and international standards organizations, are now available which effectively maintain quality levels for some dental materials. Such specifications normally give details for the testing of certain products, the method of calculating the results of the minimum permissible result which is acceptable. Although such specifications play a useful part they should not be seen as indicating total suitability since the tests carried out often do not cover critical aspects of the use of a material. For example, many materials fail by a fatigue mechanism in practice, but few specifications involve fatigue testing.

Laboratory evaluations: Laboratory tests, some of which are used in standard specifications, can be used to indicate the suitability of certain materials. For example, a simple solubility test can indicate the stability of a material in aqueous media – a very important property for filling materials.

It is important that methods used to evaluate materials in the laboratory give results which can be correlated with clinical experience. For example, when upper dentures fracture along the midline they do so through bending. Hence a bending or transverse strength test is far more meaningful for denture base materials than a compression test.

Clinical trials: Although laboratory tests can provide important and useful data on materials the ultimate test is the randomised controlled clinical trial and the verdict of practitioners after a period of use in general practice. Many materials produce good results in the laboratory, only to be found lacking when subjected to clinical use. The majority of manufacturers carry out extensive clinical trials of new materials, normally in co-operation with a university or hospital department prior to releasing a product for use by general practitioners.

2.1 Introduction

Many factors must be taken into account when considering which properties are relevant to the successful performance of a material used in dentistry. The situation in which the material is to be used and the recommended technique for its manipulation define the properties which characterise the material. Laboratory tests used to evaluate materials often duplicate conditions which exist in situ. This is not always possible and sometimes not desirable since one aim of in vitro testing is to predict in a rapid laboratory test what may happen in the mouth over a number of months or years. Many tests used to evaluate dental materials involve the measurement of simple properties such as compressive strength or hardness which have been shown to correlate with clinical performance.

Many materials used in dentistry are supplied as two or more components which are mixed together and undergo a chemical reaction, during which the mechanical and physical properties may change dramatically. For example, many impression materials are supplied as fluid pastes which begin to set when mixed together. The set material may be a rigid solid or a flexible rubber depending upon the chemical nature of the product.

The acceptance of such a product by the dentist depends upon the properties of the unmixed paste, the properties during mixing and setting and the properties of the set material (Table 2.1). This classification of properties applies to virtually all groups of materials.

Properties of unmixed materials: Manufacturers formulate materials which give optimal performance as evaluated by their quality assurance programme and clinical trials. It is known however, that certain products deteriorate during storage

and as a result may perform poorly. Such materials are said to have limited shelf life. Some materials have an extended shelf life if refrigerated during storage. One technique commonly employed to predict stability is to carry out accelerated ageing by storing samples at elevated temperature, commonly 60ºC, followed by evaluation of material properties.

Containers used for materials generally have a batch number stamped or printed onto them from which the date of manufacture can be obtained. Thus, for materials with limited shelf life it is possible to ascertain the date at which one would expect the properties to deteriorate.

Properties of materials during mixing, manipulation and setting: Properties of materials during mixing, manipulation and setting are considered together since they mainly involve a consideration of rheological properties and the way in which these change as a function of time during setting.

For materials of two or more components which set by a chemical reaction, thorough mixing is essential in order to achieve homogeneous distribution of properties throughout the material. The ease of mixing depends on factors such as the chemical affinity of the components, the viscosity, both of the components and the mixed material, the ambient temperature, the method of dispensation and the method of mixing.

Several methods of dispensation exist among materials used in dentistry. Some involve the mixing of powder and liquid components, others the mixing of two pastes, while others involve paste and liquid components. When the mixing of two pastes is required, the manufacturer often gives a good colour contrast between the two pastes. The achievement of a thorough mix of the two components can be judged by the attainment

Shelf life, expiry date or date of manufacture given by manufacturer.

Test that certain key properties are within acceptable limits after a period of storage.

Test reproducibility of proportioning. Test effect of proportions on properties. Test speed and completeness of mix.

Test rheological properties and ‘tackiness’.

Measure working time and setting time using meaningful tests.

Measure dimensional change during setting. Measure heat absorbed or evolved using

thermometry.

Measure mechanical properties such as strength, hardness, abrasion resistance.

Evaluate resistance to fluids such as saliva and dietary liquids.

Evaluate colour, translucency and gloss.

of a homogeneous colour with no streaks. When powder and liquid or paste and liquid are mixed, the achievement of a thorough mix is less certain. The components are mixed for a recommended time and/or until a recommended consistency is reached.

A growing number of materials are mixed mechanically. This method removes uncertainty and gives a more reproducible result.

The use of encapsulated materials which are mixed mechanically is becoming very popular. These offer the dual advantages of easier and more reproducible mixing coupled with pre-set proportions of components within the capsules.

Certain products have specified manipulative requirements which will be referred to later. For many applications, materials should be in a relatively fluid state at the time they are introduced into the patient’s mouth but should undergo rapid setting involving a change to a more rigid or rubbery form. From the commencement of mixing, two important times can be defined which have an important bearing on the acceptability of materials. The first is the working time, defined as the time available for mixing and manipulating a material. For example, an impression material should be seated in the mouth before the end of the working time otherwise setting will have proceeded sufficiently for the viscosity to have

increased considerably. The other time which characterises setting is the setting time. This, like working time, is to some extent arbitrary since it is defined as the time taken for a material to have reached a certain level of rigidity or elasticity. It is known that many materials continue setting for a considerable time after the apparent setting and optimum properties may not be achieved until several hours later.

Properties of the set material: The properties of the unmixed material and those during mixing and setting are important and may influence the practitioner’s selection. Generally, it is the properties of the set material which indicate the suitability of a product for any application. For example, in the case of a filling material, the method of dispensation, viscosity of the mixed material, working time and setting time control the ease of handling of the product, but the durability of the material in the oral environment depends on factors such as strength, solubility, abrasion resistance, etc. The properties of the set material can be conveniently divided into the following categories: mechanical properties, thermal properties, chemical properties, biological properties and miscellaneous other physical properties. Naturally, the properties relevant to any one material will depend on the application.