![]() We have to find a workable compromise between these properties. It will be found that these requirements cannot be met fully in practical materials, and to some extent are mutually incompatible. The implication here is that the impression should be removed in one ‘snap’, a quick operation that does not allow time for flow. The principal material requirement in this respect is its permanent deformation after stress, as shown, for example, by the compression set test ( 4§8). This must only be a temporary deformation, so that the original dimensions are recovered perfectly, but with the least convenient force being used to avoid pain for, or injury to, the patient. Thus, in order to remove an impression from an undercut it must be deformed, stretched, to allow the crown of the tooth to be slid out. The cervix of the tooth is narrower in at least one direction than the crown, that is, there are ‘undercuts’. Human teeth are not smoothly conical, but ‘waisted’. The reasons for this combination of properties are entirely due to the usual morphology of oral structures. ![]() ![]() The second required characteristic is that of elasticity, in conjunction with a low elastic modulus and large elastic range ( 1§2.1). For example, an error of 10% when a 1 mm distance is considered may not seem very much, but clearly over 100 mm such an error would be laughable – more than the width of a tooth. This requirement is more critical for larger devices. This means that both small scale detail and larger scale dimensional accuracy must be attained, and particularly over the full size of the region being reproduced, in other words: no distortion. All angles and distances must be preserved. The most important characteristic for an impression material is fairly obvious: it must accurately reproduce the entire surface upon which the device to be made will fit. The earliest mould material was in fact wax, soft enough that the shape of the teeth could be impressed into it, hence the term impression, even though very little pressure is used with most modern materials. For the model to be made, leaving aside the question of from what, a mould is required. The problem then is one of making the model. This used to be done when full dentures were carved from materials such as ivory, but it was quickly realized that a model of the mouth made the task much easier. Since many of the restorative devices that are used in dental treatment need to be fabricated outside of the mouth, ranging from inlays to full dentures, there are obvious problems in getting them to fit if reliance is placed on trial and error methods. A knowledge of these structures and the chemistry is essential in order to make an intelligent selection. Again, compromise is necessary to balance the various beneficial and detrimental properties.įlexible impression materials play a crucial role in the fabrication of many dental devices, from full dentures to inlays, from orthodontic appliances to implanted prostheses. The many sources of permanent deformation after exposure to a stress mean that perfection is unattainable in practice. These two types are sensitive to water gain and loss. Two polysaccharide materials, agar and alginate, are strongly dependent on hydrogen bonding for network formation alginate relies on a chelation mechanism as well. Polysulphide, polyether and silicone impression materials all give covalently-bonded networks when set, although each has its own particular dimensional stability problems arising from their chemistry. The properties are also dependent on the nature of the filler. However, the large strains that these materials must endure may result in a structural breakdown called strain-softening. The good as well as the bad points of each must be taken into account in the selection of the type of product and its manipulation for the task in hand.Īll flexible mould materials are polymeric and must be cross-linked to be rubber-like, that is, three-dimensional random networks, and they are markedly non-Hookean in their deformation behaviour this can be traced to features of their structure.įillers are again often important in order to obtain adequate viscosity in the unset material, and appropriate stiffness when it is set. This chapter describes a range of common types of dental impression material and how they meet – or fail to meet – those demands. No perfect solution has been found, although many attempts have been made. It includes severe requirements for flow, setting in a reasonable time and perfect elasticity as well as absence of dimensional changes on setting and after. The problem of designing a mould material for making dental models is formidable.
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