12: Other dental cements

Chapter 12 Other dental cements

Introduction

This chapter deals with the other, more traditional, cements that dentists have used for many years. While their use has diminished to some extent in recent years as a result of the development of new materials, traditional cements are still available and widely used. It is therefore important that their composition, uses and handling characteristics are still understood by the dental team.

Essentially, these cements are varying permutations of a combination of an acidic liquid and a basic powder (Table 12.1), which, when mixed together, form a salt plus water. The acids and bases react to form different materials. The permutations of these interactions are illustrated in Figure 12.1. Glass ionomer cement has already been discussed in detail in Chapter 9. Silicate cement is no longer used in modern dentistry and therefore there is no need to consider it further. The other cement permutations are:

Table 12.1 Acids and bases participating in the formation of traditional dental cements

Acids Bases
Phosphoric acid Zinc oxide
Poly(acrylic acid) Magnesium oxide
Maleic acid Copper oxide
Eugenol Fluoro-alumino-silicate glass
Mellitic acid  

As they are derived from similar building blocks, these products have common principles of use and share many properties as might be expected.

Zinc oxide eugenol cement has further been modified with the addition of other active ingredients, marketed as a steroid/antibiotic cement. This modified cement and other materials such as setting calcium hydroxide cements and cavity varnishes are also discussed in this chapter.

General Structure

Zinc phosphate, zinc polycarboxylate and zinc oxide eugenol cements have the same structure (Figure 12.2). This is because these products have the same base (zinc oxide) and the different cements are produced by the reaction of this base with different acids. In each case, only the surface of the zinc oxide particles will react with the acid when mixed. The set cement is therefore composed of unreacted cores of zinc oxide powder surrounded by the matrix of the reaction product, namely zinc phosphate, zinc polycarboxylate or zinc eugenolate, respectively.

Zinc Phosphate Cements

Properties

Exothermic reaction and factors affecting the speed of set

The chemical reaction of zinc phosphate cement is the most exothermic of all the dental cements and so there is a potential risk that the heat produced during the setting reaction could cause pulpal inflammation. There are two methods by which manufacturers attempt to reduce this:

Heat treatment of the powder causes granulation which, as the name suggests, converts the product into granules. The product is then sintered with other less reactive oxides (Figure 12.3) before being ground to a fine powder. The smaller the particles are ground, the greater is their reactivity.

As well as varying the particle size, the reactivity of the powder also depends on the degree of calcination and its composition.

Mixing on a cooled glass slab permits the heat produced in the reaction to dissipate more easily and slows the chemical reaction, providing the dentist with a longer time for manipulation of the cement. Care must be taken not to cool the slab below the dew point or water condensation will form and affect the properties of the cement (Figure 12.4). Further regulation of the rate of setting is possible by varying the rate of incorporation of the powder to the liquid. The slower the incorporation of the powder fraction, the more slowly the cement will set. This is partly due to the fact that there is a reduction in temperature rise observed as each increment of cement powder starts to react at a different time. All of these factors may be used in the clinic to control the setting time of the cement.

Indications and contraindications

Indications and contraindication are set out in Table 12.5.

Table 12.5 Indications and contraindications of zinc phosphate cement

Indications Contraindications
Definitive cementation of inlays, metal-based crowns, metal-based bridges and orthodontic bands Definitive cementation of all ceramic crowns and bridges
As a base When in very close proximity to the pulp without another intermediate material such as calcium hydroxide
Temporary restorations where adequate retention is present  

Commercially available products

Commercially available products are shown in Table 12.6.

Table 12.6 Some zinc phosphate cements currently available on the market

Product name Manufacturer
De Trey Zinc Dentsply
Hy-Bond Zinc Phosphate Cement Shofu
Zinc Cement SS White
Zinc Phosphate Cement Heraeus

Zinc Polycarboxylate (Polyalkenoate/Polyacrylate) Cements

Chemical constituents

When zinc oxide powder is mixed with poly(acrylic acid) liquid, zinc polycarboxylate cement is produced, this being the primary chemical reaction. The cement is also referred to as zinc polyacrylate or strictly speaking, in modern chemical terminology, zinc polyalkenoate.

Most products are presented as a powder, which is primarily zinc oxide (Table 12.7). The powder is mixed with the liquid, a viscous solution of poly(acrylic acid). Some manufacturers, however, vacuum dry the poly(acrylic acid) and add it to the zinc oxide powder. In these products the setting reaction is initiated by the addition of water to the powder.

The advantage of blending the poly(acrylic acid) and zinc oxide together in one composition is that the component will be in optimized proportions for use. The disadvantage is that the powder must be kept completely dry or the setting reaction will commence prematurely. In climates where the humidity is high this can be a major problem even with a desiccant being incorporated into the storage bottle cap (see Figure 9.19, p. 114).

In those presentations where the poly(acrylic acid) is in an aqueous form there is a risk that loss of water from the solution will cause thickening of the mixture making mixing more difficult. It also adversely affects the concentration of the acid solution. It is essential that the liquid container is kept sealed at all times except during dispensation.

Properties

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 12: Other dental cements
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