Chapter 12 Other dental cements
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:
|Phosphoric acid||Zinc oxide|
|Poly(acrylic acid)||Magnesium oxide|
|Maleic acid||Copper oxide|
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.
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
Zinc phosphate cements have been used in dentistry for many decades. The material is generally supplied as zinc oxide powder and phosphoric acid liquid. Other chemicals are added to both the powder and liquid to modify the properties of the product (see Table 12.2).
These chemicals derive from the tin ion, stannum (Sn). When this ion reacts with the fluoride ion, stannous fluoride (SnF2) and stannic fluoride (SnF4) are produced. The difference between the two is that tin exists in two valent states. These compounds are often added to dental cements to convey caries resistance to the tooth in which they are placed as they leach fluoride. Stannous fluoride is more commonly used. Occasionally, there is a reference to ‘tannin fluoride’. This refers to a mixture of tannic acid and a fluoride salt, which used to be known commercially as the HY agent. It purports to cause remineralization of dentine, but has not been mentioned in the literature for the past decade. Addition of any of these materials to zinc phosphate cement is at the expense of the cement matrix formation and so will weaken the cement. Their inclusion does, however, impart some cariostatic properties to the cements.
Setting reaction and structure
The setting reaction of zinc phosphate cements is a two-stage process. In the first part of the reaction, the zinc oxide in the powder reacts with the phosphoric acid in the liquid to form zinc phosphate and water. This newly formed zinc phosphate reacts with more zinc oxide, forming hopeite. This compound is hydrated zinc phosphate. Aluminium when present prevents crystallization and permits the formation of an amorphous cement, which is very similar in structure to the glass ionomer cement (see Chapter 9).
This matrix is almost insoluble but the set cement is porous, making it highly permeable as water which was not consumed in the reaction forms globules within the material. The water produced during the setting reaction is in both free and bound form within the cement. While the water exists in its free form the cement is weaker and more soluble. As the cement matures (the maturation phase), water is bound more strongly into the cement, leading to a stronger and less soluble cement.
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.
Fig. 12.3 (A) Zinc oxide powder produced from zinc oxalate as a result of sintering. (B) Note the reduction in size and the alteration in shape achieved by raising the temperature to below the melting point of the solid.
(From Prosser and Wilson 1982, J of Biomedical Materials Research 16, 585–98).
Densify: to heat a product so that its particles coalesce but no ingredients are lost in the process. This can be compared to melting an Aero bar (Nestlé). None of the chocolate is lost, but as its constituents come closer together as a result of the heating, its volume decreases.
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.
Depending on the size of the particles of zinc oxide, the cement lute gained with this material is the thinnest of all the currently available luting agents. Generally quoted figures state that the cement lute is approximately 25 μm (range 15–40 μm).
Mode of retention
Zinc phosphate cements do not adhere to either tooth tissue or restorative materials. They function by grouting the potential space between the cast and tooth preparation. The cement forms tags between the micro-irregularities on the two surfaces being luted. For this reason these surfaces should not be polished. In fact, sandblasting the fitting surface of the cast restoration prior to luting will increase retention.
This material has a low solubility. However, any change in the water content of the liquid component can adversely affect the set cement. That is, there is crystallization of the matrix, which leads to a weakening of the cement structure.
While the solubility of the cement is low, it further reduces with age as the cement matures and the water becomes bound into the matrix. The set cement starts to erode in an acid medium when the pH drops below 4.5, and the erosion becomes more marked with increasing acidity. In the clinical scenario, the set cement exposed to the oral environment will be subjected to cyclical changes in pH, which frequently falls below 4.5 (see Chapter 1).
The material’s viscosity increases as setting starts. To enable full seating of the cast restoration and to achieve the optimal film thickness, the dentist must either maintain the seating pressure on the cast restoration or vent the restoration, especially if it is a full crown.
Inclusion of fluoride into the cement may impart some cariostatic properties. This is borne out clinically as little caries is seen under inlays when the cement has washed out over time. The addition of the fluoride salt does, however weaken the cement because it is present as inclusions which disrupt the matrix.
The mechanical properties of the final cement are dependent on the powder/liquid ratio used. The mechanical properties increase with increased powder content until a point is reached where the material will not mix to a homogeneous mass. The more powder that is incorporated, the thicker the cement lute that is produced. The consistency of the mixed cement produced will depend on the clinical indication. Table 12.3 sets out the effects of some handling variables on the properties of the final cement.
Zinc phosphate, zinc polycarboxylate and zinc oxide eugenol cements have a common base, zinc oxide. However, the powder supplied by the manufacturer usually has a different chemical composition depending on the material. In other words, the powder is not composed solely of zinc oxide. These powders should therefore not be used interchangeably as mixing and matching them will produce a substandard product.
Effect of the acid
Phosphoric acid is highly acidic and for this reason there has been concern that its acidity may have detrimental effects on the pulp, such as pulpal inflammation and possibly pulpal death. Clinically this would manifest as postoperative sensitivity or pain. The more liquid that is used, the greater is the initial acidity of the cement. In very runny mixes the pH could be as low as 2. Stiffer mixes produce an initial pH of about 3. It is known that the pH approaches neutrality with time. However, the more liquid used, the longer it takes for the pH to rise. Generally the pH is higher than 4 at 60 minutes after mixing and returns to neutral after 24 hours. The moisture in dentine can have a buffering effect.
There is still no evidence to confirm whether the incidence of pulpal death or postoperative pain is higher with these cements when compared with other cements. This is not particularly surprising in that the acidic nature of the cement is not very different from that of the etching gels used currently as part of the adhesive process with resin-based filling materials. The pulpal reactions which have been reported are more likely to be attributable to the porosity of the cement and consequential bacterial leakage. However, if the clinician is concerned, they may choose to apply a cavity varnish (see p. 181) as a barrier in an attempt to reduce or eliminate postoperative sensitivity. Alternatively, another material could be selected as a base.
Advantages and disadvantages
|Easy to mix||Possibly irritant to the pulp|
|Sharp set||Does not bond to tooth tissue or restorative materials|
|Acceptable properties for purpose||Brittle|
|Cheap||No antibacterial effects|
|Long successful track record||Soluble in the mouth|
The set material is opaque and thus it may shine through all ceramic restorations, which can compromise the aesthetic result. Another luting material may therefore be more appropriate for use in this situation if this is a potential concern.
Indications and 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
|De Trey Zinc||Dentsply|
|Hy-Bond Zinc Phosphate Cement||Shofu|
|Zinc Cement||SS White|
|Zinc Phosphate Cement||Heraeus|
It is very important that the cement is mixed correctly as the properties of the cement will be determined by the reaction between the powder and liquid. The recommended method of mixing zinc phosphate cement to lute a cast restoration is illustrated in Box 12.1 and should be followed precisely.
The bottle containing the liquid should only be open to the atmosphere for the shortest possible time necessary to dispense the liquid. If the stopper is left off for any extended period of time, water will either be lost or gained depending on the humidity and temperature of the surrounding environment. This will adversely affect the setting reaction and the final physical properties of the cement. If the liquid is cloudy or crystals are present in the bottle, the material should be discarded, as the concentration of the acid will have changed and will no longer be optimal.
The dental nurse should not continue to mix the cement after its viscosity has started to increase. This is because it means that the zinc phosphate matrix is starting to form (i.e. the material is beginning to set). Further mixing at this stage will significantly weaken the cement by disrupting the salt matrix formation.
Zinc Polycarboxylate (Polyalkenoate/Polyacrylate) Cements
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.
Powder manufacturing process
The most common method of preparing the powder is to heat the two main ingredients for 9 hours to sinter the constituents. This is similar to the manufacture of zinc oxide powder for the zinc phosphate cements. The sintered mass is ground to reduce particle size and then reheated for a further 8–12 hours. Pigments are added to control shades.
Other chemicals (such a stannous or strontium fluoride) have been added to some products in an attempt to confer the benefits of fluoride leaching. The fluoride release is transitory and usually stops within 30 days. Stannous fluoride also enhances the mixing and mechanical properties and the set of the cement.
Setting reaction and structure
When the zinc oxide and poly(acrylic acid) come into contact, a salt (zinc polyacrylate) matrix is formed only on the surface of the zinc oxide particles. Poly(acrylic acid) chains cross-link through the zinc ions of the zinc oxide. The set material has cores of zinc oxide within the zinc polyacrylate matrix binding the unreacted zinc oxide cores together as previously described.
The mixed cement goes through a rubbery phase as it is setting and it should not be disturbed during this phase. This is because the adhesive bond of the cement will be ruptured. There is a risk that microleakage will then occur if the cement has been pulled away from the tooth surface. The cement should be left undisturbed until it can be fractured away cleanly from the margin of a cemented restoration.
The compressive strength of zinc polycarboxylate is less than that of zinc phosphate cement. Once set, the cement has a lower modulus and is therefore slightly more elastic and less likely to fracture under heavy load. While it appears thick after mixing, the cement is more fluid and it exhibits shear thinning under load allowing restorations to be seated completely under pressure.
Poly(acrylic acid) is a weak acid with a relatively high molecular weight. As such it will not diffuse readily along the dentinal tubules and is rapidly immobilized in them. The cement’s pH returns to neutral within a short period of time after mixing and the cement is less acidic than zinc phosphate. The biological compatibility of the cement with the pulp is regarded as being excellent. There is some release of zinc fluoride and poly(acrylic acid) but these do not appear to affect the tissues in clinical use.
Mixing these cements has proved to be difficult as the viscous nature of the poly(acrylic acid) when in liquid form often results in less powder being incorporated into the mix than is recommended by the manufacturer. This in turn results in inferior properties. One reason that the materials which contain vacuum-dried poly(acrylic acid) were produced was to improve the mixing properties of the cements. This has enhanced the mixing properties to some extent but these cements are still notoriously difficult to mix. The material’s viscosity increases as setting starts and the material stretches if disturbed with a probe before setting is complete. It returns to its original position but the seal between restoration and tooth will be compromised. This effect is termed rebound.
Film thickness has been reported to vary from 20 to 100 μm but this is dependent on the plasticity of the cement at the time of placement. The longer the material has been mixed, the lower the plasticity and hence the thicker the cement lute. The minimum cement lute thickness is governed by the particle size of the cement powder.
Solubility of the cement is relatively low, ranging from 0.1% to 0.6% after 24 hours. One of the disadvantages of the addition of stannous fluoride is that it tends to increase the dissolution. This process appears to start at the site of the zinc oxide particles and the solubility increases with increasing concentrations of magnesium oxide in the powder.